Fourth assessment report by the IPCC

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The fourth assessment report ( English Fourth Assessment Report , AR4 ) of the Intergovernmental Panel on Climate Change (IPCC) of the United Nations appeared in the year 2007. The IPCC reports, the scientific knowledge of the climate change and in particular global warming summarize, are published every five to six years. They are widely regarded as a consensus position within climatological experts regarding the influence of humans on the earth's climate. The first three reports appeared in 1990, 1995 and 2001 ( Third Assessment Report, Third Assessment Report, TAR ). The current report is the fifth assessment report by the IPCC from 2013 and 2014.

On 2 February 2007, the Summary for Policymakers was (Summary for Policymakers) Working Group I (Working Group I) on the "Scientific Foundations" (Physical Science Basis) published following a plenary session of the IPCC in Paris. The summary of the report of Working Group II on " Impacts, Adaptation and Vulnerability" followed on April 6, that of Working Group III on " Mitigation of Climate Change" on May 4, 2007 A summary of the full report ("Synthesis volume") was published on November 17, 2007.

Some estimated mean values ​​for the warming of the earth's surface through 2100

Results of the individual working groups

Working group I: "Scientific principles"

Overview of the origins of the participants in Working Group 1 on the Fourth Assessment Report of the IPCC
The drivers of global warming since 1750 and their net effect on the Earth's heat balance in the analysis of the IPCC Fourth Assessment Report
Comparison of observed CO 2 emissions from fossil fuels and the IPCC scenarios
The four scenario families of the Fourth Assessment Report of the IPCC and the forecast increase in the global average temperature by 2100
AR4 Summary (PDF; 3.9 MB)
oriented (economically oriented)
oriented (ecologically oriented)
(homogeneous world)
High growth
(Groups: A1T; A1B; A1FI)
2.4-6.4 ° C
Global sustainability 1.1–2.9 ° C
(heterogeneous world)
economic development

2.0–5.4 ° C

1.4-3.8 ° C
Global warming from 1980–1999 to 2090–2099 according to the A1B scenario

The report of Working Group I deals with the physical principles of climate change. It gives an overview of the understanding of the natural sciences that prevailed in mid-2006, as it emerges from the relevant scientific literature. The eleven chapters deal with changes in the atmospheric composition, knowledge of past climate changes ( paleoclimate ), climate models and their predictions, the observed changes in nature due to global warming and the determination of the underlying climatic factors.

A total of 1043 different scientists from 48 different countries and 7 international law organizations worked on this partial report . These include 152 lead authors, 498 contributing authors, 26 review editors and 626 reviewers, some of whom have contributed to several chapters under different roles. In total, more than 6,000 peer-reviewed specialist articles were cited by the authors and editors and more than 30,000 comments from the reviewers were taken into account. The Executive Summary for Policy Makers has been approved by the governments of 113 states.

The report emphasizes the role of humans in the currently observable climate change . The energy balance of the climate system is changed by changes in the concentration of greenhouse gases and aerosols in the atmosphere, the nature of the land surface and solar radiation. The quantitative estimates of the respective influence (see figure) have improved compared to the third assessment report. With a stated probability of over 90 percent, the most important cause of global warming is "very likely" human emissions of the greenhouse gas carbon dioxide (CO 2 ); followed by the less important gases methane (CH 4 ), nitrous oxide (N 2 O) and others. In addition, there are other factors of lesser importance, including the natural fluctuation of solar activity .

The average temperature of the near-earth atmosphere rose by 0.74 ° C (± 0.18 ° C) in the hundred-year linear trend between 1906 and 2005 (the corresponding value from the Third Assessment Report for the period 1901–2000 was still 0.6 ° C (± 0.2 ° C)). Eleven of the last twelve years (the years 1995–2006) are among the twelve warmest since the beginning of area-wide temperature measurements in 1850. The trend over the past 50 years is a measured warming of 0.13 ° C (± 0.03 ° C ) per decade almost twice as high as for the past 100 years.

The effects of global warming associated with this rise in temperature , which the report lists, include, among other things, a rise in sea levels of 17 centimeters in the 20th century - since 1993 even 3.1 millimeters per year; melting glaciers , the reduction in the snow-covered surface of the earth by 5 percent since 1980, the significantly accelerated decline in sea ice in recent years; more frequent heavy rain; increasing rainfall in northern Europe and eastern North and South America; increasing drought in the Mediterranean , the Sahel, South Africa and parts of South Asia; increasing heat waves and more violent tropical storms.

