| The four SRES scenario families [1] of the Fourth Assessment Report vs. projected global average surface warming until 2100 | ||
More economic focus | More environmental focus | |
| Globalisation (homogeneous world) | A1 rapid economic growth (groups: A1T; A1B; A1Fl) 1.4–6.4 °C | B1 global environmental sustainability 1.1–2.9 °C |
| Regionalisation (heterogeneous world) | A2 regionally oriented economic development 2.0–5.4 °C | B2 local environmental sustainability 1.4–3.8 °C |
Climate Change 2007, the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCC), was published in 2007 and is the fourth in a series of reports intended to assess scientific, technical and socio-economic information concerning climate change, its potential effects, and options for adaptation and mitigation. [2] The report is the largest and most detailed summary of the climate change situation ever undertaken, produced by thousands of authors, editors, and reviewers from dozens of countries, citing over 6,000 peer-reviewed scientific studies. People from over 130 countries contributed to the IPCC Fourth Assessment Report, which took six years to produce. [2] Contributors to AR4 included more than 2,500 scientific expert reviewers, more than 800 contributing authors, and more than 450 lead authors. [2]
"Robust findings" of the Synthesis report include: [3]
Like previous assessment reports, it consists of four reports:
Global warming projections from AR4 [3] are shown below. The projections apply to the end of the 21st century (2090–2099), relative to temperatures at the end of the 20th century (1980–1999). Add 0.7 °C to projections to make them relative to pre-industrial levels instead of 1980–1999. (UK Royal Society, 2010, p=10[ citation needed ]). [5] Descriptions of the greenhouse gas emissions scenarios can be found in Special Report on Emissions Scenarios.
| Emissions scenario | Best estimate (°C) | "Likely" range (°C) |
|---|---|---|
| B1 | 1.8 | 1.1 – 2.9 |
| A1T | 2.4 | 1.4 – 3.8 |
| B2 | 2.4 | 1.4 – 3.8 |
| A1B | 2.8 | 1.7 – 4.4 |
| A2 | 3.4 | 2.0 – 5.4 |
| A1FI | 4.0 | 2.4 – 6.4 |
"Likely" means greater than 66% probability of being correct, based on expert judgement. [4]
| Intergovernmental Panel on Climate Change |
|---|
 |
The report was released in four principal sections:
The full WGI report [6] was published in March 2007, and last updated in September of that year. It includes a Summary for Policymakers (SPM), which was published in February 2007, and a Frequently Asked Questions section.
This section of the report, Climate Change 2007: The Physical Science Basis, assessed current scientific knowledge of "the natural and human drivers of climate change" as well as observed changes in climate. It looked at the ability of science to attribute changes to different causes, and made projections of future climate change.
It was produced by 676 authors (152 lead authors, 26 review editors, and 498 contributing authors) from 40 countries, then reviewed by over 625 expert reviewers. More than 6,000 peer-reviewed publications were cited. [10]
Before being approved, the summary was reviewed line by line by representatives of 113 governments during the 10th session of WGI, in January to February 2007. [11]
On the issue of global warming and its causes, the SPM states that:
Very likely and likely mean "the assessed likelihood, using expert judgment" are over 90% and over 66%, respectively. [14]
The report notes many observed changes in the Earth's climate including atmospheric composition, global average temperatures, ocean conditions, and other climate changes.
Carbon dioxide, methane, and nitrous oxide are all long-lived greenhouse gases.
Cold days, cold nights, and frost events have become less frequent. Hot days, hot nights, and heat waves have become more frequent. Additionally:
The SPM documents increases in wind intensity, decline of permafrost coverage, and increases of both drought and heavy precipitation events. Additionally:
Table SPM-2 lists recent trends along with certainty levels for the trend having actually occurred, for a human contribution to the trend, and for the trend occurring in the future. In relation to changes (including increased hurricane intensity) where the certainty of a human contribution is stated as "more likely than not" footnote f to table SPM-2 notes "Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies."
AR4 describes warming and cooling effects on the planet in terms of radiative forcing—the rate of change of energy in the system, measured as power per unit area (in SI units, W/m2). The report shows in detail the individual warming contributions (positive forcing) of carbon dioxide, methane, nitrous oxide, halocarbons, other human warming factors, and the warming effects of changes in solar activity. Also shown are the cooling effects (negative forcing) of aerosols, land-use changes, and other human activities. All values are shown as a change from pre-industrial conditions.
