Global warming will result in the acceleration of the water cycle, which will lead to a higher variability of precipitation. Extreme rainfall events are likely to occur more often and to become more extreme. As a result, floods are likely to become more frequent and potentially more severe in most regions around the world (Milly et al, 2002). Flood frequency and severity will especially increase in regions where more rainfall is predicted; but in areas where a reduction of mean annual rainfall is expected, extreme rainfall events and floods might still increase due to more extreme events. As explained in detail in the case study on Thailand (see Chapter 9), not all floods are necessarily negative, and they are often a part of the normal seasonal cycle. Floods can, for example, be essential for fisheries and agriculture. Floods often become a disaster when they are unusual in timing and/or severity.

Changes in snowmelt patterns and glacier melting can also increase flood risks. Due to the melting of glaciers, new lakes are created in mountainous areas. Accumulation of water in these lakes can cause a sudden discharge of large volumes of water and debris. These glacial lake outburst floods (GLOFs) can result in large-scale disasters, causing death and damage to agricultural systems and infrastructure. Especially in the Himalayas, GLOFs have already increased in number and are a major concern for the future.

Many large urban areas are located in regions that are vulnerable to flooding because they are either located along the coast or along major river systems. One of the problems in these urban regions is that the most vulnerable people with the least access to resources live in areas that are most prone to flooding. Often, slums in major cities in developing countries are built on floodplains where regulated development is prohibited. However, this is not only a problem in the developing world; in New Orleans, the poorer neighbourhoods were also affected the most by the floods caused by Hurricane Katrina in 2005. Due to the fact that many growing urban centres are in areas that are vulnerable to floods, damage caused by floods is predicted to increase rapidly if current flood management policies are not changed.

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