Research: Wildfires in Sierra Nevada driven by past land use

Changes in human uses of the land have had a large impact on fire activity in California’s Sierra Nevada since 1600, according to research by a University of Arizona researcher and her colleagues.

Above: Indian Canyon Fire near Edgemont, SD, 2016. Photo by Bill Gabbert.

By Mari N. Jensen, University of Arizona College of Science

Forest fire activity in California’s Sierra Nevada since 1600 has been influenced more by how humans used the land than by climate, according to new research led by University of Arizona and Penn State scientists.

For the years 1600 to 2015, the team found four periods, each lasting at least 55 years, where the frequency and extent of forest fires clearly differed from the time period before or after.

However, the shifts from one fire regime to another did not correspond to changes in temperature or moisture or other climate patterns until temperatures started rising in the 1980s.

“We were expecting to find climatic drivers,” said lead co-author Valerie Trouet, a UA associate professor of dendrochronology. “We didn’t find them.”

Instead, the team found the fire regimes corresponded to different types of human occupation and use of the land: the pre-settlement period to the Spanish colonial period; the colonial period to the California Gold Rush; the Gold Rush to the Smokey Bear/fire suppression period; and the Smokey Bear/fire suppression era to present.

“The fire regime shifts we see are linked to the land-use changes that took place at the same time,” Trouet said.

“We knew about the Smokey Bear effect — there had been a dramatic shift in the fire regime all over the Western U.S. with fire suppression. We didn’t know about these other earlier regimes,” she said. “It turns out humans — through land-use change — have been influencing and modulating fire for much longer than we anticipated.”

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Study concludes climate change has doubled acres burned in western U.S.

Above: Acres burned in the United States, 1986 through 2015. Data from NIFC, compiled by Bill Gabbert.

A new study released yesterday concludes that human-caused climate change is responsible for nearly doubling the number of acres burned in western United States wildfires during the last 30 years.

Fires in western forests began increasing abruptly in the 1980s, as measured by area burned, the number of large fires, and length of the fire season. The increases have continued, and recently scientists and public officials have in part blamed human-influenced climate change. The new study is perhaps the first to quantify that assertion. “A lot of people are throwing around the words climate change and fire–specifically, last year fire chiefs and the governor of California started calling this the ‘new normal,’ ” said lead author John Abatzoglou, a professor of geography at the University of Idaho. “We wanted to put some numbers on it.”

Warm air can hold more moisture. As the temperature rises the relative humidity decreases. Low humidity withdraws more moisture out of live and dead plants as well as soil. Plants are the fuel for wildfires and lower moisture means fires can burn more rapidly and with increased intensity and resistance to control. Average temperatures in forested parts of the U.S. West have gone up about 2.5 degrees F since 1970, and are expected to keep rising. The resulting drying effect is evident in the rise of more fires.

Jasper Fire south dakota
Jasper Fire pyrocumulus, about two hours after the fire started, August 24, 2000 west of Custer, South Dakota. NPS photo by Bill Gabbert.

The overall increase in fire since the 1980s is about twice what the researchers attribute to climate change; the rest is due to other factors, they say. One has been a long-term natural climate oscillation over the Pacific Ocean that has steered storms away from the western United States. Another: firefighting itself. By constantly putting out fires, authorities have allowed areas they “saved” to build up more dry fuel, which later ignites, causing ever more catastrophic blazes, the researchers say. The costs of fire fighting have risen sharply in step; last year the federal government alone spent more than $2.1 billion. “We’re seeing the consequence of very successful fire suppression, except now it’s not that successful anymore,” said Abatzoglou.

The authors isolated the effects of climate warming from other factors by looking at eight different systems for rating forest aridity; these included the Palmer Drought Severity Index, the MacArthur Forest Fire Danger Index and the Canadian Forest Fire Danger Rating System. They then compared such measurements with observations of actual fires and large-scale climate models that estimate manmade warming. The crunched data showed that 55 percent of the increase in fuel aridity expected to lead to fires could be attributed to human-influenced climate change. Climate’s role in increasing such aridity has grown since 2000, the researchers say, and will continue to do so.