Climate sensitivity is defined as the amount of global average surface warming following a doubling of carbon dioxide concentrations. [16] It is likely to be in the range of 2 to 4.5 °C, with a best estimate of about 3 °C. [16] This range of values is not a projection of the temperature rise we will see in the 21st century, since the future change in carbon dioxide concentrations is unknown, and factors besides carbon dioxide concentrations affect temperature. [16]
Model projections are made based on an analysis of various computer climate models running within the different scenarios that were established in 2000 in the Special Report on Emissions Scenarios (the "SRES scenarios"). As a result, predictions for the 21st century are as shown below.
Scenario-specific projections are based on analysis of multiple runs by multiple climate models, using the various SRES Scenarios. "Low scenario" refers to B1, the most optimistic scenario family. "High scenario" refers to A1FI, the most pessimistic scenario family.
There are six families of SRES scenarios, and AR4 provides projected temperature and sea level rises (excluding future rapid dynamical changes in ice flow) for each scenario family. [20]
In the weeks before publication of the first report, controversy broke out about the report's projections of sea-level change, which in the new report was estimated at less than previous estimates. The now-published text gives a warning that the new estimation of sea-level could be too low: "Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise." The mid-points of the sea level rise estimates are within ±10% of those from the TAR; but the range has narrowed.
Lord Rees, the president of the Royal Society, said, "This report makes it clear, more convincingly than ever before, that human actions are writ large on the changes we are seeing, and will see, to our climate. The IPCC strongly emphasises that substantial climate change is inevitable, and we will have to adapt to this. This should compel all of us—world leaders, businesses and individuals—towards action rather than the paralysis of fear. We need both to reduce our emissions of greenhouse gases and to prepare for the impacts of climate change. Those who would claim otherwise can no longer use science as a basis for their argument." [21]
U.S. Energy Secretary Samuel Bodman told a news conference that the report was "sound science" and "As the president has said, and this report makes clear, human activity is contributing to changes in our earth's climate and that issue is no longer up for debate." [22] Kurt Volker, Principal Deputy Assistant Secretary for European and Eurasian Affairs, said, "We support the recent IPCC report, in which U.S. scientists played a leading role." [23]
Based on the report, 46 countries in a "Paris Call for Action" read out by French President Chirac, have called for the creation of a United Nations Environment Organization (UNEO), which is to have more power than the current United Nations Environment Programme (UNEP), and is to be modelled after the more powerful World Health Organization. The 46 countries included the European Union nations, but notably did not include the United States, China, Russia, and India, the top four emitters of greenhouse gases. [24]
Working Group II's Summary for Policymakers [25] was released on 6 April 2007. The full report was released 18 September 2007.
WGII states that "evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases".
Some observed changes have been associated with climate change at varying levels of confidence.
With a high confidence (about an 8 in 10 chance to be correct) WGII asserts that climate change has resulted in:
With a very high confidence (about a 9 in 10 chance to be correct) WGII asserts that climate change is affecting terrestrial biological systems in that:
WGII also states that the ocean has become more acidic because it has absorbed human-caused carbon dioxide. Ocean pH has dropped by 0.1, but how this affects marine life is not documented.
WGII acknowledges some of the difficulties of attributing specific changes to human-caused global warming, stating that "Limitations and gaps prevent more complete attribution of the causes of observed system responses to anthropogenic warming." but found that the agreement between observed and projected changes was "Nevertheless ... sufficient to conclude with high confidence that anthropogenic warming over the last three decades has had a discernible influence on many physical and biological systems."
WGII describes some of what might be expected in the coming century, based on studies and model projections.
It is projected with high confidence that:
It is projected with high confidence that:
It is projected with medium confidence (about 5 in 10 chance to be correct) that globally, potential food production will increase for temperature rises of 1–3 °C, but decrease for higher temperature ranges.
