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Climate Model Problems: III. Deserts
Volume 11, Number 2: 9 January 2008

In this third of our series of editorials concerning climate model problems, we focus on the study of Mahowald (2007), who used simulations produced by 17 different climate models in conjunction with the BIOME4 vegetation model to convert precipitation and temperature changes derived from the suite of climate models for the AD 1880-2080 period into predictions of simultaneous changes in the sizes of earth's desert areas. This she did both with and without the inclusion of two very important plant physiological phenomena: the aerial fertilization effect and the anti-transpiration effect of atmospheric CO2 enrichment, the first of which phenomena enhances plant growth in response to rising atmospheric CO2 concentrations and the second of which simultaneously reduces plant water loss, with the net effect that plant water use efficiency (the amount of CO2 converted into biomass per unit of H2O transpired in the process) rises significantly as the air's CO2 content rises.

So what did Mahowald learn from this exercise?

She reports, first of all, that "in the global mean, the models predict drying of the desert regions due to warmer temperatures with an increase in greenhouse gases." And with no CO2 fertilization or anti-transpiration effects in the BIOME4 model, she finds that this climatic phenomenon leads to "an increase in desert regions globally." However, when she includes the two CO2-induced biological effects in the simulations, she finds that the two phenomena produce "a decrease in desert area, as the higher carbon dioxide levels allow the plants to respond to the increased aridity more effectively."

In discussing her findings, Mahowald notes that "predicting precipitation in a general circulation [model] is very difficult, and the models do not agree what will happen in the future," as we have also discussed in our editorials of 26 December 2007 and 2 January 2008. This problem, in her words, "leads to a wide disagreement from the models on which [desert] areas will increase or decrease in the future," and she says that "unfortunately it is not known which model (if any) is correct." However, we do know that all desert areas will be benefited by the ongoing rise in the air's CO2 content, which will provide a powerful ameliorating effect that works against warming- and drying-induced desertification tendencies that might otherwise prevail in the absence of the aerial fertilization and anti-transpiration effects of atmospheric CO2 enrichment.

Looking to the future, Mahowald makes a most important point in her final sentence, stating that "desert area changes, and especially [changes in] desert dust itself, may significantly change climate (Yoshioka et al., 2007), and these impacts should be considered in future studies." In fact, declining desert area due to atmospheric CO2 enrichment is a multi-faceted feedback phenomenon that has several climate-relevant impacts in addition to changed amounts of airborne desert dust, such as changed surface albedo, changed surface-to-air evaporative and convective heat fluxes, and altered emissions of plant-derived aerosols. What is more, declining desert area is only one of many multi-faceted feedbacks to climate that occur in response to atmospheric CO2 enrichment over both land and water (see Feedback Factors (Biophysical) in our Subject Index), most of which are not included in today's climate models. Consequently, knowing that including the aerial fertilization and anti-transpiration effects of the expected rise in the air's CO2 content in the group of climate models studied by Mahowald totally overpowered the predicted climatic effects of the increase in atmospheric CO2 on the area of the world's deserts, one can begin to understand how the proper inclusion of the many other biophysical effects of atmospheric CO2 enrichment might nullify even the basic climatic effects that are currently predicted by state-of-the-art climate models.

Sherwood, Keith and Craig Idso

Mahowald, N.M. 2007. Anthropocene changes in desert area: Sensitivity to climate model predictions. Geophysical Research Letters 34: 10.1029/2007GL030472.

Yoshioka, M., Mahowald, N., Conley, A., Collins, W., Fillmore, D. and Coleman, D. 2007. Impact of desert dust radiative forcing on Sahel precipitation: Relative importance of dust compared to sea surface temperature variations, vegetation changes and greenhouse gas warming. Journal of Climate 20: 1445-1467.