Assessing the drivers of wildfire trends in the American West these days can be akin to Hercule Poirot’s task on the Orient Express, on which there was one murder with 12 final suspects — all of whom were guilty.

For western [wild]fire (it’s hard to see how the wild part of that word applies any more, given how many human factors are involved), the suspects are a century of accumulated “fire debt” from fire suppression efforts, development and road construction, natural fluctuations in drought and heat on many time scales, spreading invasive tinder-like grasses and the building influence of greenhouse-driven global warming (see Andrew Freedman’s detailed exploration of the latter point).

The verdict, of course is all of the above, but — as you’ll hear below — many experts see a prime culprit.

And of course there’s no single situation. The scale and severity of fires in the Southwest and Colorado are affected by a different mix than those, like the huge raging Rim Fire around and in Yosemite National Park, in the Sierras.

Photo

Photo

Here’s a closer look at the still-spreading Yosemite conflagration from several perspectives.

First comes fire and forest policy. The two images at the right show the same sequoia grove in Yosemite in 1890 and 1960. They were sent by Stephen J. Pyne, the Arizona State historian and author of “Fire, Nature and Culture” and other fine books.

Here’s Pyne’s description of the images and their significance:

The upper image shows a grove of Yosemite’s Big Trees as they looked in 1890 (note the fire scars on several). Written reports from the earliest visitors describe the scene as being burned almost annually. The fire regime since the 1850s has been disrupted sufficiently that some conifers have encroached. After 70 years of fire exclusion, the conifers have moved in to the point that it is hard to see the sequoias but more critically the new woods have created a fuel structure that could threaten the survival of the ancients by carrying fire into their crowns. If untreated, imagine another 50 years of such changes…. The region is prone to big explosive fires. In my recent research I had to revisit the 1961 season. The Harlow fire reportedly burned 20,000 acres in two hours, took out 105 structures, and killed two people. What I think we’re seeing now is a combination of size and intensity — ecological momentum, if you will. Everything is aligned, and everything is exaggerated.

Here’s a different visual showing the same pattern, but seen through four centuries of fire-frequency data in the Yosemite region gleaned from hundreds of tree-ring and wood samples:

Photo

You can’t avoid seeing a fingerprint of fire-suppression policies in the region now ablaze. The image was sent by Thomas W. Swetnam, the director of the Laboratory of Tree-Ring Research at the University of Arizona, and is from an important 2001 report, “Fire History Along Elevational Transects in the Sierra Nevada, California.” At the bottom of this post, I’ve included Swetnam’s long and valuable discussion of the factors involved.

Firefighting policies have shifted, but far too late to avoid more massive events like the Rim Fire, as is made clear in a sobering statistic in “With Rim Fire Near, A Look Yosemite’s History with Fire,” a fine piece by Ker Than for National Geographic:

By 1970, scientists realized that the fire suppression methods that had been official policy in Yosemite since the park’s inception were actually doing more harm than good. The lack of natural fires caused by lightning had led to overgrown and unhealthy forests that made them more vulnerable to larger, more dangerous fires. As a result, the National Park Service has for the past 40 years conducted carefully managed “prescribed burns” to clear unsafe accumulations of dead wood and for ecological restoration purposes. The effects of these fires still fall far short of what scientists think once occurred naturally, however. It’s estimated that an average of 16,000 of Yosemite’s 747,000 acres may have burned under natural conditions in the park each year. Since the 1970s, prescribed fires have burned between 12,200 and 15,600 acres per decade.

So there is an order of magnitude gap between the planned burning and the natural pattern.

As promised above, here’s Swetnam’s full comment on the human factors in the West’s fire history, and future:

The striking drop-off in surface fires occurrence in the late nineteenth/early 20th century is very common throughout the western US in pine dominant and mixed confer forests…. We have now compiled more than 800 fire scar and tree-age based fire histories from British Columbia to northern Mexico, and the drop off in surface fire regimes shows clearly in most of these sites within the forest types named above, but with variable timing depending on land use history. Intense livestock grazing, especially by huge herds of sheep, was the initial cause of the frequent fire regime disruption in many places. The exceptions prove the rule. In remote mountains of Mexico (including in Baja) fire frequencies decline later (after 1930s or 1950s) or continue burning, uninterrupted, and in a number of places in the west there are earlier fire frequency drop-offs. Typically the drop-offs coincide temporally with the beginning of heavy livestock grazing. Surface fuels amounts (especially grasses) and continuity, unbroken by trails, roads, etc., were critical in allowing the spread of frequent surface fires. After the livestock numbers dropped (drastically after the first world war) the effect of active fire suppression by government agencies became the key reason for continued low fire frequencies. There are many exceptions to these patterns, with great differences in the “natural” fire regimes across elevation gradients and vegetation types across the west. Livestock grazing and fire suppression effects, for example, have arguably had minimal effects on changes in vegetation and fuel dynamics in many shrublands (e.g., southern California chaparral) or in higher elevation spruce-fir forests. These types did NOT burn with frequent surface fires in the past, and so land management activities have had relatively little to do with fuel and fire regime changes in the modern era. Infrequent, large, high severity fires in these types of landscapes is not an ecological disaster, rather it is a continuation of an ancient process that most native species are well adapted to in those places. Although fuel/forest changes since the surface fire regime disruption 100-150 years ago is striking, the recent very large, very high intensity/severity fires in pine-dominant and mixed conifer forests can not be attributed solely or even primarily to these changes. Many Southwestern forests, for example, were already choked by thousands of small trees and dead fuel accumulations by the 1950s and 1960s (e.g., CF Cooper, Ecological Monographs 1960). Some large high severity fires occurred in these areas during the 1950s drought, and subsequent droughts, but the largest total areas burned in forests were an order of magnitude smaller than the largest fires today. The combined effects of warming and extreme dryness of the atmosphere and fuels, combined with the extraordinary fuel accumulations are “exaggerating” these fires (as Steve Pyne says). See the Park Williams et al. Nature Climate Change paper (relevant to the Southwest). And there are other things going on too, in varying degrees of importance, depending on places and vegetation types. These include the spread of highly flammable invasive grasses in many areas, and fire fighting tactics, strategies and policies are also paying a role in many cases, especially in the [wildland-urban interface]. It’s a highly diverse, interacting and “wicked”set of causes and problems, and what exactly is the primary, secondary, etc. cause(s) is not entirely clear in all cases.

