Role of Global Warming in Present and Future Hurricanes

Summary.  The day that Hurricane Harvey made landfall in Texas the water temperature in the Gulf of Mexico was about 3-7°F higher than the average for 1961-1990.  This is important, because warmer water releases more moisture into the air than cooler water, feeding heavier rainfall.  This contributed to the extreme, unprecedented flooding in the Houston area caused by Harvey.  More moisture also leads to stronger winds in storms.

Climate models project that if humanity continues to burn fossil fuels without restraint the added carbon dioxide produced will lead to sharply higher global average temperatures.  These will produce more frequent and intense extreme weather and climate events, which bring serious socioeconomic harms to society.  One model study of storm activity along the Texas coast finds that the probability of an event will triple, from 6% per year to 18% per year, by the end of this century if emissions continue unabated.

Climate scientists have been warning of major climate consequences from man-made greenhouse gas emissions for almost three decades.  Those predictions have not changed, indeed have only improved, as scientific capabilities grew.  If humanity had responded earlier, the costs of action would have been lower or spread over longer times.  In the absence of past action at the scale needed, now is the time to act.
 

Introduction.  Climate scientists understand that the long-term global average temperature will continue to increase largely in response to the increased concentration of carbon dioxide (CO₂) and other greenhouse gases (GHGs) in the atmosphere.  CO₂ is increasing since it is the combustion product of humanity’s burning of fossil fuels (i.e., fuels based on carbon: coal, petroleum products and natural gas).  Other GHGs likewise arise from human activity.  CO₂ is especially significant since, once emitted into the air, it resides there for centuries; it continues accumulating without balancing effects that remove it from the atmosphere (after about one-third of it dissolves into the ocean).

The greenhouse effect originating from these excess GHGs raises the global average temperature.  The temperature will remain elevated in coming centuries as the excess GHGs continue residing in the atmosphere. 

One effect of higher temperatures at the surface of lakes and oceans is that more water evaporates as the water temperature rises, by about 4% per degree F (about 7% per degree C).  In addition, evaporation of water vapor requires the input of heat; as a result the surrounding air momentarily cools off.  Conversely, as water vapor condenses, such as in cloud and raindrop formation, heat is released, warming the surrounding air momentarily.  These temperature changes lead to local winds. 

Storms such as hurricanes sweep over ocean water and entrap large amounts of water vapor.  When the vapor condenses the liquid falls to the ground as rain.  As this activity intensifies strong winds result.  Climate scientists foresee that as the earth warms, storms such as hurricanes will potentially carry more water vapor and generate stronger winds than in earlier decades.

Continued GHG emissions will lead to a higher incidence of extreme hurricanes.

The United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC) pubished its Fifth Assessment Report, Part 1, in 2013.  It includes climate model projections of the relationship between the excess CO₂ accumulated in the atmosphere from human activity and the predicted increase in the global average temperature resulting from the added CO₂. 

The models were run by assuming four CO₂ emission scenarios up to the year 2100: the most stringent ends GHG emissions beyond 2050, while the least stringent continues current use and unabated future growth in use of fossil fuels.  The results are shown in the graphic below, showing the dependence of the global average temperature on the atmospheric CO₂ level, including the historical record of global average temperature from 1880 to 2010 in the lower left of the image.

 

Historical record of global annual temperature increase above the average for 1861-1880 (vertical axis) as a function of historical atmospheric accumulated weight of excess carbon dioxide, due to human use of fossil fuels, above the level in 1870 (black; lower left).  The circles mark decades from 1870 to 2100.  Future model projections of the same temperature-carbon dioxide dependencies are shown from 2010 to 2100, based on four scenarios describing the stringency of policy used to limit future emissions (dark blue, most stringent; light blue, next less stringent; orange, weak limits on emissions; red, continued emissions from unabated use of fossil fuels).
Source: Intergovernmental Panel on Climate Change, Fifth Assessment Report, Working Group 1, Summary for Policymakers. http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf .

