Researchers estimate 10 percent increase in rainfall during extreme events for every 1-degree Celsius of warming
By Summit Voice
SUMMIT COUNTY — Researchers at MIT say extreme rainfall in the Earth’s tropical regions appear to be more sensitive to global warming than other parts of the world. While they don’t fully understand the mechanism for that higher sensitivity, they estimate that rainfall amounts during extreme weather events — monsoons, thunderstorms and tropical cyclones — are likely to increase by 10 percent for every 1-degree Celsius increase in temperatures.
“The study includes some populous countries that are vulnerable to climate change … and impacts of changes in rainfall could be important there,” said Paul O’Gorman, assistant professor of Atmospheric Science at MIT.
In general, most climate models agree that a warming atmosphere hold more water vapor. When storm systems develop, the increased humidity prompts heavier rain events that become more extreme as the climate warms.
The challenge has been to quantify that effect. For the most part, existing models do a decent job of simulating rainfall outside the tropics — for instance, in mid-latitude regions such as the United States and Europe,” O’Gorman said. In those regions, the models agree on the rate at which heavy rains intensify with global warming.
However, when it comes to precipitation in the tropics the models often don’t agree. The reason may come down to resolution, according to O’Gorman, who explained that most models simulate weather systems by dividing the globe into a grid, with each square on the grid representing a wide swath of ocean or land. Large weather systems that span multiple squares, such as those that occur in the United States and Europe in winter, are relatively easy to simulate. In contrast, smaller, more isolated storms that occur in the tropics may be trickier to track.
So the MIT researchers zoomed in on extreme rainfall between the latitudes of 30 degrees north and 30 degrees south — just above and below the Equator. Using satellite observations from the past 20 years, O’Gorman compared the observations to results from 18 different climate models over a similar 20-year period.
“That’s not long enough to get a trend in extreme rainfall, but there are variations from year to year,” O’Gorman says. “Some years are warmer than others, and it’s known to rain more overall in those years.”
This year-to-year variability is mostly due to El Niño — a tropical weather phenomenon that warms the surface of the Eastern Pacific Ocean. El Niño causes localized warming and changes in rainfall patterns and occurs independent of global warming.
Looking through the climate models, which can simulate the effects of both El Niño and global warming, O’Gorman found a pattern. Models that showed a strong response in rainfall to El Niño also responded strongly to global warming, and vice versa. The results, he says, suggest a link between the response of tropical extreme rainfall to year-to-year temperature changes and longer-term climate change.
O’Gorman then looked at satellite observations to see what rainfall actually occurred as a result of El Niño in the past 20 years, and found that the observations were consistent with the models in that the most extreme rainfall events occurred in warmer periods. Using the observations to constrain the model results, he determined that with every 1 degree Celsius rise under global warming, the most extreme tropical rainfall would become 10 percent more intense — a more sensitive response than is expected for nontropical parts of the world.
“Unfortunately, the results of the study suggest a relatively high sensitivity of tropical extreme rainfall to global warming,” O’Gorman said. “But they also provide an estimate of what that sensitivity is, which should be of practical value for planning.”
“However, it is important to note that computer projections indicate that although the rainfall increases in the wettest regions — or similarly, the wet season — the drier parts of the tropics … will become drier still,” Allan says. “So policymakers may have to plan for more damaging flooding, but also less reliable rains from year to year.”
Results from the study are published online this week in the journal Nature Geoscience.
Filed under: climate and weather, Environment, global warming, tropical storms and hurricanes Tagged: | atmospheric science, climate change, El Nino, extreme rainfall events, extreme weather, global warming, Nature Geoscience, tropics