Regional climate models

Most of the likely changes that we have presented have been on the scale of continents. Can more specific information be provided about change for smaller regions? In Chapter 5 we referred to the limitation of global circulation models (GCMs) in the simulation of changes on the regional scale arising from the coarse size of their horizontal grid - typically 300 km or more.30 Also in Chapter 5 we introduced the regional climate model (RCM) which typically possesses a resolution of 50 km and can be 'nested' in a global circulation model. Examples are shown in Figures 6.13 and 7.9 of the improvement achieved by RCMs in the simulation of extremes and in providing regional detail that in many cases (especially for precipitation) shows substantial disagreement with the averages provided by a GCM.

Regional models are providing a powerful tool for the investigation of detail in patterns of climate change. In the next chapter the importance of such detail will be very apparent in studies that assess the impacts of climate change.

Table 6.2 Estimates of confidence in observed and projected changes in extreme weather and climate events

Confidence in observed changes (latter half of the twentieth century)

Changes in phenomenon

Confidence in projected changes (during the twenty-first century)

Very likelya

Higher maximum temperatures and more hot days over nearly all land areas

Very likely

Very likely Very likely

Likely, over most areas

Likely, over many northern hemisphere mid- to high-latitude land areas

Higher minimum temperatures, fewer cold days and frost days over nearly all land areas Reduced diurnal temperature range over most land areas Increase of heat indexb over land areas

More intense precipitation events

Very likely Very likely

Very likely, over most areas Very likely, over most areas

Likely, increases in total area affected over many land areas

Increased summer continental drying and associated risk of drought

Likely, in most sub-tropical areas & many mid-latitude continental areas (very likely, over Mediterranean, south Australia, New Zealand)

Likely, trends towards greater storm intensity, no trend in frequency

Tropical cyclones

Likely, increase in peak wind and precipitation intensities

Likely, net increase in intensity and poleward shift in track over many northern hemisphere land areas

Extra-tropical cyclones

Likely, increase in intensity over many areas (e.g. North Atlantic, Central Europe and southern New Zealand)

a See Note 1 of Chapter 4 for explanation of likely, very likely, etc.

b Heat index is a combination of temperature and humidity that measures effects on human comfort.

However, it is important to realise that, even if the models were perfect, because much greater natural variability is apparent in local climate than in climate averaged over continental or larger scales, projections on the local and regional scale are bound to be more uncertain than those on larger scales.

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