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The Science
On this page:
• Climate Basics
• Statement by Prof. Wolfgang Berger
• To Learn More
Climate Basics
Warming of the climate system is unequivocal... Most of the observed increase in global average temperatures since the mid-20th century is very [90%] likely due to the observed increase in anthropogenic greenhouse gas concentrations.
--Intergovernmental Panel on Climate Change, February, 2007
The scientists' words are measured, but clear. We are changing the climate. The potential result is a poorer, less secure world for our children. How poor and how insecure depends on what we do in the next few decades.
We summarize below how we are changing the climate, what the consequences may be, what we can do about it, and how strong is the scientific evidence that climate change is real and human caused.
How are we changing the climate? We are warming the earth by increasing the concentrations of heat-trapping greenhouse gases such as carbon dioxide and methane in the air. Most of this increase comes from burning coal, oil and gas. Some of it comes from clearing forests, raising cattle and other activities.
This global warming affects all aspects of climate: Wind patterns; ocean currents; where, when and how much it rains; how much precipitation falls as rain and how much as snow; soil moisture; sea levels; the saltiness and acidity of the oceans.
What are some potential consequences? Climate change can affect our health, our economy and the natural world in many ways [1]. Possible effects include:
• More droughts and floods [2].
• Reduced water supplies in many areas [3] including Southern California [4].
• Crop yields reduced immediately in some of the world's poorest areas, with net loss of global crop productivity as warming increases [5].
• Increases in malnutrition, cardio-respiratory disease, tropical diseases and mortality from floods, storms, fires and droughts [6].
• Sea-level rise, resulting in coastal erosion, property losses and loss of low-lying fertile lands now home to hundreds of millions [7].
• As many as 150 million refugees from drought, flooding and famine by 2050 [8].
• Mass extinction: 20 to 30 % of the planet's species committed to extinction as early as 2050, and 40 to 70% as early as 2100 [9].
What can we do? The effects of climate change get worse as the planet gets warmer. How warm it gets depends on how much we allow greenhouse-gas concentrations and global temperatures to rise.
It is too late to prevent global warming altogether. We are already seeing climate changes that appear to be exacerbating hunger and conflict in desperately poor countries, and damaging vulnerable ecosystems in the Arctic and on coral reefs. We are already committed to significant further warming, even with major efforts to reduce our greenhouse-gas emissions.
Nevertheless, by making those efforts, we can avoid even worse effects, such as mass extinctions and loss of agricultural productivity worldwide [10]. To avoid the worst effects of climate change, scientists recommend reducing global greenhouse-gas emissions to half of today's levels by 2050 [11].
We can achieve part of that reduction by preserving forests, raising fewer cattle, and changing agricultural practices to produce less methane and nitrous oxide. However, the biggest part of our greenhouse-gas emissions comes from burning coal, oil and natural gas. The biggest part of the solution, therefore, has to come from using less energy, and producing energy without burning those fossil fuels.
Is it real? There is a strong scientific consensus that climate change is real and human-caused. The most comprehensive assessment of the scientific evidence is that of the Intergovernmental Panel on Climate Change (IPCC), a panel of hundreds of climate scientists convened by the World Meteorological Organization and the United Nations. The IPCC's conclusions are supported by the U.S. National Academy of Sciences, the American Meteorological Association, the American Geophysical Union, and the national science academies of at least 18 other countries [12].
How strong is the evidence? Today's scientific understanding of climate change is based on many independent lines of evidence. For example, at least five methods have been used to estimate the response of global temperatures to heating and cooling effects such as greenhouse gases, solar brightness, and how much of the earth's surface is covered by snow and ice that reflects sunlight. These methods include [13]:
• Computer models that simulate the climate system.
• Geological studies of past climates, including the cycles of the Ice Ages.
• Correlations between measured solar brightness and global temperature.
• Effects of volcanic eruptions, which produce soot that temporarily cools the planet.
• The temperature rise observed so far, in comparison to the heating effect of increased greenhouse gases, the cooling effect of aerosol pollution and the heat soaked up by the oceans as they respond slowly to global warming.
The approximate agreement between all of these methods is part of the reason why scientists are confident in their understanding of climate change.
These examples are only a small fraction of the work that has been done. Modern climate science is a resilient network of mutually reinforcing observations, calculations and comparisons between different methods. Projections of human-caused climate change are grounded in this comprehensive scientific understanding.
