Figure 3.2. Annual anomalies of maximum and minimum temperatures and DTR (°C) relative to the 1961 to 1990 mean, averaged for the 71% of global land areas where data are available for 1950 to 2004. The smooth curves show decadal variations (see Appendix 3.A). Adapted from Vose et al. (2005a).
Box 3.2: The Dimming of the Planet and Apparent Conflicts in Trends of Evaporation and Pan Evaporation Several reports have defined the term ‘global dimming’ (e.g., Cohen et al., 2004). This refers to a widespread reduction of solar radiation received at the surface of the Earth, at least up until about 1990 (Wild et al., 2005). However, recent studies (Alpert et al., 2005; Schwartz, 2005) found that dimming is not global but is rather confined only to large urban areas. At the same time there is considerable confusion in the literature over conflicting trends in pan evaporation and actual evaporation (Ohmura and Wild, 2002; Roderick and Farquhar, 2002, 2004, 2005; Hobbins et al., 2004; Wild et al., 2004, 2005) although the framework for explaining observed changes exists (Brutsaert and Parlange, 1998). Surface evaporation, or more generally evapotranspiration, depends upon two key components. The first is available energy at the surface, especially solar radiation. The second is the availability of surface moisture, which is not an issue over oceans, but which is related to soil moisture amounts over land. Evaporation pans provide estimates of the potential evaporation that would occur if the surface were wet. Actual evaporation is generally not measured, except at isolated flux towers, but may be computed using bulk flux formulae or estimated as a residual from the surface moisture balance. The evidence is strong that a key part of the solution to the paradox of conflicting trends in evaporation and pan evaporation lies in changes in the atmospheric circulation and the hydrological cycle. There has been an increase in clouds and precipitation, which reduce solar radiation available for actual and potential evapotranspiration but also increase soil moisture and make the actual evapotranspiration closer to the potential evapotranspiration. An increase in both clouds and precipitation has occurred over many parts of the land surface (Dai et al., 1999, 2004a, 2006), although not in the tropics and subtropics (which dominate the global land mean; Section 184.108.40.206). This reduces solar radiation available for evapotranspiration, as observed since the late 1950s or early 1960s over the USA (Liepert, 2002), parts of Europe and Siberia (Peterson et al., 1995; Abakumova et al., 1996), India (Chattopadhyay and Hulme, 1997), China (Liu et al., 2004a) and over land more generally (Wild et al., 2004). However, increased precipitation also increases soil moisture and thereby increases actual evapotranspiration (Milly and Dunne, 2001). Moreover, increased clouds impose a greenhouse effect and reduce outgoing LW radiation (Philipona and Dürr, 2004), so that changes in net radiation can be quite small or even of reversed sign. Recent re-assessments suggest increasing trends of evapotranspiration over southern Russia during the last 40 years (Golubev et al., 2001) and over the USA during the past 40 or 50 years (Golubev et al., 2001; Walter et al., 2004) in spite of decreases in pan evaporation. Hence, in most, but not all, places the net result has been an increase in actual evaporation but a decrease in pan evaporation. Both are related to observed changes in atmospheric circulation and associated weather. It is an open question as to how much the changes in cloudiness are associated with other effects, notably impacts of changes in aerosols. Dimming seems to be predominant in large urban areas where pollution plays a role (Alpert et al., 2005). Increases in aerosols are apt to redistribute cloud liquid water over more and smaller droplets, brightening clouds, decreasing the potential for precipitation and perhaps changing the lifetime of clouds (e.g., Rosenfeld, 2000; Ramanathan et al., 2001; Kaufman et al., 2002; see Sections 2.4 and 7.5). Increases in aerosols also reduce direct radiation at the surface under clear skies (e.g., Liepert, 2002), and this appears to be a key part of the explanation in China (Ren et al., 2005). Another apparent paradox raised by Wild et al. (2004) is that if surface radiation decreases then it should be compensated by a decrease in evaporation from a surface energy balance standpoint, especially given an observed increase in surface air temperature. Of course, back radiation from greenhouse gases and clouds operate in the opposite direction (Philipona and Dürr, 2004). Also, a primary change (not considered by Wild et al., 2004) is in the partitioning of sensible vs. latent heat at the surface and thus in the Bowen ratio. Increased soil moisture means that more heating goes into evapotranspiration at the expense of sensible heating, reducing temperature increases locally (Trenberth and Shea, 2005). Temperatures are affected above the surface where latent heating from precipitation is realised, but then the full dynamics of the atmospheric motions (horizontal advection, adiabatic cooling in rising air and warming in compensating subsiding air) come into play. The net result is a non-local energy balance.