Neue Studie der George Washington University: Modelle zu kalifornischen Waldbränden überschätzen Rolle des Klimawandels

Immer wenn irgendwo der Wald brennt, ist der Schuldige schnell gefunden. Es muss wohl der Klimawandel sein, wer denn sonst. Eine Forschergruppe um Michael L. Mann (das ist nicht der Hockeystick-Mann, der Michael E. Mann heißt) veröffentlichte am 28. April 2016 in PLOS One eine explosive Studie, die Unerhörtes fand: Das individuelle menschliche Verhalten hat neben dem Klimawandel ebenfalls einen sehr starken Einfluss auf die Entstehung von Waldbränden. In Kalifornien gehen 90% aller Waldbrände auf menschliche Ursachen zurück. Die Forscher fordern daher stärkere Bemühungen, das Verhalten der Menschen zu beeinflussen, z.B. keine brennenden Zigarettenstummel mehr aus dem Wagen zu werfen. Im Folgenden die entsprechende Pressemitteilung der George Washington University zur Veröffentlichung:

It’s Not Just Climate Change: Study Finds Human Activity Is a Major Factor Driving Wildfires

Study Weighs Human Influence in Wildfire Forecast Through 2050

A new study examining wildfires in California found that human activity explains as much about their frequency and location as climate influences.  The researchers systematically looked at human behaviors and climate change together, which is unique and rarely attempted on an area of land this large. 

The findings suggest many models of wildfire predictions do not accurately account for human factors and may therefore be misleading when identifying the main causes or drivers of wildfires. The newest model proportionately accounts for climate change and human behavioral threats and allows experts to more accurately predict how much land is at risk of burning in California through 2050, which is estimated at more than 7 million acres in the next 25 years. 

The paper, “Incorporating Anthropogenic Influences into Fire Probability Models: Effects of Human Activity and Climate Change on Fire Activity in California,” appears Thursday in PLOS ONE. 

Climate change affects the severity of the fire season and the amount and type of vegetation on the land, which are major variables in predicting wildfires. However, humans contribute another set of factors that influence wildfires, including where structures are built, and the frequency and location of ignitions from a variety of sources—everything from cigarettes on the highway to electrical poles that get blown down in Santa Ana winds. As a result of the near-saturation of the landscape, humans are currently responsible for igniting more than 90 percent of the wildfires in California.

“Individuals don’t have much control over how climate change will affect wildfires in the future. However, we do have the ability to influence the other half of the equation, those variables that control our impact on the landscape,” said Michael Mann, assistant professor of geography at the George Washington University and lead author of the study. “We can reduce our risks by disincentivizing housing development in fire-prone areas, better managing public land and rethinking the effectiveness of our current firefighting approach.”

The researchers found that by omitting the human influence on California wildfires, they were overstating the influence of climate change. The authors recommend considering climate change and human variables at the same time for future models. 

“There is widespread agreement about the importance of climate on wildfire at relatively broad scales. At more local scales, however, you can get the story quite wrong if you don’t include human development patterns,” said Max Moritz, a co-author and a University of California Cooperative Extension specialist based at UC Berkeley. “This is an important finding about how we model climate change effects, and it also confirms that getting a handle on where and how we build our communities is essential to limiting future losses.”

Between 1999 and 2011, California reported an average of $160 million in annual wildfire-related damages, with nearly 13,000 homes and other structures destroyed in so-called state responsibility areas—fire jurisdictions maintained by California, according to Dr. Mann. During this same period, California and the U.S. Forest Service spent more than $5 billion on wildfire suppression.

In a model from 2014 that examined California wildfires’ destruction over the last 60 years, Dr. Mann estimated that fire damage will more than triple by 2050, increasing to nearly half a billion dollars annually. “This information is critical to policymakers, planners and fire managers to determine wildfire risks,” he said.


Das Ergebnis passt gut zu einer Studie eines Teams um Scott Anderson, die im Dezember 2013 in The Holocene erschien. Die Autoren fanden, dass die Waldbrandentwicklung Kaliforniens während der letzten Jahrtausende eng an die Besiedlungsgeschichte gekoppelt war. Hier die Kurzfassung:

Holocene and historical vegetation change and fire history on the north-central coast of California, USA
Pollen, non-pollen palynomorphs (NPPs), and charcoal particle stratigraphies are used to determine environmental change at Glenmire, Point Reyes Peninsula, northcentral coastal California, over the last c. 6200 years. Pollen was not preserved in early Holocene sediments when climate was drier than present. However, groundwater tables rose after c. 6200 cal. BP, allowing for greater subsequent preservation of organic matter. Middle and late Holocene environments were a mosaic of vegetation types, including mixed conifer forest with coastal scrub grassland prior to c. 4000 cal. BP. Subsequently, hardwoods such as alder (Alnus) and coastal scrub (e.g. Artemisia, Baccharis) expanded until c. 2200 cal. BP, followed by tanoak (Lithocarpus densiflorus), Douglas fir (Pseudotsuga menziesii), and coast redwood (Sequoia sempervirens). With increasing amounts of oak (Quercus), this mosaic of vegetation types continued to dominate until the arrival of Euro-Americans in the early to mid-1800s. The fire history is probably tied closely to human settlement, since natural ignitions are rare. Elevated charcoal amounts coincide with increased sedentism of the native populations by about 3500 cal. BP. Increased sedentism may have caused a more intense and constant use of the coastal environment around Glenmire. For the most recent centuries, we compared historical records of explorations, Spanish Mission establishment, consolidation of the native Coast Miwok population, ranching by Mexican nationals, and dairying by Americans at the height of California’s gold rush with the paleoecological record. The Glenmire record thus documents changing fire use following the ad 1793 fire suppression proclamation; declines in native forest species; introductions of non-native species, including those associated with livestock grazing and land disturbance; and an increase in coprophilous fungi (NPPs) associated with the presence of large numbers of sheep and cattle, among other changes. During the historical period, the sedimentary record of historical fires closely matches the nearby fire-scar tree-ring record.

