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Fact No	Subject	Verb	Object	Weight
1	Human activities	have led	to large increases in heat-trapping gases over the past century	1
2	global warming of the past 50 years	is	due primarily to human-induced increases in heat-trapping gases	1
3	Human "fingerprints"	have been identified	in many other aspects of the climate system, including changes in ocean heat content, precipitation, atmospheric moisture, and Arctic sea ice	7
4	climate	is	to those emissions	5
5	Global temperatures	are projected	to continue to rise over this century	1
6	This introduction to global climate change	has been happening	to the world's climate and why	9
7	this report	focuses	on climate change impacts in the United States	5
8	impacts	requires	understanding of the global climate system	4
9	this report	understanding	changes	1
10	changes	have been observed	in global climate over the past century	5
11	Antarctic ice core	extending back	800,000 years	1
12	air bubbles	trapped	in an Antarctic ice core	1
13	nature and causes of these changes	have been	comprehensively chronicled in a variety of recent reports, such as those by the Intergovernmental Panel on Climate Change	5
14	This section	does not intend	to documents the Earth's changing carbon dioxide concentration	1
15	Temperature-related data but rather to provide a brief synthesis, make clear that these variations have played a central role in determining the global and to integrate more recent	work	with climate	4
16	present carbon	dioxide	concentration of the assessments of the IPCC, CCSP, and others	1
17	Human activities	have led	to large increases in heat-trapping gases over the past century	1
18	Earth's climate	depends	on the functioning of a natural "greenhouse effect	5
19	This effect	is	result of heat-trapping gases	1
20	This effect	is	also known as greenhouse gases	1
21	This effect	is	water vapor, carbon dioxide, ozone, methane, and nitrous oxide, which absorb heat radiated from the Earth's surface and lower atmosphere and then radiate much of the energy back toward the surface	2
22	average surface temperature of the Earth	would be	about 60°F colder	1
23	human activities	have been releasing	additional heat-trapping gases, intensifying the natural greenhouse effect, thereby changing the Earth's climate	5
24	Climate	is influenced	by a variety of factors, both human-induced and natural	4
25	principal factor	causing	warming over the past 50 years	1
26	increase in the carbon dioxide concentration	has been	principal factor	1
27	concentration	has been building up	in the Earth's atmosphere since the beginning of the industrial era in the mid-1700s, primarily due to the burning of fossil fuels	1
28	Human activities	increased	emissions of other greenhouse gases, such as methane, nitrous oxide, and halocarbons	2
29	emissions	causing surface temperatures	to rise	1
30	emissions	are thickening	causing surface temperatures	1
31	emissions	are thickening	blanket of heat-trapping gases in Earth's atmosphere	2
32	Carbon dioxide concentration	has increased	due to the use of fossil fuels in electricity generation, transportation, and industrial and household uses	1
33	Deforestation	provides	source of carbon dioxide	1
34	about 20 percent	resulted	from deforestation and associated agricultural practices	1
35	about 80 percent of human-induced carbon dioxide emissions	came	from the burning of fossil fuels	1
36	concentration of carbon dioxide in the atmosphere	has increased	by roughly 35 percent since the start of the industrial revolution	1
37	Methane concentration	has increased	mainly as a result of agriculture	1
38	Methane concentration	has increased	raising livestock	1
39	About 70 percent of the emissions of atmospheric methane	are	now related to human activities	1
40	Halocarbon emissions	come	from the release of certain manufactured chemicals to the atmosphere	1
41	presence in the atmosphere	was found	to cause stratospheric ozone depletion	1
42	international regulations	designed	to protect the ozone layer	1
43	abundance of these gases in the atmosphere	decreasing	as a result of international regulations	1
44	Continued decreases in ozone-deplet-ing halocarbon emissions	are expected	to reduce their relative influence on climate change in the future	6
45	halocarbon Ozone	produced	atmosphere by chemical reactions	1
46	halocarbon Ozone	destroyed	in the atmosphere by chemical reactions	1
47	halocarbon Ozone	is	greenhouse gas	1
48	human activities	have increased	ozone concentration through the release of gases such as carbon monoxide, hydrocarbons, and nitrogen oxides	2
49	chemical reactions	produce	ozone in the presence of sunlight	1
50	excess ozone in the troposphere	causes	respiratory illnesses and other human health problems	1
51	layer above the troposphere, ozone	protects	life on Earth from exposure to excessive ultraviolet radiation from the Sun	1
52	halocarbons	have caused	ozone hole over Antarctica	1
53	halocarbons	destroy	ozone in the stratosphere	1
54	Changes in the stratospheric ozone layer	have contributed	to changes in wind patterns and regional climates in Antarctica	2
55	Water vapor	is	most important and abundant greenhouse gas in the atmosphere	1
56	Human activities	produce	only a very small increase in water vapor through irrigation and combustion processes	1
57	surface warming	caused	by human-produced increases in other greenhouse gases	1
58	warmer climate	allows	atmosphere to hold more moisture	4
59	warmer climate	increases	evaporation	4
60	surface warming	leads	to an increase in atmospheric water vapor	1
61	human activities	produce	additional local and regional effects	1
62	others	increase	warming	1
63	warming	caused	by greenhouse gases	1
64	tiny particles	called	aerosols	1
65	One such influence on climate	is caused	by tiny particles	5
66	burning of coal	produces	emissions of sulfur-containing compounds	1
67	sulfate aerosol" particles	reflect	incoming sunlight away from the Earth	1
68	sulfate aerosol" particles	causing	cooling influence at the surface	1
69	Sulfate aerosols	tend	to make clouds more efficient at reflecting sunlight, causing an additional indirect cooling effect	1
70	type of aerosol	referred	to as soot or black carbon	1
71	type of aerosol	absorbs	incoming sunlight and traps heat in the atmosphere	1
72	warming	caused	by increased levels of greenhouse gases	1
73	aerosols	mask	increase the warming	1
74	warming	caused	by heat-trapping gases	1
75	sum of these aerosol effects	offsets	warming	1
76	gases and particles	remain	in the atmosphere	1
77	elevated concentrations of aerosols	persist	for days to weeks	1
78	atmospheric concentration of carbon dioxide	remains	elevated for thousands of years, and that of methane for decades	1
79	climate effects of reductions in emissions of carbon dioxide and other long-lived gases	do not become	apparent for at least several decades	6
80	geographic patterns of their climatic influence and the resulting surface temperature responses	are	quite different	1
81	reductions in emissions of short-lived compounds	can have	rapid, but complex effect	1
82	greatest emissions of short-lived pollutants in summertime by late this century	are projected	to come from Asia	1
83	strongest climate response	is projected	to be over the central United States	4
84	heat	is reflected	absorbed by the surface	1
85	Human activities	changed	land surface in ways that alter	1
86	changes	include	cutting and burning of forests, the replacement of other areas of natural vegetation with agriculture and cities, and large-scale irrigation	1
87	transformations of the land surface	can cause	warming or cooling	1
88	important natural factors	influence	climate: the Sun and volcanic eruptions	4
89	net effect of these changes	been	slight cooling of the Earth's surface over the past Two	2
90	human influences on climate	have become	increasingly obvious	4
91	Sun's energy output (as measured by satellites since 1979)	has followed	historical 11-year cycle of small ups and downs, but with no net increase	1
92	short-term cooling effects on climate	lasting	2 to 3 years	5
93	two major volcanic eruptions of the past 30 years	have had	short-term cooling effects on climate	5
94	net effect on climate	been	slight cooling influence over this period	5
95	natural factors	can not explain	warming of recent decades	1
96	Slow changes in Earth's orbit around the Sun and its tilt toward or away from the Sun	are	only important on timescales from thousands to many tens of thousands of years	1
97	Slow changes in Earth's orbit around the Sun and its tilt toward or away from the Sun	are	also a purely natural influence on climate	6
98	climate changes	have occurred	over the last century	5
99	climate changes	are not	solely caused by the human and natural factors	5
100	fluctuations in climate	occur	even in the absence of changes in human activities, the Sun, or volcanoes	5
101	One example	is	El Niño phenomenon, which has important influences on many aspects of regional and global climate	4
102	effects on climate	occur	at the same time as the effects of human activities, the Sun, and volcanoes	5
103	other modes of variability	have been identified	by climate scientists	5
104	carbon dioxide	is emitted	to the atmosphere	1
105	it	is absorbed	by the oceans and taken up by vegetation	1
106	carbon dioxide	emitted	by human activities in the last 50 years	1
107	About 45 percent of the carbon dioxide	is	now stored in the oceans and vegetation	1
108	rest	has remained	increasing the atmospheric concentration	1
109	rest	has remained	in the air	1
