Climate models predict that one of the consequences of CO2-induced global warming is an enhancement of the world's hydrologic cycle, which is projected to result in increased precipitation and floods. At the same time, however, the models also predict longer and more severe droughts due to increased evapotranspiration driven by rising temperatures. Given such predictions, notable floods and droughts experienced around the globe in recent years have led climate alarmists to proclaim they are due to global warming. But is this attribution correct? We here explore this question as it pertains to drought across the world as a whole.
Svensson et al. (2005) examined 20th-century river flow data for a group of 21 stations distributed around the globe, which they obtained from the Global Runoff Data Centre in Koblenz, Germany. Individual record lengths of the 21 stations varied from 44 to 100 years, with an average of 68 years; and the three researchers' analyses of the data consisted of computing trends in both high flows and low flows using Mann-Kendall and linear regression methods. This work revealed, in the case of high flows, that there were slightly more stations exhibiting significant negative trends (reduced flooding) than significant positive trends (increased flooding), while with respect to low-flows, nearly all stations showed increasing trends, approximately half of which were significant at the 90% level, indicative of a general trend of decreasing drought throughout the world.
Huntington (2006) reviewed the current state of science regarding historical trends in hydrologic variables, including precipitation, runoff, soil moisture and a number of other water-related parameters. In doing so, he found that on a globally-averaged basis, "precipitation over land increased by about 2% over the period 1900-1998 (Dai et al., 1997; Hulme et al., 1998)." He also reported that "an analysis of trends in world continental runoff from major rivers from 1910-1975 found an increase in runoff of about 3% (Probst and Tardy, 1987)," and that a reanalysis of those trends for the period 1920-1995 "confirmed an increase in world continental runoff during the 20th century (Labat et al., 2004)." Last of all, and as a result of these observations, Huntington further reported that "summer soil moisture content has increased [italics added] during the last several decades at almost all sites [italics added] having long-term records in the Global Soil Moisture Data Bank (Robock et al., 2000)."
Narisma et al. (2007) analyzed "global historical rainfall observations to detect regions that have undergone large, sudden decreases in rainfall [that] are statistically significant at the 99% level, are persistent for at least ten years, and ... have magnitudes that are [mostly] 10% lower than the climatological normal (1901-2000 rainfall average)." Working with the gridded high-resolution (0.5 x 0.5 degrees of latitude and longitude) global precipitation data set of Mitchell et al. (2004), which covers the period 1901-2000, they identified 30 drought episodes throughout the world that satisfied these stringent criteria during the 20th century, among which were the sudden and prolonged Sahel drought of Africa in the late 1960s, the United States Dust Bowl of the 1930s and Southwest drought of the 1950s (which also affected parts of Mexico), the strong and persistent droughts that occurred in northeast China in the 1920s, in Kazakhstan and regions of the former Soviet Union in the late 1930s, in southeast Australia in the late 1930s, and in southern Africa and eastern Europe in the 1980s, as well as the World War II droughts of 1937-1945 and the droughts that occurred over large regions of East India and Bangladesh in the 1950s.
With respect to the temporal distribution of the 30 severe and persistent droughts identified by Narisma et al., seven of them occurred during the first two decades of the 20th century (1901-1920), seven occurred during the next two decades (1921-1940), eight during the middle two decades of the century (1941-1960), but only five during the next two decades (1961-1980), and a mere three during the final two decades of the century (1981-2000), which is not at all what one would have expected if the climate-alarmist thesis propounded by Al Gore and the world's climate alarmists is correct.
So just what is the situation here? The scientists who performed the analysis note that the 30 major droughts they identified were "mostly located in semi-arid and arid regions" that "are naturally prone [italics added] to large fluctuations." And it's as simple as that. The 30 major droughts of the 20th century were likely natural in all respects; and, hence, they are "indicative of what could also happen in the future," as Narisma et al. state in their concluding paragraph. And, sooner or later, happen they will. Consequently, the next time a serious drought grips some part of the world, and Al Gore and his followers blame it on the "carbon footprints" of you and your family, ask them why the exact opposite of what their hypothesis predicts occurred over the entire course of the 20th century, i.e., why, when the earth warmed -- and at a rate and to a degree they claim was unprecedented over the past two millennia -- severe regional drought occurrence actually declined.
