WARMOCEANWATERGAVE HARVEY A BURST OF ENERGY, EXPERTS ON CLIMATE SAY
draw any connection between global warming and any extreme event,” Diffenbaugh said. “What has happened since, particularly in the past five years, is an explosion of research by multiple groups that are working very hard to pose and test hypotheses of how global warming is possibly influencing individual events.”
In theory, the role of climate change in weather patterns, particularly heat waves, is fairly straightforward. Greenhouse gases, such as carbon dioxide and methane, prevent sunlight from escaping into space, and raise temperatures.
These higher temperatures play out in a number of ways. Warmer air, for example, means more evaporation, which puts more moisture in the atmosphere. So when storms or hurricanes develop, there’s more water for these systems to wring out. Higher temperatures are also melting ice caps and raising sea levels, meaning bigger storm surges and additional flooding.
In the case of Harvey, which slammed into southeast Texas early last week and dropped more than 50 inches of rain in some places, extraordinarily warm ocean water gave the hurricane an added burst of energy.
Climate experts trying to isolate the role of global warming on an Atlantic Ocean storm track that has always spawned vicious hurricanes have offered a number of appraisals. The consensus seems to be that 10 to 15 percent of Harvey’s rainfall was due to human-caused climate change.
The estimates are based on vast wells of historical weather data that scientists use to model various scenarios — with and without the influence of global warming.
These models have improved from being able to detect the influence of climate change across a number of extreme events and over large swaths of land to pinpointing its role in smaller, shorter events, such as wildfires and big storms. And the results are coming back faster and faster.
“The confidence in one that you spend many years on is better than one you’d do on the back of an envelope. But we do them on the back of an envelope,” said Michael Wehner, senior staff scientist at Lawrence Berkeley National Laboratory.
Wehner’s team has looked at dozens of extreme weather episodes, sometimes drawing up models just as an event unfolds, as was the case with California’s recent blast of heat. With one of the fastest computers on the planet, the team deduced that climate change made the event four to seven times more likely and raised temperatures 2 to 4 degrees.
Other models require more time, like Wehner’s examination ofa2015heatwaveinindiaand Pakistan that killed nearly 4,500 people. Wehner concluded in a paper published last year that human activities made the event as much as 1,000 times more likely.
“We don’t make statements like ‘climate change caused that event,’ ‘’ he stressed. “We make one or two statements: How does climate influence the magnitude and how has climate change affected the chances of that event occurring?”
In most cases, both the probability and severity of extreme weather have gone up substantially with global warming, Wehnerhasfound.
Diffenbaugh, at Stanford, has done similar work with the California drought. His models suggest climate change fostered conditions that made the historic five-year dry spell twice as likely.
Calculating the likelihood of extreme events, he said, can be done so accurately now that it can help cities and states plan for long-term changes in weather, at least to a point.
“If you’re asking about the temperature in my driveway on a particular date next year or three years from now, there’s a lot of noise,” Diffenbaugh said. “But what we are able to do is calculate the statistical probabilities.”
Some researchers say they’ve begun to go even further.
Michael Mann, a professor of atmospheric science at Pennsylvania State University, published a paper in March describing a climate-caused mechanism behind extreme weather events, which could eventually help with forecasting once the link is better understood.
According to his research, the normally mobile jet stream high in the atmosphere is locking into place at the same time that floods, droughts and heat waves are wreaking havoc below.
“Some of my colleagues are trying to operationalize this sort of analysis,” Mann said. “Such predictive capabilities would presumably lead to more resilience on the part of stakeholders and impacted populations.”