110	carbon	tends	to be released, for instance, from thawing permafrost	1
111	more carbon release	leads	to more warming which leads to further release, and so on	1
112	carbon	tends	initiating a feedback loop	1
113	it	is known	from long records of Earth's climate history	4
114	carbon dioxide	observed	in the atmosphere	1
115	increasing emissions of carbon dioxide	are	primary cause of the increased concentration of carbon dioxide	2
116	growth rate	increased	from 1.3 percent per year in the 1990s to 3.3 percent per year between 2000 and 2006.21	1
117	greater fraction	remaining	in the atmosphere	1
118	ocean	absorbs	carbon dioxide from the atmosphere	1
119	process	referred	to as ocean acidification	1
120	carbon dioxide concentrations	continue	to increase	1
121	ocean acidification	is	essentially irreversible over a time scale of centuries	1
122	living things	create	shells and skeletons, with substantial negative consequences for coral reefs, mollusks, and some plankton species important to ocean food chains	1
123	ocean acidification	affects	process of calcification	1
124	estimated change in the average temperature of Earth's surface	is based	on measurements from thousands of weather stations, ships, and buoys around the world	3
125	Global average surface air temperature	has increased	substantially since 1970.26	1
126	Global average surface air temperature	has increased	satellites	1
127	measurements	are	independently compiled, analyzed, and processed by different research groups	1
128	instruments	used	to measure temperature, the measurement times and locations, the local environment around the measuring site, and such factors as satellite orbital drift	1
129	growth of cities	can cause	localized "urban heat island" effects	1
130	number of research groups around the world	have produced	estimates of global-scale changes in surface temperature	2
131	warming trend	is	apparent in all of these temperature records	1
132	warming trend	is confirmed	by other independent observations, such as the melting of Arctic sea ice, the retreat of mountain glaciers on every continent,27 reductions in the extent of snow cover, earlier blooming of plants in spring, and increased melting of the Greenland and Antarctic ice sheets	1
133	snow and ice	reflect	Sun's heat	1
134	this melting	causes	resulting in another feedback loop	1
135	this melting	causes	more heat to be absorbed, which causes more melting	1
136	temperature measurements above the surface	have been made	by weather balloons since the late 1940s, and from satellites since 1979	1
137	measurements	show	warming of the troposphere, consistent with the surface warming	1
138	They	reveal	cooling in the stratosphere	1
139	atmospheric temperature	would be expected	to change in response to increasing greenhouse gas concentrations and the observed depletion of stratospheric ozone	2
140	This pattern of tropospheric warming and stratospheric cooling	agrees	with our understanding	1
141	Precipitation	is not distributed	evenly over the globe	1
142	average distribution	is governed	primarily by atmospheric circulation patterns, the availability of moisture, and surface terrain effects	1
143	first two of these factors	are influenced	by temperature	1
144	human-caused changes in temperature	are expected	to alter precipitation patterns	2
145	Observations	show	that such shifts are occurring	1
146	Changes	have been observed	in the amount, intensity, frequency, and type of precipitation	1
147	Pronounced increases in precipitation over the past 100 years	have been observed	in eastern North America, southern South America, and northern Europe	1
148	Decreases	have been seen	in the Mediterranean, most of Africa, and southern Asia	1
149	Changes in the geographical distribution of droughts and flooding	have been	complex	1
150	northern regions and mountainous areas	are experiencing	falling as rain rather than snow	1
151	northern regions and mountainous areas	are experiencing	more precipitation	1
152	Widespread increases in heavy precipitation events	have occurred	even in places where total rain amounts have decreased	1
153	more water vapor	evaporating	from the world's oceans and land surface	1
154	changes	are associated	with the fact that warmer air holds	1
155	This increase in atmospheric water vapor	is	primarily due to human influences	1
156	This increase in atmospheric water vapor	has been observed	from satellites	1
157	sea level	rose	by roughly 8 inches over the past century	1
158	rate	observed	over the past century	1
159	Satellite data available over the past 15 years	show	rising at a rate roughly double the rate	1
160	Satellite data available over the past 15 years	show	sea level	1
161	global warming	causes	to rise	1
162	global warming	causes	sea level	1
163	Warming	has been	directly linked to human influences	1
164	Warming	has been observed	in each of the world's major ocean basins	1
165	warming	leads	to the melting of glaciers and ice sheets, which raises sea level by adding water to the oceans	1
166	rate of retreat	has increased	in the past decade	1
167	Glaciers	have been retreating	worldwide for at least the last century	1
168	increased precipitation	has outpaced	melting	1
169	Only a few glaciers	advancing	in locations that were well below freezing	1
170	progressive disappearance of glaciers	has	implications not only for the rise in global sea level, but also for water supplies in certain densely populated regions of Asia and South America	1
171	Earth	has	major ice sheets on Greenland and Antarctica	1
172	ice sheets	calving	of icebergs	1
173	ice sheets	contributing	to sea-level rise	1
174	ice sheets	losing	ice volume by increased melting	1
175	Greenland Ice Sheet	been experiencing	record amounts of surface melting, and a large increase in the rate of mass loss in the past decade	1
176	it	would raise	sea level by about 20 feet	1
177	Antarctic Ice Sheet	consists	of two portions, the West Antarctic Ice Sheet and the East Antarctic Ice Sheet	1
178	West Antarctic Ice Sheet	contains	enough water to raise global sea levels by about 16 to 20 feet	1
179	it	would raise	global sea level by about 200 feet	1
180	past climate records	increases	in sea level	5
181	past climate records	provide	precedent for very significant decreases in ice volume	5
182	Complete melting of these ice sheets over this century or the next	is thought	to be virtually impossible	1
183	global warming of the past 50 years	is	due primarily to human-induced increases in heat-trapping gases	1
184	Human "fingerprints"	have been identified	in many other aspects of the climate system, including changes in ocean heat content, precipitation, atmospheric moisture, and Arctic sea ice	7
185	IPCC Second Assessment Report44	concluded	that "the balance of evidence suggests a discernible human influence on global climate	5
186	number of national and international assessments	have come	to much stronger conclusions about the reality of human effects on climate	4
187	Recent scientific assessments	find	that most of the warming of the Earth's surface over the past 50 years has been caused by human activities	1
188	scientific evidence for a human influence on global climate	has accumulated	over the past several decades, from many hundreds of studies	5
189	No single study	is	smoking gun	1
190	evidence	supporting	conclusion that human activity is the primary driver of recent warming	1
191	any single study or combination of studies	undermined	large body of evidence	1
192	climate system	responds	to increases in greenhouse gases	5
193	other human and natural factors	influence	climate	4
194	first line of evidence	is	basic physical understanding of how greenhouse gases trap heat	1
195	second line of evidence	is	from indirect estimates of climate changes over the last 1,000 to 2,000 years	5
196	records	are obtained	from living things and their remains	1
197	lesson from these data	is	that global surface temperatures over the last several decades are clearly unusual, in that they were higher than at any time during at least the past 400 years	1
198	recent temperature rise	is	clearly unusual in at least the last 1,000 years	1
199	climate	would be expected	to change in response to human activities	4
200	third line of evidence	is based	on the broad, qualitative consistency between observed changes in climate and the computer model simulations	5
201	climate models	are run	with historical increases in greenhouse gases	5
202	they	show	gradual warming of the Earth and ocean surface, increases in ocean heat content and the temperature of the lower atmosphere, a rise in global sea level, retreat of sea ice and snow cover, cooling of the stratosphere, an increase in the amount of atmospheric water vapor, and changes in large-scale precipitation and pressure patterns	4
203	other aspects of modeled climate change	are	in agreement with observations	5
204	factor	affects	climate	4
205	person	has	unique fingerprint	1
206	factor	produces	unique pattern of climate response	4
207	Fingerprint studies	allow	detailed comparisons of modeled and observed climate change patterns	5
208	Fingerprint studies	exploit	unique signatures	1
209	Scientists	rely	to attribute observed changes in climate to a particular cause or set of causes	5
210	Scientists	rely	on such studies	1
211	climate changes	have occurred	since the start of the Industrial Revolution	5
212	climate changes	are	due to a complex mixture of human and natural causes	5
213	importance of each individual influence in this mixture	changes	over time	1
214	multiple Earths	would allow	experimenter to change one factor at a time on each Earth	1
215	multiple Earths	helping	to isolate different fingerprints	1
216	individual factors	affect	climate	4
217	climate models	are used	to study	4
218	climate model simulations of the last century	include	all of the major influences on climate, both human-induced and natural	9
219	they	can reproduce	important features of observed climate change patterns	5
220	human influences	are removed	from the model experiments	1
221	results	suggest	that the surface of the Earth would actually have cooled slightly over the last 50 years	1
222	observed warming over the last half-century can-not	caused	primarily by human factors	1
223	clear message from fingerprint studies	is	that the observed warming over the last half-century can-not be explained by natural factors	1
224	fingerprint of human effects on climate	has been identified	by looking at a slice through the layers of the atmosphere, and studying the pattern of temperature changes from the surface up through the stratosphere	6
225	increases in carbon dioxide	cause	warming at the surface and in the troposphere, but lead to cooling of the stratosphere	1
226	models	calculate	that the human-caused depletion of stratospheric ozone has had a strong cooling effect in the stratosphere	1
227	most of the observed temperature change	had been	due to an increase in solar output rather than an increase in greenhouse gases	3
228	Earth's atmosphere	would have warmed	throughout its full vertical extent, including the stratosphere	1
229	tern of atmospheric temperature changes, with its pronounced cooling in the stratosphere	is	therefore inconsistent with the hypothesis that changes in the Sun can explain the warming of recent decades	2
230	direct satellite measurements of solar output	show	slight decreases during the recent period of warming	1
231	earliest fingerprint work51	focused	on changes in surface and atmospheric temperature	1
232	Scientists	applied	fingerprint methods to a whole range of climate variables,50,52 identifying human-caused climate signals in the heat content of the oceans,38,39 the height of the tropopause53	5
233	Studies	published	after the appearance of the IPCC Fourth Assessment Report in 2007	1
234	Studies	found	human fingerprints in the increased levels of atmospheric moisture35,36	1
235	changes in temperature, ice extent, moisture, and circulation patterns	fit	together in a physically consistent way, like pieces in a complex puzzle	2
236	message from this entire body of work	is	that the climate system is telling a consistent story of increasingly dominant human influence	4
237	this type of fingerprint work	is shifting	emphasis	1
238	clear and compelling scientific evidence	supports	case for a pronounced human influence on global climate	4
239	Much of the recent attention	is	now on climate changes at continental and regional scales,64,65 and on variables that can have large impacts on societies	5
240	scientists	have established	causal links between human activities and the changes in snowpack, maximum and minimum temperature, and the seasonal timing of runoff over mountainous regions of the western United States	1
241	Human activity	have made	substantial contribution to ocean surface temperature changes in hurricane formation regions	2
242	Researchers	are beginning	to tie changes in the distribution and seasonal behavior of plant and animal species to human-caused changes in temperature and precipitation	1
243	Researchers	looking	beyond the physical climate system	4
244	all models	predicted	that with a rise in greenhouse gases, the troposphere would be expected to warm more rapidly than the surface	2
245	Observations from weather balloons, satellites, and surface thermometers	seemed	to show the opposite behavior	1
246	This issue	was	stumbling block in our understanding of the causes of climate change	5
247	Research	showed	that there were large uncertainties in the satellite and weather balloon data	1
248	uncertainties in models and observations	are	properly accounted for	1
249	newer observational data sets (with better treatment of known problems)	are	in agreement with climate model results	5
250	aspects of precipitation,54,77 and patterns of surface pressure,56	appear	to be proceeding much more rapidly than models have projected	1
251	observed changes	studied	to date	1
252	heat-trapping gases	resulting	from human activities	1
253	climate system	would be expected	to respond to the increase in heat-trapping gases	5
254	most of the observed changes	are	also consistent with our scientific understanding	1
255	bottom-line conclusion from climate fingerprinting	is	that most of the observed changes studied to date are consistent with each other	4
256	Scientists	are	sometimes asked whether extreme weather events can be linked to human activities	1
257	Scientific research	has concluded	that human influences on climate are indeed changing the likelihood of certain types of extreme events	4
258	it	would have been	in the absence of human-induced climate change	5
259	analysis of the European summer heat wave of 2003	found	that the risk of such a heat wave is now roughly four times greater	1
260	analyses of human-caused changes in the risks of extreme events	rely	on information from climate models, and on our understanding of the physics of the climate system	5
261	models	used	in this work	1
262	All of the models	have imperfections	in their representation of the complexities of the "real world" climate system	5
263	models	are	extremely useful, for a number of reasons	1
264	current generation of climate models	portrays	important aspects of today's weather patterns and climate	9
265	Models	are	routinely tested against many observations of Earth's climate system	5
266	fingerprint work	shows	that models capture not only our present-day climate, but also key features of the observed climate changes over the past cent ur y.	5
267	large-scale observed cli- mate changes (such as the warming of the surface and troposphere, and the increase in the amount of moisture in the atmosphere)	are driven	by very basic physics, which is well-represented in models	2
268	climate models	can be used	to predict changes in climate that can be verified in the real world	10
269	Examples	include	short-term global cooling subsequent to the eruption of Mount Pinatubo and the stratospheric cooling with increasing carbon dioxide	1
270	models	are	only tools that exist for trying to understand the climate changes likely to be experienced over the course of this century	5
271	No period in Earth's geological history	provides	exact analogue for the climate conditions that will unfold in the coming decades	5
272	climate	is	to those emissions	5
273	Global temperatures	are projected	to continue to rise over this century	1
274	continued warming of the planet	is projected	over the next few decades due to past emissions	1
275	Choices	will influence	amount of future warming	1
276	higher levels	will result	in more warming, and more severe impacts on society and the natural world	1
277	Lower levels of heat-trapping emissions	will yield	less future warming	1
278	IPCC	developed	set of scenarios in a Special Report on Emissions Scenarios	1
279	None of these scenarios	called	lower	1
280	implementation of policies	limit	climate change or to stabilize atmospheric concentrations of heat-trapping gases	5
281	differences among these scenarios	are	due to different assumptions about changes in population, rate of adoption of new technologies, economic growth, and other factors	1
282	emissions	can change	less than those scenarios imply	1
283	IPCC emission scenarios	do not encompass	full range of possible futures	1
284	highest emissions scenario	developed	by the IPCC er	1
285	Recent carbon dioxide emissions	are	fact, above the highest emissions scenario	2
286	United States and 191 other countries	are	signatories	1
287	Framework Convention on Climate Change	calls	for stabilizing concentrations of greenhouse gases in the atmosphere at a level that would avoid dangerous human interference with the climate system	10
288	exactly constitutes such interference	is	subject to interpretation	1
289	variety of research studies	suggest	would lead to severe, widespread, and irreversible impacts	1
290	Reducing emissions of carbon dioxide	would reduce	warming over this century and beyond	1
291	Implementing sizable and sustained reductions in carbon dioxide emissions	would be	more effective than reductions of the same size	1
292	Implementing sizable and sustained reductions in carbon dioxide emissions	reduce	pace and the overall amount of climate change	6
293	Reducing emissions of some shorter-lived greenhouse gases, such as methane, and some types of particles, such as soot	would begin	to reduce the warming influence within weeks to decades	2
294	graphs	show	emissions scenarios and resulting carbon dioxide concentrations for three IPCC scenarios90,91 and one stabilization scenario	2
295	Scenarios of Future Carbon Dioxide Global Emissions and Concentrations The stabilization scenario	is aimed	at stabilizing the atmospheric carbon dioxide concentration at roughly 450 parts per million	3
296	Resulting temperature changes	depend	on atmospheric concentrations of greenhouse gases and particles and the climate's sensitivity to those concentrations	6
297	concerns	have been raised	about dangerous human interference with the climate system	4
298	450 ppm stabilization target	has	potential to keep the global temperature rise at or below about 3.5°F from pre-industrial levels and 2°F above the current average temperature, a level	1
