6.2.2 The spatial distribution of surface temperature and precipitation changes

The increase in global mean temperature by 2010 is associated with a warming in all regions according to the multi-model average (Fig. 6.9). The regional pattern is similar in all the scenarios, with a larger change over the land than over the ocean. This is due to the larger thermal inertia of the ocean and to the increase in latent heat loss that mitigates the temperature changes there. The amplitude of changes is particularly low over the high latitude oceans because of the deeper mixed layer and the contact with colder deep water which has not recently been exposed to surface warming. Additionally, in some models, changes in ocean currents may be responsible for the very small warming, or even a small cooling predicted for the North Atlantic (see Section 6.2.3). The changes simulated for the Arctic are also much larger than at mid-latitudes, partly because of the temperature/albedo feedback and other feedbacks related to the cryosphere (see Section 4.2.3). Changes in heat transport, clouds and the water-vapour feedback also play a role in this behaviour.

Global warming is associated with a global increase in precipitation (see Fig. 6.10). This is because of the greater evaporation over the ocean and the larger water-holding capacity of the atmosphere in a warmer world, as described by the Clausius-Clapeyron equation. Depending on the model and the scenario, the predicted increase by 2100 is between 1 and 8% compared to the late 20^th century. More importantly, this increase in precipitation is far from being spatially uniform and varies strongly over the seasons. At high latitudes, both the multi-model mean and the large majority of individual models predict an increase in precipitation in both winter and summer. An increase in precipitation is also predicted over the tropical oceans, and in the regions influenced by the summer monsoon in South Asia. By contrast, precipitation is predicted to decrease over many subtropical areas and in particular regions such as tropical Central America and the Caribbean, and the Mediterranean.

These changes in the annual and seasonal mean temperature and precipitation are important elements of the projected climate for the 21^st century. However, many other characteristics of the atmospheric state, such as wind intensity or cloud amount, are also expected to change. For instance, nearly all the models simulate an increase in the intensity of the westerly winds over the Southern Ocean in the near future, which can also be related to an increase in the Southern Annular Mode (SAM) index (see Section 5.2.3). A particularly sensitive point is the change in the probability of extreme events (such as major storms and heat waves) in a warmer climate. Such extreme events are difficult to model and the available time series are usually too short for reliable predictions to be made for rare events. However, some simple arguments suggest that even a small change in the mean temperature greatly increases, for example, the probability of experiencing a temperature above a particular threshold, and thus of an increase in the number of very hot days (Fig. 6.11). A rise in average temperature also decreases the probability of the temperature falling below a particular level, and so decreases the probability of cold days. This simple reasoning is in agreement with model results which suggest an increase in heat waves in summer and a decline in the incidence of frosts in many regions.

Fig. 6.11 is based on a simple shift in the distribution but the shape of the distribution can also change. Indeed, some studies suggest that the future climate will also be more variable in some regions (corresponding to a wider distribution on Fig. 6.11 and thus even more frequent extremes), although those results are not always robust between the different models.