Thursday, March 23, 2017

Global Warming Is Increasing Rainfall Rates

The world is warming because humans are emitting heat-trapping greenhouse gases.  We know this for certain; the science on this question is settled.  Humans emit greenhouse gases, those gases should warm the planet, and we know the planet is warming.  All of those statements are settled science.

Okay so what?  Well, we would like to know what the implications are.  Should we do something about it or not?  How should we respond?  How fast will changes occur?  What are the costs of action compared to inaction?  These are all areas of active research.

Part of answering these questions requires knowing how weather will change as the Earth warms.  One weather phenomenon that directly affects humans is the pattern, amount, and intensity of rainfall and the availability of water.  Water is essential wherever humans live, for agriculture, drinking, industry, etc.  Too little water and drought increases risk of wild fires and can debilitate societies.  Too much water and flooding can occur, washing away infrastructure and lives.

It’s a well-known scientific principle that warmer air holds more water vapor.  In fact the amount of moisture that can be held in air grows very rapidly as temperatures increase.  So, it’s expected that in general, air will get moister as the Earth warms – provided there is a moisture source.  This may cause more intense rainfalls and snow events, which lead to increased risk of flooding. 

But warmer air can also more quickly evaporate water from surfaces.  This means that areas where it’s not precipitating dry out more quickly.  In fact, it’s likely that some regions will experience both more drought and more flooding in the future (just not at the same time!).  The dry spells are longer and with faster evaporation causing dryness in soils.  But, when the rains fall, they come in heavy downpours potentially leading to more floods.  The recent flooding in California – which followed a very intense and prolonged drought – provides a great example.

Okay so what have we observed?  It turns out our expectations were correct.  Observations reveal more intense rainfalls and flooding in some areas.  But in other regions there’s more evaporation and drying with increased drought.  Some areas experience both.

Some questions remain.  When temperatures get too high, there’s no continued increase in intense rain events.  In fact, heavy precipitation events decrease at the highest temperatures. There are some clear reasons for this but for brevity, regardless of where measurements are made on Earth, there appears to be an increase of precipitation with temperature up until a peak and thereafter, more warming coincides with decreased precipitation. 

A new clever study by Dr. Guiling Wang from the University of Connecticut and her colleagues has looked into this and they’ve made a surprising discovery.  Their work was just published in Nature Climate Change.  They report that the peak temperature (the temperature where maximum precipitation occurs) is not fixed in space or time.  It is increasing in a warming world. 

The idea is shown in the sketch below.  Details vary with location but, as the world warms, there is a shift from one curve to the next, from left to right.  The result is a shift such that more intense precipitation occurs at higher temperatures in future, while the drop-off moves to even higher temperatures.

Read more at Global Warming Is Increasing Rainfall Rates

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