299	Scenarios that stabilize carbon dioxide below 450 ppm (not shown in the figure)	stabilize	carbon dioxide below 450 ppm	1
300	Scenarios that stabilize carbon dioxide below 450 ppm (not shown in the figure)	offer	increased chance of avoiding dangerous climate change	5
301	Carbon dioxide	is not	only greenhouse gas of concern	1
302	Concentrations of other heat-trapping gases like methane and nitrous oxide and particles like soot	have	to be stabilized at low enough levels	1
303	Concentrations of other heat-trapping gases like methane and nitrous oxide and particles like soot	have	to prevent global temperatures from rising higher than the level	1
304	other gases	are added	offsetting cooling effects of sulfate aerosol particles	1
305	analyses	suggest	that stabilizing concentrations around 400 parts per million of "equivalent carbon dioxide" would yield about an 80 percent chance of avoiding exceeding the 2°F above present temperature threshold	1
306	concentrations	stabilized	at 400 parts per million roughly a century later	1
307	concentrations	peaked	as high as 475 parts per million	1
308	Reductions in sulfate aerosol particles	would necessitate	lower equivalent carbon dioxide targets	1
309	All climate models	project	that human-caused emissions of heat-trapping gases will cause further warming in the future	6
310	global average temperature	is projected	to rise by 2 to 11.5°F by the end of this century90	1
311	climate	is	to past and future emissions	5
312	actual warming in 2100 will be closer to the low or the high end of this range	depends	primarily on two factors: first, the future level of emissions of heat-trapping gases, and second	2
313	range of possible outcomes	has been explored	using a range of different emissions scenarios, and a variety of climate models that encompass the known range of climate sensitivity	5
314	Changing precipitation patterns	observed	patterns of change, with overall increases in the global average but substantial shifts in where and how precipitation falls	2
315	dry belt	lies	just outside the tropics expands	1
316	dry belt	receives	less rain	1
317	dry belt	poleward	96,97	1
318	higher latitudes	are projected	to receive more precipitation	1
319	Increases in tropical precipitation	are projected	during rainy seasons	1
320	Certain regions, including the U.S. West	are expected	to become drier	1
321	precipitation	becoming	less frequent but more intense	1
322	widespread trend toward more heavy downpours	is expected	to continue, with precipitation	1
323	More precipitation	is expected	to fall as rain rather than snow	1
324	models	will be	increased risk of more intense, more frequent, and longer-lasting heat waves	1
325	rare extreme events	are becoming	more common In a warmer future climate, models project	4
326	European heat wave of 2003	is	example of the type of extreme heat event that is likely to become much more common	1
327	greenhouse gas emissions	continue	to increase, by the 2040s more than half of European summers	1
328	greenhouse gas emissions	continue	hotter than the summer of 2003, and by the end of this century, a summer	1
329	greenhouse gas emissions	will be	hotter than the summer of 2003, and by the end of this century, a summer	1
330	Increased extremes of summer dryness and winter wetness	are projected	for much of the globe, meaning a generally greater risk of droughts and floods	1
331	precipitation	tends	to be concentrated into heavier events, with longer dry periods in between	1
332	Models	project	general tendency for more intense but fewer storms overall outside the tropics, with more extreme wind events and higher ocean waves in a number of regions in association with those storms	1
333	Models	project	shift of storm tracks toward the poles in both hemispheres	1
334	Changes in hurricanes	project	be-cause	1
335	Changes in hurricanes	are	difficult	1
336	Higher ocean temperatures	lead	to stronger storms with higher wind speeds and more rainfall	1
337	changes in wind speed and direction with height	are	also projected to increase in some regions	1
338	more research	is required	on these issues	1
339	It	appears	that stronger, more rain-produc-ing tropical storms and hurricanes are generally more likely	1
340	More discussion of Atlantic hurricanes	affect	United States	1
341	More discussion of Atlantic hurricanes	appears	on page 34 in the National Climate Change section	5
342	future sea-level rise	presents	special challenges	1
343	Sea level	will continue	to rise	1
344	Sea level	will continue	Projecting	1
345	models	project	sea-level rise	1
346	models	include	processes	1
347	Scientists	have	well-developed understanding of the contributions of thermal expansion and melting glaciers to sea-level rise, so	1
348	contributions to past and future sea-level rise from ice sheets	are	less well understood	1
349	Recent observations of the polar ice sheets	show	that a number of complex processes control the movement of ice to the sea, and thus affect the contributions of ice sheets to sea-level rise	2
350	processes	producing	substantial loss of ice mass.	1
351	it	predict	future contributions to sea-level rise	1
352	2007 assessment by the IPCC	projected	rise of the world's oceans from 8 inches to 2 feet by the end of this century	1
353	2007 assessment by the IPCC	could not quantify	contributions to sea-level rise due to changes in ice sheet dynamics, and thus	2
354	More recent research	quantify	potential contribution to sea-level rise from the accelerated flow of ice sheets to the sea27,42 or to estimate future sea level based on its observed relationship to temperature	2
355	average estimates under higher emissions scenarios	are	for sea-level rise between 3 and 4 feet by the end of this century	1
356	resulting estimates	exceed	IPCC	1
357	sea-level rise	expected	over this century	1
358	important question	is	upper bound of sea-level rise	1
359	it	have	rise of sea level higher than about 6.5 feet by the end of this century	1
360	sea level	experienced	at any particular location along the coast	1
361	changes in sea level	depend	increase in the global average sea level, but also on changes in regional currents and winds	2
362	changes in sea level	depend	proximity to the mass of melting ice sheets, and on the vertical movements of the land due to geological forces	2
363	consequences of sea-level rise at any particular location	depend	on the amount of sea-level rise relative to the adjoining land	1
364	parts of the U.S. coast	are undergoing	uplift	1
365	Not all changes in the climate	are	gradual	5
366	climate	found	in ice cores, tree rings, and other natural records	5
367	long record of climate	show	that Earth's climate patterns have undergone rapid shifts from one stable state to another within as short a period as a decade	9
368	Examples of such changes	are	abrupt shifts in drought frequency and duration	1
369	other regions including the Midwest and Great Plains	had	kinds of abrupt shifts in the past	1
370	Southwest	may be	at greatest risk for this kind of change	1
371	other regions including the Midwest and Great Plains	could experience	them again in the future	1
372	Rapid ice sheet collapse with related sea-level rise	is not	well understood or modeled and that poses a risk for the future	1
373	Rapid ice sheet collapse with related sea-level rise	is	type of abrupt change	2
374	it	lubricates	ice previously frozen to the rock below	1
375	Recent observations	show	that melting on the surface of an ice sheet produces water that flows down through large cracks that create conduits through the ice to the base of the ice sheet	1
376	ice	meets	sea	1
377	interaction with warm ocean water	can lead	to sudden losses in ice mass and accompanying rapid global sea-level rise	1
378	ice sheets	will respond	in the future	1
379	scientists	are not	yet confident that they can project	1
380	Observations	indicate	that ice loss has increased dramatically over the last decade	1
381	abrupt release of methane	occur	within 100 years	1
382	warming	will accelerate	potentially increasing the rate of global temperature rise	1
383	third major area of concern regarding possible abrupt change	involves	operation of the ocean currents that transport vast quantities of heat around the globe	2
384	One branch of the ocean circulation	is	in the North Atlantic	1
385	cold water	flows	back from the North Atlantic to the tropics in the ocean's deep layers	1
386	warm water	flows	creating a "conveyor belt" for heat	1
387	warm water	flows	northward from the tropics to the North Atlantic in the upper layer of the ocean	1
388	Recent findings	indicate	that it is very likely that the strength of this North Atlantic circulation will decrease over the course of this century in response to increasing greenhouse gases	1
389	warming	increases	melting of glaciers and ice sheets and the resulting runoff of freshwater to the sea	1
390	This additional water	makes	it less dense than sea water	1
391	This additional water	is	virtually salt-free	1
392	Increased precipitation	contributes	fresh, less-dense water to the ocean	1
393	less surface water	sink	reducing the conveyor belt's transport of heat	1
394	less surface water	is	dense enough	1
395	strength of this circulation	will decrease	leading to a reduction in heat transfer to the North Atlantic	1
396	It	is considered	very unlikely that this circulation would collapse entirely during the next 100 years or so	1
397	potential consequences of such an abrupt event	would be	severe	1
398	rise	expected	from thermal expansion and melting glaciers and ice sheets	1
399	Impacts	include	sea-level rise around the North Atlantic of up to 2.5 feet	1

