Earthâs climate is changing because humans are increasing the concentrations of greenhouse gases in the atmosphere by burning fossil fuels and through other activities. The increase in greenhouse gases is warming the planet and driving other observed climate trends, including increases in the frequency and severity of many types of extreme weather events. Future changes and impacts depend largely on the choices humans make about future greenhouse gas emissions.
Human activities are changing the climate. The evidence for warming across multiple aspects of the Earth system is incontrovertible, and the science is unequivocal that increases in atmospheric greenhouse gases are driving many observed trends and changes (KM 3.1). There are more greenhouse gases in the atmosphere primarily because humans have burned and continue to burn fossil fuels for transportation and energy generation.1 Industrial processes, deforestation, and agricultural practices also increase greenhouse gases in the atmosphere.1 As a result of increases in the atmospheric concentrations of these heat-trapping gases, the planet is on average about 2°F (1.1°C) warmer than it was in the late 1800s.2,3,4,5 No natural processes known to science could have caused this long-term temperature trend. The only credible explanation for the observed warming is human activities (Ch. 3).
AuthorsMarvel, K., W. Su, R. Delgado, S. Aarons, A. Chatterjee, M.E. Garcia, Z. Hausfather, K. Hayhoe, D.A. Hence, E.B. Jewett, A. Robel, D. Singh, A. Tripati, and R.S. Vose, 2023: Ch. 2. Climate trends. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.CH2
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Climate change is happening now in the United States. Including Alaska, the continental US has been warming about 60% faster than the planet as a whole since 1970. This temperature change has driven increases in the frequency and severity of some extreme events, consistent with the scientific understanding of climate change (Ch. 3). There has always been extreme weather, which occurs even in an unchanged climate due to the natural variability of the Earth system. However, recent advances in attribution science (KM 3.3) mean that the role of climate change in some extreme events can now be quantified in real time.6,7 For example, climate change made the record-breaking Pacific Northwest heatwave of June 2021 2° to 4°F hotter,8 and in 2017, Hurricane Harveyâs rainfall was estimated to be about 15%â20% heavier than it would have been without human-caused warming.9,10,11
Climate change is already affecting people in the United States. Extreme heat was estimated to be responsible for more than 700 deaths per year between 2004 and 2018,12 although some estimates put heat-related mortality closer to 1,300 deaths annually.13,14 Disasters are now coming more frequently and causing more damage. In the 1980s, the country experienced, on average, one (inflation-adjusted) billion-dollar weather disaster every four months.15 Now there is one, on average, every three weeks.15
Disaster risk in a complex society such as the United States is never determined simply by extreme weather events. It also depends strongly on exposure (who or what lies in the path of hazards) and vulnerability (their ability to cope with hazards). Climate change interacts with existing social, political, and economic structuresâincreases in property values as well as increased development in hazard hotspots16 have also contributed to the increase in billion-dollar disastersâand exacerbates existing inequalities. Certain groups are more vulnerable to extreme events due to socioeconomic or demographic factors. Americans over 65 are several times more likely to die of heat-related cardiovascular disease than younger people, while Black Americans die from heat-related diseases at a rate twice that of the general population.17 The extreme rainfall brought by Hurricane Harvey increased the flooded area in the Greater Houston area by 14%,18 which led to 32% more homes flooded in Harris County,19 with a disproportionate impact on low-income Hispanic neighborhoods. The spatial distribution of climate impacts partially reflects current and past policy choices: low-income neighborhoods, including those historically affected by redlining or other discriminatory policies, can be as much as 12°F hotter during heatwaves than wealthier neighborhoods in the same city20 and are at a substantially higher risk of flooding.21
FOCUS ON Western WildfiresClimate change is leading to larger and more severe wildfires in the western United States, bringing acute and chronic impacts both near and far from the flames. Wildfires have significant public health, socioeconomic, and ecological implications for the Nation.