To Learn More
Intergovernmental Panel on Climate Change. The IPCC is a panel of hundreds of climate scientists from around the world, who have been brought together by the United Nations Environment Program and the World Meteorological Organization. Their 2007 Fourth Assessment Report, based on a review of thousands of scientific articles, is the most comprehensive assessment of what is known about climate change.
RealClimate.org. Click "Start Here" for Links to sources for anybody -- beginner, expert or in between -- who wants to know more about climate change. Then, type keywords into their search function for commentary by real climate scientists on many of the controversies surrounding the science of climate change.
Sources for this page
This information on this web page comes from the sources listed below. Many of the citations are from the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). These citations use the following abbreviations:
WG1: Working Group 1
WG2: Working Group 2
TS: Technical Summary
SPM: Summary for Policy Makers
[1] Varied effects of climate change:
IPCC AR4, WG2, TS, p. 44: Box TS.5 summarizes many impacts of climate change.
[2] More droughts and floods:
IPCC AR4, WG2, TS, p. 35: "Increased precipitation intensity and variability is projected to increase the risk of floods and droughts in many areas."
[3] Reduced water supplies in many areas:
IPCC AR4, WG2, TS, p. 36. Estimates global warming will put 1 to 2 billion people at increased risk of water stress by 2050.
IPCC AR4, WG2, TS, p. 35: "More than one-sixth of the world’s population live in glacier- or snowmelt-fed river basins and will be affected by a decrease in water volume stored in glaciers and snowpack, an increase in the ratio of winter to annual flows, and possibly a reduction in low flows caused by decreased glacier extent or melt-season snow water storage."
[4]. Reduced water supplies in Southern California:
IPCC AR4, WG2, TS, p. 36: "Semi-arid and arid areas are particularly exposed to the impacts of climate change on freshwater (high confidence). Many of these areas (e.g., Mediterranean Basin, western USA, southern Africa, north-eastern Brazil, southern and eastern Australia) will suffer a decrease in water resources due to climate change."
TP Barnett and D. W. Pierce, "When will Lake Mead go dry?" Water Resources Research 44, W03201 ( 2008).
Scripps Institution of Oceanography, "Lake Mead Could Be Dry by 2021," press release, Feb. 12, 2008.
W. Dunham, "Scientists see looming water crisis in western U.S.," Reuters, Jan. 31, 2008.
[5] Immediate crop loss in poor areas, global net loss as temperature increases:
IPCC AR4, WG2, SPM, p. 6: "Crop productivity is projected to increase slightly at mid- to high latitudes for local mean temperature increases of up to 1-3°C depending on the crop, and then decrease beyond that in some regions. At lower latitudes, especially seasonally dry and tropical regions, crop productivity is projected to decrease for even small local temperature increases (1-2°C), which would increase risk of hunger. Globally, the potential for food production is projected to increase with increases in local average temperature over a range of 1-3°C, but above this it is projected to decrease."
IPCC AR4, WG2, SPM, p 8: "Agricultural production, including access to food, in many African countries and regions is projected to be severely compromised by climate variability and change. The area suitable for agriculture, the length of growing seasons and yield potential, particularly along the margins of semi-arid and arid areas, are expected to decrease. This would further adversely affect food security and exacerbate malnutrition in the continent. In some countries, yields from rain-fed agriculture could be reduced by up to 50% by 2020."
[6] Health effects of global warming:
IPCC AR4, WG2, SPM, p. 7:
"Projected climate change-related exposures are likely to affect the health status of millions of people, particularly those with low adaptive capacity, through:
• increases in malnutrition and consequent disorders, with implications for child growth and development;
• increased deaths, disease and injury due to heat waves, floods, storms, fires and droughts;
• the increased burden of diarrhoeal disease;
• the increased frequency of cardio-respiratory diseases due to higher concentrations of ground level ozone related to climate change; and,
• the altered spatial distribution of some infectious disease vectors."
IPCC AR4, WG2, TS, Figure TS.9, p. 43. Figure summarizes estimated sign and magnitude of above health effects.
[7] Loss of low-lying fertile lands:
IPCC AR4, WG2, SPM, p. 8: "Towards the end of the 21st century, projected sea-level rise will affect low-lying coastal areas with large populations."