1

Human activities

have led

to large increases in heat-trapping gases over the past century

1

2

global warming of the past 50 years

is

due primarily to human-induced increases in heat-trapping gases

1

3

Human "fingerprints"

have been identified

in many other aspects of the climate system, including changes in ocean heat content, precipitation, atmospheric moisture, and Arctic sea ice

7

4

climate

is

to those emissions

5

Global temperatures

are projected

to continue to rise over this century

1

6

This introduction to global climate change

has been happening

to the world's climate and why

9

7

this report

focuses

on climate change impacts in the United States

5

8

impacts

requires

understanding of the global climate system

4

9

this report

understanding

changes

1

10

changes

have been observed

in global climate over the past century

5

11

Antarctic ice core

extending back

800,000 years

1

12

air bubbles

trapped

in an Antarctic ice core

1

13

nature and causes of these changes

have been

comprehensively chronicled in a variety of recent reports, such as those by the Intergovernmental Panel on Climate Change

5

14

This section

does not intend

to documents the Earth's changing carbon dioxide concentration

1

15

Temperature-related data but rather to provide a brief synthesis, make clear that these variations have played a central role in determining the global and to integrate more recent

work

with climate

4

16

present carbon

dioxide

concentration of the assessments of the IPCC, CCSP, and others

1

17

Human activities

have led

to large increases in heat-trapping gases over the past century

1

18

Earth's climate

depends

on the functioning of a natural "greenhouse effect

5

19

This effect

is

result of heat-trapping gases

1

20

This effect

is

also known as greenhouse gases

1

21

This effect

is

water vapor, carbon dioxide, ozone, methane, and nitrous oxide, which absorb heat radiated from the Earth's surface and lower atmosphere and then radiate much of the energy back toward the surface

2

22

average surface temperature of the Earth

would be

about 60°F colder

1

23

human activities

have been releasing

additional heat-trapping gases, intensifying the natural greenhouse effect, thereby changing the Earth's climate

5

24

Climate

is influenced

by a variety of factors, both human-induced and natural

4

25

principal factor

causing

warming over the past 50 years

1

26

increase in the carbon dioxide concentration

has been

principal factor

1

27

concentration

has been building up

in the Earth's atmosphere since the beginning of the industrial era in the mid-1700s, primarily due to the burning of fossil fuels

1

28

Human activities

increased

emissions of other greenhouse gases, such as methane, nitrous oxide, and halocarbons

2

29

emissions

causing surface temperatures

to rise

1

30

emissions

are thickening

causing surface temperatures

1

31

emissions

are thickening

blanket of heat-trapping gases in Earth's atmosphere

2

32

Carbon dioxide concentration

has increased

due to the use of fossil fuels in electricity generation, transportation, and industrial and household uses

1

33

Deforestation

provides

source of carbon dioxide

1

34

about 20 percent

resulted

from deforestation and associated agricultural practices

1

35

about 80 percent of human-induced carbon dioxide emissions

came

from the burning of fossil fuels

1

36

concentration of carbon dioxide in the atmosphere

has increased

by roughly 35 percent since the start of the industrial revolution

1

37

Methane concentration

has increased

mainly as a result of agriculture

1

38

Methane concentration

has increased

raising livestock

1

39

About 70 percent of the emissions of atmospheric methane

are

now related to human activities

1

40

Halocarbon emissions

come

from the release of certain manufactured chemicals to the atmosphere

1

41

presence in the atmosphere

was found

to cause stratospheric ozone depletion

1

42

international regulations

designed

to protect the ozone layer

1

43

abundance of these gases in the atmosphere

decreasing

as a result of international regulations

1

44

Continued decreases in ozone-deplet-ing halocarbon emissions

are expected

to reduce their relative influence on climate change in the future

6

45

halocarbon Ozone

produced

atmosphere by chemical reactions

1

46

halocarbon Ozone

destroyed

in the atmosphere by chemical reactions

1

47

halocarbon Ozone

is

greenhouse gas

1

48

human activities

have increased

ozone concentration through the release of gases such as carbon monoxide, hydrocarbons, and nitrogen oxides

2

49

chemical reactions

produce

ozone in the presence of sunlight

1

50

excess ozone in the troposphere

causes

respiratory illnesses and other human health problems

1

51

layer above the troposphere, ozone

protects

life on Earth from exposure to excessive ultraviolet radiation from the Sun

1

52

halocarbons

have caused

ozone hole over Antarctica

1

53

halocarbons

destroy

ozone in the stratosphere

1

54

Changes in the stratospheric ozone layer

have contributed

to changes in wind patterns and regional climates in Antarctica

2

55

Water vapor

is

most important and abundant greenhouse gas in the atmosphere

1

56

Human activities

produce

only a very small increase in water vapor through irrigation and combustion processes

1

57

surface warming

caused

by human-produced increases in other greenhouse gases

1

58

warmer climate

allows

atmosphere to hold more moisture

4

59

warmer climate

increases

evaporation

4

60

surface warming

leads

to an increase in atmospheric water vapor

1

61

human activities

produce

additional local and regional effects

1

62

others

increase

warming

1

63

warming

caused

by greenhouse gases

1

64

tiny particles

called

aerosols

1

65

One such influence on climate

is caused

by tiny particles

5

66

burning of coal

produces

emissions of sulfur-containing compounds

1

67

sulfate aerosol" particles

reflect

incoming sunlight away from the Earth

1

68

sulfate aerosol" particles

causing

cooling influence at the surface

1

69

Sulfate aerosols

tend

to make clouds more efficient at reflecting sunlight, causing an additional indirect cooling effect

1

70

type of aerosol

referred

to as soot or black carbon

1

71

type of aerosol

absorbs

incoming sunlight and traps heat in the atmosphere

1

72

warming

caused

by increased levels of greenhouse gases

1

73

aerosols

mask

increase the warming

1

74

warming

caused

by heat-trapping gases

1

75

sum of these aerosol effects

offsets

warming

1

76

gases and particles

remain

in the atmosphere

1

77

elevated concentrations of aerosols

persist

for days to weeks

1

78

atmospheric concentration of carbon dioxide

remains

elevated for thousands of years, and that of methane for decades

1

79

climate effects of reductions in emissions of carbon dioxide and other long-lived gases

do not become

apparent for at least several decades

6

80

geographic patterns of their climatic influence and the resulting surface temperature responses

are

quite different

1

81

reductions in emissions of short-lived compounds

can have

rapid, but complex effect

1

82

greatest emissions of short-lived pollutants in summertime by late this century

are projected

to come from Asia

1

83

strongest climate response

is projected

to be over the central United States

4

84

heat

is reflected

absorbed by the surface

1

85

Human activities

changed

land surface in ways that alter

1

86

changes

include

cutting and burning of forests, the replacement of other areas of natural vegetation with agriculture and cities, and large-scale irrigation

1

87

transformations of the land surface

can cause

warming or cooling

1

88

important natural factors

influence

climate: the Sun and volcanic eruptions

4

89

net effect of these changes

been

slight cooling of the Earth's surface over the past Two

2

90

human influences on climate

have become

increasingly obvious

4

91

Sun's energy output (as measured by satellites since 1979)

has followed

historical 11-year cycle of small ups and downs, but with no net increase

1

92

short-term cooling effects on climate

lasting

2 to 3 years

5

93

two major volcanic eruptions of the past 30 years

have had

short-term cooling effects on climate

5

94

net effect on climate

been

slight cooling influence over this period

5

95

natural factors

can not explain

warming of recent decades

1

96

Slow changes in Earth's orbit around the Sun and its tilt toward or away from the Sun

are

only important on timescales from thousands to many tens of thousands of years

1

97

Slow changes in Earth's orbit around the Sun and its tilt toward or away from the Sun

are

also a purely natural influence on climate

6

98

climate changes

have occurred

over the last century

5

99

climate changes

are not

solely caused by the human and natural factors

5

100

fluctuations in climate

occur

even in the absence of changes in human activities, the Sun, or volcanoes

5

101

One example

is

El Niño phenomenon, which has important influences on many aspects of regional and global climate