It is projected with very high confidence that:
US negotiators managed to eliminate language calling for cuts in greenhouse gas emissions, according to Patricia Romero Lankao, a lead author from the National Center for Atmospheric Research (NCAR). The original draft read: "However, adaptation alone is not expected to cope with all the projected effects of climate change, and especially not over the long run as most impacts increase in magnitude. Mitigation measures will therefore also be required." The second sentence does not appear in the final version of the report. [26]
China objected to wording that said "based on observed evidence, there is very high confidence that many natural systems, on all continents and in most oceans, are being affected by regional climate changes, particularly temperature increases". When China asked that the word "very" be stricken, three scientific authors balked, and the deadlock was broken only by a compromise to delete any reference to confidence levels. [26]
Working Group III's Summary for Policymakers (SPM) [27] was published on 4 May 2007 at the 26th session of the IPCC. [28] The full WG III report was published online in September 2007. [29]
The IPCC convened in Bangkok on April 30 to start discussions on the draft Summary, with the participation of over 400 scientists and experts from about 120 countries. [30] At the full IPCC meeting on May 4, agreement was reached by the larger gathering of some 2,000 delegates. One of the key debates concerned a proposal to limit concentrations of greenhouse gases in the atmosphere to between 445 parts per million and 650 parts per million to avoid dangerous climate change, with pressure from developing countries to raise the lower limit. Despite this, the figures from the original proposal were incorporated into the Summary for Policymakers. [31] The Summary concludes that stabilization of greenhouse gas concentrations is possible at a reasonable cost, with stabilization between 445 ppm and 535 ppm costing less than 3% of global GDP. [32]
The WG III report analyses mitigation options for the main sectors in the near-term, addressing also cross-sectorial matters such as synergies, co-benefits, and trade-offs. It also provides information on long-term mitigation strategies for various stabilization levels, paying special attention to implications of different short-term strategies for achieving long-term goals. [33]
The Summary for Policymakers concludes that there was a high level of agreement and much evidence that "there is substantial economic potential for the mitigation of global greenhouse gas emissions over the coming decades, that could offset the projected growth of global emissions or reduce emissions below current levels", [34] taking into account financial and social costs and benefits. [35] The technologies with the largest economic potential within this timescale are considered to be: [36]
| Sector | Key mitigation technologies and practices currently commercially available | Key mitigation technologies and practices projected to be commercialized before 2030 |
| Energy Supply | Improved supply and distribution efficiency; fuel switching from coal to gas; nuclear power; renewable heat and power (hydropower, solar, wind, geothermal and bioenergy); combined heat and power; early applications of CCS (e.g. storage of removed CO2 from natural gas) | Carbon Capture and Storage (CCS) for gas, biomass and coal-fired electricity generating facilities; advanced nuclear power; advanced renewable energy, including tidal and waves energy, concentrating solar, and solar PV. |
| Transport | More fuel efficient vehicles; electric vehicle; hybrid vehicles; cleaner diesel vehicles; biofuels; modal shifts from road transport to rail and public transport systems; non-motorised transport (cycling, walking); land-use and transport planning | Second generation biofuels; higher efficiency aircraft; advanced electric and hybrid vehicles with more powerful and reliable batteries |
| Buildings | Efficient lighting and daylighting; more efficient electrical appliances and heating and cooling devices; improved cook stoves, improved insulation; passive and active solar design for heating and cooling; alternative refrigeration fluids, recovery and recycle of fluorinated gases | Integrated design of commercial buildings including technologies, such as intelligent meters that provide feedback and control; solar PV integrated in buildings |
| Industry | More efficient end-use electrical equipment; heat and power recovery; material recycling and substitution; control of non-CO2 gas emissions; and a wide array of process-specific technologies | Advanced energy efficiency; CCS for cement, ammonia, and iron manufacture; inert electrodes for aluminium manufacture |
| Agriculture | Improved crop and grazing land management to increase soil carbon storage; restoration of cultivated peaty soils and degraded lands; improved rice cultivation techniques and livestock and manure management to reduce CH 4 emissions; improved nitrogen fertilizer application techniques to reduce N 2O emissions; dedicated energy crops to replace fossil fuel use; improved energy efficiency | Improvements of crop yields |
| Forestry/forests | Afforestation; reforestation; forest management; reduced deforestation; harvested wood product management; use of forestry products for bio-energy to replace fossil fuel use | Tree species improvement to increase biomass productivity and carbon biosequestration. Improved remote sensing technologies for analysis of vegetation/ soil carbon sequestration potential and mapping land use change |
