After the stifling hot temperatures parked over the Pacific Northwest in late June, an international group of 27 climate scientists – including Cornell’s Flavio Lehner – concluded July 7 that the heat wave was “virtually impossible without human-caused climate change.”

“The recent Pacific Northwest heat wave was extreme in the context of what this region has experienced before,” said Lehner, assistant professor in the College of Agriculture and Life Sciences. “The Pacific Northwest episode was so extreme that it did not fit our standard modeling approaches. To put this into human terms, this event should not have been possible.”

Lehner, who is in the Department of Earth and Atmospheric Sciences, creates model simulations to reconcile high temperatures as a result of climate change and hydrology. He aims to understand large-scale climate variability and regional climate impacts such as droughts, heatwaves or wildfire.

On June 28, Seattle soared to an all-time high temperature of 108 degrees Fahrenheit, while Portland, Oregon sizzled at 116 F on the same day, setting that city’s all-time high temperature record, according to the National Weather Service.

The heat wave’s observed temperatures were so dangerous that they landed far outside the range of historically recorded temperatures, according to the group. This made it difficult for the scientists to quantify the heat wave’s rarity.

The report, “Rapid Attribution Analysis of the Extraordinary Heatwave on The Pacific Coast of the U.S. and Canada June 2021,” was released July 7 by World Weather Attribution, a group of climate scientists including Lehner, a co-author on the report.

Examining the heat wave through the lens of more than 30 climate models, the group showed that heat wave frequency and severity is increasing.

The World Weather Attribution group estimated the severe heat was a one in a 1,000-year event and the heat wave would have been 150 times more rare without anthropogenic climate change.

But climate change is not only about rising temperatures, Lehner said. It also has to do with water.

“Water is quite relevant,” he said. “Throughout all of the heat, you have to think about water resources and droughts. You have to be aware about the sensitivity of the rivers and the water basins in a changing climate. We talk prominently about temperature, but precipitation also changes.”

To that end, Lehner has become the principal investigator for a new $540,000 federal grant (July 2021) from NOAA to develop hydrologic metrics to evaluate and comprehend water sensitivity biases in Earth system models.

“Scientists develop climate models and study the real world. It’s always a struggle to compare the two,” Lehner said. “We're always looking for better ways to evaluate the climate models to understand whether they are useful in telling us something about the future processes. We’ll be examining runoff sensitivity for rivers and other water resources.”

As the principal investigator, Lehner will be working with scientists Andrew Wood and David Lawrence, both of the National Center for Atmospheric Research, Boulder, Colorado, who will be co-principal investigators.

In essence, Lehner and his colleagues will sort out climate factors in water runoff and develop metrics that can help us assess the climate models and reduce the uncertainties around future climate projections.

“When temperatures rise over a watershed, what happens to runoff in that region?” Lehner said. “In an era of climate change, will there be more or less runoff?”

The Department of Earth and Atmospheric Sciences is shared jointly by CALS and the College of Engineering.