Influential global change model predicts greater warming

Results from the most comprehensive simulations to date of how the global climate is likely to change by 2100 were published online on May 5 in the Journal of Climate (DOI 10.1175/2009JCLI2863.1). Andrei Sokolov of the Massachusetts Institute of Technology’s (MIT’s) Joint Program on the Science and Policy of Global Change and his colleagues predict that the median global climate warming will be 5.2 °C by the end of the century. This is more than double previous predictions made with the same widely respected model.

The authors say that the model, MIT’s Integrated Global System Model (IGSM), incorporates the most comprehensive formal treatment of both emissions scenarios and scientific uncertainty compared to other similar models. For example, the IGSM features significantly more chemical and biological detail than the atmosphere–ocean general circulation models (AOGCMs) used in the Intergovernmental Panel on Climate Change’s (IPCC’s) influential Fourth Assessment Report released in 2007.

Since 2003, when its first projections predicting a median rise of 2.4 °C were published, the IGSM has been significantly modified. For example, the new version incorporates a full set of anthropogenic and natural climate factors to evaluate changes that can upset the earth’s atmospheric energy balance; these are called forcings. The newly added 20th-century volcanic-eruptions data play an important role. However, the authors say that the primary basis for the differences in the predictions relates to changes in the input parameters for the model’s earth systems and economic components.

Previously, the global greenhouse gas emissions scenarios were based solely on projections of growth in the gross domestic products of the world’s nations. The model now incorporates details such as estimates of total available fossil resources, particularly coal and shale, and the effects of technological changes, such as moves toward increasing energy efficiency.

The new research involved 400 runs of different model variations believed to have an equal probability of being correct, on the basis of present observations and knowledge. The modelers did not simulate the effects of policy changes in the model runs discussed in the paper.

The authors say that the greenhouse gas concentrations in their simulations are somewhat higher than the AOGCMs in the IPCC’s Fourth Assessment Report. This is mainly because the IGSM treats the terrestrial ecosystem differently and takes into account the increase in the natural CH4 and N2O emissions caused by surface warming, they say.

However, Sokolov stresses that the model’s projections “are conditional on the . . . input climate parameters.” For example, if the researchers use alternative data for 20th-century changes in the deep ocean’s heat content, the projected surface warming decreases to 4.1 °C, he points out.

more from EST