The changing water cycle - Grantham Note 5

Authors:  Dr Simon Buckle and Dr Flora Whitmarsh

Overview

• Water vapour is the main component of the Earth’s natural greenhouse effect, which keeps the surface of the planet about 33°C warmer than it would be otherwise.
• The availability of water is critical for agriculture and is one of the greatest pressures on freshwater resources. Extremes in precipitation – either high or low – can be hugely damaging.
• With climate change, warming of the lowest part of the atmosphere allows the maximum level of water vapour there to increase. This in turn acts to amplify the greenhouse gas warming, as does the melting of ice and snow in a warmer climate.
• The net impact of warming on clouds depends on many detailed aspects of changes to the atmospheric circulation and has so far been one of the main uncertainties in projections of climate change. The IPCC’s Fifth Assessment Report (AR5) says that the feedback from clouds is likely to be positive.
• Global precipitation is expected to increase as the climate warms, perhaps by between 1-3 per cent per degree Celsius of warming. The contrast in precipitation between wet and dry regions and between wet and dry seasons will increase, although there may be regional exceptions.
• Globally for short-duration events, AR5 expects a shift to more intense individual storms and fewer weak storms as temperatures increase. Extreme precipitation events over land will very likely be more intense and more frequent.
• Regional to global-scale decreases in soil moisture and increased risk of drought in presently dry regions are expected by the end of the century in a high emissions scenario. Soil moisture drying would likely be most prominent in the Mediterranean, Southwest US and southern African regions.
• Monsoons are likely to affect greater areas and the monsoon season is likely to lengthen in many regions, with a likely intensification of monsoon precipitation.

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