(The graphic below is from the study.)

wildfires climate change

The researchers found that anthropogenic climate change accounted for about 55% of observed increases in fuel aridity from 1979 to 2015 across western US forests, highlighting both anthropogenic climate change and natural climate variability as important contributors to increased wildfire potential in recent decades.

Mr. Abatzoglou and coauthor Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, say they do not account for some factors that could be offshoots of climate warming, and thus they may be understating the effect. These include millions of trees killed in recent years by beetles that prefer warmer weather, and declines in spring soil moisture brought on by earlier snowmelt. There is also evidence that lighting may increase with warming.

The study does not cover western grasslands. These have seen more fires too, but there is little evidence that climate plays a role there, said Mr. Abatzoglou; rather, the spread of highly flammable invasive grasses appears to be the main driver.

Mike Flannigan, a fire researcher at the University of Alberta, said that previous studies have tried to understand the effects of climate on fires in parts of Canada, but that nothing had been done for the United States on this scale. “What’s great about this paper is that it quantifies this effect, and it does it on a national scale,” he said.

Worldwide, wildfires of all kinds have been increasing, often with a suspected climate connection. Many see a huge fire that leveled part of the northern city of Fort McMurray, Alberta, this May as the result of a warming trend that is drying out northern forests. Fires have even been spreading beyond, into the tundra, in places where blazes have not been seen for thousands of years. That said, fires are not expected to increase everywhere. “Increased fire in a lot of places agrees with the projections,” said Jeremy Littell, a research ecologist with the U.S. Geological Survey in Anchorage, Alaska. “But in many woodlands, the relationship between climate and fire is not as tidy.”

Many scientists studying the issue believe the growth in U.S. western fires will continue for many years. Mr. Williams and others say that eventually, so many western forests will burn, they will become too fragmented for fires to spread easily, and the growth in fire will cease. But, he says, “there’s no hint we’re even getting close to that yet. I’d expect increases to proceed exponentially for at least the next few decades.” In the meantime, he said, “It means getting out of fire’s way. I’d definitely be worried about living in a forested area with only one road in and one road out.”

Linking wildfires to climate change — it’s complicated

This is a rather surprising 2-minute video about attributing wildfires to climate change. It was produced by the National Center for Atmospheric Research and the University Corporation for Atmospheric Research.

Tuesday’s weather in Fort McMurray was 8 degrees above record high for the date

Hot weather Alberta

“#ymmfire” refers to the airport code for the Fort McMurray International Airport in Alberta. The entire population of the city, more than 80,000, was ordered to evacuate when it became obvious it was going to be overrun by the fire.

More information about the fire.

Below is an excerpt from a May 4, 2016 article at Slate about the weather in Canada:

…Canada’s northern forests have been burning more frequently over recent decades as temperatures there are rising at twice the rate of the global average. A 2013 analysis showed that the boreal forests of Alaska and northern Canada are now burning at a rate unseen in at least the past 10,000 years. The extreme weather of recent months is also closely linked with the ongoing record-setting El Niño conditions in the Pacific Ocean, which tends to bring a warmer and drier winter to this part of Canada. Last month, Canadian officials mentioned the possibility of “large fires” after over-winter snowpack was 60 to 85 percent below normal and drought conditions worsened.

This week, a strong atmospheric blocking pattern—a semi-stable extreme arrangement of the jet stream—reinforced an unseasonable heat wave and helped temperatures reach 90 degrees Fahrenheit (32.2 degrees Celsius) on Tuesday in Fort McMurray, 40 degrees Fahrenheit above normal, eight degrees above the daily record high, 15 degrees warmer than Houston, and the same temperature as Miami. While fleeing, some evacuees had to turn on their air conditioners…

New York Times: wildfire seasons are longer

Yesterday the New York Times published an article titled, “Wildfires, Once Confined to a Season, Burn Earlier and Longer”.

Here is an excerpt:

…Fires, once largely confined to a single season, have become a continual threat in some places, burning earlier and later in the year, in the United States and abroad. They have ignited in the West during the winter and well into the fall, have arrived earlier than ever in Canada and have burned without interruption in Australia for almost 12 months.