| Waste | Landfill methane recovery; waste incineration with energy recovery; composting of organic waste; controlled waste water treatment; recycling and waste minimization | Biocovers and biofilters to optimize CH 4 oxidation |
The IPCC estimates that stabilizing atmospheric greenhouse gases at between 445–535 ppm CO2 equivalent would result in a reduction of average annual GDP growth rates of less than 0.12%. Stabilizing at 535 to 590 ppm would reduce average annual GDP growth rates by 0.1%, while stabilization at 590 to 710 ppm would reduce rates by 0.06%. [37] There was high agreement and much evidence that a substantial fraction of these mitigation costs may be offset by benefits to health as a result of reduced air pollution, and that there would be further cost savings from other benefits such as increased energy security, increased agricultural production, and reduced pressure on natural ecosystems as well as, in certain countries, balance of trade improvements, provision of modern energy services to rural areas and employment. [38]
The IPCC considered that achieving these reductions would require a "large shift in the pattern of investment, although the net additional investment required ranges from negligible to 5–10%".They also concluded that it is often more cost effective to invest in end-use energy efficiency improvement than in increasing energy supply. [39]
In terms of electricity generation, the IPCC envisage that renewable energy can provide 30 to 35% of electricity by 2030 (up from 18% in 2005) at a carbon price of up to US$50/t, and that nuclear power can rise from 16% to 18%. They also warn that higher oil prices might lead to the exploitation of high-carbon alternatives such as oil sands, oil shales, heavy oils, and synthetic fuels from coal and gas, leading to increasing emissions, unless carbon capture and storage technologies are employed. [40]
In the transport sector there was a medium level of agreement and evidence that the multiple mitigation options may be counteracted by increased use, and that there were many barriers and a lack of government policy frameworks. [41]
There was high agreement and much evidence that, despite many barriers (particularly in the developing countries), new and existing buildings could reduce emissions considerably, and that this would also provide other benefits in terms of improved air quality, social welfare and energy security. [42]
The IPCC reported that the effectiveness of mitigation efforts over the next two or three decades would have a large impact on the ability to stabilize atmospheric greenhouse gases at lower levels, and that the lower the ultimate stabilization levels, the more quickly emissions would need to peak and decline. [43] For example, to stabilize at between 445 and 490 ppm (resulting in an estimate global temperature 2 to 2.4 °C above the pre-industrial average) emissions would need to peak before 2015, with 50 to 85% reductions on 2000 levels by 2050. [44]
There was high agreement and much evidence that stabilization could be achieved by 2050 using currently available technologies, provided appropriate and effective incentives were put in place for their development, acquisition, deployment and diffusion, and that barriers were removed. [45] For stabilization at lower levels the IPCC agreed that improvements of carbon intensity need to be made much faster than has been the case in the past, and that there would be a greater need for efficient public and private research, development and demonstration efforts and investment in new technologies during the next few decades. [46] The IPCC points out that government funding in real absolute terms for most energy research programmes has been flat or declining for nearly 20 years, and is now about half the 1980 level. [47] Delays in cutting emissions would lead to higher stabilization levels and increase the risk of more severe climate change impacts, as more of the current high-emission technologies would have been deployed. [48]
Among the measures that might be used, there was high agreement and much evidence that policies that put a price on the cost of carbon emissions could provide incentives for consumers and producers. Carbon prices of 5 to 65 US$/tCO2 in 2030 and 15 to 130 US$/tCO2 by 2050 are envisaged for stabilization at around 550 ppm by 2100. [49]
A draft version of the Synthesis Report, said to be subject to final copyedit, was published on 16 November 2007. [50]
The Synthesis Report goes one step further [than the first three Climate Change 2007 Working Group Reports]: it is the decisive effort to integrate and compact this wealth of information into a readable and concise document explicitly targeted to the policymakers.
The Synthesis Report also brings in relevant parts some material contained in the full Working Group Reports over and above what is included in the Summary for Policymakers in these three Reports. It is designed to be a powerful, scientifically authoritative document of high policy relevance, which will be a major contribution to the discussions at the 13th Conference of the Parties in Bali during December 2007. In fact, this Conference was postponed to December to allow the IPCC Synthesis Report to come out first.
The six topics [51] addressed in the Synthesis Report are:
The "Convention" mentioned in Topic 5 is the UN Framework Convention on Climate Change (UNFCCC).
The key findings from the AR4 Synthesis Report will be discussed Wednesday 13 December 2007 [52] at the United Nations Climate Change Conference (UNFCCC COP 13—CMP 3) in Bali, Indonesia, which takes place 3–14 December (see UNFCCC home page [53] ).
