Extreme Wildfire Events and Global Warming

Summary: Wildfires have occurred in the western United States with increasing frequency and greater severity in recent decades.  This has been associated with increased average temperatures, lower precipitation and lower humidity in the affected areas.  The stress on trees from these conditions make them prone to bark beetle infestation as well, leading to extensive loss of trees from the pests, even more pronounced than from wildfires.  Similar patterns arise elsewhere in world as well. 

These events, as well as extremes of rainfall, major flooding, and diminished yields of important staple crops, are all attributable to global warming, and are predicted to become worse as more greenhouse gases accumulate leading to even greater warming of the atmosphere.  Humanity is faced with choices concerning global warming.  We may choose preventive investments in policies to minimize the worsening of global warming.  Otherwise we may choose simply to respond to the damages inflicted by extreme events, which would not contribute to putting in place measures to minimize warming.

Introduction.  Worldwide long-term average temperatures have been increasing since the industrial revolution began, and have been especially pronounced since the middle of the 20^th century.  This warming is associated with mankind’s ever-increasing use of fossil fuels for energy, resulting in emissions of greenhouse gases into the atmosphere.  Worldwide, the Intergovernmental Panel for Climate Change has assigned a high probability that greenhouse gas emissions are a major contributor to the warming of the planet.  Warming brings about many consequences detrimental to human wellbeing.

Wildfires in the Western U. S.  Wildfires in the western U. S. (West) have been widely covered in the press in recent years, giving an appearance that they have actually become more frequent, and more damaging, than in earlier decades.  According to A. L. Westerling and coworkers (Science 2006, Vol. 313, pp. 940-943; see Note 1), as of 2006 fighting wildfires in the West costs over US$1 billion per year; this does not count property damage or the value of the destroyed timber.

Generally, possible causes for increased wildfire activity include human activity and land use changes, uncharacterized climate changes (such as global warming), and the known long-term climate patterns, the Pacific Decadal Oscillation, the El Niño/La Niña cycle, and recent droughts in 2000 and 2002.  Land use has drawn water from natural sites for use by humans at remote locations, especially the rapidly growing cities in the region, promoting aridity in the natural settings.  While the El Niño/La Niña cycle is well known, Westerling and coworkers state there appears to be no correlation of the cycle with wildfires in the 20^th century.

Wildfire Activity Has Intensified in Recent Decades.  Westerling and coworkers examined wildfires greater than about 1000 acres on federal lands from 1970 to 2003.  They found that after the mid-1980’s the frequency of fires was almost four times greater than the average frequency from 1970 to 1986, an abrupt change in the pattern, and that the total area consumed was more than 6 ½ times greater.  Higher temperatures during spring and summer correlated highly with the frequency increase, and the season for reported fires also grew longer by more than 2 months.  In addition, years with high temperatures correlated with earlier melting of winter snows, so that both factors work to decrease available moisture, leading to drier combustible fuel, during the season of prevalent fires.  Years with early melting had five times as many wildfires as years with late snowmelt.  Summer temperatures during 1987-2003 were 0.87ºC (1.57ºF) warmer than for 1970-1986, and were the warmest since recordkeeping in the region began in 1895.  A similar trend is noted for Canada from 1920 to 1999 (N. P. Gillett et al., Geophys. Res. Lett., 2004, Vol. 31, L18211).  Although the authors did not seek any relationship of their historical analysis of past events with greenhouse gas-induced global warming, they do point out that projections of future warming due to increasing amounts of atmospheric greenhouse gases reinforce the recent trends of more and larger forest wildfires.

Effects of Warming and Aridity on Forest Growth in the American Southwest.  Williams and coworkers (Proc. Natl. Acad. Sci., 2010, Vol. 107 pp. 21289–21294) (see Note 2) assessed local forest growth year-by-year at 1,097 locations in the U. S. by recording tree ring intervals.  Wide ring intervals indicate favorable growth conditions, and narrow intervals reflect poor conditions.  These were compared to predictions of growth based on modeled variations in the climate during the 20^th century, including effects of temperature, precipitation and relative humidity.  The modeled predictions correlate with high statistical significance with the observed tree ring growth patterns, indicating that the model is very likely appropriate.  The forests of the American southwest are particularly sensitive to climatic variations.  As trees are stressed, not only are they more prone to ignition and burning, but they also become more susceptible to insect pests such as bark beetles.  The authors estimate that up to 18% of southwestern forests died or were lost from 1997 to 2008, about one-fourth to fires and the remainder to bark beetle infestations. They believe that these losses are due to the extreme conditions of aridity and high temperature that prevailed over this period.

