By Dr. Aiguo Dai, Ph.D.
Professor, State University of New York, Albany
The largest impacts of human-induced climate change in the coming decades will likely come from large percentage changes in weather and climate extremes, as these extreme events will change at much faster rates than the mean climate does. Among the projected changes of extreme events, increased risk of drought, heat waves and wild fires is a major concern, as climate models predict higher summer temperatures, increased evaporative demands, fewer rainy days, and drier soils over many land areas.
In fact, large drying trends may have already occurred since the 1950s over many land areas such as most of Africa, South and East Asia, Southern Europe, eastern Australia, and parts of South and Central America, as shown by historical records of streamflow, precipitation and drought index (Fig. 1). The rising temperature since the late 1970s has enhanced the drying over many land areas, although natural, decadal precipitation variations (e.g., associated with decadal variations in sea-surface temperatures in the Pacific) may have contributed considerably to the historical drying trend over some of these regions such as the U.S., South Asia, and eastern Australia.
As atmospheric CO2 and other greenhouse gases continue to rise, the impact of global warming will, however, become increasingly dominant over natural variations in the coming decades. The averaged soil moisture changes projected by climate models (Figure 2) suggest wide-spread drying in the Americas, Europe, Australia, and southern Africa. One might think 5-10% reduction in decadal-mean soil moisture content is not a big deal, as it can vary from year to year at much larger amplitudes. However, such a mean shift can lead to much larger increases (e.g., more than doubling by the 2090s) in global drought areas, as shown by Figure 3. The reason is that droughts are located on the left-side tail of the probability distribution of soil moisture at a given location, and the probability for drought can increase greatly for even just a small shift in the mean.
This future increased risk of drought is further illustrated (Figure 4) by the self-calibrated Palmer Drought Severity Index (PDSI) with potential evapotranspiration estimated using the Penman-Monteith equation and calculated using the surface meteorological data from the climate models. While the PDSI is designed to measure droughts under current climate conditions and thus its quantitative interpretation for future climates needs to be cautious, the combination of the soil moisture (Figures 2-3) and PDSI (Figure 4) changes does suggest a dire projection of increased risk of severe droughts.