A further increase in global warming is expected for the future , for the range of which various scenarios are set up that operate with different assumptions such as population development or economic growth . The resulting emission scenarios can be assigned to one of the four characteristic scenario families A1, B1, A2, B2. The letters and numbers designate the scenario families: The letter A denotes business-oriented scenarios, the letter B environmentally-oriented scenarios, the number 1 scenarios with a focus on global development and the number 2 scenarios with a focus on regional development. See also the table opposite, which shows the combinations of these scenario families.

The A1 scenario family is divided into the following three scenario groups, which describe the different directions of technological changes in the energy sector :

  1. Scenario Group A1FI =  f ossil i ntensiv ;
  2. Scenario group A1T = non-fossil energy sources;
  3. Scenario A1B group = ( b alanced) balanced use of fossil and non-fossil energy sources.

In the best scenario B1, the increase in the average temperature by the decade 2090–2100 is 1.8 ° C (with a likely range of 1.1–2.9 ° C), in the worst case A1FI 4.0 ° C (2.4 ° C) -6.4 ° C). At higher latitudes, the temperature is likely to rise more sharply than near the equator.

The sea level rises in accordance with the underlying trends until the end of the 21st century by at least 18-38 cm and a maximum of 26-59 cm, with some complicated assumptions about the future behavior of ice sheets were used as a basis. The report expressly points out that the models currently used do not include the full effects of changes in the ice sheet fluxes or uncertainties in the climate-carbon cycle feedback, as the relevant basis was lacking in the published literature at the time of writing. A higher rise in sea level cannot therefore be ruled out.

Working group II: "Impact, Adaptation, Vulnerabilities"

Working group II compiled the current state of knowledge on the effects of climate change . These include both the effects on the environment that are already recognizable today and the expected future effects.

Numerous effects can already be seen today. These include the decline in the ice and snow cover in the cold regions of the world, as a result of which glacial lakes are becoming more numerous and larger and rivers fed by glaciers carry more water. Spring (and with it events such as leaf unfolding and bird migration) begins earlier and earlier; the areas of distribution of animal and plant species are shifting towards the poles or higher up in the mountains. Of the 29,000 data series evaluated, over 89 percent show a result that would be expected in the event of global warming; Changes accumulate where temperatures have increased the most. Therefore, natural causes for the changes are considered very unlikely (probability <10 percent).

Compared to the Third Assessment Report, knowledge about the future effects of climate change has also increased. It is expected that there will be more rainfall in high latitudes and humid tropical areas; less in dry regions. Both floods from heavy rain and periods of drought will increase. As a result, the adaptability of many ecosystems is overwhelmed; the damage will have a negative impact on biodiversity and the services provided by the ecosystems (such as water supply). Droughts and floods will reduce food production, especially in low latitudes, leading to increasing hunger in Africa and some regions of Asia. Coasts and coastal ecosystems such as mangroves will also be exposed to particular risks . Floods from rising sea levels will affect millions of people, especially in the great deltas of Africa and Asia and on small islands. Human health will also be directly affected, for example by increasing diarrheal diseases as a result of floods and the spread of pathogens.

The regional distribution of the impact is also better known. Africa and Asia will be particularly hard hit, especially since poor countries have no funds for protective measures. Africa will suffer particularly from a food shortage, Asia from flooding in the great river deltas .

Adaptation measures are indispensable even with the best climate protection, since the greenhouse gases already released will lead to a further warming of 0.6 ° C in the next century. The spectrum of measures ranges from technological measures (such as buildings to protect the coast) to changes in behavior (such as resource-saving consumption) to political measures (such as planning decisions); However, adaptation measures alone will not be able to control the effects of climate change.

Working group III: "Mitigation of Climate Change"

Working group III summarized the current state of research on technical, economic and social aspects of climate protection. The main focus here is the technical and economic potential for reducing emissions, possible political measures for climate protection and the connection between sustainability and climate protection.