Read MoreClimate change has other wide-ranging consequences for peopleâs health and well-being (KM 15.1) and the land and ocean ecosystems on which we depend (Chs. 8, 10). The 2021 Pacific Northwest heatwave, which resulted in more than 1,400 heat-related fatalities, also led to widespread die-offs of shellfish and other marine organisms (Box 10.1), tree and crop damage, and other impacts on the regionâs ecosystems.15,22 Western wildfires, made more severe by climate change (Focus on Western Wildfires), have destroyed towns and infrastructure and contributed to an increase in the frequency and persistence of high levels of air pollution across the US West (Chs. 14, 15).23 These extreme events occur against a changing backdrop as climate change pushes aspects of the Earth system into a ânew normal.â
Long-term warming trends are associated with shifts in other aspects of the climate system. For example, both drought in the western US24 and heavier precipitation and increased flood risk across much of the US25 are linked to rising temperatures (KM 3.5). Sea level rise threatens the coasts (Ch. 9; Figure A4.10) and makes storm surges higher. Scientists cannot rule out the possibility of still more dramatic shifts if certain tipping elements trigger rapid and irreversible changes. While immediate and aggressive reductions in greenhouse gas emissions can mitigate future warming (KM 32.2) and reduce the risk of exceeding tipping points, temperatures will continue to increase until emissions of carbon dioxide reach net zero. When or if warming stops, long-term responses to the temperature changes that have already occurred will continue to drive changes for decades. Put simply, communities across the country are built for a climate that no longer exists.
Climate Is Changing, and Scientists Understand WhyIt is unequivocal that human activities have increased atmospheric levels of carbon dioxide and other greenhouse gases. It is also unequivocal that global average temperature has risen in response. Observed warming over the continental United States and Alaska is higher than the global average . Long-term changes have been observed in many other aspects of the climate system . The Earth system is complex and interconnected, which means changes in faraway regions are to affect the United States .
Extreme Events Are Becoming More Frequent and SevereObservations show an increase in the severity, extent, and/or frequency of multiple types of extreme events. Heatwaves have become more common and severe in the West since the 1980s . Drought risk has been increasing in the Southwest over the past century , while at the same time rainfall has become more extreme in recent decades, especially east of the Rockies . Hurricanes have been intensifying more rapidly since the 1980s and causing heavier rainfall and higher storm surges . More frequent and larger wildfires have been burning in the West in the past few decades due to a combination of climate factors, societal changes, and policies .
How Much the Climate Changes Depends on the Choices Made NowThe more the planet warms, the greater the impactsâand the greater the risk of unforeseen consequences . The impacts of climate change increase with warming, and warming is to continue if emissions of carbon dioxide do not reach net zero . Rapidly reducing emissions would limit future warming and the associated increases in many risks . While there are still uncertainties about how the planet will react to rapid warming and catastrophic future scenarios that cannot be ruled out, the future is largely in human hands.
AcknowledgmentsThe authors would like to thank Talia Resnick and Annika Larson for research assistance, including discussions about report structure, literature review, and bibliography management.
Process DescriptionMost team members were selected from the pool of nominations received via the public call for authors; others were identified through extended networks to ensure diverse representation across multiple axes. The following areas of expertise were identified as crucial for Chapter 2:
Author meetings were held virtually biweekly. Consensus was built by referring to the literature and leveraging the specific expertise of chapter authors. Engagement with other chapters occurred through formal presentations at the April 2022 chapter leadership meeting and one-on-one meetings between chapter lead authors.
It is unequivocal that human activities have increased atmospheric levels of carbon dioxide and other greenhouse gases. It is also unequivocal that global average temperature has risen in response. Observed warming over the continental United States and Alaska is higher than the global average . Long-term changes have been observed in many other aspects of the climate system . The Earth system is complex and interconnected, which means changes in faraway regions are to affect the United States .
Read about Confidence and Likelihood
The evidence base for human-caused increases in greenhouse gases (GHGs) is extensive and includes satellite and ground-based observations, solid theoretical understanding, and coherent measurements across multiple systems. Evidence for changes in aerosols includes long-term satellite and ground-based observations. Evidence for warming and other long-term climate changes has been extensively documented across multiple variables. Observations at smaller scales are noisier and regional signals more difficult to separate from natural internal climate variability.
Observational surface temperature records are available from a wide variety of scientific groups (e.g., Hansen et al. 2010; Vose et al. 2021; Morice et al. 2021; Rohde and Hausfather 20202,3,4,5). These temperature records combine land surface temperature data from weather stations with ocean sea surface temperature records from sources including ships and buoys. These records are corrected for inhomogeneities introduced by changes in measurement techniques over time and use various different interpolation techniques to estimate temperature anomalies between measurement locations.
