Hot and dry. These are the watchwords for large fires.

While every fire needs a spark to ignite and fuel to burn, it's the hot and dry conditions in the atmosphere that determine the likelihood of a fire starting, its intensity and the speed at which it spreads.

Over the past several decades, as the world has increasingly warmed, so has its potential to burn.

Since 1880, the world has warmed by 1.9 degrees Fahrenheit, with the five warmest years on record occurring in the last five years.

Since the 1980s, the wildfire season has lengthened across a quarter of the world's vegetated surface, and in some places like California, fire has become nearly a year-round risk. The year 2018 presented California's worst wildfire season on record, on the heels of a devasting 2017 fire season.

In 2019, wildfires have already burned 2.5 million acres in Alaska in an extreme fire season driven by high temperatures, which have also led to massive fires in Siberia.

Whether started naturally or by people, fires worldwide and the resulting smoke emissions and burned areas have been observed by NASA satellites from space for two decades. Combined with data collected and analyzed by scientists and forest managers on the ground, researchers at NASA, other U.S. agencies and universities are beginning to draw into focus the interplay between fires, climate and humans.

"Our ability to track fires in a concerted way over the last 20 years with satellite data has captured large-scale trends, such as increased fire activity, consistent with a warming climate in places like the western U.S., Canada and other parts of Northern Hemisphere forests where fuels are abundant," said Doug Morton, chief of the Biospheric Sciences Laboratory at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Where warming and drying climate has increased the risk of fires, we’ve seen an increase in burning."

A hotter, drier world

High temperatures and low humidity are two essential factors behind the rise in fire risk and activity, affecting fire behavior from its ignition to its spread.

Even before a fire starts they set the stage, said Jim Randerson, an Earth system scientist at the University of California, Irvine who studies fires both in the field and with satellite data.

He and his colleagues studied the abundance of lightning strikes in the 2015 Alaskan fire season that burned a record 5.1 million acres. Lightning strikes are the main natural cause of fires.

The researchers found an unusually high number of lightning strikes occurred, generated by the warmer temperatures that cause the atmosphere to create more convective systems — thunderstorms — which ultimately contributed to more burned area that year.

Hotter and drier conditions also set the stage for human-ignited fires. "In the Western U.S., people are accidentally igniting fires all the time," Randerson said. "But when we have a period of extreme weather, high temperatures, low humidity, then it’s more likely that typical outdoor activity might lead to an accidental fire that quickly gets out of control and becomes a large wildfire."

For example, in 2018 sparks flying from hammering a concrete stake into the ground in 100-degree Fahrenheit heat and sparks from a car's tire rim scraping against the asphalt after a flat tire were the causes of California's devastatingly destructive Ranch and Carr Fires, respectively.

These sparks quickly ignited the vegetation that was dried out and made extremely flammable by the same extreme heat and low humidity, which research also shows can contribute to a fire's rapid and uncontrollable spread, said Randerson. The same conditions make it more likely for agricultural fires to get out of control.

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