by Bill Schlesinger
Thirty years ago, a NASA scientist, Dave Rind, published a paper showing the enhanced likelihood of droughts with global warming. His view was prescient. Rind reasoned that with global warming the land surface would warm faster than the oceans. Since most of the world’s rain is formed by evaporation from the oceans, rainfall would not increase fast enough to make up for increased evaporation from the land surface, which would dry out. The effect would be most dramatic towards the central portion of large continents, such as in the Great Plains of the United States.
The intervening years have allowed us to refine Rind’s predictions, but the ongoing drought in the Southwestern U.S. is certainly consistent with this model. An analysis of tree-ring records shows that the drought during the period of 2000 to 2018 has not been seen since 1500 and it is the second driest period since 800 A.D. About 47% of the recent drought is attributable to changes in climate from human activities.
The recent drought during a period of global warming has a mirror image in regional wet conditions in the Southwest during the last glacial period, when the Earth’s climate was much cooler. We can certainly anticipate that the southwestern drought will intensify as global warming from human activities increases in the coming decades. Several past civilizations have disappeared as a result of the onset of drought conditions.
Drought has direct economic impacts seen in reduced crop yields, loss of grazing lands, and a greater frequency of forest fires—already seen in California in recent years. Drought tips the balance between semi-arid grassland systems, which have typically been used as rangelands, and shrub deserts which cannot support cattle. Often these changes are associated with the loss and redistribution of soil nutrients, so that if prior climatic conditions return, the landscape is slow to recover.
When plant productivity declines on a landscape due to dry conditions, we call the process desertification. Deserts cannot desertify, but the lands are the borders of deserts can become more desert-like—the essence of desertification. The region south of the Sahara desert of Africa is vulnerable to desertification, which would displace millions of people in migration.
Desertification joins the long list of costs associated with global warming, including sea-level rise, vulnerability to exotic infectious disease, and crop failures. These impacts show why it is dumb and misplaced to ignore the prediction and the current onset of global changes in climate due to human activity. The current generation should not leave the bill for its behavior and these costs to be borne by future generations. That is the essence of unsustainable.
References
Ault, T. R. 2020. On the essentials of drought in a changing climate. Science 368: 256-260.
Bradford, J.B., D.R. Schlaepfer, W.K. Lauenroth, and K.A. Palmquist. 2020. Robust ecological drought projections for drylands in the 21st century. Global Change Biology 26; 3906-3919
Klemm, T., D.D. Briske, and M.C. Reeves. 2020. Vulnerability of rangeland beef cattle production to climate-induced fluctuations in the U.S. Great Plains. Global Change Biology doi: 10.1111/gcb.15202
Rind, D., R. Goldberg, J. Hansen, C. Rosenzweig, and R. Ruedy. 1990. “Potential Evapotranspiration and the Likelihood of Future Drought.” Journal of Geophysical Research—Atmospheres 95: 9983–10004.
Schlesinger, W.H., J.F. Reynolds, G.L. Cunningham, L.F. Huenneke, W.M. Jarrell, R.A. Virginia, and W.G. Whitford. 1990. Biological feedbacks in global desertification. Science 247: 1043-1048.
Schlesinger, W. H., J. A. Raikes, A. E. Hartley, and A. E. Cross. 1996. “On the Spatial Pattern of Soil Nutrients in Desert Ecosystems.” Ecology 77: 364–74.
Williams, A.P. and 8 others. 2020. Large contribution from anthropogenic warming to an emerging North American megadrought. Science 368: 314-318.