Greenhouse gas emissions rose by 70 percent between 1970 and 2004; that of the most important greenhouse gas, carbon dioxide, by about 80 percent. Energy supply (+ 145 percent) and transport (+120 percent) had the largest share in this increase. If current policy is not changed, we can expect a further increase of 25 to 90 percent by 2030 - depending on the assumption about economic and technological developments (for carbon dioxide even 45 to 110 percent).

However, there are a number of economic measures that could reduce greenhouse gas emissions. Which measures are economical depends on the price that is attached to a ton of carbon dioxide; 6 billion tons of carbon dioxide could even be saved by 2030 through measures that also save money. A whole range of measures is needed to reduce greenhouse gas emissions. There is no such thing as one key measure or technology. A core area is efficient energy generation, distribution and use; for example through combined heat and power , renewable energy sources , replacing coal with gas, more efficient vehicles, lighting and electricity use, better thermal insulation and heat recovery; but this also includes measures such as shifting traffic from road to rail, improved use of nitrogen fertilizers and better methods of growing rice in agriculture and (re) afforestation and use of forest products as a substitute for fossil fuels. The IPCC believes that nuclear power expansion will remain limited due to security concerns, the risk of nuclear proliferation and the unsolved waste problem .

Other technologies will become marketable in the next few decades, such as second generation biofuels , cheaper solar power generation and better renewable energy sources. The measures that would limit the concentration of greenhouse gases to a level that would raise the temperature by a maximum of 2 to 2.4 ° C would, in the most expensive case, reduce annual global economic growth by 0.12 percent - albeit with considerable regional growth Differed. These costs are offset by the consequential costs of climate change and other benefits, such as lower air pollution. To achieve this goal, the turnaround in greenhouse gas emissions would have to be initiated by 2015 at the latest.

By 2050, greenhouse gas emissions would have to fall by 50 to 80 percent if the temperature rise is to be limited to 2 to 2.4 ° C. In addition to energy efficiency, carbon-free energy sources and the separation of carbon dioxide from the exhaust gases of power plants play a role; the costs would also correspond to the 0.12 percent of economic growth mentioned above.

The most important instrument of climate policy for the IPCC is a price for the emission of greenhouse gases - see emissions trading . A price of 20 to 50 US dollars per ton would make many climate protection measures economical; Subsidies for fossil fuels, however, are harmful to climate protection. Governments could promote efficient energy use with tax breaks or strict standards; Technology transfer helps to enable other countries to use modern technology.

Climate protection is a component of sustainable development; and sustainable development helps to protect the climate, for example by protecting forests. With lower emissions of air pollutants and more efficient use of energy, climate protection improves human health and increases the security of energy supplies. But there can also be conflicts or trade-offs between goals, for example between climate protection and sustainability (e.g. cultivating plants for energy production can come at the expense of food security). Compromises are necessary here to avoid problems; Goals need to be operationalized .

Synthesis report

The synthesis report summarizes the essential evaluations of the individual working groups and offers an integrated view of climate change as the final part of the fourth assessment report. Accordingly, the main findings of the report are:

There is no doubt about global warming, evidence of which is rising temperatures, widespread melting of ice and snow, and rising sea levels.

Global warming is already having recognizable effects on many natural systems, such as the early start of spring or the change in the range of animal and plant species towards the poles or at higher altitudes.

The causes of global warming are very likely (probability> 90 percent) in the man-made increase in the concentration of greenhouse gases in the atmosphere. The concentrations of carbon dioxide and methane are not only much higher than before the start of industrialization, but also higher than in the past 650,000 years of the earth's history. From 1970 to 2004, greenhouse gas emissions rose by 70 percent and carbon dioxide by as much as 80 percent.

If the current policy is continued, the concentrations of greenhouse gases will continue to rise. How much the concentrations increase depends on the assumptions about economic and technological developments, which were examined in various scenarios. The range of possible resulting temperature increases by the end of the 21st century is 1.1 to 6.4 ° C.

The possible consequences include increasing extreme weather conditions such as hot days, heat waves and heavy rain. Tropical storms are getting worse; Precipitation increases at high latitudes and decreases at low latitudes. Semi-arid areas will suffer from increasing water shortages. Africa (severe damage to agriculture in many countries) and Asia (flooding in the densely populated large deltas) will be particularly affected.

