Is climate change wreaking weather havoc? Evolving science seeks answers

When San Francisco hit 103 degrees in June 2000, a new high after more than a century of record-keeping, the forces of global warming were likely at work. But scientists weren’t ready to go there.

Climate change had barely become a household term, and teasing out its complex role in single events that are largely at the mercy of natural weather variability was unthinkable.

Seventeen years later, as the Bay Area recovers from another round of record heat that pushed San Francisco to a new pinnacle of 106 degrees — and as a deadly lineup of storms pounds the southern United States and the Caribbean — climate researchers are making these critical but elusive connections.

There’s still no simple answer to the question, “Was that hurricane caused by climate change?” But scientists can now often say whether an event was more likely, and more severe, due to the warming planet.

A team of experts at Lawrence Berkeley National Laboratory, among the pioneers of the evolving science of extreme event attribution, estimated that human-caused climate change probably raised temperatures in California by as much as 4 degrees last week.

Similar accounting has been done for the California drought and strings of wildfires across the West, as well as the catastrophic hurricanes Harvey and Irma, whose devastation continues to unfold.

The ability to quantify the influence of global warming on a single event is important not only because it brings urgency to the threat of climate change, but because it could help communities understand exactly what they’re up against. Those seeking stronger responses to global warming have been frustrated by the information gap.

While scientists can’t blame climate change for causing any one weather system, studies have found that past heat waves in both the U.S. and abroad were so unlikely in the absence of global warming that there was little other explanation for what drove them. Eventually, the research could find that climate change is the primary engine behind some disasters.

“Scientifically we’ve come a long way,” said Noah Diffenbaugh, an Earth system scientist at Stanford University who is among those tracking the fingerprints of climate change on weather.

“Around 2000 or so, you mostly would have heard from the scientific community that we can’t 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 several appraisals. Some suggest 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 series 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 occurs, 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 of a 2015 heat wave in India and 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 many cases, both the probability and severity of extreme weather have gone up substantially with global warming, Wehner has found.

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.”

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander