not as coarse as continents, but which at the same time are still assumed to provide robust projections (Solomon and Qin 2007).

At the regional level, the seasonality of changes is also important, since changes in annual averages do not uniquely determine the way in which the frequency or intensity of extreme weather events might change in the future. For instance, in Europe, where the annual mean temperature is likely to increase, the largest warming is projected to be likely in winter in Northern Europe and in summer in the Mediterranean area (Christensen and Hewitson 2007). Levels of confidence in projections of changes in frequency and intensity of extreme events, in particular regional-level statements concerning heat waves, heavy precipitation and droughts, can be estimated using different sources of information including observational data and model simulations. For extreme rainfall events, it is expected that these will be unrelated to changes in average rainfall. Average rainfall amount depends on the vertical temperature gradient of the atmosphere which in turn depends on how quickly the top of the atmosphere can radiate energy into space. This is expected to change only slightly with changes in carbon dioxide concentrations. On the other hand, extreme precipitation depends on how much water the air can hold, which increases exponentially with temperature. Thus it is reasonable to expect that in a warmer climate, short extreme rainfall events could become more intense and frequent, even in areas that become drier on average. Some studies have found that in regions that are relatively wet already, extreme precipitation will increase, while areas that are already dry are projected to become drier due to longer dry spells (Fussel 2009). The spatial extent of areas with severe soil moisture deficits and frequency of short-terms drought is expected to double until the late 21<sup>st</sup> Century, and long term droughts are projected to become three times more common, in particular in the Mediterranean, West African, Central Asian and Central American regions (Fussel 2009).

Uncertainties in climate projections are not only due to the fact that the future trajectory of socio-economic development is inherently unknown. In Figure 9, this uncertainty is represented by different colors corresponding to the different SRES scenarios. Uncertainty is also a consequence of the incomplete knowledge of the climate system, and the limitations of the computer models used to generate projections (part of this uncertainty is represented by the different color bands for each of the SRES scenarios). The relative and absolute importance of different sources of uncertainties depends on the spatial scale, the lead time of the projection, and the variable under consideration. Over shorter time scales, for many systems the natural variability of the climate system and other non-climatic risks are expected to have a higher impact than climate change. For example, in the next few years, changes in urbanization and urban development could increase significantly the risk of flooding independently of climate change. On longer time scales, climate change might play a more significant role.

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