The warming would continue even after the end of the 21st century, even if greenhouse gas concentrations remained the same, since changes in the climate system once set in motion will only find a new equilibrium after centuries. This also applies to the rise in sea level, the long-term extent of which cannot yet be quantified. The same applies to the possible occurrence of abrupt or irreversible consequences.

Adaptation to climate change must and can be improved; However, adaptation alone will not be enough to cope with the consequences, especially since the possibilities for adaptation vary greatly from region to region. It is precisely the regions that are most affected that are least able to protect themselves.

However, there are numerous possibilities to keep the actual warming through measures against climate change lower than calculated in the scenarios. The most important technologies in the fight against climate change are those for efficient energy use and the use of low-carbon and low-carbon energy sources. Governments can help achieve this by putting a price on greenhouse gas emissions; this makes the necessary measures more economical. Even with ambitious goals, the costs for climate protection would correspond to a reduction in average growth of no more than 0.12 percent.


Public reaction

The IPCC's Fourth Assessment Report was the subject of criticism even before the general report was published because the executive summary was published before the actual report, so that the detailed evidence for the statements was not available at the same time. It was assumed that the underlying report, which was more than a thousand pages long, was adapted to the summary. This is not the case, however, because according to the rules of the IPCC , the statements in the summaries for decision-makers must be justified in the underlying reports and changes that go beyond grammatical or editorial content are not permitted. This can also be understood by comparing the drafts published later. The IPCC justifies the delayed publication of the full report with the time required for editorial changes and printing.

Spiegel Online wrote that the political influence had made the report less sharp. The governments of the USA, China, Russia and Saudi Arabia in particular have downplayed the anthropogenic contribution to global warming and its possible consequences, making it look weaker than what some scientists had called for. According to the IPCC rules mentioned above, the scientists involved agree to the amendments proposed by the government delegates at the plenary session. In addition, the draft versions of the scientists are documented in various places.

Fueled by the deniers of climate change, the hacker incident at the University of East Anglia's climate research center and the errors in the report section on impacts, adaptation and vulnerabilities , climate research, the IPCC and its Fourth Assessment Report came under public criticism, especially in 2010. Several studies by various institutions did not reveal any evidence of scientific misconduct. The fundamental statements of the IPCC on climate change were not affected by the allegations and were confirmed again by the following Fifth IPCC Assessment Report.

Errors in the report section on impact, customization and vulnerabilities

In January 2010 it turned out that an information on the melting of the glaciers in the Himalayas in the report section of Working Group II was incorrect. The IPCC report said that the glaciers in the Himalayas would melt completely by 2035. The source was a study from the 1990s that stated that non-polar glaciers would melt by 2350. A Dutch journalist also found a second mistake in the same month: The area that is threatened by rising sea levels in the Netherlands (26%) was stated as too high at 55%, as the risk of flooding from the Rhine and Meuse (29%) was included. However, the Dutch environmental authority PBL took responsibility for this error. She had sent the IPCC the text from which the incorrect information came. The PBL emphasized that the lead authors of the IPCC could not be blamed for relying on the authority's information.

After a parliamentary debate on January 29, the Dutch Environment Minister Jacqueline Cramer instructed the PBL to undertake a comprehensive assessment of the 32 conclusions (listed in the synthesis report) on the regional effects of global warming and the underlying chapters of Working Group II. The PBL report published on July 5, 2010 came to the conclusion that the conclusions in the synthesis report were not undermined by errors found. However, several conclusions contained statements that had no basis in the chapters or the sources cited therein. There were also isolated inadmissible generalizations as well as a lack of transparency and credibility of sources. So concluded the IPCC by declining revenues in millet , groundnut and cowpea in Niger to declining yields of crops in the Sahel , and declining cattle productivity in Argentina to declining livestock productivity in South America . Overall, slight deficiencies were found in five and serious deficiencies in three of the 32 conclusions.

When analyzing the individual chapters on regional impacts, in addition to the errors affecting the Himalayas and the Netherlands, another error was found: The IPCC report states that the productivity of West African anchovy fisheries could be reduced by 50–60%. In the source cited by the IPCC, however, it says that extreme wind and water turbulence would decrease by 50–60%; Effects on productivity in anchovy fisheries have not been quantified. In addition to six other minor errors and inaccuracies in the statements, the PBL report came to some critical comments regarding the representations of individual matters. For example, it has been criticized that the forecasted additional deaths from heat in Australia did not mention the impact of global warming - according to the underlying forecasts, a significant proportion of these deaths are caused by demographic developments alone.