4

102

effects on climate

occur

at the same time as the effects of human activities, the Sun, and volcanoes

5

103

other modes of variability

have been identified

by climate scientists

5

104

carbon dioxide

is emitted

to the atmosphere

1

105

it

is absorbed

by the oceans and taken up by vegetation

1

106

carbon dioxide

emitted

by human activities in the last 50 years

1

107

About 45 percent of the carbon dioxide

is

now stored in the oceans and vegetation

1

108

rest

has remained

increasing the atmospheric concentration

1

109

rest

has remained

in the air

1

110

carbon

tends

to be released, for instance, from thawing permafrost

1

111

more carbon release

leads

to more warming which leads to further release, and so on

1

112

carbon

tends

initiating a feedback loop

1

113

it

is known

from long records of Earth's climate history

4

114

carbon dioxide

observed

in the atmosphere

1

115

increasing emissions of carbon dioxide

are

primary cause of the increased concentration of carbon dioxide

2

116

growth rate

increased

from 1.3 percent per year in the 1990s to 3.3 percent per year between 2000 and 2006.21

1

117

greater fraction

remaining

in the atmosphere

1

118

ocean

absorbs

carbon dioxide from the atmosphere

1

119

process

referred

to as ocean acidification

1

120

carbon dioxide concentrations

continue

to increase

1

121

ocean acidification

is

essentially irreversible over a time scale of centuries

1

122

living things

create

shells and skeletons, with substantial negative consequences for coral reefs, mollusks, and some plankton species important to ocean food chains

1

123

ocean acidification

affects

process of calcification

1

124

estimated change in the average temperature of Earth's surface

is based

on measurements from thousands of weather stations, ships, and buoys around the world

3

125

Global average surface air temperature

has increased

substantially since 1970.26

1

126

Global average surface air temperature

has increased

satellites

1

127

measurements

are

independently compiled, analyzed, and processed by different research groups

1

128

instruments

used

to measure temperature, the measurement times and locations, the local environment around the measuring site, and such factors as satellite orbital drift

1

129

growth of cities

can cause

localized "urban heat island" effects

1

130

number of research groups around the world

have produced

estimates of global-scale changes in surface temperature

2

131

warming trend

is

apparent in all of these temperature records

1

132

warming trend

is confirmed

by other independent observations, such as the melting of Arctic sea ice, the retreat of mountain glaciers on every continent,27 reductions in the extent of snow cover, earlier blooming of plants in spring, and increased melting of the Greenland and Antarctic ice sheets

1

133

snow and ice

reflect

Sun's heat

1

134

this melting

causes

resulting in another feedback loop

1

135

this melting

causes

more heat to be absorbed, which causes more melting

1

136

temperature measurements above the surface

have been made

by weather balloons since the late 1940s, and from satellites since 1979

1

137

measurements

show

warming of the troposphere, consistent with the surface warming

1

138

They

reveal

cooling in the stratosphere

1

139

atmospheric temperature

would be expected

to change in response to increasing greenhouse gas concentrations and the observed depletion of stratospheric ozone

2

140

This pattern of tropospheric warming and stratospheric cooling

agrees

with our understanding

1

141

Precipitation

is not distributed

evenly over the globe

1

142

average distribution

is governed

primarily by atmospheric circulation patterns, the availability of moisture, and surface terrain effects

1

143

first two of these factors

are influenced

by temperature

1

144

human-caused changes in temperature

are expected

to alter precipitation patterns

2

145

Observations

show

that such shifts are occurring

1

146

Changes

have been observed

in the amount, intensity, frequency, and type of precipitation

1

147

Pronounced increases in precipitation over the past 100 years

have been observed

in eastern North America, southern South America, and northern Europe

1

148

Decreases

have been seen

in the Mediterranean, most of Africa, and southern Asia

1

149

Changes in the geographical distribution of droughts and flooding

have been

complex

1

150

northern regions and mountainous areas

are experiencing

falling as rain rather than snow

1

151

northern regions and mountainous areas

are experiencing

more precipitation

1

152

Widespread increases in heavy precipitation events

have occurred

even in places where total rain amounts have decreased

1

153

more water vapor

evaporating

from the world's oceans and land surface

1

154

changes

are associated

with the fact that warmer air holds

1

155

This increase in atmospheric water vapor

is

primarily due to human influences

1

156

This increase in atmospheric water vapor

has been observed

from satellites

1

157

sea level

rose

by roughly 8 inches over the past century

1

158

rate

observed

over the past century

1

159

Satellite data available over the past 15 years

show

rising at a rate roughly double the rate

1

160

Satellite data available over the past 15 years

show

sea level

1

161

global warming

causes

to rise

1

162

global warming

causes

sea level

1

163

Warming

has been

directly linked to human influences

1

164

Warming

has been observed

in each of the world's major ocean basins

1

165

warming

leads

to the melting of glaciers and ice sheets, which raises sea level by adding water to the oceans

1

166

rate of retreat

has increased

in the past decade

1

167

Glaciers

have been retreating

worldwide for at least the last century

1

168

increased precipitation

has outpaced

melting

1

169

Only a few glaciers

advancing

in locations that were well below freezing

1

170

progressive disappearance of glaciers

has

implications not only for the rise in global sea level, but also for water supplies in certain densely populated regions of Asia and South America

1

171

Earth

has

major ice sheets on Greenland and Antarctica

1

172

ice sheets

calving

of icebergs

1

173

ice sheets

contributing

to sea-level rise

1

174

ice sheets

losing

ice volume by increased melting

1

175

Greenland Ice Sheet

been experiencing

record amounts of surface melting, and a large increase in the rate of mass loss in the past decade

1

176

it

would raise

sea level by about 20 feet

1

177

Antarctic Ice Sheet

consists

of two portions, the West Antarctic Ice Sheet and the East Antarctic Ice Sheet

1

178

West Antarctic Ice Sheet

contains

enough water to raise global sea levels by about 16 to 20 feet

1

179

it

would raise

global sea level by about 200 feet

1

180

past climate records

increases

in sea level

5

181

past climate records

provide

precedent for very significant decreases in ice volume

5

182

Complete melting of these ice sheets over this century or the next

is thought

to be virtually impossible

1

183

global warming of the past 50 years

is

due primarily to human-induced increases in heat-trapping gases

1

184

Human "fingerprints"

have been identified

in many other aspects of the climate system, including changes in ocean heat content, precipitation, atmospheric moisture, and Arctic sea ice

7

185

IPCC Second Assessment Report44

concluded

that "the balance of evidence suggests a discernible human influence on global climate

5

186

number of national and international assessments

have come

to much stronger conclusions about the reality of human effects on climate

4

187

Recent scientific assessments

find

that most of the warming of the Earth's surface over the past 50 years has been caused by human activities

1

188

scientific evidence for a human influence on global climate

has accumulated

over the past several decades, from many hundreds of studies

5

189

No single study

is

smoking gun

1

190

evidence

supporting

conclusion that human activity is the primary driver of recent warming

1

191

any single study or combination of studies

undermined

large body of evidence

1

192

climate system

responds

to increases in greenhouse gases

5

193

other human and natural factors

influence

climate

4

194

first line of evidence

is

basic physical understanding of how greenhouse gases trap heat

1

195

second line of evidence

is

from indirect estimates of climate changes over the last 1,000 to 2,000 years

5

196

records

are obtained

from living things and their remains

1

197

lesson from these data

is

that global surface temperatures over the last several decades are clearly unusual, in that they were higher than at any time during at least the past 400 years

1

198

recent temperature rise

is

clearly unusual in at least the last 1,000 years

1

199

climate

would be expected

to change in response to human activities

4

200

third line of evidence

is based

on the broad, qualitative consistency between observed changes in climate and the computer model simulations

5

201

climate models

are run

with historical increases in greenhouse gases

5

202

they

show

gradual warming of the Earth and ocean surface, increases in ocean heat content and the temperature of the lower atmosphere, a rise in global sea level, retreat of sea ice and snow cover, cooling of the stratosphere, an increase in the amount of atmospheric water vapor, and changes in large-scale precipitation and pressure patterns

