Global Warming's Link to Wildfires Is Murkier Than It Seems

The other day I couldn’t give my 8-year-old son a clean explanation, so I looked into it and put together what I found here.

Here’s how the exchange went, by the way.

Daughter (5): “A fire ant!” Son (8): “Ah, that’s not a fire ant. That’s a red ant. Fire ants are an extinct species, you know.” Me: “Not extinct — eradicated.” Son (8): “Oh, right. Because they… hurt people… they’re trying to wipe them out! (proud face)” Me: “Right, exactly.” Son (8): “By the way, I heard koalas recently became an endangered species.” Me: “Oh, that’s right. There’s a proposal for that going on right now. Do you know why?” Son (8): “Because there was a big wildfire in Australia (proud face)” Me: “That’s right.” Son (8): “Wildfires happened in America too, didn’t they? Why do they happen?” Me: “Hmm, in Japan I’ve heard it’s mostly things like campfires or carelessly discarded cigarettes, but I wonder how it is in Australia or California.” Son (8): “Does global warming have anything to do with it?” Me: “Hmm, I wonder. To begin with, the spark that naturally ignites a wildfire… and how that relates to warming…”

I trailed off without being sure, so I decided to look into it properly.

My wife wrapped up that conversation with, “Warming, warming, it’s all because of warming. Dinner’s ready.”

What exactly is the spark behind a wildfire?

There’s no smoke without fire — though that’s not quite the phrase — but a fire doesn’t start without some kind of spark.

Looking at the statistics on forest fires in the Fiscal 2019 White Paper on Fire Service (Reiwa 1) https://www.fdma.go.jp/publication/hakusho/r1/assets_c/2019/03/1-1-10hyo.html

“Bonfires” top the list of causes, but there’s an even larger category of “unknown” causes above it. Some of these might be arson cases where the culprit simply hasn’t been caught, but presumably some portion is “spontaneous combustion.” If that’s the case, does spontaneous combustion occur at a rate that can’t be ignored even in a relatively humid country like Japan?

Curious, I also went looking for statistics from California, famous for its frequent wildfires, and the official data I managed to find was from the US Fire Administration. There, Natural is listed at 3.2% (the “unknown” category being so small compared to Japan is striking). Here’s an easy-to-read table. https://www.usfa.fema.gov/data/statistics/#causesNR And here’s the detailed report. https://www.usfa.fema.gov/downloads/pdf/statistics/nonres_bldg_fire_estimates.pdf And this one appears to be statistics put out by California’s insurance association. https://www.iii.org/fact-statistic/facts-statistics-wildfires#:~:text=2019%3A%20In%202019%20there%20were,Interagency%20Fire%20Center%20(NIFC).&text=The%20Mendocino%20Complex%20Fire%20broke,date%2C%20with%20459%2C000%20acres%20burned.

Things like “Power Line” and “Lightning.” Looking at past large-scale fires, a fair share of “unknown cause” (which might include spontaneous combustion) shows up there too.

The following article from the Los Angeles Times states that “84% is human-caused and 16% is lightning.” https://www.latimes.com/california/story/2019-10-29/how-do-wildfires-start

It also notes that arson is, if anything, a rare case.

I couldn’t find an official statistic specifically identifying causes for California, but in the end, it seems that in the United States, wildfires from spontaneous combustion are generally understood to be rare.

The situation in Australia, however, appears to be different.

There’s an analysis of the large-scale fires of 2019-2020 by CSIRO, Australia’s national research organization. It’s very well organized and easy to read. https://www.csiro.au/en/research/natural-disasters/bushfires/2019-20-bushfires-explainer The cause of the major 2019-2020 fires was lightning, and apparently half of Australia’s fires are caused by lightning. There’s also apparently a mechanism by which a fire itself can be a distant contributing factor to the next lightning strike elsewhere. That’s how forest fires are said to chain together. Here’s an easy-to-understand illustration from the BBC. https://ichef.bbci.co.uk/news/800/cpsprodpb/1407F/production/_110374028_pyrocumulonimbus_640_eng-nc.png

Incidentally, when people generally talk about “spontaneous combustion,” the mechanism is said to be “friction between leaves.” Intuitively, that sounds like it would take quite a bit of persistence. It smells like a rare case to me.

