Judy Woodruff: Firefighters are battling tonight against a wildfire that has become the fifth largest fire in California history. It has been a brutal fire year throughout the state. Scientists are trying to better understand the role that climate change and influence that it has, in hopes of preventing and dealing with tough fire years to come. Miles O'Brien has our report for our segment on the Leading Edge of science.

Miles O'brien: Long before the devastation in California, scientists had built a strong case linking a changing climate to more wildfires, a lot more.

Park Williams: Since 1984, the area that burns in any given year is up by over 300 percent. If we look at forests in particular, the amount of area that burns in any given year is up by over 1000 percent.

Miles O'brien: Park Williams is a bio-climatologist at Lamont-Doherty Earth Observatory of Columbia University.

Park Williams: We use tree rings to understand the history of the environment and especially of drought and temperature.

Miles O'brien: And that helps him understand how drought and temperatures are linked to fire. A narrow ring means a tree grew less that season, possibly signaling a drought. And those skinny rings often correlate to scars left by fire.

Park Williams: Drought years, you have a higher probability of fire. And temperature promotes drought. And so, as temperatures have been rising, we have seen drought intensifying. And, as a result, we have seen increased fire.

Miles O'brien: His colleague at Lamont-Doherty, climate scientist Radley Horton, says the link between fires and climate change is well documented.

Radley Horton: I think the science is pretty solid to indicate that wildfire risk is likely to increase in the future due to climate change.

Miles O'brien: Climate change creates conflagrations for a handful of reasons.

Radley Horton: I think exhibit A has to be the increase in temperature that we have observed. In California, we have seen about a 1.5-degree increase in temperature over the last century.

Park Williams: What we have seen, especially in forested areas, is that, as we turn up the temperature, even by one or two degrees, then fire responds in a large and measurable way, because the vegetation dries out.

Miles O'brien: Along with the increased temperatures comes a greater likelihood of less rainfall in Southern California and other subtropical regions. The driver for all of this is an atmospheric circulation pattern called the Hadley cell. Here is how it works. Warm, moist air at the equator causes heavy precipitation. The air rises to more than 30,000 feet, where it then flows toward the poles. Then the air gets cool and dry, descends, and then heats up.

Radley Horton: With climate change, we expect a stronger branch of rising air over the tropics. There's a lot of suggestion that the sinking branch of that Hadley cell could become stronger, which would tend to make things even drier.

Miles O'brien: Normally, California's winter rains begin in October and November. But, this year, it's been a hot and dry fall and summer.

Radley Horton: The overall trend with climate change, we think, is going to be towards drier winters and, for sure, warmer weather that's going to increase the amount of evaporation. You would effectively need more rainfall just to maintain the fire risk that you had in the past and not see it go up.

Miles O'brien: But a fire of this magnitude also requires moisture as well, in advance, to create the fuel. And, last winter, California got a huge amount of precipitation.

Park Williams: And that has led to an abnormal amount of vegetation growing in the hills around the cities of Los Angeles and the surrounding areas. And it's now that vegetation that's burning.

Miles O'brien: The unusually wet winter followed by the long stretch of hot, dry weather needed one more thing: hot, dry winds whipping down from the high desert toward the ocean. In the north, they call these winds Diablo, in Southern California, Santa Ana.

Park Williams: The air gets compressed. And when we compress air, it warms. And when we compress dry air, it warms really rapidly. And so, these winds that are coming out across coastal Southern California are very warm and very dry. And that's a perfect recipe for intense wildfire.

Radley Horton: Those air masses could get even drier in the future as temperatures rise, which would give us more fuel for fires.

Miles O'brien: Human beings have another role to play, besides their impact on the climate. Our cities are fast encroaching into forest and grassland into a place called the wildland-urban interface.

Park Williams: People are wanting to live in nature. So, they're creeping up into the foothills of the coastal mountains. And this is where we're seeing a lot of the trouble with fire interacting with people. People accidentally start fires. They're living in forests or in heavily vegetated areas, and these areas are very prone to fire. And so these people are living in potential disaster areas.

Radley Horton: There's a broader conversation we need to have, I think, about helping to get people out of harm's way, making sure that we're pricing risk to reflect the true risk. That's a difficult conversation. We need to make sure that vulnerable members of the population, those with less economic resources, aren't left behind, as we have that discussion about how to make sure people are safe and that their assets are protected.

Miles O'brien: And it's part of a larger conversation about our changing climate, which California Governor Jerry Brown has already begun.

Gov. Jerry Brown: This is kind of the new normal. With climate change, some scientists are saying Southern California is literally burning up, and burning up as maybe a metaphor or a description, but not just in the fire right here, but what we can expect over the next years and decades.

Miles O'brien: Whether it's this winter's California fires, this summer's devastating hurricanes, or routine blue sky flooding in Miami and Norfolk, the evidence is stacking up. Global warming, and its deadly consequences, are here and now. For the PBS NewsHour, I'm Miles O'Brien.