3.4.4.2 Surface Radiation
The energy balance at the surface requires net radiative heating to be balanced by turbulent energy fluxes and thus determines the evolution of surface temperature and the cycling of water, which are key parameters of climate change (see Box 7.1). In recent years, several studies have focused on observational evidence of changing surface radiative heating. Reliable SW radiative measurement networks have existed since the 1957–1958 International Geophysical Year.
A reduction in downward solar radiation (‘dimming’) of about 1.3% per decade or about 7 W m–2 was observed from 1961 to 1990 at land stations around the world (Gilgen et al., 1998; Liepert, 2002). Additional studies also found declines in surface solar radiation in the Arctic and Antarctic (Stanhill and Cohen, 2001) as well as at sites in the former Soviet Union (Russak, 1990; Abakumova et al., 1996), around the Mediterranean Sea (Aksoy, 1997; Omran, 2000), China (Ren et al., 2005), the USA (Liepert, 2002) and southern Africa (Power and Mills, 2005). Stanhill and Cohen (2001) claim an overall globally averaged reduction of 2.7% per decade but used only 30 records. However, the stations where these analyses took place are quite limited in domain and dominated by large urban areas, and the dimming is much less at rural sites (Alpert et al., 2005) or even missing altogether over remote areas, except for identifiable effects of volcanic eruptions, such as Mt. Pinatubo in 1991 (Schwartz, 2005). At the majority of 421 analysed sites, the decline in surface solar radiation ended around 1990 and a recovery of about 6 W m–2 occurred afterwards (Wild et al., 2004; 2005). The increase in surface solar radiation (‘brightening’) agrees with satellite and surface observations of reduced cloud cover (Wang et al., 2002b; Wielicki et al., 2002a; Rossow and Dueñas, 2004; Norris, 2005b; Pinker et al., 2005), although there is evidence that some of these changes are spurious (see Section 3.4.3). In addition, the satellite-observed increase in surface radiation noted by Pinker et al. (2005) occured primarily over ocean, whereas the increase observed by Wild et al. (2005) was restricted to land stations.
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 3.3.2.2). 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.
From 1981 to 2003 over central Europe, Philipona and Dürr (2004) showed that decreases in surface solar radiation from increases in clouds were cancelled by opposite changes in LW radiation and that increases in net radiative flux were dominated by the clear-sky LW radiation component relating to an enhanced water vapour greenhouse effect. Alpert et al. (2005) provided evidence that a significant component of the reductions may relate to increased urbanisation and anthropogenic aerosol concentrations over the period (see also Section 7.5). This has been detected in solar radiation reductions for polluted regions (e.g., China; Luo et al., 2001), but cloudiness changes must also play a major role, as shown for European sites and the USA (Liepert, 2002; Dai et al., 2006). In the USA increasing cloud optical thickness and a shift from cloud-free to more cloudy skies are the dominating factors compared to the aerosol direct effects. Possible causes of the 1990s reversal are reduced cloudiness and increased cloud-free atmospheric transparency due to the reduction of anthropogenic aerosol concentrations and recovery from the effects of the 1991 eruption of Mt. Pinatubo. See Box 3.2 for more discussion and a likely explanation of these aspects.