A leading culprit is climate change. Drier winters mean less moisture on the land, and warmer springs are pulling the moisture into the air more quickly, turning shrub, brush and grass into kindling. Decades of aggressive policies that called for fires to be put out as quickly as they started have also aggravated the problem. Today’s forests are not just parched; they are overgrown.

In some areas, “we now have year-round fire seasons, and you can say it couldn’t get worse than that,” said Matt Jolly, a research ecologist for theUnited States Forest Service. “We expect from the changes that it can get worse.”…

British Columbia using assisted migration to help forests keep up with climate change

range of Western Larch
The range of Western Larch, Larix occidentalis, sometimes called Western tamarack. From Natural Resources Canada.

Warming caused by climate change is moving the suitable habitat for some plant species farther north in the northern hemisphere. A plant that was once comfortable in one location may be finding it is becoming too warm for it to thrive.

British Columbia, unlike the other Canadian provinces, has changed their rules about replanting forests, hoping to ensure that adapted tree varieties can keep up with the moving habitats. Critics say assisted migration, as it is called, has sometimes produced disastrous results in the past when species were placed in new environments.

Western larch
Western Larch, a deciduous conifer. Photo from Montana Outdoors.

Vice’s Motherboard web site has a fascinating article by Stephen Buranyi on the subject. Here is an excerpt:

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“The Western Larch can live for hundreds of years and grow to over 200 feet, but the oldest Larch trees in northern British Columbia’s Bulkley Valley are only about four feet tall. In fact, the nearest full grown Western Larch is nearly 900 kilometers south by the US border, which has been the Larch’s natural range for thousands of years. These are the first trees of their kind to be planted so far north.

If the disastrous history of invasive species has taught us anything, it’s that it’s often difficult to predict the consequences of such a change. Ecologists and conservationists generally caution against moving a species outside of the areas they naturally live—a process known as assisted migration—and governments generally agree with this take. Across North America there are strict prohibitions against the large scale movement of living populations.

But for the past seven years the province of BC has allowed millions of trees to be planted toward the northernmost reaches of their natural range and beyond. The government is working with scientists who predict that our climate is changing so quickly that, 50 years from now, when the trees are fully grown, the conditions in the trees’ new homes will actually be more like their old ones.

“It restores the tree to the environment for which they are best suited,” said Greg O’Neill, an adaptation and climate change scientist with the BC government, who helped design and implement the province’s assisted migration program. But while BC scientists think that they’ve acted just in time to prepare their forests for the future, no other province appears ready to adopt assisted migration as a strategy anytime soon.

Many trees are what ecologists call foundational species—organisms whose removal would cause enormous disruption in the ecosystem. Trees are a sort of infrastructure for forests; they bind the soil, retain water, and provide food and shelter. Just like the infrastructure unpinning cities, it takes years to establish a tree population, and they are virtually impossible to move.

And yet, because BC’s northern regions are warming at nearly twice the average rate, much of the province’s 55 million hectares of forest may find that their homes have moved north without them. A 2006 paper from the University of British Columbia applied a climate based model to forest ecosystems and showed that some species ranges could shift by up to 100 kilometers each decade.

Rules in BC require that, as trees are cut down, planters use seeds from the same area to re-plant, preserving the genetic character of the forest. O’Neill and his colleagues produced a forestry report in 2008 that drew on the projected range expansions due to climate change, and their own extensive experiments testing various tree species in different climates. They suggested that the province instead expand the distance seeds could be moved uphill, to track with global warming. Later that year the Chief Forester’s Standards for Seed Use were changed for the majority of BC’s commercial tree species to reflect the suggestions in the report.

According to O’Neill, “these were the first policy changes that addressed climate change in forestry.”

Then, in 2010 the standards were changed again, to allow Western Larch to be planted hundreds of kilometres away from its current range. “That had been a long-standing paradigm that no-one dared transgress,” said O’Neill. One ecologist had even called BC’s migration plans “a little scary.”

It’s difficult to overstate how deeply rooted the aversion to moving nature is for many biologists. In 2009 assisted migration was called “planned invasion” in a report that listed our really awful, truly just stupendously bad track record with species that unexpectedly turn invasive…”