An excellent Associated Press feature by Tracie Cone and Brian Skoloff describes the contribution of past warped fire policies, and notes how a shift toward managed burning of forests around Yosemite may be a critical factor slowing this blaze as it plays out. Here’s an excerpt:

Federal forest ecologists say historic policies of fire suppression to protect Sierra timber interests left a century’s worth of fuel in the fire’s path. “That’s called making the woodpile bigger,” said Hugh Safford, an ecologist with the U.S. Forest Service in California. Two years of drought and a constant slow warming across the Sierra Nevada also worked to turn the Rim Fire into an inferno. For years, forest ecologists have warned that Western wildfires will only get worse. “Every year, the summer temperatures are a little warmer, hence the conditions for burning are a little more auspicious,” said Safford. “People can deny it all they want, but it’s happening. Every year, the fuels are a little bit drier.” The Rim Fire’s exponential growth slowed only after hitting areas that had burned in the past two decades, and Safford says that shows the utility of prescribed and natural burns that clear brush and allow wildfires to move rapidly without killing trees. “If you look at the Sierra Nevada as a whole, by far the largest portion hasn’t seen a fire since the 1910s and 1920s, which is very unnatural,” said Safford, who has authored several papers on the increasing wildlife severity across California’s mountain ranges. “This one isn’t stopping for a while.” Since a 1988 fire impacted nearly one third of Yellowstone National Park, forestry officials have begun rethinking suppression policies. Yosemite has adopted an aggressive plan of prescribed burns while allowing backcountry fires caused by lightning strikes to burn unimpeded as long as they don’t threaten park facilities. “Yosemite is one of the biggest experimental landscapes for prescribed fire and it’s going to pay off,” Safford said. “The Rim Fire is starting to hit all those old fire scars.”

But of course, as Ker Than reported, that effort is clearly too little, too late. The Associated Press story ends with an important discussion of the role of fire in the ecosystem in this region:

The Rim Fire is the first of any ecological significance in about a decade in the area stretching from the Sequoia National Forest south of Yosemite to north of Lake Tahoe, said Chad Hanson, a forest ecologist and environmental activist who has published a number of papers on the significance and increasing rarity of post-fire habitat in the Sierra Nevada. Eventually, the forest will come back. “Because we are in such tremendous deficit of this post-fire habitat type, especially in this area, the Rim Fire is a good thing ecologically,” Hanson said. “This is not destruction, this is ecological restoration.”

Chad Hanson of the Earth Island Institute posted a long piece focused on the ecological normalcy and benefits of big fires in the Sierras, “The Ecological Importance of California’s Rim Fire.” (Others, including Swetnam, say he’s far too tilted toward the “let nature take its course” perspective.)

However humans respond, it’s clear that the dawning age of megafires in the West is still in its early stages given the buildup of fuel over a century and the trends in climate. Brandon Keim at Wired, quoting “pyrogeographer” Jennifer Marlon of Yale University, said it best in a feature in July 2012 focused on the Southwest:

Much of the West is now a giant tinderbox, literally ready to combust. Yet thanks to fire suppression, the consequences have been postponed for decades. “When you look at the long record, you see fire and climate moving together over decades, over centuries, over thousands of years,” said pyrogeographer Jennifer Marlon of Yale University, who earlier this year co-authored a study of long-term fire patterns in the American West. “Then, when you look at the last century, you see the climate getting warmer and drier, but until the last couple decades the amount of fire was really low. We’ve pushed fire in the opposite direction you’d expect from climate,” Marlon said. The fire debt is finally coming due.

So what about the role of human-driven global climate change? Read Freedman at the Climate Central link above. But also circle back in the next couple of days*. I posed the “whodunit?” question, Agatha Christie style, to Swetnam, Pyne and several other fire and forest experts, as if they were members of a murder-trial jury. I’ll post their exchange.

Circling back to Yosemite and the continuing drama there, I encourage you to explore this interactive map:

I originally wrote that I’ll post the climate section Friday but it’ll take another day or two.