The SPM states that "Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."[ citation needed ]
The following table is based on Chapter 2 of the Working Group 1 Report. [54]
| Mole fractions and their changes | Radiative forcing | |||
|---|---|---|---|---|
| Species | 2005 | Change since 1998 | 2005 (W m−2) | 1998 (%) |
| CO2 | 379 ± 0.65 μmol/mol | +13 μmol/mol | 1.66 | +13 |
| CH4 | 1,774 ± 1.8 nmol/mol | +11 nmol/mol | 0.48 | – |
| N2O | 319 ± 0.12 nmol/mol | +5 nmol/mol | 0.16 | +11 |
| CFC-11 | 251 ± 0.36 pmol/mol | −13 | 0.063 | −5 |
| CFC-12 | 538 ± 0.18 pmol/mol | +4 | 0.17 | +1 |
| CFC-113 | 79 ± 0.064 pmol/mol | −4 | 0.024 | −5 |
| HCFC-22 | 169 ± 1.0 pmol/mol | +38 | 0.033 | +29 |
| HCFC-141b | 18 ± 0.068 pmol/mol | +9 | 0.0025 | +93 |
| HCFC-142b | 15 ± 0.13 pmol/mol | +6 | 0.0031 | +57 |
| CH3CCl3 | 19 ± 0.47 pmol/mol | −47 | 0.0011 | −72 |
| CCl4 | 93 ± 0.17 pmol/mol | −7 | 0.012 | −7 |
| HFC-125 | 3.7 ± 0.10 pmol/mol | +2.6 | 0.0009 | +234 |
| HFC-134a | 35 ± 0.73 pmol/mol | +27 | 0.0055 | +349 |
| HFC-152a | 3.9 ± 0.11 pmol/mol | +2.4 | 0.0004 | +151 |
| HFC-23 | 18 ± 0.12 pmol/mol | +4 | 0.0033 | +29 |
| SF6 | 5.6 ± 0.038 pmol/mol | +1.5 | 0.0029 | +36 |
| CF4 (PFC-14) | 74 ± 1.6 pmol/mol | – | 0.0034 | – |
| C2F6 (PFC-116) | 2.9 ± 0.025 pmol/mol | +0.5 | 0.0008 | +22 |
The New York Times reported that "the leading international network of climate scientists has concluded for the first time that global warming is 'unequivocal' and that human activity is the main driver, 'very likely' causing most of the rise in temperatures since 1950". [55]
The same newspaper wrote: "The Intergovernmental Panel on Climate Change said the likelihood was 90 percent to 99 percent that emissions of heat-trapping greenhouse gases like carbon dioxide, spewed from tailpipes and smokestacks, were the dominant cause of the observed warming of the last 50 years. In the panel's parlance, this level of certainty is labeled 'very likely'. Only rarely does scientific odds-making provide a more definite answer than that, at least in this branch of science, and it describes the endpoint, so far, of a progression." [56]
The Associated Press summarized the position on sea level rise: [57]
On sea levels, the report projects rises of 7 to 23 inches by the end of the century. An additional 3.9 to 7.8 inches are possible if recent, surprising melting of polar ice sheets continues.
The Fourth Assessment Report has been the subject of criticism. Skeptics of anthropogenic global warming contend that their claims are not sufficiently incorporated in the report. Others regard the IPCC as too conservative in its estimates of potential harm from climate change. The report has also been criticized for inclusion of an erroneous date for the projected demise of the Himalayan glaciers.
Related to the subject of global warming in general, the IPCC Fourth Assessment Report has been discussed by various bodies such as government officials, special interest groups and scientific organizations; see the article "Politics of global warming" for a thorough discussion of the politics surrounding the phenomenon, and the positions of the various parties involved.
The United Nations appointed an independent board of scientists to "review the workings of the world's top climate science panel" which reported in September 2010; [58] see Intergovernmental Panel on Climate Change#InterAcademy Council review in 2010.
Several science academies have referred to and/or reiterated some of the conclusions of AR4. These include:
The Netherlands Environmental Assessment Agency (PBL, et al., 2009; [65] 2010) [66] has carried out two reviews of AR4. These reviews are generally supportive of AR4's conclusions. [67] [68] PBL (2010) [68] make some recommendations to improve the IPCC process. A literature assessment by the US National Research Council (US NRC, 2010) [69] concludes:
Climate change is occurring, is caused largely by human activities, and poses significant risks for—and in many cases is already affecting—a broad range of human and natural systems [emphasis in original text]. ... This conclusion is based on a substantial array of scientific evidence, including recent work, and is consistent with the conclusions of recent assessments by the U.S. Global Change Research Program ..., the Intergovernmental Panel on Climate Change's Fourth Assessment Report ..., and other assessments of the state of scientific knowledge on climate change.
Some errors have been found in the IPCC AR4 Working Group II report. Two errors include the melting of Himalayan glaciers (see later section), and Dutch land area that is below sea level. [70]
A paragraph in the 2007 Working Group II report ("Impacts, Adaptation and Vulnerability"), chapter 10 included a projection that Himalayan glaciers could disappear by 2035:
Glaciers in the Himalaya are receding faster than in any other part of the world (see Table 10.9) and, if the present rate continues, the likelihood of them disappearing by the year 2035 and perhaps sooner is very high if the Earth keeps warming at the current rate. Its total area will likely shrink from the present 500,000 to 100,000 km2 by the year 2035 (WWF, 2005).