4

203

other aspects of modeled climate change

are

in agreement with observations

5

204

factor

affects

climate

4

205

person

has

unique fingerprint

1

206

factor

produces

unique pattern of climate response

4

207

Fingerprint studies

allow

detailed comparisons of modeled and observed climate change patterns

5

208

Fingerprint studies

exploit

unique signatures

1

209

Scientists

rely

to attribute observed changes in climate to a particular cause or set of causes

5

210

Scientists

rely

on such studies

1

211

climate changes

have occurred

since the start of the Industrial Revolution

5

212

climate changes

are

due to a complex mixture of human and natural causes

5

213

importance of each individual influence in this mixture

changes

over time

1

214

multiple Earths

would allow

experimenter to change one factor at a time on each Earth

1

215

multiple Earths

helping

to isolate different fingerprints

1

216

individual factors

affect

climate

4

217

climate models

are used

to study

4

218

climate model simulations of the last century

include

all of the major influences on climate, both human-induced and natural

9

219

they

can reproduce

important features of observed climate change patterns

5

220

human influences

are removed

from the model experiments

1

221

results

suggest

that the surface of the Earth would actually have cooled slightly over the last 50 years

1

222

observed warming over the last half-century can-not

caused

primarily by human factors

1

223

clear message from fingerprint studies

is

that the observed warming over the last half-century can-not be explained by natural factors

1

224

fingerprint of human effects on climate

has been identified

by looking at a slice through the layers of the atmosphere, and studying the pattern of temperature changes from the surface up through the stratosphere

6

225

increases in carbon dioxide

cause

warming at the surface and in the troposphere, but lead to cooling of the stratosphere

1

226

models

calculate

that the human-caused depletion of stratospheric ozone has had a strong cooling effect in the stratosphere

1

227

most of the observed temperature change

had been

due to an increase in solar output rather than an increase in greenhouse gases

3

228

Earth's atmosphere

would have warmed

throughout its full vertical extent, including the stratosphere

1

229

tern of atmospheric temperature changes, with its pronounced cooling in the stratosphere

is

therefore inconsistent with the hypothesis that changes in the Sun can explain the warming of recent decades

2

230

direct satellite measurements of solar output

show

slight decreases during the recent period of warming

1

231

earliest fingerprint work51

focused

on changes in surface and atmospheric temperature

1

232

Scientists

applied

fingerprint methods to a whole range of climate variables,50,52 identifying human-caused climate signals in the heat content of the oceans,38,39 the height of the tropopause53

5

233

Studies

published

after the appearance of the IPCC Fourth Assessment Report in 2007

1

234

Studies

found

human fingerprints in the increased levels of atmospheric moisture35,36

1

235

changes in temperature, ice extent, moisture, and circulation patterns

fit

together in a physically consistent way, like pieces in a complex puzzle

2

236

message from this entire body of work

is

that the climate system is telling a consistent story of increasingly dominant human influence

4

237

this type of fingerprint work

is shifting

emphasis

1

238

clear and compelling scientific evidence

supports

case for a pronounced human influence on global climate

4

239

Much of the recent attention

is

now on climate changes at continental and regional scales,64,65 and on variables that can have large impacts on societies

5

240

scientists

have established

causal links between human activities and the changes in snowpack, maximum and minimum temperature, and the seasonal timing of runoff over mountainous regions of the western United States

1

241

Human activity

have made

substantial contribution to ocean surface temperature changes in hurricane formation regions

2

242

Researchers

are beginning

to tie changes in the distribution and seasonal behavior of plant and animal species to human-caused changes in temperature and precipitation

1

243

Researchers

looking

beyond the physical climate system

4

244

all models

predicted

that with a rise in greenhouse gases, the troposphere would be expected to warm more rapidly than the surface

2

245

Observations from weather balloons, satellites, and surface thermometers

seemed

to show the opposite behavior

1

246

This issue

was

stumbling block in our understanding of the causes of climate change

5

247

Research

showed

that there were large uncertainties in the satellite and weather balloon data

1

248

uncertainties in models and observations

are

properly accounted for

1

249

newer observational data sets (with better treatment of known problems)

are

in agreement with climate model results

5

250

aspects of precipitation,54,77 and patterns of surface pressure,56

appear

to be proceeding much more rapidly than models have projected

1

251

observed changes

studied

to date

1

252

heat-trapping gases

resulting

from human activities

1

253

climate system

would be expected

to respond to the increase in heat-trapping gases

5

254

most of the observed changes

are

also consistent with our scientific understanding

1

255

bottom-line conclusion from climate fingerprinting

is

that most of the observed changes studied to date are consistent with each other

4

256

Scientists

are

sometimes asked whether extreme weather events can be linked to human activities

1

257

Scientific research

has concluded

that human influences on climate are indeed changing the likelihood of certain types of extreme events

4

258

it

would have been

in the absence of human-induced climate change

5

259

analysis of the European summer heat wave of 2003

found

that the risk of such a heat wave is now roughly four times greater

1

260

analyses of human-caused changes in the risks of extreme events

rely

on information from climate models, and on our understanding of the physics of the climate system

5

261

models

used

in this work

1

262

All of the models

have imperfections

in their representation of the complexities of the "real world" climate system

5

263

models

are

extremely useful, for a number of reasons

1

264

current generation of climate models

portrays

important aspects of today's weather patterns and climate

9

265

Models

are

routinely tested against many observations of Earth's climate system

5

266

fingerprint work

shows

that models capture not only our present-day climate, but also key features of the observed climate changes over the past cent ur y.

5

267

large-scale observed cli- mate changes (such as the warming of the surface and troposphere, and the increase in the amount of moisture in the atmosphere)

are driven

by very basic physics, which is well-represented in models

2

268

climate models

can be used

to predict changes in climate that can be verified in the real world

10

269

Examples

include

short-term global cooling subsequent to the eruption of Mount Pinatubo and the stratospheric cooling with increasing carbon dioxide

1

270

models

are

only tools that exist for trying to understand the climate changes likely to be experienced over the course of this century

5

271

No period in Earth's geological history

provides

exact analogue for the climate conditions that will unfold in the coming decades

5

272

climate

is

to those emissions

5

273

Global temperatures

are projected

to continue to rise over this century

1

274

continued warming of the planet

is projected

over the next few decades due to past emissions

1

275

Choices

will influence

amount of future warming

1

276

higher levels

will result

in more warming, and more severe impacts on society and the natural world

1

277

Lower levels of heat-trapping emissions

will yield

less future warming

1

278

IPCC

developed

set of scenarios in a Special Report on Emissions Scenarios

1

279

None of these scenarios

called

lower

1

280

implementation of policies

limit

climate change or to stabilize atmospheric concentrations of heat-trapping gases

5

281

differences among these scenarios

are

due to different assumptions about changes in population, rate of adoption of new technologies, economic growth, and other factors

1

282

emissions

can change

less than those scenarios imply

1

283

IPCC emission scenarios

do not encompass

full range of possible futures

1

284

highest emissions scenario

developed

by the IPCC er

1

285

Recent carbon dioxide emissions

are

fact, above the highest emissions scenario

2

286

United States and 191 other countries

are

signatories

1

287

Framework Convention on Climate Change

calls

for stabilizing concentrations of greenhouse gases in the atmosphere at a level that would avoid dangerous human interference with the climate system

10

288

exactly constitutes such interference

is

subject to interpretation

1

289

variety of research studies

suggest

would lead to severe, widespread, and irreversible impacts

1

290

Reducing emissions of carbon dioxide

would reduce

warming over this century and beyond

1

291

Implementing sizable and sustained reductions in carbon dioxide emissions

would be

more effective than reductions of the same size

1

292

Implementing sizable and sustained reductions in carbon dioxide emissions

reduce

pace and the overall amount of climate change

6

293

Reducing emissions of some shorter-lived greenhouse gases, such as methane, and some types of particles, such as soot

would begin

to reduce the warming influence within weeks to decades

2

294

graphs

show

emissions scenarios and resulting carbon dioxide concentrations for three IPCC scenarios90,91 and one stabilization scenario

2

295

Scenarios of Future Carbon Dioxide Global Emissions and Concentrations The stabilization scenario

is aimed

at stabilizing the atmospheric carbon dioxide concentration at roughly 450 parts per million

3

296

Resulting temperature changes

depend

on atmospheric concentrations of greenhouse gases and particles and the climate's sensitivity to those concentrations

6

297

concerns

have been raised

about dangerous human interference with the climate system

4

298

450 ppm stabilization target

has

potential to keep the global temperature rise at or below about 3.5°F from pre-industrial levels and 2°F above the current average temperature, a level

