Rainfall and Drought
Trends & Patterns: NASA Research
The distribution of the world's rainfall is shifting as our climate changes. Wet areas may become wetter, dry areas drier, storms more intense, leading to more chaotic weather around the world. According to the Intergovernmental Panel on Climate Change (IPCC, 2011), an increase in the average global temperature is very likely to lead to changes in precipitation and atmospheric moisture, including shifts towards more extreme precipitation during storms.
As the lower atmosphere (the troposphere) becomes warmer, evaporation rates increase, which leads to an increase in the amount of moisture circulating. When the troposphere has more moisture, more intense precipitation occurs, thus potentially triggering more flooding over land. Conversely in other areas, warmer temperatures may lead to increased drying accelerating the onset of drought.
While the models used by climate scientists generally agree on how different parts of the Earth will warm, there is much less agreement about where and how precipitation will change.
Move evaporation and more water vapour
There are some basic physical processes that inform scientists' expectations of how precipitation will respond in a warming world. With higher temperatures comes greater evaporation and surface drying, potentially contributing to the intensity and duration of drought.
However, as the air warms its water-holding capacity increases, particularly over the oceans. But this increased moisture will not fall evenly across the planet. Some areas will see increased precipitation, while other areas are expected to see less due to shifting weather patterns and other factors.
On average, warming is expected to result in dry areas becoming drier and wet areas becoming wetter, especially in mid- and high-latitude areas. (This is not always true over land, however, where the effects of warming are a bit more complex.)
The average of the models shows large increases in precipitation near the equator, particularly in the Pacific Ocean. They also show more precipitation in the Arctic and Antarctic, where cold temperatures currently limit how much water vapour the air can hold.
The Mediterranean region is expected to have around 20% less precipitation by 2100, with similar reductions also found in southern Africa. Western Australia, Chile, and Central America/Mexico may all become around 10% drier.
However, the simple picture painted by the average of all the models shown above hides profound differences. There are actually relatively few areas that all the models agree will become wetter or drier. Climate models are not perfect and projections of future average precipitation changes may become more consistent as models improve.