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