1

299

Scenarios that stabilize carbon dioxide below 450 ppm (not shown in the figure)

stabilize

carbon dioxide below 450 ppm

1

300

Scenarios that stabilize carbon dioxide below 450 ppm (not shown in the figure)

offer

increased chance of avoiding dangerous climate change

5

301

Carbon dioxide

is not

only greenhouse gas of concern

1

302

Concentrations of other heat-trapping gases like methane and nitrous oxide and particles like soot

have

to be stabilized at low enough levels

1

303

Concentrations of other heat-trapping gases like methane and nitrous oxide and particles like soot

have

to prevent global temperatures from rising higher than the level

1

304

other gases

are added

offsetting cooling effects of sulfate aerosol particles

1

305

analyses

suggest

that stabilizing concentrations around 400 parts per million of "equivalent carbon dioxide" would yield about an 80 percent chance of avoiding exceeding the 2°F above present temperature threshold

1

306

concentrations

stabilized

at 400 parts per million roughly a century later

1

307

concentrations

peaked

as high as 475 parts per million

1

308

Reductions in sulfate aerosol particles

would necessitate

lower equivalent carbon dioxide targets

1

309

All climate models

project

that human-caused emissions of heat-trapping gases will cause further warming in the future

6

310

global average temperature

is projected

to rise by 2 to 11.5°F by the end of this century90

1

311

climate

is

to past and future emissions

5

312

actual warming in 2100 will be closer to the low or the high end of this range

depends

primarily on two factors: first, the future level of emissions of heat-trapping gases, and second

2

313

range of possible outcomes

has been explored

using a range of different emissions scenarios, and a variety of climate models that encompass the known range of climate sensitivity

5

314

Changing precipitation patterns

observed

patterns of change, with overall increases in the global average but substantial shifts in where and how precipitation falls

2

315

dry belt

lies

just outside the tropics expands

1

316

dry belt

receives

less rain

1

317

dry belt

poleward

96,97

1

318

higher latitudes

are projected

to receive more precipitation

1

319

Increases in tropical precipitation

are projected

during rainy seasons

1

320

Certain regions, including the U.S. West

are expected

to become drier

1

321

precipitation

becoming

less frequent but more intense

1

322

widespread trend toward more heavy downpours

is expected

to continue, with precipitation

1

323

More precipitation

is expected

to fall as rain rather than snow

1

324

models

will be

increased risk of more intense, more frequent, and longer-lasting heat waves

1

325

rare extreme events

are becoming

more common In a warmer future climate, models project

4

326

European heat wave of 2003

is

example of the type of extreme heat event that is likely to become much more common

1

327

greenhouse gas emissions

continue

to increase, by the 2040s more than half of European summers

1

328

greenhouse gas emissions

continue

hotter than the summer of 2003, and by the end of this century, a summer

1

329

greenhouse gas emissions

will be

hotter than the summer of 2003, and by the end of this century, a summer

1

330

Increased extremes of summer dryness and winter wetness

are projected

for much of the globe, meaning a generally greater risk of droughts and floods

1

331

precipitation

tends

to be concentrated into heavier events, with longer dry periods in between

1

332

Models

project

general tendency for more intense but fewer storms overall outside the tropics, with more extreme wind events and higher ocean waves in a number of regions in association with those storms

1

333

Models

project

shift of storm tracks toward the poles in both hemispheres

1

334

Changes in hurricanes

project

be-cause

1

335

Changes in hurricanes

are

difficult

1

336

Higher ocean temperatures

lead

to stronger storms with higher wind speeds and more rainfall

1

337

changes in wind speed and direction with height

are

also projected to increase in some regions

1

338

more research

is required

on these issues

1

339

It

appears

that stronger, more rain-produc-ing tropical storms and hurricanes are generally more likely

1

340

More discussion of Atlantic hurricanes

affect

United States

1

341

More discussion of Atlantic hurricanes

appears

on page 34 in the National Climate Change section

5

342

future sea-level rise

presents

special challenges

1

343

Sea level

will continue

to rise

1

344

Sea level

will continue

Projecting

1

345

models

project

sea-level rise

1

346

models

include

processes

1

347

Scientists

have

well-developed understanding of the contributions of thermal expansion and melting glaciers to sea-level rise, so

1

348

contributions to past and future sea-level rise from ice sheets

are

less well understood

1

349

Recent observations of the polar ice sheets

show

that a number of complex processes control the movement of ice to the sea, and thus affect the contributions of ice sheets to sea-level rise

2

350

processes

producing

substantial loss of ice mass.

1

351

it

predict

future contributions to sea-level rise

1

352

2007 assessment by the IPCC

projected

rise of the world's oceans from 8 inches to 2 feet by the end of this century

1

353

2007 assessment by the IPCC

could not quantify

contributions to sea-level rise due to changes in ice sheet dynamics, and thus

2

354

More recent research

quantify

potential contribution to sea-level rise from the accelerated flow of ice sheets to the sea27,42 or to estimate future sea level based on its observed relationship to temperature

2

355

average estimates under higher emissions scenarios

are

for sea-level rise between 3 and 4 feet by the end of this century

1

356

resulting estimates

exceed

IPCC

1

357

sea-level rise

expected

over this century

1

358

important question

is

upper bound of sea-level rise

1

359

it

have

rise of sea level higher than about 6.5 feet by the end of this century

1

360

sea level

experienced

at any particular location along the coast

1

361

changes in sea level

depend

increase in the global average sea level, but also on changes in regional currents and winds

2

362

changes in sea level

depend

proximity to the mass of melting ice sheets, and on the vertical movements of the land due to geological forces

2

363

consequences of sea-level rise at any particular location

depend

on the amount of sea-level rise relative to the adjoining land

1

364

parts of the U.S. coast

are undergoing

uplift

1

365

Not all changes in the climate

are

gradual

5

366

climate

found

in ice cores, tree rings, and other natural records

5

367

long record of climate

show

that Earth's climate patterns have undergone rapid shifts from one stable state to another within as short a period as a decade

9

368

Examples of such changes

are

abrupt shifts in drought frequency and duration

1

369

other regions including the Midwest and Great Plains

had

kinds of abrupt shifts in the past

1

370

Southwest

may be

at greatest risk for this kind of change

1

371

other regions including the Midwest and Great Plains

could experience

them again in the future

1

372

Rapid ice sheet collapse with related sea-level rise

is not

well understood or modeled and that poses a risk for the future

1

373

Rapid ice sheet collapse with related sea-level rise

is

type of abrupt change

2

374

it

lubricates

ice previously frozen to the rock below

1

375

Recent observations

show

that melting on the surface of an ice sheet produces water that flows down through large cracks that create conduits through the ice to the base of the ice sheet

1

376

ice

meets

sea

1

377

interaction with warm ocean water

can lead

to sudden losses in ice mass and accompanying rapid global sea-level rise

1

378

ice sheets

will respond

in the future

1

379

scientists

are not

yet confident that they can project

1

380

Observations

indicate

that ice loss has increased dramatically over the last decade

1

381

abrupt release of methane

occur

within 100 years

1

382

warming

will accelerate

potentially increasing the rate of global temperature rise

1

383

third major area of concern regarding possible abrupt change

involves

operation of the ocean currents that transport vast quantities of heat around the globe

2

384

One branch of the ocean circulation

is

in the North Atlantic

1

385

cold water

flows

back from the North Atlantic to the tropics in the ocean's deep layers

1

386

warm water

flows

creating a "conveyor belt" for heat

1

387

warm water

flows

northward from the tropics to the North Atlantic in the upper layer of the ocean

1

388

Recent findings

indicate

that it is very likely that the strength of this North Atlantic circulation will decrease over the course of this century in response to increasing greenhouse gases

1

389

warming

increases

melting of glaciers and ice sheets and the resulting runoff of freshwater to the sea

1

390

This additional water

makes

it less dense than sea water

1

391

This additional water

is

virtually salt-free

1

392

Increased precipitation

contributes

fresh, less-dense water to the ocean

1

393

less surface water

sink

reducing the conveyor belt's transport of heat

1

394

less surface water

is

dense enough

1

395

strength of this circulation

will decrease

leading to a reduction in heat transfer to the North Atlantic

1

396

It

is considered

very unlikely that this circulation would collapse entirely during the next 100 years or so

1

397

potential consequences of such an abrupt event

would be

severe

1

398

rise

expected

from thermal expansion and melting glaciers and ice sheets

1

399

Impacts

include

sea-level rise around the North Atlantic of up to 2.5 feet

1


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