Most of the regional projections of reductions in precipitation in the twenty-first century are associated with areas adjacent to these sub-tropical highs. The poleward expansion of the sub-tropical high pressure regions, combined with the general tendency towards reduction in sub-tropical precipitation, creates robust projections of a reduction in precipitation on the poleward edges of the sub-tropics.In addition to overall global average precipitation increase, there are large regional variations, areas with decreases in average precipitation, changes in its seasonal distribution and a general increase in the spatial variability of precipitation, contributing for instance to a reduction of rainfall in the sub-tropics and an increase at high latitudes and parts of the tropics.Three broad characteristics of precipitation changes are as follows.20

In Figure 6.7 are shown projected changes in the distribution of precipitation as global warming increases. The likely effect of this on precipitation extremes will be discussed in the next section. A characteristic therefore of anthropogenic climate change due to the increase of greenhouse gases will be a more intense hydrological cycle. In fact, model projections indicate increases in precipitation broadly related to surface temperature increases of about 3% per degree Celsius.19 Further, since the largest component of the energy input to the atmospheric circulation comes from the release of latent heat as water vapour condenses, the energy available to the atmosphere's circulation will increase in proportion to the atmospheric water content. Since the water-holding capacity of the atmosphere increases by about 6.5% per degree Celsius,18 the increases in precipitation as surface temperature rises can be expected to be substantial. With warming at the Earth's surface there is increased evaporation from the oceans and also from many land areas leading on average to increased atmospheric water vapour content and therefore also on average to increased precipitation. An even more important indicator of climate change is precipitation. So far we have been presenting results solely for atmospheric temperature change. A reduction in diurnal temperature range over land in most seasons and most regions night-time lows increase more than daytime highs.Little seasonal variation of the warming in low latitudes or over the southern circumpolar ocean.Minimum warming around Antarctica and in the northern North Atlantic which is associated with deep oceanic mixing in those areas.Across the world torrential rain and flooding has increased, and sights such as these in Canada have been more commonplace in recent years. Generally greater surface warming of land areas than of the oceans typically by about 40% compared with the global average, greater than this in northern high latitudes in winter (associated with reduced sea-ice and snow cover) and southern Europe in summer less than 40% in south and southeast Asia in summer and in southern South America in winter.Reference to Chapter 4 indicates that many of these characteristics are already being found in the observed record of the last few decades. Listed below are some of the broad features on the continental scale that characterise the projected temperature changes more detailed patterns are illustrated in Figure 6.6. We can, for instance, expect substantial differences between the changes over large land masses and over the ocean land possesses a much smaller thermal capacity and so can respond more quickly.

With respect to regional change, it is important to realise that, because of the way the atmospheric circulation operates and the interactions that govern the behaviour of the whole climate system, climate change over the globe will not be at all uniform. It is in the regional or local changes that the effects and impacts of global climate change will be felt.Ģ090-99 Figure 6.6 Projected pattern average of surface temperature changes in ☌ for the twenty-first century - the period 2090-99 compared with 1980-99 - for the SRES scenario A1B, from multi-model AOGCM averages. In terms of regional implications, however, a global average conveys rather little information. So far we have been presenting global climate change in terms of likely increases in global average surface temperature that provide a useful overall indicator of the magnitude of climate change.

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