Interessant auch eine Trendanalyse aus der kalifornischen Sierra Nevada von Chad Hanson und Dennis Odion, die im Zeitraum von 1984-2010 auf Basis von Satellitenbildern keinen Trend in der Waldbrandhäufigkeit feststellen konnte. Die Autoren schlussfolgern, dass der Einfluss des Klimawandels auf die Waldbrandentstehung im Vergleich zu menschlichen Auslösern gering ist. Hier der Abstract der Studie, die 2013 im International Journal of Wildand Fires erschien:

Is fire severity increasing in the Sierra Nevada, California, USA?
Research in the Sierra Nevada range of California, USA, has provided conflicting results about current trends of high-severity fire. Previous studies have used only a portion of available fire severity data, or considered only a portion of the Sierra Nevada. Our goal was to investigate whether a trend in fire severity is occurring in Sierra Nevada conifer forests currently, using satellite imagery. We analysed all available fire severity data, 1984–2010, over the whole ecoregion and found no trend in proportion, area or patch size of high-severity fire. The rate of high-severity fire has been lower since 1984 than the estimated historical rate. Responses of fire behaviour to climate change and fire suppression may be more complex than assumed. A better understanding of spatiotemporal patterns in fire regimes is needed to predict future fire regimes and their biological effects. Mechanisms underlying the lack of an expected climate- and time since fire-related trend in high-severity fire need to be identified to help calibrate projections of future fire. The effects of climate change on high-severity fire extent may remain small compared with fire suppression. Management could shift from a focus on reducing extent or severity of fire in wildlands to protecting human communities from fire.

Schließlich sei noch auf eine Studie aus dem Juli 2015 hingewiesen, in der ein ganz unerwarteter weiterer Schuldiger für die Entstehung von Waldbränden in Kalifornien genannt wird: Luftmassenintrusionen aus der Stratosphäre. Die American Geophysical Union (AGU) gab hierzu die folgende Pressemitteilung heraus:

Stratospheric accomplice for Santa Ana winds and California wildfires
Southern Californians and writers love to blame the hot, dry Santa Ana winds for tense, ugly moods, and the winds have long been associated with destructive wildfires. Now, a new study finds that on occasion, the winds have an accomplice with respect to fires, at least: Natural atmospheric events known as stratospheric intrusions, which bring extremely dry air from the upper atmosphere down to the surface, adding to the fire danger effects of the Santa Anas, and exacerbating some air pollution episodes.

The findings suggest that forecast models with the capacity to predict stratospheric intrusions may provide valuable lead time for agencies to issue air quality alerts and fire weather warnings, or to reallocate firefighting resources before these extreme events occur. “The atmosphere could give us an early warning for some wildfires,” said Andrew Langford, a research chemist at the National Oceanic and Atmospheric Administration’s (NOAA) Earth System Research Laboratory in Boulder, Colorado, and lead author of the study.

Researchers at NOAA’s National Environmental Satellite, Data, and Information Service (NESDIS) and the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU-Boulder coauthored the work, which has been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union. The authors took a detailed look at the May 2013 “Springs Fire” that burned 100 square kilometers (25,000 acres) about 75 kilometers (50 miles) northwest of Los Angeles. The researchers used a NOAA forecast model that incorporates satellite observations of ozone, wind data, and other atmospheric information to detect the occurrence of the intrusions.

The analysis showed that in the early hours before the Springs Fire, a tongue of air characteristic of the stratosphere–extremely dry and very high in ozone from the stratosphere’s ozone layer–reached to the surface in southern California and extended as far south as Baja California. The researchers found that ground-based monitoring stations near the fire’s origin also confirmed the telltale signs of the intrusion right before the fire broke out: A large drop in relative humidity and a rise in ozone. As the day went on, a combination of factors accelerated the fire: Low humidity, persistent high winds, dry condition of the grasses and other vegetation, clear skies and bright sunlight, and very warm surface temperatures. A few days later, cloudy skies, a drop in temperature, a shift in winds, and widespread rainfall helped extinguish the fire.

The stratospheric intrusion also had another downside during the Springs Fire: It added ozone from the upper atmosphere to the urban and fire-related pollution produced in the lower atmosphere. On the second and third days of the fire, this helped to push levels of ozone–which can harm people’s lungs and damage crops–over the federal ozone limit at 24 monitoring sites across southern California. Monitors as far away as Las Vegas also saw a spike in ozone on the third day of the fire. The observed exceedances of the ozone standard were unusual for the region for that time period, suggesting that the stratospheric intrusions were a contributing factor.

“Stratospheric intrusions are double trouble for Southern California,” said Langford. “We knew that the intrusions can add to surface ozone pollution. Now we know that they also can contribute to the fire danger, particularly during La Niña years when deep intrusions are more frequent, as recently shown by our NOAA colleagues at the Geophysical Fluid Dynamics Laboratory. The good news is that with models and observations, we can get an early warning from the atmosphere in some cases.”

The authors note that stratospheric intrusions have previously been implicated in the explosive development of wildland fires in New Jersey and Michigan, but have not previously been connected to fires in southern California or to the Santa Ana winds. The frequent occurrence of stratospheric intrusions above the west coast during the fall, winter, and spring suggests that similar circumstances may have played a role in other major southern California fires, including the series of destructive fires that burned more than 3,000 square kilometers (more than 800,000 acres) in October of 2003, and burned about 4,000 square kilometers (nearly a million acres) in October of 2007, say the authors.