Fires of the present and past

It’s no secret that we’ve been having some unusual weather recently throughout the western United States. I wrote previously that we should and do expect fire in California; that fire is a natural feature of the ecosystem here and there are many plants (and presumably animals) evolved to take advantage of periodic and spontaneous wildfires. On the other hand, some of this could be unprecedented, at least over a very long time.

This is an image from my backyard in Santa Barbara, California, taken just before noon today. No filters were applied to the image; ordinarily, the sky should be bright and blue. The haze is a mixture of fog (it’s a bit humid) and smoke, mostly from the wildfires burning in Northern California, several hundred kilometers away. The sepia hue is probably due to nanoscale aerosols scattering higher energy sunlight; it’s just less dense here than it is over Northern California where we’re seeing dim, reddened skies.

The only way to determine this for sure is to turn to paleofire reconstructions. These are pre-historic records assembled by scientists to measure and compare intensities of wildfires based on the charcoal they produced and left behind, usually accumulated in lake sediments. (Fires from the past several hundred or perhaps a thousand years can be reconstructed with a good degree of areal precision using tree rings. But for very long fire histories, and therefore to determine a genuine natural fire baseline against which to compare modern wildfire conditions, we need to use other proxies.) Charcoal is a nice proxy for several reasons, but tops is that it’s inherently non-reactive, unlike other proxies that fall prey to post-depositional chemical changes (known as diagenesis). However, because charcoal can persist on the landscape indefinitely before it is incorporated, and effectively entombed, in sediment, it’s not always clear what a charcoal signal means. (I got a laugh out of one of my committee members, a soil scientists, when I once stated frankly that “charcoal is a great proxy, if we can figure out what it means”.) In addition to its indefinite persistence, it can also get launched high into the air and fly quite some distance from its source. We attempt to account for these characteristics with statistical models and field measurements of charcoal from modern fires, but individual reconstructions of are inconsistent quality.

I produced a few records from mountain lakes in Utah, soon to be published. Though we couldn’t develop a good record for the past century or so, I’m pretty confident with what we produced for the period before that. (It would be nice to get a look at the recent history as well, because then we could make direct comparisons. Other methods show the present to be so “weird” as compared to the past that it’s difficult to even imagine an adequate comparison.) One of my records, from a small pond in Cedar Breaks National Monument, near Cedar City, shows increased fire frequency about a thousand years ago, during the Medieval Climate Anomaly when we know conditions were warm and dry like they are presently, but also coincides with a very well-dated volcanic cinder cone eruption only a few kilometers away as well as the most intense period of Fremont Native American occupations nearby. Another of my records, from a higher lake in Manti La Sal National Forest, to the east of Salina, is well constrained by radiocarbon dating to more than 6000 years ago. Crucially, it resolves a period of dramatically increased fire activity about 4200 years ago, coincident with a global 4.2 kiloyear event that appears in other records, including in the ancient Near East where it is associated with sociocultural transformation in the Akkadian Empire.

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