Lightning and warming

Is there a relationship between lightning and warming?

In 2014, a paper published in Science by researchers at UC Berkeley drew a lot of attention.

Projected increase in lightning strikes in the United States due to global warming https://science.sciencemag.org/content/346/6211/851 According to this paper, a 1-degree rise in temperature increases the probability of lightning by 12%, and they projected that lightning would increase by 50% over the course of this century.

The logic goes like this: water vapor in the atmosphere is like fuel for lightning, and warming is known to increase water vapor; once ignition occurs, it spreads in a chain reaction and triggers a great deal of lightning. Increased lightning also raises NOx levels in the atmosphere, which is itself a problem.

They had published research in 2011 on the correlation between rainfall, cloud buoyancy, and lightning, and this hypothesis builds on that. Updrafts increase water vapor and ice particles in the atmosphere, which triggers lightning. When they checked this hypothesis against data held by the US Weather Service, they were able to predict lightning with 77% accuracy. They then combined this model with 11 models projecting future climate to produce the lightning forecast through 2100 mentioned at the outset.

You can read more detail at this link. https://news.berkeley.edu/2014/11/13/lightning-expected-to-increase-by-50-percent-with-global-warming/ This report has been cited in quite a lot of places and appears to carry a fair amount of authority (there was a Japanese-language article about it here https://www.afpbb.com/articles/-/3031736).

A completely opposite report, on the other hand, appeared in Nature in 2018.

A projected decrease in lightning under climate change https://www.nature.com/articles/s41558-018-0072-6

This is a paper by researchers at the University of Edinburgh, University of Leeds, and Lancaster University, who compared the conventionally used CTH (Cloud-Top Height) approach with a new IFLUX (upward cloud Ice FLUX) approach that overcomes the limitations of the older approach. Under the IFLUX approach, they confirmed a 15% decrease in lightning per 1-degree rise. They also argue that without a model that properly accounts for ice clouds and small-scale physical phenomena, it will be difficult to accurately predict future lightning.

The debate over lightning and warming has not been settled.

Whether a fire spreads is the real question

Let’s get back on track.

The difference between “ignition” and “catching fire” is whether something sets itself alight or catches fire from something else, and here’s a page showing example ignition point and flash point values for common everyday materials. https://www.hakko.co.jp/qa/qakit/html/h01080.htm

The only material generally capable of “igniting” at ordinary temperatures is “yellow phosphorus.” Phosphorus’s ignition point is 30°C, so it can genuinely ignite spontaneously under everyday climate conditions. That’s the kind of thing you have to handle with care. Aside from yellow phosphorus, though, there’s really nothing else that spontaneously ignites at room temperature, whether it’s hot or cold.

A spark can occur as a natural phenomenon just as readily as lightning can. In prehistoric and primitive times too, forest fires presumably broke out and then died down naturally. So when discussing the effects of human activity or changes in weather conditions, the focus should mainly be on whether some kind of interference or change is making forest fires more prone to spreading.

That brings us to the question of “flash point.” A spark alone, smoldering a little, just fizzles out on its own. A fire only becomes a real fire once it catches something else alight. The likelihood of a large-scale fire occurring is, in other words, thought to be determined by how much of the surrounding area is filled with substances that are likely to catch fire.

Looking at the flash points in the table mentioned earlier, “kerosene,” “heavy oil,” and “diesel oil” are the ones that become more likely to catch fire as temperatures rise. From this we can infer that the real issue might be whether kerosene or heavy oil is present somewhere in concentrated form, and whether those substances catch fire.