Long-term changes have been observed in many other aspects of the climate system. Seasonal average and extreme precipitation changes are widely documented using observations, and changes are consistent with our physical understanding. The evidence base for ocean changes includes long-term surface and subsurface ocean observations of temperature, salinity, oxygen, and pH in the coastal and open ocean and satellite data.
Paleoclimate evidence includes multiple proxy-based reconstructions and modeling.
Sea level rise over the industrial era has been measured with local tide gauges and satellite altimetry (since the 1970s). Changes in the processes contributing to sea level rise (ocean thermal expansion, glacier and ice sheet melt, and terrestrial freshwater discharges) have been independently measured using in situ techniques in the ocean (e.g., floats, ship-based measurements) and on ice sheets, as well as remotely (e.g., satellite gravimetry and interferometry). Changes in sea ice and lake ice over the past several decades at the poles have been extensively documented from satellites, including visible imagery, altimetry, and microwave backscatter.
Drought has many definitions including meteorological drought, agricultural drought, snow drought, and soil moisture drought; soil moisture projections depend on depth, with surface layers more responsive to short-term temperature changes, while deeper root-zone moisture changes on longer timescales. Moreover, drought can be defined on timescales ranging from several weeks to multidecadal megadroughts. The level of uncertainty in drought changes in several regions depends on the definitions and metrics used and the sources of measurements. Surface-based measurements of soil moisture are limited, and reliable satellite-based measurements of soil moisture are less than a decade long.
Uncertainty in Global Surface Temperature ReconstructionsIn recent years, different groups producing global surface temperature records have somewhat converged in methodological approach, drawing on a larger set of collected weather station data256,257 and using more granular interpolation approaches rather than simple latitude/longitude grid-cell averaging.5 Published global surface temperature uncertainties on an annual basis range from ±0.13°C to ±0.2°C in the 1850s when records are more sparse to ±0.03°C to ±0.09°C at present across different surface temperature datasets, with differences between datasets driven by the number of measurements included, the spatial interpolation approach, and the method of uncertainty calculation.4
Uncertainty in GHG Emissions Estimates from Inventories and ModelsGHG emissions estimates are typically derived using either a âbottom-upâ or a âtop-downâ approach.258,259 The bottom-up approach uses a combination of activity data and emissions factors alongside empirical or process-based models to estimate the flux exchange between the different compartments of the landâoceanâatmosphere system. A primary advantage of the bottom-up approach is that it allows for explicit characterization of emissions and removals into specific sectors identified in the 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories.79,260 However, bottom-up emissions estimates can have significant uncertainty when the activity data or emissions factors are not well quantified or when process-based models are not well characterized due to missing processes or uncertain parameterization. On the other hand, the top-down approach aims to utilize the information from atmospheric greenhouse gas observations and atmospheric transport model to infer information about the distribution of emissions and removals at the surface of the Earth. For example, recent advancements in atmospheric CO2 observations from satellites and top-down modeling approaches have allowed insights into CO2 emissions and removals at the national scale.261 However, uncertainties in the modeling framework, spatial and temporal observational gaps, and uncertainties in the data may result in large uncertainties in the emissions estimates derived from the top-down approaches. Within the larger carbon cycle science community, various efforts are underway (e.g., REgional Carbon Cycle Assessment and Processes, Phase 2) to increase the level of agreement between estimates from these two approaches, thereby yielding more robust knowledge of GHG emissions.262
Uncertainty in Arctic Connections to Midlatitude Weather ExtremesUncertainties in the influence of the Arctic on midlatitude weather extremes remain due to lack of consistency in model responses and observations, particularly for the winter season. Several advances in the physical understanding of how Arctic processes could influence midlatitude extremes in various seasons have been made since the publication of the Fourth National Climate Assessment,263 yet the mechanisms continue to be a subject of debate in the scientific community.