IPCC AR4, WG2, SPM, p. 7:
"Those densely-populated and low-lying areas where adaptive capacity is relatively low, and which already face other challenges such as tropical storms or local coastal subsidence, are especially at risk. The numbers affected will be largest in the mega-deltas of Asia and Africa while small islands are especially vulnerable"
IPCC AR4, WG2, TS, p. 40: "Three key hotspots of societal vulnerability are: (i) deltas (see Figure TS.8), especially the seven Asian megadeltas with a collective population already exceeding 200 million; (ii) low-lying coastal urban areas, especially those prone to subsidence; and (iii) small islands, especially coral atolls."
IPCC AR4, WG2, TS, p. 40-41: "Without action, the highest sea-level scenarios combined with other climate change (e.g., increased storm intensity) are about as likely as not to make some low-lying islands and other low-lying areas (e.g., in deltas and megadeltas) uninhabitable by 2100."
[8] Many millions of climate refugees:
Sachs JD, Scientific American Magazine, June 1, 2007, Climate change refugees. Reviews findings from IPCC AR4, WG2. Does not give specific figure for projected number of refugees, but discusses several areas of the globe where hundreds of millions of people are potentially at risk for displacement due to climate change.
IPS News, May 23, 2007, Climate Change: U.N. Braces for New Breed of Refugees: "Gorlick also pointed out the different estimates of recent statistics on environmental refugees: 50 million more by the end of this decade (United Nations University); 150 million by 2050 (Oxford University); 50 million by 2060 in Africa alone (the U.N. Environment Programme in Nairobi) and; one billion displaced globally by 2050 (Christian Aid)."
Greenpeace website, accessed 1/5/08. Warming to cause 150 million environmental refugees by 2050. Link Cites 1993 article by Norman Myers of Oxford University (Total Environment Refugees Foreseen, N. Myers, BioScience, v. 43 (11), December 1993.) Discusses in detail how many refugees Myers thinks will come from each of several at-risk regions.
climate.org website, posted August 2007. Link
Discusses climate refugee problem, giving references to other sources.
BBC, 24 January, 2000, West warned on climate refugees: Link
"Speaking to the BBC Radio 4 environment programme Costing the Earth, Mrs. Choudhury said the Intergovernmental Panel on Climate Change (IPCC) was predicting that 17.5% of Bangladesh's landmass could be lost beneath the waves... Approximately 20 million people will become ecological refugees [from Bangladesh alone]."
Reuters, January 18, 2007, Global warming to multiply world's refugee burden: Link
"People displaced by global warming -- the Christian Aid agency has predicted there will be one billion by 2050 -- could dwarf the nearly 10 million refugees and almost 25 million internally displaced people already fleeing wars and oppression."
Christian Science Monitor, June 21, 2007, Global warming may uproot millions. Link
[9] Extinctions:
IPCC conclusions, Summary by Union of Concerned Scientists: "Up to 30 percent of plant and animal species could face extinction if the global average temperature rises more than ~3 to ~5°F (1.5 to 2.5°C)[4] relative to the 1980–1999 period. Many projections suggest the low end of this temperature range could be reached by mid-century." Webpage, "Union of Concerned Scientists, Findings of the IPCC Fourth Assessment Report: Climate Change Impacts," available at UCS website. Link
IPCC AR4, WG2, SPM, p. 6: "Approximately 20-30% of plant and animal species assessed so far are likely to be at increased risk of extinction if increases in global average temperature exceed 1.5-2.5 oC. ...For increases in global average temperature exceeding 1.5-2.5°C and in concomitant atmospheric carbon dioxide concentrations, there are projected to be major changes in ecosystem structure and function, species’ ecological interactions, and species’ geographic ranges, with predominantly negative consequences for biodiversity, and ecosystem goods and services e.g., water and food supply."
IPCC AR4, WG2, TS, p. 37: See Figure TS.6, which summarizes ecological changes and temperature thresholds at which they may occur. Note "major extinctions worldwide" around 4 degrees C temperature rise, a level that could be reached by 2100 in rapid-emissions-growth scenario. Note also "20-30% of species committed to extinction" around 2 deg. C, a level that would be reached between 2050 (for a rapid-growth scenario) and 2100 (for a scenario in which greenhouse emissions rise, then fall so that atmospheric greenhouse gas concentrations level off by 2100).
