Global warming drives more extreme rainfall

Summer rainstorm in the Rocky Mountains. @bberwyn photo.

New study pinpoints regional patterns in changes

Staff Report

Basic physics tells us a warmer atmosphere can hold more moisture, and that, at some point, that moisture will condense and fall as rain. That’s why climate scientists are certain that global warming will lead to more extreme rainfall, as has already been documented in various parts of the world the past few decades.

A new study now helps quantify the impact of warming and also reveals regional patterns that will help people prepare. According to the researchers with MIT and the Swiss Federal Institute of Technology, the most extreme rain events in most regions of the world will increase in intensity by 3 to 15 percent, depending on region, for every degree Celsius that the planet warms.

Under a business-as-usual greenhouse gas scenario, an average global temperature increase of 4 degrees Celsius during the next hundred years means much of North America and Europe would experience increases in the intensity of extreme rainfall of abut 25 percent. Some areas, including parts of the Asian monsoon region would experience greater increases, while there will be smaller increases in the Mediterranean, South Africa and Australia.

There are a few regions that are projected to experience a decrease in extreme rainfall as the world warms, mostly located over subtropical oceans that lie just outside the tropical, equatorial belt.

The regional variations patterns in rainfall can be explained by  changes in the strength of local winds that influence the intensity of a region’s most extreme rainstorms, according to the findings published in the journal Nature Climate Change.

“There is interest around the world in the question of whether to adjust codes to adapt to a changing climate and precipitation, particularly for flooding,” said MIT climate scientist Paul O’Gorman. “We found there are regional variations in the projected precipitation response because of changes in winds, and of course if you’re interested in the impacts of precipitation extremes, you’d want to know what’s happening in your region.”

Global observations have verified the increase in extreme rainfall, but  projecting how extreme storms will change on a more specific, regional scaleis more challenging. Climate data is not equally available in all countries, or even continents, and the signal of climate change is masked by weather noise to a greater extent on the regional scale.

The researchers started with a global look, then scaled it down to a 100 kilomoeter by 200 kilometer grid. For each cell, they  identified the maximum daily rainfall per year and compared this to the average global temperature for that year.

All the models predicted that the highest increases in extreme rainfall will occur over parts of the Asian monsoon region such as India and over parts of the equatorial Pacific, with more moderate increases in North America, Central America, the Mediterranean, and Australia.

O’Gorman said that, while the spatial pattern of change was robust across the models, the magnitude of the change was much more uncertain in tropical regions, and higher-resolution modeling is needed to narrow down this uncertainty.

The researchers also found decreases in extreme rainfall amounts over subtropical ocean regions, where the overlying atmosphere is generally dry, producing relatively weak storm systems.

“It’s kind of striking,” O’Gorman said. “Almost everywhere, there’s an increase in precipitation extremes, except for these ocean regions.”

He suggested this may be partly due to the ongoing expansion of the tropics. As the climate has warmed in past decades, researchers have noted that the climate at the equator has spread towards the poles, creating a much wider tropical belt. As the tropics and the Hadley cell continue to expand, this would affect the pattern of extreme precipitation, especially in the subtropics.

“The subtropics are generally dry, and if you move the region of descending air poleward, you would get some regions with increases, and others with decreases [in extreme rainfall],” O’Gorman said. “However we found that this only explained half of the decreases from changes in winds, so it’s still something of a mystery as to why you get a decrease in precipitation extremes there.”

O’Gorman is currently investigating whether the duration of extreme rainfall events changes with increasing temperatures, which could have practical implications for determining the resilience of buildings and infrastructure.

“Given an extreme precipitation event, how long does it last, say in hours, and does that time change with climate warming?” O’Gorman said. “We think the intensity of an event changes, and if the duration also changes, that could be significant too.”

Swiss researcher and co-author Stephan Pfahlsaid changes in updrafts are key to understanding regional variations. ”

A clearer understanding of the reasons for stronger or weaker updrafts is extremely important. We now know where we need to focus in order to further reduce the uncertainties in regional projections,” he said.

Particularly in the equatorial Pacific or in the Asian monsoon region, strong increases in upward wind velocities produce even heavier downpours, while they tend to yield a decrease in extreme precipitation over many parts of the subtropical oceans, where vertical wind velocities are weak, transporting only low amounts of moisture upward.  According to the models, these upward wind velocities will decrease further in a warmer climate in future, so that extreme precipitation will become weaker and less common.

Over Central Europe, the increase in atmospheric moisture content is the dominant factor and leads to much heavier downpours. The new analysis shows that the upward wind velocities will hardly change, except in the summer, even assuming global warming of up to four degrees by the end of this century. Across the Mediterranean, however, changes in updrafts could be critical. They will probably become weaker, thus reducing the frequency and strength of extreme precipitation.

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