Uncertainty in Drought Projections and DefinitionsDrought projections are complicated by definitional ambiguity and the use of many standard metrics. For example, there is ambiguity in the definition of âflash drought,â with more than 20 unique definitions present in the literature.264 Moreover, agricultural drought depends not just on precipitation and temperature but also on evaporation and transpiration from the land surfaceâprocesses that are projected to change in a warmer world (Ch. 3). Metrics such as the standardized precipitation index or the Palmer Drought Severity Index that rely on meteorological values may yield different projections than indices that take into account land changes (such as precipitation minus evaporation).265
It is unequivocal that global temperatures are increasing, and scientists are virtually certain that the planet has warmed between 1.1° and 1.2°C since the beginning of the industrial revolution, based on multiple observational datasets. There is very high confidence that this warming is driven by human-caused GHG emissions, which have increased by over 47% since 1850 based on modeling studies and theoretical understanding. There is very high confidence that changes outside the boundaries of the United States affect the Nationâs climate because scientists understand the mechanisms by which melting in Antarctica and Greenland affect sea level in the US.67 The links between tropical warming and atmospheric river intensity are due to well-understood atmospheric thermodynamics.98,99 A wide range of detection and attribution studies (discussed in Ch. 3 and summarized in Eyring et al. 2021266) establish that long-term changes due to climate change have been observed in many aspects of the climate system.
Observations show an increase in the severity, extent, and/or frequency of multiple types of extreme events. Heatwaves have become more common and severe in the West since the 1980s . Drought risk has been increasing in the Southwest over the past century , while at the same time rainfall has become more extreme in recent decades, especially east of the Rockies . Hurricanes have been intensifying more rapidly since the 1980s and causing heavier rainfall and higher storm surges . More frequent and larger wildfires have been burning in the West in the past few decades due to a combination of climate factors, societal changes, and policies .
Read about Confidence and Likelihood
Extreme events are rare by definition, but multiple datasets125,267 indicate they are increasing. The authors have a solid theoretical understanding of how some events (heatwaves, downpours) should increase in a warming world (Ch. 3). Others (e.g., agricultural drought) depend on multiple interacting physical processes.189,268 Event attribution now allows us to assign a quantifiable fraction of attributable risk to climate change (Ch. 3).
A wide variety of observational evidence exists for the occurrence of different storm types. Trained weather observers and storm spotters create storm reports across the country for severe hail, winds, and tornadoes. The National Weather Service (NWS) WSR-88D radar network maintains nationwide surveillance observations of precipitation, winds, and storm occurrence. The NWS additionally conducts storm damage surveys for high-impact events. NOAA and Air Force Hurricane Hunters conduct surveillance flights into tropical cyclones expected to impact US interests. NOAA geostationary satellite observations maintain a record of cloud properties and lightning occurrence. However, the length and representativeness of each data source are variable; storm reporting relates to population density and exposure, and the availability of trained observers impacts record quality, especially for transient phenomena such as hail.
There is growing evidence that the impacts of climate change are, and will be, distributed unequally across US populations due to differences in both exposure and vulnerability. However, there are gaps in understanding the community-level impacts of projected changes in extreme events. Vulnerability at this level is in part a function of our investments (capital, operations, and management) in the built environment and natural resource functionality that serve to buffer these impacts (e.g., stormwater conveyance and levees to reduce flooding, water storage to relieve water shortages during drought). There is a lack of systematic assessment of these assets and other facets of vulnerability across the United States.
New literature has emerged documenting changes in certain types of compounding extremes such as heat and drought, but the limited observed record hinders quantifying long-term trends in several other compound extremes. Several frameworks for studying various compound extremes have emerged as well, and the physical understanding of certain compound extreme events such as heat/drought, heat/humidity, and coastal wind/precipitation/flooding has been documented, yet the understanding of the physical drivers of many other compound extremes is still emerging. Therefore, there are gaps in methodological advances, advances in understanding of their physical drivers, and studies quantifying projections in compound extreme risks.
The lack of homogenized daily and hourly temperature datasets limits our ability to reliably assess the evolution of extreme heat events over century-scale periods, although the availability of modern reanalysis products has increased agreement in changes in extreme heat events over the past 50 years.
