Regional patterns of climate change

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. In terms of regional implications, however, a global average conveys rather little information. What is required is spatial detail. It is in the regional or local changes that the effects and impacts of global climate change will be felt.

2090-99

2090-99

Figure 6.6 Projected pattern average of surface temperature changes in °C for the twenty-first century - the period 2090-99 compared with 1980-99 - for the SRES scenario A1B, from multi-model AOGCM averages.

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. 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. 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. Reference to Chapter 4 indicates that many of these characteristics are already being found in the observed record of the last few decades.

• 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.

Across the world torrential rain and flooding has increased, and sights such as these in Canada have been more commonplace in recent years.

• Minimum warming around Antarctica and in the northern North Atlantic which is associated with deep oceanic mixing in those areas.

• Little seasonal variation of the warming in low latitudes or over the southern circumpolar ocean.

• A reduction in diurnal temperature range over land in most seasons and most regions; night-time lows increase more than daytime highs.

So far we have been presenting results solely for atmospheric temperature change. An even more important indicator of climate change is precipitation. 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. 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. 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. A characteristic therefore of anthropogenic climate change due to the increase of greenhouse gases will be a more intense hydrological cycle. The likely effect of this on precipitation extremes will be discussed in the next section.

In Figure 6.7 are shown projected changes in the distribution of precipitation as global warming increases. Three broad characteristics of precipitation changes are as follows.20

• 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.

• 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. Most of the regional projections of reductions in precipitation in the twenty-first century are associated with areas adjacent to these sub-tropical highs. For instance, southern Europe, Central America, southern Africa and Australia are likely to have drier summers with increasing risk of drought.

• A tendency for monsoonal circulations to result in increased precipitation due to enhanced moisture convergence, despite a tendency towards weakening of the monsoonal flows themselves. However, many aspects of tropical climatic responses remain uncertain.

Much natural climate variability occurs because of changes in, or oscillations between, persistent climatic patterns or regimes. The Pacific-North Atlantic Anomaly (PNA - which is dominated by high pressure over the eastern Pacific and western North America and which tends to lead to very cold winters in the

(a) December January February

(a) December January February

Figure 6.7 Projected relative changes in precipitation in per cent for the period 2090-99 relative to 1980-99, for the SRES scenario A1B, from multi-model AOGCM averages, for (a) December to February and (b) June to August. White areas are where fewer than 66% of the models agree in the sign of the change and stippled areas are where more than 90% of the models agree in the sign of the change.

Figure 6.7 Projected relative changes in precipitation in per cent for the period 2090-99 relative to 1980-99, for the SRES scenario A1B, from multi-model AOGCM averages, for (a) December to February and (b) June to August. White areas are where fewer than 66% of the models agree in the sign of the change and stippled areas are where more than 90% of the models agree in the sign of the change.

eastern United States), the North Atlantic Oscillation (NAO - which has a strong influence on the character of the winters in northwest Europe) and the El Niño events mentioned in Chapter 5 are examples of such regimes. Important components of climate change in response to the forcing due to the increase in greenhouse gases can be expected to be in the form of changes in the intensity or the frequency of established climate patterns illustrated by these regimes.21 There is little consistency at the present time between models regarding projections of many of these patterns. However, recent trends in the tropical Pacific for the surface temperature to become more El Niño-like (see Table 4.1 on page 73-5), with the warming in the eastern tropical Pacific more than that in the western tropical Pacific and with a corresponding eastward shift of precipitation, are projected to continue by many models.22 The influence of increased greenhouse gases on these major climate regimes, especially the El Niño, is an important and urgent area of research.

A complication in the interpretation of patterns of climate change arises because of the differing influence of atmospheric aerosols as compared with that of greenhouse gases. Although in the projections based on SRES scenarios the influence of aerosols is less than in those based on the IS 92 scenarios published by the IPCC in its 1995 Report their projected radiative forcing is still significant. When considering global average temperature and its impact on, for instance, sea-level rise (see Chapter 7) it is appropriate in the projections to use the values of globally averaged radiative forcing. The negative radiative forcing from sulphate aerosol, for instance, then becomes an offset to the positive forcing from the increase in greenhouse gases. However, because the effects of aerosol forcing are far from uniform over the globe (Figure 3.7), the effects of increasing aerosol cannot only be considered as a simple offset to those of the increase in greenhouse gases. The large variations in regional forcing due to aerosols produce substantial regional variations in the climate response. Detailed regional information from the best climate models is being employed to assess the climate change under different assumptions about the increases in both greenhouse gases and aerosols.

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