The historical data show that in 2010 the global average temperature was about 0.9°C (1.6°F) higher than in 1870.  The modeling shows that the most stringent scenario (dark blue) projects a temperature increase above the 1870 level of about 1.8°C (3.2°F) by 2050-2100.  On the other hand, the scenario based on unconstrained continued use of carbon-containing fuels (red) foresees that the global average temperature in 2100 will be about 4.7°C (8.5°F) above the 1870 temperature.  Such a drastic increase in global temperature will lead to periods of time, and/or regions of the earth’s surface, experiencing one or more of fierce heat waves; extreme storms that may be more frequent, or have more intense rainfall and winds; droughts; wildfires; and pronounced increases in sea levels.

Hurricane Harvey pummeled Houston and neighboring regions with torrential rainfall in August 2017.  While the hurricane would likely have happened anyway, rainfall was more intense because the water of the Gulf of Mexico was warmer than in the past.  This is seen in heat map for the Gulf, shown below for the day that Harvey made landfall.

               Source: http://www.climatesignals.org/node/7158 (accessed late August 2017).

 

(The legend under the heat map is the one appearing on the web site from which the map was copied.)  The map makes clear that the excess heat in the Gulf of Mexico abutting the Texas coast, shown by the color code bar at the right, was a factor in the extreme rainfall and flooding generated by the storm. As explained in the Introduction, warmer water leads to more moisture evaporating into the storm.   A second factor was that the hurricane lingered over the Houston area for several days.  The total rainfall from the storm at Cedar Bayou was 51.88 in (1318 mm), perhaps the highest in the region.

The likelihood of a return event of a hurricane like Harvey increases 3-fold in the unrestrained emission scenario described above for the first graphic.  K. Emanuel published an analysis analysis of hurricane rainfall properties by modeling previous storms impacting Texas.  This was carried out historically for the period 1980-2016, and with the unrestrained scenario for 2081-2100.  He developed results assuming a storm with 500 mm (19.7 in) of rainfall, much less than the local maximum cited above for Cedar Bayou.  The likelihood of such precipitation is evaluated at 6% per year for 2017, and increases three-fold to 18% per year by the final decades of this century if fossil fuel use remains unconstrained. Emanuel also found that for the period 1981-2000 the historical likelihood is modeled as 1% per year.  Thus, his modeling shows that global warming has already increased hurricane/storm likelihoods in recent decades by a factor of six, and for the century-long interval from the end of the 20^th century to the end of 21^st century by 18-fold. 

Conclusion

Climate scientists have been warning for almost three decades of the hazards arising from increased levels of greenhouse gases in the atmosphere.  The increase in atmospheric CO₂ from 1958 to the present is shown below.

Atmospheric concentration of CO₂ in parts of CO₂ per million parts of air; ppm).

Source: www.CO2.earth.

 

From the first IPCC Assessment Report in 1990 to the fifth in 2013-2014, the forecasts of future climate trends and harmful events have not changed.  It was already understood in1990 that manmade global warming imperiled our society’s wellbeing.  What has changed is first, an increase in the CO₂ level from about 355 ppm in 1990 to 402 ppm in August 2016 (see the graphic just above); and second, increased certainty in the predictions of the effects of higher levels of greenhouse gases on the earth’s climate, arising from dramatic increases in data available, sophistication of climate models employed, and the computational power of modern supercomputers.

The flooding from the hurricanes that struck the Caribbean and southeastern U. S. in the summer of 2017 is just one example of the types of extreme events that climate scientists have foreseen over the past decades.  Others include heat waves, droughts, forest wildfires and sea level rise.  In the earlier decades they were only predictions, dismissed by many.  But by today the warnings have come to pass; extreme events will increase in occurrence and severity as warming worsens.

The harms and damages inflicted by extreme events have major economic and societal consequences.  The need arises to reconstruct damaged homes and facilities and to undertake projects that increase resiliency in the face of future climate threats.  These costs ultimately fall on the population at large, for example from increased insurance premiums and higher taxes.  Had earlier action been undertaken it is likely that such societal costs would have been lower, or at least spread out over longer time frames. 

As of today, however, much of the response is on an emergency basis, i.e. as the response to unforeseen disasters.  The U.S. in particular, as well as the world at large, should accept the reality of the climate change threat.  We must make the investments now that are needed to minimize further greenhouse gas emissions and to adapt to the threats already with us.



© 2017 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