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

Decreased Worldwide Crop Yields Are Tied to Global Warming

[Note: This post is revised from the original version posted May 10, 2011 by the addition of the section on the article by Zhao and Running.]

Summary.  Climate scientists model global climate trends showing varying regions of increased temperature in some regions, and increased rainfall or aridity in differing regions of the world.  Recently rigorous statistical analyses have shown that increased global temperatures are responsible for extreme rainfall and flooding events.  In May 2011 Lobell and coworkers showed that increased average global temperatures over the period 1980-2008 are responsible for decreased crop yields of the staple crops maize and wheat.  The resulting shortages lead to significant worldwide modeled increases in prices for these commodities.  This results in increased hardship for those populations of the world living at or near poverty.  This economic argument provides a strong incentive to develop new economic activity by investing in renewable, carbon-free sources of energy.

Introduction.  The global average temperature, measured over the face of the earth over year-long intervals, has been increasing since the beginning of the industrial revolution.  The trend has become more pronounced in recent decades, and is forecast to continue increasing even more strongly in the future in the absence of action to reverse the trend.  This increase in temperature correlates with, and is due to, the increased burning of fossil fuels for energy by the nations of the world.  Fossil fuels, coal, oil and natural gas, are all carbon-based and emit carbon dioxide (CO₂), a greenhouse gas, into the earth’s atmosphere in direct proportion to the amount of fuel that is burned (although the different fuels yield differing amounts of CO₂ on combustion). 

Global Warming and Extreme Weather. Climate scientists, on the basis of elaborate computational models of long-term climate trends covering the entire planet, predict varying degrees of temperature increases, and changes in rainfall and snowfall patterns, for different regions of the globe.  Some regions may become warmer and more arid, while others may experience more rainfall.  The phenomena are likely to produce extreme weather events, whose consequences can be very costly to the populations subjected to their damaging effects.  For this reason global warming has a strong potential for causing severe economic distress in ways that nations will not have prepared for.

Until recently we have only treated news reports of extreme weather events as anecdotes, without necessarily saying they might have been due to, or made worse by, global warming.  Examples that come to mind include drought and increased forest fire activity in the American west, increased incidence of strong hurricanes in the Caribbean region, the devastating monsoon flooding in Pakistan in the summer of 2010, and droughts in Russia and Siberia in recent summers leading to pronounced reductions in wheat harvests across the region.

Extreme Weather Is Directly Caused by Global Warming.  Recently climate scientists have conducted rigorous statistical analyses specifically to determine whether extreme events, whose probabilities of occurrence are necessarily very small, are correlated with global warming.  Two peer-reviewed reports that were published in the authoritative journal Nature in February 2011 were able to draw exactly such conclusions.  In one report (see Note 1), a decades-long study of temperature and precipitation patterns across the entire Northern hemisphere, from 1951 to 1999, concluded that long-term patterns of increased precipitation were statistically linked with the increase in global temperature caused by human activity. In the second report (see Note 1), a particular devastating flood in a region of England in October-November of 2000 likewise could be directly attributed to man-made global warming determined over the period 1957-1999 preceding the flood.  In a tutorial fashion, a post on this blog seeks to make understandable how changes in atmospheric temperature can contribute to changes in precipitation patterns, while acknowledging that other factors also enter into an understanding of these changed patterns.

In this post we present a new report concerning the role of global warming in reduced crop yields affecting the harvest of important staple foods around the world.

Global Warming Reduces Harvest Yields of Staple Crops.  The important journal Science published a peer-reviewed report by Lobell, Schlenker and Costa-Roberts online on May 5, 2011 (10.1126/science.1204531; see Note 2), in which the authors examined whether any correlation is found between temperature, rainfall amount and crop yields for the four staples maize (corn), wheat , soybeans and rice.  They analyzed temperature and rainfall patterns during the growing seasons locally at all places on Earth for which records of raising any of these crops exists, over the period 1980-2008, using data from 1960-2000 as a reference (see Note 3).  Models were created for this analysis that characterized the relationship between the weather-related variables and crop yields.  The authors found that, for all regions yielding these crops, except in the United States, the time-averaged global yields of maize and wheat declined, by 3.8% and 5.5%, respectively, compared to models that did not incorporate the time trends observed for temperature and rainfall.  The results for soybeans and rice had regions with increased yields and regions with decreased yields, which largely compensated one another over the globe, resulting in no clear trend.

Over the period 1980-2008 the measured concentration of CO₂ in the atmosphere increased from 339 to 386 parts per million (ppm; volumes of CO₂ per 1,000,000 volumes of air).  Experiments by other researchers suggest that this increase in CO₂ could have produced a “CO₂ fertilizer effect”, increasing yields by about 3% over the measured time period, because plants use the extra CO₂ in the air to grow faster.  The results show that, if a positive CO₂ fertilizer effect occurred, it was exceeded by the negative effect of the warming of the planet.  The authors also point out that the decreased yields prevailed over any improvements in agricultural technology that may have been implemented over the 28 year period examined.

