3.3.6 Summary

Substantial uncertainty remains in trends of hydrological variables because of large regional differences, gaps in spatial coverage and temporal limitations in the data (Huntington, 2006). At present, documenting interannual variations and trends in precipitation over the oceans remains a challenge. Global precipitation averages over land are not very meaningful and mask large regional variations. Precipitation generally increased over the 20th century from 30°N to 85°N over land, and over Argentina, but notable decreases have occurred in the past 30 to 40 years from 10°S to 30°N. Salinity decreases in the North Atlantic and south of 25°S suggest similar precipitation changes over the ocean (Sections 5.3.2 and 5.5.3). Runoff and river discharge generally increased at higher latitudes, along with soil moisture, consistent with precipitation changes. River discharges in many tropical areas of Africa and South America are strongly affected by ENSO, with greater discharges from the Paraná River after the 1976–1977 climate shift but lower discharges from some major African rivers since then.

However, the PDSI suggests there has likely been a large drying trend since the mid-1950s over many land areas, with widespread drying over much of Africa, southern Eurasia, Canada and Alaska. In the SH, there was a drying trend from 1974 to 1998, although trends over the entire 1948 to 2002 period are small. Seasonal decreases in land precipitation since the 1950s are the main cause for some of the drying trends, although large surface warming during the last two to three decades has also likely contributed to the drying. Based on the PDSI data, very dry areas (defined as land areas with a PDSI of less than –3.0) have more than doubled in extent since the 1970s, with a large jump in the early 1980s due to an ENSO-induced precipitation decrease over land and subsequent increases primarily due to surface warming.

Hence, the observed marked increases in drought in the past three decades arise from more intense and longer droughts over wider areas, as a critical threshold for delineating drought is exceeded over increasingly widespread areas. Overall, consistent with the findings of Huntington (2006), the evidence for increases in both severe droughts and heavy rains (Section 3.8.2) in many regions of the world makes it likely that hydrologic conditions have become more intense.