We’ve heard wildfires are “caused by” “exacerbated by” “primarily” “significantly” by climate change in various stories and op-eds. So let’s trace these back to the studies.
Human-caused climate change is a significant contributor to the increasing size and intensity of, and damage from, western U.S. wildfires.
In the western United States human-caused climate change caused more than half the increase in forest fuel aridity (how dry and flammable vegetation is) since the 1970s and has approximately doubled the cumulative area burned in forest fires since 1984.
Now many of us might ask, how could you know that “across the western US?” Could there have been more vegetation due to lack of fire suppression, that led to less water for each plant and hence dryness of vegetation? Haven’t suppression tactics changed over that period? If logging is bad for fires, as some claim, then there’s been much logging since 1984, how does that factor in? How could these estimates possibly be considered “facts?”
But first let’s take an aside to point out two concepts that may get lost in stories.
1. Not everything about climate is AGW or anthropogenic climate change. As we’ve seen via Matthew’s graphs of PDO, there is much climate variability that occurs naturally. E.g., in my own part of the country about 100 years ago there was a serious drought that led to the Dust Bowl. So just because I’m experiencing heat, it’s not necessarily AGW.
2. And not all climate change is about carbon, nor even all greenhouse gases (GHGs). Land use changes such as albedo, irrigation, urban heat islands and so on can also affect climate, in complex and interactive ways we don’t yet understand. So we can’t go directly to “reducing carbon (alone) will solve the problem.”
To find the contribution of AGW, we have to look at attribution studies. These studies run climate models and try to find the fingerprint of AGW by what sounds like comparing the model results to real world observations of some kind. I recommend taking the time to read this one. Sure I don’t get all the details of CMIPs and so on, but we can all understand what data goes in, what comes out, and we can read the caveats, and the conclusions. We can also examine the almost magical transitions between the paper (careful), the interviews (not quite as careful) and the headlines e.g. “caused by.”
Here’s a typical exchange of scientists:
Swain with UCLA and other scientists earlier this year published a study that said climate change has doubled the number of extreme-risk days for California wildfires.
It said temperatures statewide rose 1.8 degrees Fahrenheit since 1980, while precipitation dropped 30%. That doubled the number of autumn days that offer extreme conditions for the ignition of wildfires (Climatewire, April 3).
The heat is expected to get worse with time. Climate models estimate that average state temperatures will climb 3 degrees Fahrenheit by 2050 unless the world makes sharp cuts in greenhouse gas emissions, said Michael Wehner, a senior scientist at Lawrence Berkeley National Laboratory.
Even with emissions cuts, average temperatures would rise 2 degrees by midcentury, he said.
Jon Keeley, a senior scientist at the U.S. Geological Survey Western Ecological Research Center, argued that the study from Swain and others failed to show that hotter temperatures are driving wildfires.
“Show us data that shows that level of temperature increase is actually associated with increased fire activity,” Keeley said. “They don’t show that.”
Keeley added, “We ought to be much more concerned with ignition sources than a 1- to 2-degree change in temperature.”
A big contributor to large California fires is that the state has focused on extinguishing blazes for about a century rather than allowing for controlled burns, he said. That has caused dead vegetation to accumulate.
Trump has accused California of failing to “sweep” its forests, which he has linked to fires in the state.
Keeley said that “we don’t sweep forests here in the U.S., but what we do is prescription burning. … It’s potentially the same thing. It’s modifying the fuels prior to a fire.”
Swain, the UCLA climate scientist, said global warming is affecting how big fires get and how fast they move.
“What happens when they start burning, what is the character of those fires, and is it changing?” Swain asked. “The answer is yes.”
If we look at the statements in the study, though..we get a “fingerprint for meteorological preconditions.” Not exactly “fires caused by climate change.”
Collectively, this analysis offers strong evidence for a human fingerprint on the observed increase in meteorological preconditions necessary for extreme wildfires in California.
In the present study, we do not quantify the relative role of increased urban and suburban incursion into the high-risk wildland-urban interface, nor the contribution of historical land/vegetation management practices to increasing wildfire risk or possible future climate-fire feedbacks
And sounding rather like Keeley..or any of the rest of us..
In the long-term, reduction of global greenhouse gas emissions is the most direct path to reducing this risk, though the near-term impacts of these reductions may be limited given the many sources of inertia in the climate system . Fortunately, a broad portfolio of options already exists, including the use of prescribed burning to reduce fuel loads and improve ecosystem health , upgrades to emergency communications and response systems, community-level development of protective fire breaks and defensible space, and the adoption of new zoning rules and building codes to promote fire-resilient construction
I recommend reading the conclusions for yourself.