This projection was not included in the final summary for policymakers. The IPCC has since acknowledged that the date is incorrect, while reaffirming that the conclusion in the final summary was robust. They expressed regret for "the poor application of well-established IPCC procedures in this instance". The date of 2035 has been correctly quoted by the IPCC from the WWF report, which has misquoted its own source, an ICSI report "Variations of Snow and Ice in the past and at present on a Global and Regional Scale".
Rajendra K. Pachauri responded in an interview with Science. [71]
Former IPCC chairman Robert Watson said, regarding the Himalayan glaciers estimation, "The mistakes all appear to have gone in the direction of making it seem like climate change is more serious by overstating the impact. That is worrying. The IPCC needs to look at this trend in the errors and ask why it happened". [72] Martin Parry, a climate expert [73] who had been co-chair of the IPCC working group II, said that "What began with a single unfortunate error over Himalayan glaciers has become a clamour without substance" and the IPCC had investigated the other alleged mistakes, which were "generally unfounded and also marginal to the assessment". [74]
The IPCC Fourth Assessment Report featured a graph showing 12 proxy based temperature reconstructions, including the three highlighted in the 2001 Third Assessment Report (TAR); Mann, Bradley & Hughes 1999 as before, Jones et al. 1998 and Briffa 2000 had both been calibrated by newer studies. In addition, analysis of the Medieval Warm Period cited reconstructions by Crowley & Lowery 2000 (as cited in the TAR) and Osborn & Briffa 2006. Ten of these 14 reconstructions covered 1,000 years or longer. Most reconstructions shared some data series, particularly tree ring data, but newer reconstructions used additional data and covered a wider area, using a variety of statistical methods. The section discussed the divergence problem affecting certain tree ring data. [75]
Scientific studies have investigated the causes of climate change. They have found that the main cause and driver of recent climate change is elevated levels of greenhouse gases produced by human activities. Natural forces add climate variability as well. Based on many scientific studies, it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." Studies on attribution have focused on changes observed during the period of instrumental temperature record, particularly in the last 50 years. This is the period when human activity has grown fastest and observations of the atmosphere above the surface have become available. Some of the main human activities that contribute to global warming are: (a) increasing atmospheric concentrations of greenhouse gases, for a warming effect; (b) global changes to land surface, such as deforestation, for a warming effect; and (c) increasing atmospheric concentrations of aerosols, mainly for a cooling effect.
The United Nations Framework Convention on Climate Change (UNFCCC) is an international treaty among countries to combat "dangerous human interference with the climate system", in part by stabilizing greenhouse gas concentrations in the atmosphere. It was signed in 1992 by 154 states at the United Nations Conference on Environment and Development (UNCED), informally known as the Earth Summit, held in Rio de Janeiro. Its secretariat was in Geneva at first but relocated to Bonn in 1996. The treaty entered into force on 21 March 1994. "UNFCCC" is also the name of the Secretariat charged with supporting the operation of the convention, with offices on the UN Campus in Bonn, Germany.
A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources. These equations are the basis for computer programs used to simulate the Earth's atmosphere or oceans. Atmospheric and oceanic GCMs are key components along with sea ice and land-surface components.
The IPCC Third Assessment Report (TAR), Climate Change 2001, is an assessment of available scientific and socio-economic information on climate change by the IPCC. Statements of the IPCC or information from the TAR were often used as a reference showing a scientific consensus on the subject of global warming. The Third Assessment Report (TAR) was completed in 2001 and consists of four reports, three of them from its Working Groups: Working Group I: The Scientific Basis; Working Group II: Impacts, Adaptation and Vulnerability; Working Group III: Mitigation; Synthesis Report. A number of the TAR's conclusions are given quantitative estimates of how probable it is that they are correct, e.g., greater than 66% probability of being correct. These are "Bayesian" probabilities, which are based on an expert assessment of all the available evidence.
The Second Assessment Report (SAR) of the Intergovernmental Panel on Climate Change (IPCC), published in 1995, is an assessment of the then available scientific and socio-economic information on climate change. The report was split into four parts: a synthesis to help interpret UNFCCC article 2, The Science of Climate Change, Impacts, Adaptations and Mitigation of Climate Change, Economic and Social Dimensions of Climate Change. Each of the last three parts was completed by a separate Working Group (WG), and each has a Summary for Policymakers (SPM) that represents a consensus of national representatives.
