A groundbreaking modelling study by scientists working at NASA has provided new evidence that global warming may increase the risk of extreme rainfall and drought.

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In a sobering revelation, the study shows for the first time how rising carbon dioxide levels could affect the entire range of rainfall types around the world. Rains are predicted to become more concentrated in certain regions, with others suffering longer periods without rain.

The NASA scientists set out to investigate how rainfall patterns would change as a result of global warming. Computer simulations point to increases in heavy rainfall in wetter parts of the world, while more arid land areas outside the tropics, along with regions which currently have moderate rainfall, could become drier.

The study's lead author William Lau said, "In response to carbon dioxide-induced warming, the global water cycle undergoes a gigantic competition for moisture resulting in a global pattern of increased heavy rain, decreased moderate rain, and prolonged droughts in certain regions."

The modelling predicted that, for every 1 degree Fahrenheit of carbon dioxide-induced warming, heavy rainfall will increase globally by 3.9 percent and light rain by 1 percent. However, the authors said they do not expect the total global rainfall to change overall, as moderate rainfall will decrease globally by 1.4 percent.

The tropical zones are expected to see the most significant increases in heavy rainfall; particularly in the Pacific Ocean and Asian monsoon regions. According to the modelling, which spanned periods of 140 years, Some regions outside the tropics may have no rainfall at all. The models also projected for every degree Fahrenheit of warming, the length of periods with no rain will increase globally by 2.6 percent.

Lau elaborated on one of the more worrying patterns identified in the modelling experiment. He said, "Large changes in moderate rainfall, as well as prolonged no-rain events, can have the most impact on society because they occur in regions where most people live. Ironically, the regions of heavier rainfall, except for the Asian monsoon, may have the smallest societal impact because they usually occur over the ocean."

Lau and colleagues based their analysis on the outputs of 14 climate models in simulations of 140-year periods. The simulations began with carbon dioxide concentrations at about 280 parts per million - similar to pre-industrial levels and well below the current level of almost 400 parts per million - and then increased by 1 percent per year. The rate of increase is consistent with a "business as usual" trajectory of greenhouse gasses, as described by the United Nations' Intergovernmental Panel on Climate Change.

Lau concluded that, although his modelling analysis was a poor predictor of how much rain would fall on a given location, the study painted a clear picture of more concentrated rainfall and a greater potential for dry regions to become drier. "If we look at the entire spectrum of rainfall types we see all the models agree in a very fundamental way - projecting more heavy rain, less moderate rain events, and prolonged droughts," He said.

Within the UK, Met Office analysis suggests the country is getting wetter, but that rainfall is occurring in more concentrated deluges. 30-year long-term averages show an increase in annual rainfall of about 5 percent from 1961-1990 to 1981-2010. The figures also suggest 'extreme' days of rainfall may be becoming more common, increasing the possibility of years like 2012, when many parts of the UK began the year in a state of drought, but were subsequently affected by widespread flooding in late Summer and Winter.