IPCC AR4, WG2, TS, p. 67: Caption to Table TS.4 explains that, "For extinctions, ‘major’ means ~40 to ~70% of assessed species."
IPCC AR4, WG2, TS, p. 74: Table TS.8 summarizes possible impacts of climate change, including high confidence of major extinctions at temperature rise around 4 deg. C, and medium confidence of extinctions of 20-30% of species at temperature rises around 2 deg. C.
[10] Impacts depend on degree of warming:
IPCC AR4, WG2, TS, p. 37 (Fig. TS.6), p. 74 (Table TS.8).
[11] Need to reduce greenhouse emissions to half of today's levels by 2050:
IPCC WG3, Ch. 1, Sec. 1.2.2: "Dangerous climate change" defined as global mean temperature 2 degrees C above pre-industrial level.
IPCC WG3, Ch. 1, Sec. 1.2.3: Stabilizing global temperatures at less than 2 degrees C above pre-industrial levels would require reducing global greenhouse gas emissions to half of today's levels.
See also IPCC WG3, Ch. 3, Sec. 3.5.2, for a more detailed discussion of equilibrium temperature change versus greenhouse gas emissions.
[12]Science academies supporting IPCC:
2001 National Academy of Sciences report directly affirming IPCC's essential conclusions: National Academy of Sciences Committee on the Science of Climate Change, "Climate Change Science: An Analysis of Some Key Questions," National Academy Press, Washington, DC, 2001. Link
A more recent NAS report summarizing other NAS reports on climate change. Link
American Meteorological Society, "Climate Change: An Information Statement of the American Meteorological Society. (Adopted by AMS Council on 1 February 2007) Bull. Amer. Met. Soc., 88. Link
American Geophysical Union. "AGU position on climate change: Human impacts on climate." Adopted December, 2003. Link
National science academies of 11 countries, including US National Academy of Sciences: "Joint science academies’ statement: Global response to climate change." Link
National science academies of eight more countries, plus some of the above: "The Science of Climate Change: A joint statement issued by the Australian Academy of Sciences, Royal Flemish Academy of Belgium for Sciences and the Arts, Brazilian Academy of Sciences, Royal Society of Canada, Caribbean Academy of Sciences, Chinese Academy of Sciences, French Academy of Sciences, German Academy of Natural Scientists Leopoldina, Indian National Science Academy, Indonesian Academy of Sciences, Royal Irish Academy, Accademia Nazionale dei Lincei (Italy), Academy of Sciences Malaysia, Academy Council of the Royal Society of New Zealand, Royal Swedish Academy of Sciences, and Royal Society.(UK)."Link
[13] Independent methods used to estimate climate sensitivity:
"Climate sensitivity" describes the response of the global mean temperature to "radiative forcing", that is, a change in the balance between heat input from the absorption of sunlight, and heat loss through infrared radiation back into space. When this balance is disrupted, by a change in the sun's brightness, the amount of sunlight-reflecting snow and ice, or the greenhouse gases in the atmosphere, the mean surface temperature of the earth will adjust itself (over a period of decades) until the balance of incoming and outgoing radiation is restored. "Climate sensitivity" describes how big that temperature change is, in relation to the imbalance in the incoming and outgoing heat flows:
Climate sensitivity = (Temperature change required to restore balance)
(Change in heat balance)
Several methods have been used to estimate this quantity:
Correlation of temperature with solar variability: See Camp CD and Tung KK 2007. Surface warming by the solar cycle as revealed by the composite mean difference projection, GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L14703, doi: 10.1029/2007GL030207, 2007.
For other items, see:
IPCC AR4, WG1, TS, p. 64: "Several approaches are used in this assessment to constrain climate sensitivity, including the use of AOGCMs [Atmosphere-Ocean General Circulation Models], examination of the transient evolution of temperature (surface, upper air and ocean) over the last 150 years and examination of the rapid response of the global climate system to changes in the forcing caused by volcanic eruptions (see Figure TS.25). These are complemented by estimates based upon palaeoclimate studies such as reconstructions of the NH [Northern Hemisphere] temperature record of the past millennium and the LGM [Last Glacial Maximum]. Large ensembles of climate model simulations have shown that the ability of models to simulate present climate has value in constraining climate sensitivity."
IPCC, AR 4, WG 1, TS, p. 65. See Fig. TS.25, which shows probability distributions for estimates of climate sensitivity, as derived from several different methods.