There is limited research on changes in lightning activity due to lack of a long-term observational record. Satellite-based records and lightning detection networks are not sufficiently long to allow for detecting trends. Lightning can pose major hazards to society including direct casualties, igniting wildfires, and damaging energy infrastructure.166
There is very high confidence that heatwaves globally are becoming more frequent and severe, based on multiple observational datasets. In the United States, there is high confidence that heatwaves in the West are becoming more common and severe based on observational records since 1901 (Figure 2.7). There is also very high confidence that climate change is and will continue to make rainfall extremes more intense. Basic physical understanding and climate models both provide robust explanations for the links between climate change and observed changes in these extremes: this is why the authors also have high confidence that storms are delivering more rainfall and high confidence that storm surges are becoming higher. There is very high confidence that the Southwest is experiencing more severe drought: a recent paper found the 2002â2022 multidecadal soil moisture drought was the worst in the past 1,200 years.134 The eastern region is experiencing reduced drought risk; studies suggest a transition toward more frequent extremes140 and indicate that warming may partially counteract the effects of increased precipitation. Other extremes involve more complex interactions between human and natural systems: the occurrence and impacts of wildfires depend on fire ignition and suppression practices. However, while fire risk is not solely determined by climate factors, the authors have very high confidence that the hot and dry weather conditions that elevate fire risk are becoming more common.
The more the planet warms, the greater the impactsâand the greater the risk of unforeseen consequences . The impacts of climate change increase with warming, and warming is to continue if emissions of carbon dioxide do not reach net zero . Rapidly reducing emissions would limit future warming and the associated increases in many risks . While there are still uncertainties about how the planet will react to rapid warming and catastrophic future scenarios that cannot be ruled out, the future is largely in human hands.
Read about Confidence and Likelihood
The Shared Socioeconomic Pathways (SSPs) were made available to the broader research community, replacing the old Representative Concentration Pathways (RCPs) and providing a more detailed assessment of the range of possible emissions pathways, as well as mitigation and adaptation challenges across different sets of socioeconomic assumptions.269 A subset of the SSPs served as the basis for CMIP6 scenarios used in this Assessment and the Sixth Assessment Report (AR6) of the IPCC.88
CMIP6 provides a large set of model runs to use in evaluating different future emissions pathways and global warming levels. In addition, recent work assessing multiple lines of evidence from observational data, paleoclimate evidence, and physical process modeling has helped narrow the range of climate sensitivity.231 The IPCC AR6 produced a new set of assessed warming projections based on these climate sensitivity estimates and CMIP6 models that were weighted based on their performance in reproducing historical temperatures.
The IPCC AR6 Working Group III255 report explored a wider range of âovershootâ scenarios, where global temperatures temporarily exceed 1.5°C before being reduced through the large-scale use of negative-emissions technologies. Additionally, AR6 Working Group I provided a thorough exploration of the zero-emissions commitment associated with the cessation of carbon dioxide (CO2) and other GHG emissions, building off the work of the Zero Emissions Commitment Model Intercomparison Project (ZECMIP).226
Recent literature summarized in IPCC AR6 Working Group III255 and in Hausfather and Moore (2022)218 provides a clearer sense of expected global average surface temperature outcomes under scenarios including only current policy, near-term 2030 commitments, and long-term net-zero commitments.
Major uncertainties remain surrounding the emissions trajectories implied by current policies and the plausibility of worse-than-current-policy emissions outcomes. While most current policy scenarios in the literature project relatively flat global emissions over the next few decades, there are some (e.g., in the IPCC AR6 Working Group III scenario database) in which emissions continue to increase. Similarly, large uncertainties remain when translating near-term and long-term mitigation commitments to global emissions pathways, particularly for non-CO2 GHGs and other climate forcings like aerosols.
The translation from emissions scenarios to warming outcomes is complicated by uncertainties in both the sensitivity of the climate to emissions (both the transient climate response and the equilibrium climate sensitivity) and carbon cycle feedbacks that may affect the portion of emissions that accumulate in the atmosphere. Specifically for carbon cycle feedbacks, it will be the balance between the response of land and ocean systems to future climate that will determine the strength and extent of carbon uptake by these systems, whether they may become a net source of CO2 to the atmosphere, and, consequently, the trajectory of future GHG forcings.
