But there’s something strange about these fires. Park Williams, a professor of biology and environment at Columbia University, told me that a wet spring normally makes for a calmer fire season. This spring, rainfall was adequate. And in 2017, the spring rains were enormous and excessive. Both fire seasons, therefore, should have subdued. But both seasons turned out to be anything but.

“Last year and this year we’ve seen giant outbreaks of fires in areas where you wouldn’t have expected it based on the soil-moisture balance,” Williams said in an email.

The following map, for instance, shows how soil-moisture levels in June 2018 compared to early summer moisture over the past 120 years. “The areas where we’ve been seeing big fires this year are generally dry,” he said. “But some [areas], including where the Carr fire is, are not even near record-breaking dry.”

National Soil-Moisture Map, as of June 2018

So if forests had plenty of moisture in early June, how did they become tinderboxes by late July?

“The factor that clearly made the difference in 2017, and again in 2018, is heat,” Williams said. “Last summer was record-breaking, or near record-breaking, hot across much of the West, and I believe July 2018 will break records or come close to it again this year. Even if the deep soils are wet following winter and spring, a hot and dry atmosphere seems to be able to overwhelm that effect.”

As it turned out, July 2018 was the hottest month in California ever recorded. The state as a whole was 5 degrees Fahrenheit warmer than normal. (The National Oceanic and Atmospheric Administration announced the record on Wednesday, after Williams and I corresponded.)

Death Valley also endured the hottest month ever recorded on Earth, averaging 108.1 degrees Fahrenheit (or 42.3 degrees Celsius) across day and night.

The literature suggests that wildfires are more driven by the temperature and moisture content in the air than by the moisture content in the soil, Williams said. This bodes ill for our warming world. It also means that wildfires may become harder to predict during the preceding winter and spring. “We tend to think of fire danger as being a function of the drought status of an area,” Williams said. “Even the Earth-system models used to project climate/land-cover changes for the next century do this.” But extreme air temperatures may overwhelm that effect, leading us to undercount future fire risk.

What has been “reinforced this year and last,” Williams said, is that there’s even more evidence “that a warming climate strongly promotes increases in forest fire activity in western North America.”

As if there wasn’t enough evidence of that. Last year, the National Climate Assessment—written by a panel of scientists in the military, federal civilian agencies, and private universities—reviewed the complete scientific literature on climate change and wildfires. They concluded that the number of large blazes had increased since the early 1980s. They also said the number of wildfires “is projected to further increase in those regions as the climate warms.” They warned this could induce “profound changes to certain ecosystems.”