The above concerned common everyday substances, but I also found more detailed data on flammable gases here. http://www.nohken.com/japan/technology/reference/explosive_gas.pdf

Looking at these chemical substances makes it abundantly clear that there’s no shortage of things that catch fire at room temperature. But the obvious question that arises is whether these substances are commonly found in nature.

The Fire and Disaster Management Agency also explains low-temperature fires as follows. http://nrifd.fdma.go.jp/public_info/faq/teionhakka/index.html

Wood’s inherent ignition temperature is above 200°C, but the explanation is that continued exposure to a lower temperature (around 100°C here) causes moisture to evaporate, and as porosity increases, ignition can occur. I see — this can be interpreted as meaning that if dryness is present as a precondition, this change can happen quite rapidly. In other words, this offers a physical explanation for the commonly stated phenomenon that “fires spread more easily during dry seasons.”

The Fire and Disaster Management Agency’s data also shows a clear correlation between humidity and the number of fire incidents.

Regarding fires involving fuel (i.e., some flammable substance), meanwhile, Australia’s national public broadcaster comments that “higher ambient temperatures raise fire risk by bringing conditions closer to the fuel’s flash point.” https://www.ga.gov.au/scientific-topics/community-safety/bushfire On this point, I’ve come across the argument that as forests are developed, humans bring fuel into the mountains, and because these materials aren’t managed thoroughly, forest fires become more prone to spreading. An essay from The Breakthrough Institute in Oakland was interesting on this point. https://thebreakthrough.org/issues/energy/wildfire-causes

There are several points worth summarizing.

“Rising temperatures raise summer temperatures, which brings snowmelt earlier and pushes the winter rains to the tail end of autumn (and makes them end earlier); as a result of these effects, the high-risk fire season is now 50 days longer than it was in 1979.”

The explanation isn’t that things burn because temperatures rise, but rather that the overall period of high risk — including dryness — has lengthened. I see.

“The decline of forests in the Sierra Nevadas due to dryness, and the increased likelihood of strong winds, are not factors directly tied to recent fires.”

“The movement of fossil fuels is the greatest risk. The trend of people moving their homes closer to forests in search of a lower cost of living is clearly evident from WUI surveys.”

The argument here seems to be this: forest fires, especially those with natural causes, are fundamentally a fairly common natural phenomenon that, absent human interference, would settle down naturally to a reasonable level. What’s raising the risk is that human activity has moved closer to forests, combined with poor management of fuel.

“Why California fires have become less frequent but larger over time.”

That’s the idea.

Conclusion

At this point, I think the argument strongly tying forest fires to warming (the claim that “it’s all because of warming”) objectively lacks persuasiveness. A connection can’t be ruled out, but it falls short of establishing warming as a dominant factor.

Ending on “it’s unclear” would be an unsatisfying conclusion, so let me organize things with some sense of confidence levels attached.

On ignition

• Spontaneous combustion accounts for a low share of fire causes in Japan and the US. In Australia, however, spontaneous combustion is a major factor.
• Among cases of spontaneous combustion, lightning is the cause in a fairly dominant share.
• It’s hard to imagine warming directly increasing the number of sparks (ignition temperatures are far removed from the scale of warming’s effects).

On catching fire

• The ease with which forest fires spread is affected to some degree by warming (through the promotion of porosity in wood, dead leaves, and the like).
• In California, the dry season has lengthened over the course of the year due to the effects of warming.
• There are cases where fossil fuels brought in by people, once present in nature, make fires more prone to spreading.

<Things that can’t be clearly stated>

• Lightning does appear to be related to warming, but whether that effect is positive or negative isn’t well understood at this point (either conclusion is possible depending on the climate model).
• It’s not clear whether warming is a major factor increasing the ease with which forest fires spread.

Originally published in Japanese at https://clazytech.com/2020/10/379/. Translated with LLM assistance and reviewed before publication.