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This blog is expressly directed to readers who do not have strong training or backgrounds in science, with the intent of helping them grasp the underpinnings of this important issue. I'm going to present an ongoing series of posts that will develop various aspects of the science of global warming, its causes and possible methods for minimizing its advance and overcoming at least partially its detrimental effects.

Each post will begin with a capsule summary. It will then proceed with captioned sections to amplify and justify the statements and conclusions of the summary. I'll present images and tables where helpful to develop a point, since "a picture is worth a thousand words".

Monday, March 9, 2015

Why Care about Global Warming? Droughts, Poor Harvests, and Social Upheaval

In both industrialized countries and developing countries there may be those who ask “why should we care about global warming?”  After all, many people may not feel that they are affected by warming, and may pay scant attention to reports showing the effects of extreme weather and climate events.  Also, political leaders, especially in developing countries, may place higher priorities on economic development than on greenhouse gas emissions.

Global warming has proceeded ever more rapidly over the last century.  Warming occurs by virtue of the greenhouse effect due mostly to carbon dioxide and other greenhouse-active gases added to the atmosphere by human activity.  Many recent accounts have catalogued the progressive warming and the resulting harms caused by extreme weather and climate events. 

Here we focus on new reports from the technical literature on severe droughts in various parts of the globe and on the harmful effects that they produce.

Drought is characterized by a net loss of moisture from soil.  This involves assessing the difference between precipitation, which adds moisture, and the combined effects of evaporation from the soil and loss of moisture by plant transpiration to the atmosphere.  In drought conditions more moisture leaves the soil than enters it.  It is frequently accompanied by untypically low precipitation and/or untypically high surface air temperature.

Droughts have led to important economic damages in many regions of the world and a geopolitical crisis in Syria.  This post considers four current reports of drought and its socioeconomic and geopolitical consequences.  The droughts are directly or indirectly associated in the reports with global warming brought about by humanity’s burning of fossil fuels for energy.  The reports are summarized here.  More extensive information on each is given in the Details section at the end of this post.

  • Colin Kelley and coworkers report on the worst drought in recorded history in Syria and neighboring countries just prior to the “Arab Spring”.  The drought was serious enough that large numbers of farmers left their villages and migrated to Syria’s cities.  This caused major social and political turmoil and is considered to be a contributing factor to Syria’s civil war.  Human-derived greenhouse gases contributed to the drought.
  • Moore and Lobell analyze changes in crop yields and climate change across Europe.  Large scale decreases in yields were found in many localized regions, which correlated with increased temperatures and decreased precipitation over the 20 year period studied.
  • Diffenbaugh and coworkers examine the recent drought in California, likely the worst in 1000 years.  By simulating the region’s climate in model calculations the authors find that the extra amount of greenhouse gases added by human activity likely resulted in higher temperatures and reduced precipitation in the region.  This factor also contributes to a high risk of continued severe droughts.
  • Cook and coworkers assess drought conditions in the American Southwest and Central Plains.  Assuming that unrestrained emission of greenhouse gases will continue, the risk of severe droughts in these regions is projected to be extremely high, by various measures between about 69% and 97% in the second half of this century.

Why Should We Care About Global Warming?  Earlier predictions concerning the harms that global warming inflicts on regions of the earth are already coming true, causing damages that affect socioeconomic wellbeing and can lead to political upheaval.  Projections of future warming also include worsening harms, including in various regions extremes of heat and consequent droughts, or extremes of precipitation and consequent flooding, and secondary effects on human wellbeing and health.  All these effects impose significant monetary costs both in seeking to avoid them and in adapting to their impacts.

Droughts.  The journal articles summarized here focus our attention on the effects that droughts linked to global warming have already caused in various regions of the world.  Most singularly, the record drought in Syria and ensuing agricultural failures beginning in 2007-8 led to major internal migration of displaced farmers to cities.  This is considered to be an important factor contributing to that country’s social unrest after the Arab Spring.  The U. S. Department of Defense’s report “2014 Climate Change Adaptation Roadmap” includes “[i]nstability within and among other nations” as a potential effect of climate change having military implications.  Indeed, the U. S. is now conducting military operations tangential to the Syrian civil war.

Global warming had negative impacts on agricultural productivity in Europe between 1989 and 2009.  Lower harvest yields have the potential to drive up commodity prices affecting the cost of food.  OXFAM GB published its briefing report “Extreme Weather, Extreme Prices” in 2012.  The report models projected world commodity price increases to 2030 in response to projected global warming, showing both long-term increases for staples, and the higher increases foreseen in cases of climate “shock” involving short-term responses to extreme climate events.  For wheat, maize and rice, increases between 107% and 177% are predicted by 2030, of which one-third to one-half is attributed to global warming.

