PALO ALTO >> Weird weather and climate are two separate things, but a Stanford team is finally linking them.
Using math, powerful computers and historical records, research led by Noah Diffenbaugh found that climate change has boosted the odds of extreme heat, drought, punishing rainstorms and retreating sea ice.
“The odds of hitting record-setting level of extremes have been made greater by climate warming,” caused by human emission of greenhouse gases, said Diffenbaugh, a professor of Earth system science at Stanford’s School of Earth, Energy & Environmental Sciences.
Weather is what the atmosphere does in the short-term — hour-to-hour, day-to-day. It’s what’s happening outside your window. Climate is the long-term average of weather, over decades and millennium, created by big changes in global forces, such as the global warming.
In the past, scientists typically avoided connecting individual weather events — say, the catastrophic flooding in northern India in June 2013 — to climate change, citing the challenges of teasing apart human influence from the natural variability of the weather.
The new paper, the latest in a burgeoning field of climate science called “extreme event attribution,” changes that. Published in this week’s Proceedings of the National Academy of Sciences, major goal was to test the ability of the framework to evaluate events in multiple regions of the world, and to extend beyond extreme temperature and precipitation, which have been the emphasis of most event attribution studies. It combines statistical analyses of climate observations with increasingly powerful computer models to study the influence of climate change on individual extreme weather events.
In their analysis, Diffenbaugh and a group of current and former Stanford colleagues reached two conclusions:
First, global warming boosted the odds of a record-breaking heat wave — for example, a hot day or hot month — in more than 80 percent of the surface area of the Earth that they studied.
Secondly, global warming also increased the odds of extreme wet and dry spells. For these wet and dry extremes — such as California’s 2012-2016 drought — global warming has increased the odds for these events to occur for half of the globe, they found.
This is important for a society trying to make decisions about how to manage the risks of a changing climate, said Diffenbaugh.
“Getting an accurate answer is important for everything from farming to insurance premiums, to international supply chains, to infrastructure planning,” he said.
It’s a well-established fact that global warming is happening. It also is proven that climate has been changing in many regions of the world, said Diffenbaugh. And it’s also known that the frequency and severity of extreme events has been increasing over many areas of the world.
What they did was connect the two trends. “What we found in this study was that when you look across the Earth, global warming has influenced the record-setting events in many areas for a number of different event extremes,” he said.
In 2015, Diffenbaugh’s research linked the stubborn high-pressure systems that blocked California rains to the abundance of human-caused greenhouse gases that heat the oceans.
In this new study, they used this same approach — what they call a new “four-step framework” to study global events. A major focus was Arctic sea ice, which has declined by around 40 percent during the summer season over the past three decades.
They found overwhelming statistical evidence that global warming contributed to the severity and probability of the sea ice measurements.
“The trend in the Arctic has been really steep, and our results show that it would have been extremely unlikely to achieve the record-low sea ice extent without global warming,” according to Diffenbaugh.
Their approach can be used to study not only the weather conditions at the surface, but also the meteorological “ingredients” that contribute to rare events.
“For example, we found that the atmospheric pressure pattern that occurred over Russia during the 2010 heat wave has become more likely in recent decades, and that global warming has contributed to those odds,” said co-author and former Stanford postdoc Daniel Horton, now an assistant professor at Northwestern University in Evanston, Illinois, in a prepared statement.
“If the odds of an individual ingredient are changing — like the pressure patterns that lead to heat waves — that puts a thumb on the scales for the extreme event,” he said.
While the Stanford team did not study why there is a connection, scientists with the National Center for Atmospheric Research have found that warmer oceans — which may be linked to persistent high-pressure systems — can trigger changes in how the atmosphere sweeps across our landscape.
For instance, warm oceans give rise to moister air, strengthening hurricanes and other extreme events.
“Our results suggest that the world isn’t quite at the point where every record hot event has a detectable human fingerprint, but we are getting close,” Diffenbaugh said.