The Special Report on Emissions Scenarios (SRES) is a report by the Intergovernmental Panel on Climate Change (IPCC) that was published in 2000. The greenhouse gas emissions scenarios described in the Report have been used to make projections of possible future climate change. The SRES scenarios, as they are often called, were used in the IPCC Third Assessment Report (TAR), published in 2001, and in the IPCC Fourth Assessment Report (AR4), published in 2007. The SRES scenarios were designed to improve upon some aspects of the IS92 scenarios, which had been used in the earlier IPCC Second Assessment Report of 1995. The SRES scenarios are "baseline" scenarios, which means that they do not take into account any current or future measures to limit greenhouse gas (GHG) emissions.
Climate change affects the physical environment, ecosystems and human societies. Changes in the climate system include an overall warming trend, more extreme weather and rising sea levels. These in turn impact nature and wildlife, as well as human settlements and societies. The effects of human-caused climate change are broad and far-reaching. This is especially so if there is no significant climate action. Experts sometimes describe the projected and observed negative impacts of climate change as the climate crisis.
The economic analysis of climate change explains how economic thinking, tools and techniques are applied to calculate the magnitude and distribution of damage caused by climate change. It also informs the policies and approaches for mitigation and adaptation to climate change from global to household scales. This topic is also inclusive of alternative economic approaches, including ecological economics and degrowth. In a cost–benefit analysis, the trade offs between climate change impacts, adaptation, and mitigation are made explicit. Cost–benefit analyses of climate change are produced using integrated assessment models (IAMs), which incorporate aspects of the natural, social, and economic sciences. The total economic impacts from climate change are difficult to estimate, but increase for higher temperature changes.
The Climate Change Science Program (CCSP) was the program responsible for coordinating and integrating research on global warming by U.S. government agencies from February 2002 to June 2009. Toward the end of that period, CCSP issued 21 separate climate assessment reports that addressed climate observations, changes in the atmosphere, expected climate change, impacts and adaptation, and risk management issues. Shortly after President Obama took office, the program's name was changed to U.S. Global Change Research Program (USGCRP) which was also the program's name before 2002. Nevertheless, the Obama Administration generally embraced the CCSP products as sound science providing a basis for climate policy. Because those reports were mostly issued after the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), and in some cases focused specifically on the United States, they were generally viewed within the United States as having an importance and scientific credibility comparable to the IPCC assessments for the first few years of the Obama Administration.
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to Earth's climate. The current rise in global average temperature is more rapid than previous changes, and is primarily caused by humans burning fossil fuels. Fossil fuel use, deforestation, and some agricultural and industrial practices add to greenhouse gases, notably carbon dioxide and methane. Greenhouse gases absorb some of the heat that the Earth radiates after it warms from sunlight. Larger amounts of these gases trap more heat in Earth's lower atmosphere, causing global warming.
The First Assessment Report (FAR) of the Intergovernmental Panel on Climate Change (IPCC) was completed in 1990. It served as the basis of the United Nations Framework Convention on Climate Change (UNFCCC). This report had effects not only on the establishment of the UNFCCC, but also on the first conference of the parties (COP), held in Berlin in 1995. The executive summary of the WG I Summary for Policymakers report that said they were certain that emissions resulting from human activities are substantially increasing the atmospheric concentrations of the greenhouse gases, resulting on average in an additional warming of the Earth's surface. They calculated with confidence that CO2 had been responsible for over half the enhanced greenhouse effect.
Climate change in South Asia is having significant impacts already which are expected to intensify as global temperatures rise due to climate change. The South Asia region consists of the eight countries: Afghanistan, Pakistan, India, Nepal, Bhutan, Bangladesh, the Maldives and Sri Lanka. In the 2017 edition of Germanwatch's Climate Risk Index, Bangladesh and Pakistan ranked sixth and seventh respectively as the countries most affected by climate change in the period from 1996 to 2015, while India ranked fourth among the list of countries most affected by climate change in 2015. The Indian subcontinent is one of the most vulnerable regions globally to a number of direct and indirect effects of climate change, including sea level rise, cyclonic activity, and changes in ambient temperature and precipitation patterns. Ongoing sea level rise has already submerged several low-lying islands in the Sundarbans region, displacing thousands of people.
The Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) is the fifth in a series of such reports and was completed in 2014. As had been the case in the past, the outline of the AR5 was developed through a scoping process which involved climate change experts from all relevant disciplines and users of IPCC reports, in particular representatives from governments. Governments and organizations involved in the Fourth Report were asked to submit comments and observations in writing with the submissions analysed by the panel. Projections in AR5 are based on "Representative Concentration Pathways" (RCPs). The RCPs are consistent with a wide range of possible changes in future anthropogenic greenhouse gas emissions. Projected changes in global mean surface temperature and sea level are given in the main RCP article.
The history of the scientific discovery of climate change began in the early 19th century when ice ages and other natural changes in paleoclimate were first suspected and the natural greenhouse effect was first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could change Earth's energy balance and climate. The existence of the greenhouse effect, while not named as such, was proposed as early as 1824 by Joseph Fourier. The argument and the evidence were further strengthened by Claude Pouillet in 1827 and 1838. In 1856 Eunice Newton Foote demonstrated that the warming effect of the sun is greater for air with water vapour than for dry air, and the effect is even greater with carbon dioxide.
Climate change feedbacks are effects of global warming that amplify or diminish the effect of forces that initially cause the warming. Positive feedbacks enhance global warming while negative feedbacks weaken it. Feedbacks are important in the understanding of climate change because they play an important part in determining the sensitivity of the climate to warming forces. Climate forcings and feedbacks together determine how much and how fast the climate changes. Large positive feedbacks can lead to tipping points—abrupt or irreversible changes in the climate system—depending upon the rate and magnitude of the climate change.
Climate change scenarios or socioeconomic scenarios are projections of future greenhouse gas (GHG) emissions used by analysts to assess future vulnerability to climate change. Scenarios and pathways are created by scientists to survey any long term routes and explore the effectiveness of mitigation and helps us understand what the future may hold. This will allow us to envision the future of human environment system. Producing scenarios requires estimates of future population levels, economic activity, the structure of governance, social values, and patterns of technological change. Economic and energy modelling can be used to analyze and quantify the effects of such drivers.
A Representative Concentration Pathway (RCP) is a greenhouse gas concentration trajectory adopted by the IPCC. Four pathways were used for climate modeling and research for the IPCC Fifth Assessment Report (AR5) in 2014. The pathways describe different climate change scenarios, all of which are considered possible depending on the amount of greenhouse gases (GHG) emitted in the years to come. The RCPs – originally RCP2.6, RCP4.5, RCP6, and RCP8.5 – are labelled after a possible range of radiative forcing values in the year 2100. The higher values mean higher greenhouse gas emissions and therefore higher global temperatures and more pronounced effects of climate change. The lower RCP values, on the other hand, are more desirable for humans but require more stringent climate change mitigation efforts to achieve them.
The Special Report on Global Warming of 1.5 °C (SR15) was published by the Intergovernmental Panel on Climate Change (IPCC) on 8 October 2018. The report, approved in Incheon, South Korea, includes over 6,000 scientific references, and was prepared by 91 authors from 40 countries. In December 2015, the 2015 United Nations Climate Change Conference called for the report. The report was delivered at the United Nations' 48th session of the IPCC to "deliver the authoritative, scientific guide for governments" to deal with climate change. Its key finding is that meeting a 1.5 °C (2.7 °F) target is possible but would require "deep emissions reductions" and "rapid, far-reaching and unprecedented changes in all aspects of society". Furthermore, the report finds that "limiting global warming to 1.5 °C compared with 2 °C would reduce challenging impacts on ecosystems, human health and well-being" and that a 2 °C temperature increase would exacerbate extreme weather, rising sea levels and diminishing Arctic sea ice, coral bleaching, and loss of ecosystems, among other impacts.
The Sixth Assessment Report (AR6) of the United Nations (UN) Intergovernmental Panel on Climate Change (IPCC) is the sixth in a series of reports which assess scientific, technical, and socio-economic information concerning climate change. Three Working Groups covered the following topics: The Physical Science Basis (WGI); Impacts, Adaptation and Vulnerability (WGII); Mitigation of Climate Change (WGIII). Of these, the first study was published in 2021, the second report February 2022, and the third in April 2022. The final synthesis report was finished in March 2023.
the leading international network of climate scientists has concluded for the first time that global warming is 'unequivocal' and that human activity is the main driver, 'very likely' causing most of the rise in temperatures since 1950
The Intergovernmental Panel on Climate Change said the likelihood was 90 percent to 99 percent that emissions of heat-trapping greenhouse gases like carbon dioxide, spewed from tailpipes and smokestacks, were the dominant cause of the observed warming of the last 50 years. In the panel's parlance, this level of certainty is labeled "very likely." Only rarely does scientific odds-making provide a more definite answer than that, at least in this branch of science, and it describes the endpoint, so far, of a progression.
The Fourth Assessment Report consists of the following reports from each of the three Working Groups, and a Synthesis Report. Additional reports and documents can be found at the IPCC's documents web page.
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