Global warming: Encyclopedia II - Global warming - Climate models

Global warming - Climate models

Scientists have studied this issue with computer models of the climate (see below). These models are accepted by the scientific community as being valid only after it has been shown that they do a good job of simulating known climate variations, such as the difference between summer and winter, the North Atlantic Oscillation, or El Niño. All climate models that pass these tests also predict that the net effect of adding greenhouse gases will be a warmer climate in the future. The amount of predicted warming varies by model, however, which probably reflects the way different models depict clouds differently.

As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4 °C to 5.8 °C between 1990 and 2100 [20]. They have also been used to help determine the causes of recent climate change by comparing the observed changes to those that the models predict from various natural and human derived forcing factors.

The most recent climate models can produce a good match to observations of global temperature changes over the last century. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions. Adding simulation of the ability of the environment to sink carbon dioxide suggested that rising fossil fuel emissions would decrease absorption from the atmosphere, amplifying climate warming beyond previous predictions, although "Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses" [21].

Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release the potent greenhouse gas, methane, that is trapped in large quantities in permafrost and ice clathrates [22].

Uncertainties in the representation of clouds are a dominant source of uncertainty in existing models, despite clear progress in modeling of clouds [23]. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability. Further, all such models are limited by available computational power, so that they may overlook changes related to small scale processes and weather (e.g. storm systems, hurricanes). However, despite these and other limitations, the IPCC considers climate models "to be suitable tools to provide useful projections of future climates" [24].

In December, 2005 Bellouin et al suggested in Nature that the reflectivity effect of airborne pollutants was about double that previously expected, and that therefore some global warming was being masked. If supported by further studies, this would imply that existing models underpredict future global warming. [25]

Adapted from the Wikipedia article "Climate models", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki