Woman wearing mask
A walk on an empty street in Yubileiny, Moscow region, in August 2010, as wildfires and record-setting temperatures brought thick smog, scorching heat, respiratory problems and more.

It was a grim day in Moscow on July 25, 2010. The English-language Moscow Times reported that the Russian capital had broken the 1936 temperature record for July: 98 degrees Fahrenheit.

That month, the city, where air conditioning was (and still is) rare, was an average of 32 degrees Fahrenheit hotter than a normal July.

Plumes of smoke rising from peat bogs burning in the distance wafted through Moscow’s central Presnensky district. The newspaper reported that “grass has dried down to straw,” and trees were dropping leaves and dying from dehydration.

The mega heatwave – since designated eastern Europe’s hottest episode in 500 years – affected an area as large as Mexico. By the time it abated it had caused 55,000 deaths from respiratory failure and other disorders caused by heat and air pollution from wildfires. It had destroyed one-third of Russia’s grain harvest.

Jakob Zscheischler, a senior scientist at the Climate and Environmental Physics group at the University of Bern, calls the 2010 incident a quintessential compound climate event, a weather disaster intensified by the combined effect of two or more factors. In this case, a heat spike reinforced by a prolonged drought produced the second most deadly heatwave in history after the western European heatwave of 2003. Climate change could make compound events more likely, Zscheischler says.

When it rains … it often really does pour

Researchers generally study the intensity and likelihood of heatwaves, droughts, and other adverse climate incidents in isolation, as if the incidents were independent phenomena that don’t interact with each other. Radley Horton, a Columbia University climate science professor, who had helped organize the world’s first international workshop on compound events, says that such phenomena are often not independent at all and that assuming they are can lead to unreliable results. “Relying on climate models to look at single variables could understate the risk because of the way these extreme events interact,” he said in a phone call.

Research on the 2010 Russian heatwave illustrates how taking more than one factor into account can improve estimates of the likelihood that an adverse event will recur, and of the degree to which climate change can be held responsible for it.

Though climate scientists have generally analyzed the incidence of drought and elevated heat in isolation, they have known for decades that the two phenomena go hand in hand, each making the other worse. The causal connection between them is easily explained by the well-known relationship between air and moisture in the soil, an effect called land-atmosphere feedback.

When temperature increases so too does the amount of water evaporating from the ground. Even if precipitation remains unchanged, soils become drier – a defining characteristic of drought – when temperature goes up, reducing the availability of moisture for forests, crops, and urban gardens. By the same token, soil moisture regulates regional air temperature. Wetter soil absorbs more heat. So unseasonably low rainfall dries soil and raises temperature.

When six plus three equals 13?

Authors of several papers have studied the Russian heatwave as a compound event with land-atmosphere feedback. Authors of a 2016 publication concluded that climate change increases the likelihood of a devastating heatwave by a factor of six. Low soil moisture increases the likelihood by a factor of three. The combined effect of drought and climate change raises the probability of such a heatwave by a factor of 13.

Another compound event attracting close attention is the combined impact of ocean surge and river flooding on coastal cities. Surge temporarily increases sea level when high wind pushes water against the shore. River flooding occurs when heavy rain swells waterways.

Either surge or river flooding alone can swamp a city, threatening large losses of property and life. In 2011 Hurricane Irene caused $15 billion in damage, much of it caused by raging rivers swollen by intense rainfall. In 2012, in contrast, it was storm surge that made Superstorm Sandy one of the most costly natural disasters in U.S. history. High tide in New York was nine feet above normal, inundating entire neighborhoods, flooding subway lines, and shorting out parts of the electrical grid.

When the surge of Sandy meets the rainfall of Irene …

Horton advises that disaster planners consider the possibility that a future storm could combine the offshore surge of Sandy with the heavy rainfall of Irene. Prevented by elevated tides from draining into the ocean, rivers in flood would overrun the coast at their mouths. A 2015 analysis of weather records concluded that simultaneous ocean surge and heavy rainfall has become more common at major coastal cities of the contiguous U.S. “That combination could potentially lead to greater risk and loss of life,” Horton said.

Researchers at the recent Columbia University workshop distinguished between several varieties of compound events (sometimes called correlated extremes). Not all are incidents enhanced by multiple factors occurring in synchrony such as drought-enhanced heatwaves or surge exacerbated by rain.

Another kind includes episodes occurring roughly simultaneously but far from each other. The El Niño weather pattern is a well-known example of how atmospheric conditions can change all at once in places separated by great distances. During an El Niño, drought descends on the Amazon and the U.S. Pacific Northwest, while heavier rains fall in the southeastern U.S. and British Columbia. Researchers have identified other climate phenomena, connecting far-flung parts of the world by often-poorly understood “teleconnections.” Could such events cause previously unanticipated problems? Researchers are striving to better understand and answer that question.

In a world dependent on far-flung shipments of food, fuel, and manufactured goods produced in and shipped through a small number of key centers, simultaneous climate disasters could create widespread economic and humanitarian disaster. For instance, Horton worries that drought and high heat could cause crop failure in the breadbaskets of the Ukraine, central Europe, and the U.S. “You could see big changes in crop prices and big implications on food security, and potential repercussions in terms of conflict and migration as well.”

Recent research justifies Horton’s concern. Authors of one paper published last March have teased out relationships between adverse growing conditions in the world’s top wheat-growing regions. Among other findings, they noted a suggestive relationship between hot, dry periods in the E.U. and Australia. And authors of a paper published in June 2018 concluded that global warming will dramatically increase the likelihood that the world’s four top maize producing regions will have simultaneously climate-related crop losses.

More questions now than answers = more research to come

Jane Baldwin, a postdoctoral fellow at Princeton University, is studying another kind of compound event made up of a repeated series of short, extreme incidents. She wonders if a succession of short temperature spikes, each too brief to fit the definition of a heatwave, could be as deadly as a longer spell of high heat officially designated as a heatwave. Last April she and several colleagues at Princeton published a paper showing that climate change is increasingly the likelihood of such short extremes of heat. “We should be thinking if having multiple heatwaves in sequence matters,” says Baldwin. She’s continuing her research with epidemiologists to find out.

Geoscience professor Michael Oppenheimer, at Princeton, says he thinks the Columbia University conference – which he helped put on – “signaled a broad interest in compound events in the climate community.” Zscheischler, who has written prominent papers on compound events in Nature Climate Change and Science Advances, says that so far researchers have raised more questions about the subject than they have answered. But, he added, “I’m sure there will be a lot of research in the next couple of years on the topic.”

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