Moving on, Sheffield and Wood (2008) studied "variability and trends in soil moisture and drought characteristics, globally and regionally over the second half of the twentieth century," by means of "a global soil moisture dataset derived from a model simulation of the terrestrial hydrologic cycle," which was "driven by a hybrid observation-reanalysis-based meteorological dataset." This work revealed, in their words, that "an overall increasing trend in global soil moisture, driven by increasing precipitation, underlies the whole analysis, which is reflected most obviously over the western hemisphere and especially in North America." In addition, they determined that "trends in drought characteristics are predominantly decreasing," and that "concurrent changes in drought spatial extent are evident, with a global decreasing trend of -0.021% to -0.035% per year." However, they discovered "a switch in later years to a drying trend, globally and in many regions," which they say was "concurrent with increasing temperatures." But this drying trend was not strong enough to overpower the increasing trend of global soil moisture over the entire half-century of their analysis.
Last of all, in a subsequent analysis of the same time period, Sheffield et al. (2009) used "observation-driven simulations of global terrestrial hydrology and a cluster algorithm that searches for spatially connected regions of soil moisture," to identify "296 large scale drought events (greater than 500,000 km2 and longer than 3 months) globally for 1950-2000." And in this study, as they report, "the mid-1950s showed the highest drought activity and the mid-1970s to mid-1980s the lowest activity."
In conclusion, if anthropogenic CO2 emissions, and the global warming they are blamed for producing, are supposed to generate catastrophic droughts, as the U.S. Environmental Protection Agency claims they are, it is strange indeed that we do not find any indication of this phenomenon in real-world global drought data. Could it be that Al Gore and the U.S. EPA are wrong? Very definitely.
Dai, A., Fung, I.Y. and DelGenio, A.D. 1997. Surface observed global land precipitation variations during 1900-1998. Journal of Climate 10: 2943-2962.
Hulme, M., Osborn, T.J. and Johns, T.C. 1998. Precipitation sensitivity to global warming: comparisons of observations with HadCM2 simulations. Geophysical Research Letters 25: 3379-3382.
Huntington, T.G. 2006. Evidence for intensification of the global water cycle: Review and synthesis. Journal of Hydrology 319: 83-95.
Labat, D., Godderis, Y., Probst, J.L. and Guyot, J.L. 2004. Evidence for global runoff increase related to climate warming. Advances in Water Resources 27: 631-642.
Mitchell, T.D., Carter, T.R., Jones, P.D., Hulme, M. and New, M. 2004. A comprehensive set of high-resolution grids of monthly climate for Europe and the globe: The observed record (1901-2000) and 16 scenarios (2001-2100). Tyndall Center Working Paper 55, Norwich, UK.
Narisma, G.T., Foley, J.A., Licker, R. and Ramankutty, N. 2007. Abrupt changes in rainfall during the twentieth century. Geophysical Research Letters 34: 10.1029/2006GL028628.
Probst, J.L. and Tardy, Y. 1987. Long range streamflow and world continental runoff fluctuations since the beginning of this century. Journal of Hydrology 94: 289-311.
Robock, A., Konstantin, Y.V., Srinrivasan, J.K., Entin, J.K., Hollinger, N.A., Speranskaya, N.A., Liu, S. and Nampkai, A. 2000. The global soil moisture data bank. Bulletin of the American Meteorological Society 81: 1281-1299.
Sheffield, J., Andreadis, K.M., Wood, E.F. and Lettenmaier, D.P. 2009. Global and continental drought in the second half of the twentieth century: severity-area-duration analysis and temporal variability of large-scale events. Journal of Climate 22: 1962-1981.
Sheffield, J. and Wood, E.F. 2008. Global trends and variability in soil moisture and drought characteristics, 1950-2000, from observation-driven simulations of the terrestrial hydrologic cycle. Journal of Climate 21: 432-458.
Svensson, C., Kundzewicz, Z.W. and Maurer, T. 2005. Trend detection in river flow series: 2. Flood and low-flow index series. Hydrological Sciences Journal 50: 811-824.Last updated 16 September 2009