3.3 Changes in Surface Climate: Precipitation, Drought and Surface Hydrology

3.3.1 Background

Temperature changes are one of the more obvious and easily measured changes in climate, but atmospheric moisture, precipitation and atmospheric circulation also change, as the whole system is affected. Radiative forcing alters heating, and at the Earth’s surface this directly affects evaporation as well as sensible heating (see Box 7.1). Further, increases in temperature lead to increases in the moisture-holding capacity of the atmosphere at a rate of about 7% per °C (Section 3.4.2). Together these effects alter the hydrological cycle, especially characteristics of precipitation (amount, frequency, intensity, duration, type) and extremes (Trenberth et al., 2003). In weather systems, convergence of increased water vapour leads to more intense precipitation, but reductions in duration and/or frequency, given that total amounts do not change much. The extremes are addressed in Section 3.8.2.2. Expectations for changes in overall precipitation amounts are complicated by aerosols. Because aerosols block the Sun, surface heating is reduced. Absorption of radiation by some, notably carbonaceous, aerosols directly heats the aerosol layer that may otherwise have been heated by latent heat release following surface evaporation, thereby reducing the strength of the hydrological cycle. As aerosol influences tend to be regional, the net expected effect on precipitation over land is especially unclear. This section discusses most aspects of the surface hydrological cycle, except that surface water vapour is included with other changes in atmospheric water vapour in Section 3.4.2.

Difficulties in the measurement of precipitation remain an area of concern in quantifying the extent to which global- and regional-scale precipitation has changed (see Appendix 3.B.4). In situ measurements are especially affected by wind effects on the gauge catch, particularly for snow but also for light rain. For remotely sensed measurements (radar and space-based), the greatest problems are that only measurements of instantaneous rate can be made, together with uncertainties in algorithms for converting radiometric measurements (radar, microwave, infrared) into precipitation rates at the surface. Because of measurement problems, and because most historical in situ-based precipitation measurements are taken on land leaving the majority of the global surface area under-sampled, it is useful to examine the consistency of changes in a variety of complementary moisture variables. These include both remotely-sensed and gauge-measured precipitation, drought, evaporation, atmospheric moisture, soil moisture and stream flow, although uncertainties exist with all of these variables as well (Huntington, 2006).