While recent work231 has meaningfully narrowed the potential range of climate sensitivity, there are still tail risks of outcomes where equilibrium climate sensitivity exceeds 5°C or is below 2°C per doubling of atmospheric CO2. There is also disagreement between a subset of high-sensitivity CMIP6 models and other lines of evidence supporting a narrower range of climate sensitivity.270
On timescales of less than 50 years, the most significant uncertainties in future sea level are due to regional and local variations in sea level rise and the interannual sea level variability intrinsic to the coastal ocean system. In Alaska and New England, the regional gravitational influence of glaciers and ice sheets may cause lower sea level rise or even sea level fall in the future, although the extent of these gravitational effects is highly dependent on the spatial fingerprint of glacier and ice sheet loss, which is uncertain.100 Internal variability and human-caused changes in ocean circulation appear to have a strong effect on year-to-year sea level, particularly in the US Mid-Atlantic Coast,271 but are not consistently simulated between models or included in the range of uncertainty in most sea level projections.67
On longer timescales (2100 and beyond), there are substantial uncertainties in projected sea level rise due to an incomplete understanding (and intermodel differences) of how the Greenland and Antarctic ice sheets will behave in a warmer climate. There is a consensus that past carbon emissions and even relatively moderate future global warming levels commit the planet to at least 3â6 feet of sea level rise over hundreds to thousands of years from the melting of the Greenland and Antarctic ice sheets.272 However, there are many feet of uncertainty remaining both in the already-committed sea level rise and the sea level rise that could be expected under a range of global warming levels.102,209,273,274 Ongoing research to understand how glaciers and ice sheets flow, fracture, and melt in response to climate change aims to narrow this wide range in sea level rise beyond 2100.
Projections of seasonal and extreme precipitation are widely studied and show more consistent and robust responses in extremes than average changes. The physical process link between higher temperatures and higher moisture availability in the atmosphere is well documented and understood. However, uncertainties remain in our understanding of the response of precipitation-producing systems, particularly those governed by mesoscale processes such as mesoscale convective systems and thunderstorms, which are not directly simulated in global climate models. Uncertainties, especially around how other factors that influence storm development (such as vertical wind shear and atmospheric instability) will change in future climates, link back to model uncertainty and bias at larger scales.
Uncertainty in drought projections arises from these uncertainties in precipitation. Climate models generally project drying in the US Southwest in response to elevated global warming levels, but the precipitation response is highly uncertain. The response of land vegetation also complicates drought projections. Under elevated CO2 levels, certain types of plants may become more efficient at using water due to a physiological response. This is expected to be at least partially counteracted by greening in response to elevated CO2 levels. Additionally, the vegetation response to increased heat stress, extreme precipitation, and fire risk is complex and not yet fully understood.
There is very high confidence that many impactsâboth changes to the average state and the risk of extreme eventsâwill intensify as the temperature increases. This is based on physical understanding of the underlying drivers reflected in climate models of varying complexity, including the state-of-the-art general circulation models participating in CMIP6.275 It is an unequivocal fact, backed by over 100 years of theory and observation, that warming increases with GHG emissions,276 and warming is virtually certain to continue at current levels of emissions. There is very high confidence that warming will continue at least until emissions of carbon dioxide reach net zero. The cessation of warming at the point of (net) zero CO2 emissions (called the zero-emissions commitment, or ZEC) traces back to Matthews and Caldeira (2008),277 Solomon et al. (2009),278 and Matthews and Weaver (2010),279 who were among the first to explore zero-emissions scenarios in emissions-driven climate model runs. The common conflation of constant concentration scenarios with zero-emissions scenarios has led to the misconception that substantial future warming is inevitable. In the lead-up to AR6, there was a desire by the community to further explore the robustness of ZEC results. This led to the creation of ZECMIP, where 18 different Earth system models were used to examine ZEC under a variety of emissions-reduction pathways and cumulative emissions scenarios. ZECMIP broadly found that ZEC was 0ºC ± 0.3ºC across the Earth system models examined. 226 Hence, rapidly reducing emissions would very likely limit future warming (very high confidence). It is very likely that the eventual global warming in response to a doubling of atmospheric CO2 is between 2.3° and 4.7°C and likely that the warming would be between 2.6° and 3.9°C.231 There is high confidence that catastrophic scenarios where warming exceeds 4°C cannot be ruled out due to uncertainties in climate sensitivity, carbon cycle feedbacks,238,280 and emissions scenarios.281
Likelihood Virtually Certain Very Likely Likely As Likely as Not Unlikely Very Unikely Exceptionally Unlikely 99%–100% 90%–100% 66%–100% 33%–66% 0%–33% 0%–10% 0%–1% Confidence Level Very High High Medium LowLooking for U.S. government information and services?
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