Sierra nevada

Study: Forest Fires in Sierra Nevada Driven by Past Land Use not Climate Change

Researchers from the University of Arizona and Penn State studied fire regimes in the Sierra Nevada Mountain of California for the period 1600 to 2015 and found that land use changes, not climate, were the principal controlling factors.

This result was apparently a surprise to the researchers since they set out to correlate climate with the fires.

“Initially, we did work to see if we could develop long-lead forecasts for fire in the area — six to 18 months in the future — using climate patterns such as El Niño,” said Alan H. Taylor, professor of geography, Penn State. “This would be a significant help because we could place resources in the west if forecasts indicated it would be dry and the southeast would be wet. However, the climate relationships with fire did not consistently track.”

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

The researchers used tree ring data from 29 sites, historical documents, and 20th Century records of areas burned.

From the UofA press release:

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. 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. Finding that fire activity and human land use are closely linked means people can affect the severity and frequency of future forest fires through managing the fuel buildup and other land management practices — even in the face of rising temperatures from climate change.

From the Penn State press release:

Early fires, because they were more frequent, with less fuel build-up, were “good” fires. They burned through the forest, consumed understory fuels and left the majority of trees unharmed. The Native American mosaic of burned and unburned areas prevented fires from continuously spreading.

From 1776 to 1865 the second fire regime, characterized by Spanish colonialism and the depopulation of Native Americans in the area, shows more land burned. European settlers brought diseases against which Native Americans had no immunity and the population suffered. The Spanish built a string of missions in California beginning in 1769 and relocated remaining Native Americans to the mission areas. In 1793, there was a ban on burning to preserve forage, disrupting the pre-colonial Native American burning practices. The incidence of fires became more sensitive to drought and the fire regime changed, creating the time when fires were largest and most closely coupled with climate.

The third fire period is from 1866 to 1903 and was initiated by the California gold rush, when thousands of people poured into the area. Settlement by large numbers of new immigrants began to break up the forest fuel and the creation of large herds of animals, especially sheep, removed large amounts of understory and changed the fire regime.

The fourth fire period began in 1904 and is linked to the federal government’s policy of fire suppression on government lands. The reason pre-colonial and Spanish colonial fire levels were so much higher than today is that the current fire regime is one of suppressions with an extremely low incidence of fires compared to the past. However, suppression over the last century has allowed fuel to build up on the forest floor and opened the door for “bad” fires that destroy the forest canopy and burn large areas of land.

(UofA press release, Penn State press release, paper abstract )

This finding contradicts an alarmist story printed in the Arizona Daily Star this past October (see third reference below).


See also:

Wildfires and Warming – Relationship not so clear
Claim: “Worsening Wildfires Linked to Temp Rise

Media hype about forest fires and global warming
Mega-fires in Southwest due to forest mismanagement

Sierra Nevada snow pack – the larger picture

According to a story in the Arizona Daily Star, University of Arizona researchers claim “The combination of drought and high temperatures that shrank Sierra Nevada snow pack and brought water shortages and destructive fires to California this year may have no precedent in 500 years, according to a study of tree-ring records.”

This makes for ominous headlines that will get some press. However, had the researchers gone farther into the past, they would have found that extreme drought and low snow pack conditions were more common, and all due to natural variation of the climate.

Here is what the IPCC has to say about North American droughts in their 4th Assessment Report:

Climate Change 2007: Working Group I: The Physical Science Basis The Record of Hydrologic Variability and Change in the Americas

Multiple proxies, including tree rings, sediments, historical documents and lake sediment records make it clear that the past 2 kyr [2,000 years] included periods with more frequent, longer and/or geographically more extensive droughts in North America than during the 20th century. Past droughts, including decadal-length ‘megadroughts’, are most likely due to extended periods of anomalous SST [sea surface temperature], but remain difficult to simulate with coupled ocean-atmosphere models. Thus, the palaeoclimatic record suggests that multi-year, decadal and even centennial-scale drier periods are likely to remain a feature of future North American climate, particularly in the area west of the Mississippi River.

There is some evidence that North American drought was more regionally extensive, severe and frequent during past intervals that were characterised by warmer than average NH summer temperatures (e.g., during medieval times and the mid-Holocene). There is evidence that changes in the North American hydrologic regime can occur abruptly relative to the rate of change in climate forcing and duration of the subsequent climate regime. Abrupt shifts in drought frequency and duration have been found in palaeohydrologic records from western North America. Similarly, the upper Mississippi River Basin and elsewhere have seen abrupt shifts in the frequency and size of the largest flood events. Recent investigations of past large-hurricane activity in the southeast USA suggest that changes in the regional frequency of large hurricanes can shift abruptly in response to more gradual forcing. Although the paleoclimatic record indicates that hydrologic shifts in drought, floods and tropical storms have occurred abruptly (i.e., within years), this past abrupt change has not been simulated with coupled atmosphere ocean models. Decadal variability of Central Chilean precipitation was greater before the 20th century, with more intense and prolonged dry episodes in the past. Tree-ring based precipitation reconstructions for the past eight centuries reveal multi-year drought episodes in the 14th and 16th to 18th centuries that exceed the estimates of decadal drought during the 20th century.

I understand from the Star story that the researchers used a pre-existing 500-year database of tree rings, so they cannot be accused of “cherry-picking”on purpose, although it so appears. I do not have access to the full paper, so I don’t know if the researchers tempered their claims with the longer history. In my opinion, however, based on what was reported in the Star, the researchers appear to be “crying wolf.”

Related articles:

Drought in the West
Droughts in the Southwest put in perspective

University of Arizona Scientists Find Evidence of Roman Period Megadrought