2.5.6 Tropospheric Water Vapour from Anthropogenic Sources
Anthropogenic use of water is less than 1% of natural sources of water vapour and about 70% of the use of water for human activity is from irrigation (Döll, 2002). Several regional studies have indicated an impact of irrigation on temperature, humidity and precipitation (Barnston and Schickedanz, 1984; Lohar and Pal, 1995; de Ridder and Gallée, 1998; Moore and Rojstaczer, 2001; Zhang et al., 2002). Boucher et al. (2004) used a GCM to show that irrigation has a global impact on temperature and humidity. Over Asia where most of the irrigation takes place, the simulations showed a change in the water vapour content in the lower troposphere of up to 1%, resulting in an RF of +0.03 W m–2. However, the effect of irrigation on surface temperature was dominated by evaporative cooling rather than by the excess greenhouse effect and thus a decrease in surface temperature was found. Irrigation affects the temperature, humidity, clouds and precipitation as well as the natural evaporation through changes in the surface temperature, raising questions about the strict use of RF in this case. Uncertainties in the water vapour flow to the atmosphere from irrigation are significant and Gordon et al. (2005) gave a substantially higher estimate compared to that of Boucher et al. (2004). Most of this uncertainty is likely to be linked to differences between the total withdrawal for irrigation and the amount actually used (Boucher et al., 2004). Furthermore, Gordon et al. (2005) also estimated a reduced water vapour flow to the atmosphere from deforestation, most importantly in tropical areas. This reduced water vapour flow is a factor of three larger than the water vapour increase due to irrigation in Boucher et al. (2004), but so far there are no estimates of the effect of this on the water vapour content of the atmosphere and its RF. Water vapour changes from deforestation will, like irrigation, affect the surface evaporation and temperature and the water cycle in the atmosphere. Radiative forcing from anthropogenic sources of tropospheric water vapour is not evaluated here, since these sources affect surface temperature more significantly through these non-radiative processes, and a strict use of the RF is problematic. The emission of water vapour from fossil fuel combustion is significantly lower than the emission from changes in land use (Boucher et al., 2004).
2.5.7 Anthropogenic Heat Release
Urban heat islands result partly from the physical properties of the urban landscape and partly from the release of heat into the environment by the use of energy for human activities such as heating buildings and powering appliances and vehicles (‘human energy production’). The global total heat flux from this is estimated as 0.03 W m–2 (Nakicenovic, 1998). If this energy release were concentrated in cities, which are estimated to cover 0.046% of the Earth’s surface (Loveland et al., 2000) the mean local heat flux in a city would be 65 W m–2. Daytime values in central Tokyo typically exceed 400 W m–2 with a maximum of 1,590 W m–2 in winter (Ichinose et al., 1999). Although human energy production is a small influence at the global scale, it may be very important for climate changes in cities (Betts and Best, 2004; Crutzen, 2004).