In Greece, the fires came fast and hard, sweeping through towns so quickly that residents were trapped in clouds of smoke. Some people ran toward the ocean, plunging into the water to save themselves from the flames onshore. Others tried to shelter in buildings or cars. But the fires, which raged through several towns on the outskirts of Athens, have claimed 91 lives, making it the most deadly wildfire season in Europe since 1900.

In Sweden, more than 80 fires across 100 square miles spread through the thick, normally damp northern forests. Finland and Latvia battle their own blazes.

Europe sets on fire each summer, especially around the Mediterranean. On average, about 1500 square miles of the E.U. burn every year. But last year fires burned nearly three times that much, killing 66 people in Portugal and Spain and stretching the firefighting efforts and budgets thin across the continent. “In 2018, we are observing an expansion of the areas that are at risk, with fires occurring in countries where wildfires were not so common in the past,” says Jesús San-Miguel, a researcher at the European Commission’s Joint Research Centre.

These fires across the continent have something in common: they were more likely to happen, and to burn more destructively, because of human-caused climate change. While scientists can’t attribute any single fire solely to climate change, they can say that warming conditions on the Earth—and the ways that warming affects drives more extreme weather, faster-growing, more flammable plants, and more lightning storms—set the stage for more fires and more destructive ones in the future.

How Earth Made Space for Fire

Fire is both deeply simple and incredibly complex, with origins hundreds of millions of years ago. A fire requires only three things: something to burn; oxygen to fuel the combustion; and a spark to set it all off.

For the first few billion years of Earth’s history, there was no fire. Bolts of lightning or molten volcanic particles (i.e. sparks) spewed into the air. But this early Earth atmosphere, dense with methane and ammonia (and later, carbon dioxide) and devoid of gaseous oxygen, squelched any proto-flames. And until a few hundred million years ago, there was nothing around to burn.

But once early plants and other photosynthezisers evolved, they transformed the atmosphere, turning carbon dioxide into drafts of oxygen. As a result, by about 500 million years ago, the second ingredient for fire was prepared. The third ingredient came about 30 million years later when plants made it out of the ocean and began creeping over the surface of the land. Not long after, Earth saw its first fires.

Fires have swept across Earth ever since. Despite their destruction, the planet’s ecosystems have evolved to tolerate it, or in some cases, they require fire to renew nutrients.

Wildfire Research When the firefighters are gone and the fire has been extinguished, it's up to the BAER team to make sure that the damage done has no further repercussions.

But the frequency, intensity, and extent of those fires have changed dramatically. Those changes have been closely tied to climate shifts in the past. When the planet warms up—leading the dry parts of the planet to dry out further—fires happen more frequently and burn bigger.

“When you see fire frequencies increasing in the past, there’s always evidence telling you there’s drier climate, “says Graciela Gil-Romera, a paleoecologist at Aberystwyth University in Wales who studies the history of fire. “That’s always related to enhanced fire activity.”

Spark It Up

Over the past 400 million years, oxygen concentrations have stayed high enough to fuel fires. So the other ingredients—the amount of burnable material and the number of ways to light it aflame—are the main controls on modern fire.

And the sparks seem to be increasing. Not only are humans causing more fires, both intentionally and unintentionally, but lightning strikes are expected to increase as the climate warms. Warmer, more turbulent air leads to more thunderstorms, which in turn leads to more lightning. And a recent study found that lightning-induced fires were becoming more and more common across the northern reaches of the American continent, increasing on average by two to five percent each year since 1975.

View Images A wildfire rages near the town of Rafina, near Athens, Greece, on July 23, 2018. At least 20 people have died and more have been injured as wild fires tore through woodland and villages around Athens, while blazes caused widespread damage in Sweden and other northern European nations. Photograph by ANGELOS TZORTZINIS, AFP, Getty Images

The Big Dry

But the biggest effect on how, where, and how intensely fires burn is the fuel load of burnable material. This has led Juli Pausas, an ecologist who focuses on fire at the Universitat de Valencia in Spain, to add a fourth ingredient to the fire recipe: a dry season.

“The drought is key for predicting whether the fires will happen,” says Marco Turco, a climate scientist at the University of Barcelona who is working to develop a tool to predict where and when fires might break out in coming months. In his view, climate is the most important ingredient, because it influences the flammability, type, and availability of combustible material.

The worst fire years tend to appear amid seasonal extremes, when a wet season that fuels exuberant plant growth is followed by an extremely dry season that sucks the water out of the plants and the soil.

These conditions are exactly what caused the fires in northern Europe this year. A wet winter fattened plants across the continent, and then the heat set in. A historic heat wave—which scientists say was about twice as likely to occur because of human-caused climate change—settled in over the continent.

Climate Change 101 with Bill Nye

Bill DeGroot, a scientist with the Canadian Forest Service, explains that the fire “season” has extended over the past few decades in many parts of the world, largely because warmer temperatures dry out plants more thoroughly and sooner in the year. In the western U.S., the length of time between the first and last fires of the year stretched by nearly 80 days since 1980. In Canada, where he works, the season is about a month longer.

“A fire regime is something that's changing, shifting all the time,” says DeGroot. “But this change with climate change is going to make a very significant shift in a relatively short period of time. The forest is going to look different in the future than it looks now.”

Climate change accelerates this process, as dry places tend to get drier. When the European Joint Research Centre analyzed models of present and future fire likelihood across Europe, it found that the parts of Europe that are already dry, like Spain, Greece, Turkey, will dry out further and be more susceptible to burning. Already, an average of over 3500 square miles—an area more than three times the size of Rhode Island-- burn each year across Europe. By the end of the century, that number could double.

In the western U.S., climate change has warmed and dried out the plants that blanket mountains, valleys, and plains. That extra dryness, says Park Williams, a scientist who studies the relationship between climate change and fire at Columbia University, made about twice as much area of the western U.S. ripe for catching on fire than would have been expected without climate change.

“Fire is changing right before our eyes,” Williams says.

Wildfire Photographer Mark Thiessen teams with firefighters to photograph these spectacular forces of nature.

The Sinuous Jet Stream

There’s one other way climate change is influencing the number and frequency of fires, and it has to do with the jet stream, the atmospheric highway high up in the sky that carries weather from west to east.

The shape of the jet stream, and how fast weather travels along it, is controlled by the temperature difference between the Arctic and the equator. When the temperature difference is small, weather patterns move more slowly. But climate change is causing the Arctic to warm fast, making that temperature difference smaller. As a result, weather events in the northern hemisphere stay in place for longer. When it’s wet, it stays wet, feeding and plumping plant growth. And when it’s hot, it stays hot, sucking more and more moisture out of those same plants.

The heat wave that settled in over Europe this summer is a good example of these dynamics in play, says the University of Arizona’s Valerie Trouet. “The fires in Sweden, I would say, are directly linked to the heat wave going on there and the concurrent drought,” she says.