The model was used to predict future forest growth in the period 2050-2099 compared to the period 1950-1999.  The results predict variously about 15% to as high as about 45% decrease in growth, dependent on tree species and the particular model used.  The authors discuss various options for further study, monitoring and remedial planting as these changes unfold.

Worldwide Wildfire Frequency from 850 to 2100.  Pechony and Shindell (Proc. Natl. Acad. Sci., 2010, Vol. 107 pp. 19167–19170) (see Note 2) used detailed climate models to create a worldwide pattern of wildfire frequency from 850 to 2003.  The results correlated well with geologic charcoal patterns which characterize wildfire occurrence in the past.  This agreement permitted modeling of future wildfire occurrence with confidence.

From 850 to the start of the industrial revolution at the beginning of the 19^th century fires appear to have been governed primarily by global patterns of precipitation rather than temperature; the latter was relatively constant throughout this interval.  Likewise, human population density was unchanged.  Starting with the industrial revolution wildfire frequency was more correlated with human activity.  Temperature increased dramatically over this period coupled with “unprecedented” increase in burning of fossil fuels; human population density has grown about 8-fold since 1800.  Among the causes of wildfires was clearing of forested land for agricultural use.  Starting about 1900 there was a decrease in fire activity due to human intervention and fire suppression.

Future trends were modeled using scenarios standardized by the Intergovernmental Panel on Climate Change.  In contrast to the early precipitation-modulated pattern, and the recent human intervention affecting wildfire frequency, future frequency reflects the exacerbated warming of the earth due to continued use of fossil fuels.  Thus in the future the climate will be a major factor driving wildfire frequency.  As with modeled patterns of changes in precipitation and aridity in various regions of the world, predicted patterns for the frequency of wildfires varies across the globe.  In the United States, the western region will have more wildfires, while the east will have fewer, according to the models.

Conclusions.  The reports chosen here are but a selection of many recent articles in scientific journals dealing with increased frequencies of wildfires in the U. S. and elsewhere in the world.  Climate models using recognized future scenarios for greenhouse gas emissions predict future climate changes that will likely make the West even warmer and more arid than at present.  These factors are highly likely to make the frequency and/or severity of wildfires in the region even worse than they are today, as we consider the remainder of the 21^st century.

Recent posts on this blog have described a number of other extreme climate-related events that correlate with global warming.  Two articles appearing in the journal Nature have unequivocally ascribed extreme rainfall across the Northern hemisphere up to 1999, and a single catastrophic rainfall-induced flood in England and Wales in 2000, respectively, to global warming.  These articles are even more significant because their findings relate to events that precede the first decade of the 21^st century, the hottest decade in recent history.  A Warmgloblog tutorial helps make plausible the relationship between global warming and the energetics of cloud formation and condensation of water vapor into rain.

In two other recent reports, decreased yields of the staple crops wheat and maize (corn), but not of rice and soybeans, was directly attributed to global warming in recent years; and the uptake of atmospheric carbon dioxide by growing green plants worldwide was found to be diminished in those regions experiencing droughts induced by global warming.

The various phenomena described here tie past extreme climatic events to global warming with very high to likely probabilities, depending on the authors of the different reports.  All the reports make clear correlations of future probabilities for extreme events with global warming by the very nature of the way in which the predictions were made: use of recognized climate models incorporating standardized scenarios for increases in greenhouse gas concentrations.

The adverse consequences of these various extreme events carry enormous human and societal burdens, as well as major economic costs.  Economic costs arise either from the need to recover from the events after they occur, or a perceived need to “buy insurance” before they might occur by taking steps that make adaptation easier.  Many in positions of responsibility protest against taking action to minimize the worsening of global warming because of the high financial and economic costs involved.  But it must be recognized that “there is no free lunch”.  We can choose to invest resources in a way to minimize global warming, bearing the costs while infusing our economies with the demand for new jobs.  Otherwise we can wait for extreme events to occur at times that are not predictable ahead of time, and then rush to recover and rebuild using emergency responses that do not address the basic problem.

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Note 1. Abstract is available for free online; full article is available by subscription or purchase online.  The journal is available in paper form in many public and university libraries.

Note 2. Full article is available for free online.

© 2011 Henry Auer