The importance of global warming includes the adverse effects it inflicts on humanity, especially on economic activity.  The authors have estimated the impact of the decreased yields of maize and wheat, using worldwide economic models drawn from the published literature.  They estimate that average commodity prices would increase by about 19% if the CO₂ fertilizer effect is not taken into consideration, and by about 6% if it is.

Details.  The surface of the earth was divided into grids having 5 deg of latitude and longitude on a side.  At the equator this corresponds to a grid plot about 5.8 miles (9.3 km) on a side.  For each of the four crops a grid location was included if the grid produced more than a minimum amount of the crop in question.  Temperature and rainfall records for the growing season in each grid were analyzed.  It was found that little difference existed for rainfall records between 1960-1980 and 1980-2008 for any of the crops, but that temperature increased significantly for all the crops between these two time periods.  On a projection of the earth, the temperature increased significantly in most regions depicted, but rainfall across the globe was increased modestly in some regions and decreased modestly in others.

The Supporting Online Material (Note 2) explains the mathematical modeling used in this report.

Crop trends are depicted for major country producers for each of the four staple crops.  For rice the major producers showed slight decreases but these were exceeded by 5-95% confidence limits, so that no trend was considered to be significant.  For soybeans the yields for the major producers were mostly negative, by 5-8%, but again the confidence limits were large and removed the results from significance.   The results for maize showed large, significant, decreases of about 3% to about 8% for China, Brazil and France, with narrow confidence limits.   For wheat, China, India, and France had decreased yields in the 2% to 6% range, and for Russia the decrease was about 14%, all with narrow confidence limits suggesting significance.  For maize, wheat and soybeans, the U. S. is a major producer but showed minimal change in crop yields for each. 

Global Warming Reduces Total Worldwide Use of Atmospheric Carbon Dioxide in Green Plants.  In a publication by Zhao and Running (Science Vol 329, pp. 940-943, 2010; see Note 1) the total amount of CO₂ taken up by green plants, and converted into vegetable matter, was tracked globally by area grids from 2000 to 2009.  Over the decade, year-by-year, the changes in total CO₂ taken up, while fluctuating, declined by about 1% of the total amount.  The pattern of year-by-year changes tracks remarkably precisely with the changes in atmospheric concentrations of CO₂.  On a global projection map coded by the amount of change in total CO₂ absorbed, most of the decrease, and the most dramatic decreases, occur in the Southern Hemisphere.  Comparing the Northern Hemisphere with the Southern Hemisphere year-by-year, the changes in total CO₂ taken up and a standardized measure of the severity of droughts track each other closely in the Southern Hemisphere; the Northern Hemisphere shows less striking variations.  Significantly, in addition, the global total CO₂ taken up decreases as the global temperature increases. 

This article is generally consistent with the results on crop yields described by Lobell and coworkers, above.

The authors conclude that the decrease in global total CO₂ absorbed “potentially threatens global food security and future biofuel production and weakens” the ability of vegetated land areas to absorb additional CO₂ that arises from burning fossil fuels.

Conclusion.  Contrary to earlier unsubstantiated surmises that global warming may possibly be a cause for weather extremes, the recent published scientific reports described here and in earlier posts establish with statistical rigor that this in fact is occurring.  Extreme rainfall and flooding cause major economic harms, both in human effort involved in emergency response, and in recovery and reconstruction efforts.  And in those cases mentioned anecdotally in the introduction, even if statistical causation has not been established, one can reasonably attribute at least a fraction of the economic impacts, say 20-40% for sake of discussion, to the extreme character of the event brought on by global warming.

Regardless of one’s attitudes or beliefs concerning global warming due to humanity’s burning of carbon-containing fossil fuels, the adverse economic consequences of extreme weather events provide a strong incentive to undertake remedial actions as soon as we can.  It’s better to invest in alternative energy sources in order to decarbonize our energy economy than it is to have to spend emergency relief funds on about the same scale when an extreme event occurs.  Investments in renewable energy sources and their operation provide new job opportunities that promote a vibrant economy.  To the extent that such measures would contribute to lessening global warming and its adverse consequences, human misery around the world would be considerably diminished.

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Note 1. Abstract available online free, or the full article for a fee or through personal or institutional subscription.  Many public libraries, and university libraries open to the public, receive the journal.

Note 2.  Supporting Online Material, referred to in the text of the article, is available at 

Note 3.  A simple tutorial of the scientific method used in climate science is available here.

© 2011 Henry Auer