Overall, the PBL report came to the conclusion that the conclusions of the Fourth IPCC Assessment Report were still admissible and, on the whole, well-founded. In addition to the errors, it was criticized that in the summary of the results of Working Group II in the synthesis report, negative prognoses were picked out and positive prognoses had not been named without sufficiently clarifying this “risk-oriented” selection process.


  • Harald Kohl: Humans change the climate - fourth assessment report by the IPCC. Physics in Our Time 39 (4), pp. 176-182 (2008), ISSN  0031-9252 .

Web links

Individual evidence

  1. Based on: Petra Döll, Dagmar Fuhr, Joachim Herfort, Annekathrin Jaeger, Andreas Printz, Susanne Voerkelius: Water availability and ecological, climatic and socio-economic interactions in the semi-arid north-east of Brazil . Joint project WAVES, status report of the first main phase, cross-sub-project working group scenarios, scenarios of future developments in Piauí and Ceará, February 15, 2000, PDF file (932 kB), page 17
  2. See basic assumptions of the SRES scenarios, page 106 ff. In: WBGU (Scientific Advisory Board of the Federal Government on Global Environmental Changes): World in Transition: Energy Transition to Sustainability , March 21, 2003 (PDF file, about 3.9 MB)
  3. a b IPCC Special Report on Emissions Scenarios (SRES): Chapter 4: An Overview of Scenarios ( Memento of the original of March 29, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  4. a b IPCC Fourth Assessment Report (AR4), Climate Change 2007, WG I: Summary for Policy Makers • Climate Change 2007: Scientific Basis ( Memento of the original from August 1, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and still Not checked. Please check the original and archive link according to the instructions and then remove this notice. (Page 18 of 18; PDF; 2.7 MB) @1@ 2Template: Webachiv / IABot /
  5. IPCC Working Group 1: WG1 Summary Description. (No longer available online.) December 2009, archived from the original on June 22, 2016 ; accessed on December 8, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  6. PD Jones, M. New, DE Parker, S. Martin, and IG Rigor: SURFACE AIR TEMPERATURE AND ITS CHANGES OVER THE PAST 150 YEARS, Reviews of Geophysics, 37, 2 / May 1999 pages 173–199 (PDF file, 28 pages; 7.8 MB) ( Memento from July 16, 2010 in the Internet Archive )
  7. Rahmstorf, Stefan (2007): The IPCC sea level numbers , published on March 27th by RealClimate
  8. Principles Governing IPCC Work, Appendix A, p. 4 (PDF; 92 kB) ( Memento of the original from July 1, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  9. Drafts of the IPCC Fourth Assessment Report Archived copy ( Memento of the original from December 29, 2016 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  10. ^ Spiegel Online: How politics influenced the climate report of February 2, 2007
  11. ^ Spiegel Online: Bush has UN climate report defused from April 6, 2007, and trench warfare over colorful maps from April 4, 2007
  12. For example in the material collection ( Memento of the original from April 27, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. at Harvard @1@ 2Template: Webachiv / IABot /
  13. The helpers of doubt - Before the climate summit in Cancún: Above all, the deniers of climate change felt the upswing in the past year. Science must convince more than ever. By Benjamin Reuter and Toralf Staud. In DIE ZEIT No. 48/2010 of November 25, 2010, accessed on December 29, 2016.
  14. A poor sequel (editorial) . In: Nature 480, 6, (2011), doi: 10.1038 / 480006a .
  15. Raghu Garud et al., Boundaries, breaches, and bridges: The case of Climategate . In: Research Policy 43, (2014), 60–73, doi: 10.1016 / j.respol.2013.07.007 .
  16. MyAnna Lahsen, Climategate: the role of the social sciences . In: Climatic Change 19, (2013), 547–558, doi: 10.1007 / s10584-013-0711-x .
  17. (2011): Several studies saying the researchers fraud allegations free
  19. a b c d PBL (2010): Assessing an IPCC assessment. An analysis of statements on projected regional impacts in the 2007 report ; Summary and detailed report (PDF; 1.87 MB)