This is how extreme weather events and climate change are connected

It is widely known that weather is not climate. It is also widely known that climate, and therefore climate change, can significantly affect the weather over time — not only with respect to changes in trends in important indicators like temperature and precipitation, but also with respect to the likelihoods and intensities of extreme events.

The correlation between climate and impacts is, however, most often complicated by confounding factors that include all variety of human behaviors; and that is why something called “forensic attribution” is now a growth industry. Benjamin Santer crafted an up-to-date and extensive review of the state of knowledge of forensic attribution even in its formative stages and summarized in a recent op-ed.

In 2021, when an extreme event like the recent flash flooding in western Germany occurs, the question is “How much of the observed extreme intensity can be attributed to climate change?”

As is always the case in climate change, the answer to the question is always “It depends.” But on what? Is there any way to sort out the facts about both change and the rate of change?

It turns out that a scientifically rigorous answer to this question is “Yes.” Since responding to climate change is a risk management problem, the world has accepted that the key to organize our thoughts must focus on the two fundamental components of risk — likelihood and consequence or, in the vernacular, frequency and intensity. 

Take, for example, the very recent severe flooding in Western Germany. Three months of rain fell in three hours. Flooding caused enormous damage with nearly 200 deaths and more than 300 missing as of July 20. It was the worst flooding event in more than 50 years — but that means that there had been a similar flooding event in the lifetimes of some residents. It was very unusual to see it repeat in 2021, but it was not statistically difficult to explain. 

This episode is like playing monopoly when somebody rolls 12 three times in a row — weird, but possible (0.0002148 percent probability). You might suspect that the dice had been loaded to produce high numbers, but the chance was not zero that it was just a rare event. In other words, nobody could claim without further evidence that the severity of the flooding was caused by climate change. When all is said and done, the enormous damage and the loss of life will likely be attributed to human behaviors and development decisions made over the past half-century since the last flood of this magnitude occurred.

But now, take as another example, the enormous heatwaves in the western portion of the United States. Over recorded history, there had been only three other days with temperatures that reached maxima of 106 degrees Fahrenheit in Portland, Oregon (July 2, 1942; July 30, 1965; and Aug. 10, 1981). 

Then, residents experienced three successive days above 106 degrees born during the end of June of a heat dome generated by a wildly variant jet stream trajectory.

As a third example, take Hurricane Harvey in 2017 dropping 42 inches of rain over Houston over the course of 36 hours. It created the third 500-year flood in four years. Also recall that Hurricane Dorian in 2019 stalled over Nassau in the Bahama for more than a day with similar effect. Climate change does not cause hurricanes; but Harvey’s behavior and Dorian’s behavior and Maria’s devastating swath through Puerto Rico in 2017 were typical of recent hurricanes — stalling over land because their steering currents had been weakened and so they had no place to go. Why? Likely because of shifts in the jet stream and other atmospheric currents had been caused by planetary warming that diminished the energy gradient between the poles and the equator.

To return to the monopoly metaphor: These events cannot be explained by rolling ordinary dice with unusual results. It is more like rolling 16 three or four times in a row, and therein lies the irrefutable evidence that the game has changed. In the old game, 16 is an impossible number to roll — and it’s happening more than once is still ridiculous in the new one. In the climate context, we have nearly irrefutable evidence that Mother Nature has changed the dice because, in cases like these, there is simply no other explanation.

These have all be stories about likelihood. But what about intensity (consequence)? Consider one more example. Only three of the state’s largest 20 fires (in terms of acres burned) had burned prior to 2000 — but nine of the biggest 10 had occurred since 2012.

Extremes were becoming more likely and growing larger. The year 2017 saw 9,270 fires burn a then record of 1.5 million acres. The Mendocino Complex fire the next year became the “largest wildfire in California history.”

Things were not all that unusual until 2020 when the historical mold exploded. The Complex fire would become the new largest fire in California history. It started in August. Soon thereafter, four more fires would become the third, fourth, fifth and sixth largest in history. By the beginning of October, they were all still burning with only minimal control having been established.

Wildfires have long been a natural part of the forest environment in California, of course. As time passed, damage to life and property increased markedly as more people moved into vulnerable forested areas. Changes in forest management contributed, as well. Fire suppression policies on federal land reduced the brush-clearing value of deliberately setting control blazes. But, none of these or other non-climate causes of increased fire danger have increased sufficiently over the five years to account for the erupting devastation and risk. Something else is required to explain what has been happening, and that something is climate change effecting consequence if not likelihood

Many of the fires in 2020 were caused by dry lightning, but their points of ignition appeared in the midst of a record drought. Historic heat had sustained massive infestations of bark beetles over many years resulting in large stands of dead trees covering more than 85 percent of nearly every forest in harm’s way. And the fire season had grown by 75 days a year.

The keys to quick attribution for the informed and engaged citizen are two in number – something that changed the likelihood of an extreme event quickly and/or something that changed its consequences even more quickly.

Gary Yohe, Ph.D., is the Huffington Foundation professor of Economics and Environmental Studies, Emeritus at Wesleyan University.