Saturday, January 20, 2018

Explainer:  What Climate Models Tell Us About Future Rainfall

Much of the public discussion around climate change has focused on how much the Earth will warm over the coming century.  But climate change is not limited just to temperature; how precipitation – both rain and snow – changes will also have an impact on the global population.

Raindrops hitting a picnic table; Cornwall;UK (Credit: Alamy) Click to Enlarge.
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.

More evaporation and more water vapor
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.  According to the Clausius-Clapeyron equation, the air can generally hold around 7% more moisture for every 1C of temperature rise.  As such, a world that is around 4C warmer than the pre-industrial era would have around 28% more water vapor in the atmosphere.

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.
CMIP5 RCP8.5 multimodel average percent change in total precipitation (rain and snow) between 1981-2000 and 2081-2100. Uses one run for each model, 38 models total. (Credit: Data from KNMI Climate Explorer; map by Carbon Brief) Click to Enlarge.

The figure right shows projected percentage change in precipitation between the current climate (represented by the 1981-2000 average) and the end of the century (2081-2100) in the average of all of the climate models featured in in the latest Intergovernmental Panel on Climate Change (IPCC) report (CMIP5), using the high-end warming scenario (RCP8.5).

Purple colors show areas where precipitation will increase, while orange areas indicate less future rain and snow.

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.)

Read more at Explainer:  What Climate Models Tell us About Future Rainfall

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