Scientists have found it very likely that rising greenhouse gas concentrations, if they continue unabated, will increase global average temperature above natural variability by at least 1.5° Celsius (2.7° Fahrenheit) during the 21st Century (above 1990 temperatures), with a small likelihood that the temperature rise may exceed 5° C (9°F). The projections thought most likely by many climate modelers are for greenhouse gas-induced temperature rise of approximately 2.5° to 3.5°C (4.5 to 6.3°F) by 2100. (To put the magnitude of these potential increases in context, the current global, annual mean temperature (GMT) of the Earth is approximately 14°C (57°F). The difference between the current GMT and the low point of the last Ice Age, about 21,000 years ago, was roughly 7-8°C (44-46°F).)
Future climate change may advance smoothly or sporadically, with some regions experiencing more fluctuations in temperature, precipitation, and frequency or intensity of extreme events than others. Wet regions are expected to get more precipitation and dry regions are expected to become drier. Floods, droughts, storms and other extreme weather events are projected to increase, with impacts for ecological and human systems.
A report by the U.S. Climate Change Program (CCSP) found that short-lived air pollutants, such as tropospheric ozone and black carbon aerosols could contribute as much as 20% of global warming by 2050. It found that one “climate model using projected changes in emissions and pollutant levels that occur primarily over Asia predicts significant increases in surface temperature and decreases in rainfall over the continental United States during the summertime throughout the second half of this century.”
While most policy attention has been on the long-lived GHG, such as carbon dioxide and methane, including other climate forcings, such as short-lived air pollution, offers additional opportunities to abate near-term climate change, with the co-benefits of reducing health and environmental impacts. With projected global warming, sea levels could rise by between 18 and 59 centimeters (between 7 and 23 inches) by 2100 due to expansion of oceans waters as they warm and additions of meltwater (at current rates of melting) from land-based glaciers and ice caps. The International Panel on Climate Change (IPCC) scientists were unable to include a quantitative estimate of the risks of accelerated melting or possible collapse of the Greenland or Antarctic ice sheets due to inadequacies of existing understanding of their dynamics.
Some people are concerned less about chronic damages due to slow and continuing climate change than they are about the potential for abrupt “tipping points” in the current climate system. Once the climate system reaches certain thresholds of change or tipping points, major aspects of the Earth’s climate could change abruptly and in uncertain ways. Figure 3 shows one recent set of estimates of where dangerous “tipping points” may exist in the climate system relative to potential future global temperature increases. It also shows an estimate of the likelihoods, or probability density function (the black curve), that human-related GHG emissions since 1750 have already committed the planet to degrees of warming. In other words, according to the estimates represented in this figure, it is most likely that greenhouse gas emissions from 1750 to 2005 will lead to global average warming of 1°-3°C, and potentially result in ice-free Arctic summers, major reduction of area and volume of the Hindu-Kush-Himalaya-Tibetan (HKHT) glaciers, major melting of the Greenland Ice Sheet, and so non. According to the estimates in the cited study, it is less likely that the committed warming is already greater than the “tipping points” for collapse of the Amazon rain forest and other identified components.
Figure 3. Possible “Climate-Tipping” Elements and their Possible Likelihoods for Committed Global Temperature Change that May Precipitate the Tipping
Committed Global Temperature Change that May Precipitate the Tipping. Source: 1. V. Ramanathan and Y. Feng, “On Avoiding Dangerous Anthropogenic Interference with the Climate System: Formidable Challenges Ahead,” Proceedings of the National Academy of Sciences 105, no. 38 (September 23, 2008). Notes: Seven climate system components are identified that could undergo major, potentially abrupt and catastrophic, changes, as global temperatures increase. (ENSO includes El Niño/La Niña oscillations, and thermohaline circulation is the large-scale ocean overturning and circulation driven by temperature differences between the tropics and the poles and ocean salinity/freshwater). Each “climate-tipping” component is graphed above an estimated Increases in global average temperature (compared to pre-industrial levels) that could initiate the tipping, according to T.M. Lenton et al., Tipping Elements in the Earth’s Climate System, Proceedings of the National Academy of Sciences, Vol.105:1786-1793 (2007). The estimated likelihoods (probability density function) that GHG emissions had already committed the climate system to different degrees of warming are represented by the black curve. See Ramanathan and Feng for further detail.
- Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research.[ Clarke, L., J. Edmonds, H. Jacoby, H. Pitcher, J. Reilly, R. Richels] Department of Energy, Office of Biological & Environmental Research, Washington, DC., USA. (2007)
- CCSP, 2008: Abrupt Climate Change. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research [Clark, P.U., A.J. Weaver (coordinating lead authors), E. Brook, E.R. Cook, T.L. Delworth, and K. Steffen (chapter lead authors)]. U.S. Geological Survey, Reston, VA, 459 pp. (2008)
Note: The first version of this article was drawn from Climate Change: Science Highlights by Jane Leggett, Congressional Research Service, February 23, 2009.
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