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If a tree burns in the woods and there's nobody to hear it ...

Let's go back to writing about something I actually know something about.

The headlines say climate change causes wildfires. And indeed, a new study (pdf) found a strong correlation between the increase during the 1980s in the number and length of wildfires in the western US and increased temperatures.

But before we rush off to base our wildfire policy on these findings, two grains of salt are in order: 1) explaining a phenomenon is not the same as explaining the problem associated with that phenomenon, and 2) the solution to a problem is not simply the cause applied in reverse. This post will deal only with the first issue, hopefully I'll be able to post on the second tomorrow.

By almost any measure you care to use, the most wildfire-prone state in the US is Alaska. But you hardly ever hear about Alaska in the news (I should know -- I have a Google News alert set up for "wildfire"). Why? Because hardly anybody lives in Alaska, especially in the interior where most of the fires are. Alaska has lots of wildfires, but it doesn't have much of a wildfire problem. By itself, a phenomenon in nature is morally and politically neutral. It becomes a problem, an issue to be concerned about, when it intersects with humans and the things we value*.

This becomes important in the climate change study because the authors note an important regional difference. The correlation between climate and fire is much stronger in the northwest than in the southwest. In the southwest, "land use" -- the conventional wisdom of fire suppression leading to overgrown forests -- is declared by default to be the major factor in that region's fires. The study comes to an overall conclusion similar to the northwest regional conclusion because its data is drawn from federal forest lands, of which there are far more in the northwest. (The southwest has more non-forest but still fire-prone lands such as shrublands, as well as more land under other tenure, such as Indian reservations).

Let's take, then, that the basic regional dichotomy of fire causes -- climate in the northwest, land use in the southwest -- is accurate. This is useful in itself, since fire policy ought to be tailored to the local situation rather than being based on overbroad generalizations. But we might still wonder what can we say about the overall fire problem. To do that, we have to take our understanding of the phenomenon and couple it with an understanding of the people at risk.

The biggest buzzword in fire policy today is "urban-wildland interface" or "wildland-urban interface," abbreviated UWI or WUI**. The UWI is the landscape formed when residential settlement abuts "wild" areas such as forests. There has been a great expansion in UWI in the First World over the past few decades, and most of our major recent fires (such as Southern California in 2003) have been UWI fires. So one quick and dirty measure of how many people are at risk is how many people live in the UWI in each region.

I went to this document (pdf) for estimates of the number of houses in the UWI in the northwest (WA, OR, ID, MT, and WY) and the southwest (CA, AZ, NM, NV, UT, and CO). Adding it up, we find that there are nearly 2 million UWI houses in in the northwest, but nearly 7 million in the southwest. Multiplying by the Census's figures for average household size, we get a very rough estimate of 5 million people at risk in the northwest, and 9 million in the southwest. If we measure vulnerability by population at risk, the causes of fire in the southwest are three to four times as important to the fire problem as the causes of fire in the northwest. Put another way, there would have to be three to four times as many fires in the northwest in order for that region's fire causes to be as important to the national-level fire problem.

Of course, sheer number of people in the UWI is a very crude proxy for vulnerability. You'd then have to factor in things like poverty (a quick glance at some data suggests it might be a wash in terms of regional comparisons) and race (I suspect the southwest is more diverse).

*I'm setting aside possible detrimental effects of changed fire regimes on animals and ecosystems, because most of the discussion on this topic has been very anthropocentric in this regard.

**"WUI" seems to be gaining popularity in the US, but I find "UWI" to be more euphonious both as a full phrase and as an acronym.


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