Man-made global warming is contributing to record droughts in California, and in the America Southwest and Central Plains.  California alone provides about 11% of America’s farm produce.  The New York Times reported on Feb. 27, 2015 that as the state enters its fourth year of drought the U. S. Bureau of Reclamation can provide only 15% of the water requested by farmers this year.  Only one-fifth the normal amount of water is available, due to very low winter snowpack accumulations.  The drought has already cost California  $2.2 billion in lost agricultural revenue, as well as thousands of jobs.  The Central Plains produce about 40% of the world’s corn, and 10% of its wheat.  Prolonged drought would materially impact crop yields in this region.  Climate Central states “[f]ood supplies could be disrupted and price shocks could reverberate through global markets” in response to the unprecedented droughts foreseen by Cook and coworkers when modeled by unconstrained future emissions of greenhouse gases.


The reports presented here catalog major, even unprecedented, occurrences of drought brought on by humanity’s history of greenhouse gas emissions over the past century.  They further project even more severe drought conditions when assuming continued unrestrained emissions. 

The extent of global warming and its consequences are determined by the total accumulated level of greenhouse gases in the atmosphere.  For carbon dioxide, this amount cannot be lowered by any currently available technology, so the world is already locked in to the extreme patterns we are currently experiencing.  In order to minimize further warming all nations of the world must come together to implement rapid, meaningful reductions in annual emission rates in order to keep the future accumulated greenhouse gas level as low as possible.
Drought in Syria preceded the unrest that became the civil war.  Colin Kelley and coworkers examined “[c]limate change in the Fertile Crescent and implications of the recent Syrian drought” (Proc. Natl Acad. Sci., published online before print March 2, 2015 ).  The Fertile Crescent (roughly encompassing Lebanon, Syria and Turkey, and Iraq along the Tigris and Euphrates rivers) has undergone its most severe drought since recordkeeping began.  The winter of 2007-8 was the driest in recorded history and marked the beginning of a three year drought period marked by the lowest rainfall, high to highest annual mean temperatures, and low to lowest values of a standard drought index used by climate scientists.  The drought caused serious agricultural and social dislocations in Syria, with loss of livestock and serious decreases in crop yields.  The authors found statistically significant decreases in long term winter rainfall (-13%) and measurements of groundwater content, and an increase in annual surface temperature compared to the global average, especially in the last 20 years.

These dire conditions led as many as 1.5 million people to leave the land and migrate to cities.  The migrants added to Syria’s social burdens, since its population has swelled from about 4 million in 1950 to about 22 million now.  The expanding population increases the demand for water under the drought, made worse by the effects of an earlier drought in the 1990s.  Syria also absorbed 1.2 to 1.5 million refugees from the war in Iraq just before the drought period.  These factors produced significant social unrest, just prior to the rise of the Arab Spring in other Arab countries.   

The authors conclude that these climate disruptions exceed expectations from only natural long term variability, stating “the long-term …trends and the recent increase in the occurrence of multiyear droughts and in surface temperature is consistent with … [man-made greenhouse warming]”.  This conclusion is reinforced by modeling of rainfall, which showed with statistical significance that “natural variability [combined with carbon dioxide greenhouse warming] is 2 to 3 times more likely to produce the most severe 3-year droughts than natural variability alone”.

What started as nonviolent protests in Syria against unemployment, corruption and major inequality degenerated into repression and the outbreak of the civil war which still goes on.  The authors make clear that while it is not possible to establish a direct link between the drought and these events, they believe it likely was a contributing factor.  The authors cite recent scientific publications “establishing a link between climate and conflict”.

Reduced agricultural harvests in Europe.  Frances Moore and David Lobell analyzed “[t]he fingerprint of climate trends on European crop yields” (Proc. Natl. Acad. Sci., vol. 112 no. 9, pp. 2670–2675, 2015).  The “fingerprint” refers to mapping of geographic localization of changes in yields of important staple crops, together with changes in climate (generally, temperature increases and reduced rainfall amounts) in the same regions.  These are compared with expected temperature and precipitation trends evaluated using climate models for the same regions.

The authors found that, averaged over the entire expanse of the European continent, crops yields between 1989 and 2009 fell for wheat (2.5%) and barley (3.8%), while yields of sugar beets and maize (corn) increased very slightly, compared to yields expected from models omitting the changes in climate.  The authors point out, however, that these continent-wide averages don’t represent the fingerprints well; in many regions crop yields fell (as represented by color-coded regions on the maps) by 20% to 40%, 40% to 60%, or in a few regions by more than 60%.  These results are especially pronounced for wheat and maize, and occur mostly in regions of Portugal, Spain, Italy, France and Greece.  The robust statistical methods employed in this work permit strong correlation of the changes in staple yields to the increase in temperature during the growing season over the 20 year interval studied, as well as to decreases in precipitation.

Long term drought in California.  Noah Diffenbaugh and coworkers have studied how “[man-made] warming has increased drought risk in California” (Proc. Natl. Acad. Sci., published online before print March 2, 2015 ).  They observe that California has been suffering a long term drought from December 2012 to September 2014.  It is characterized by record low precipitation and has been accompanied by extremely high temperatures, which may be the most severe set of conditions in a millennium.  The high temperature contributes to drought conditions by altering the seasonal timing of mountain snowpack melting and increasing evaporation and transpiration.

Using an objective quantitative drought index the authors conclude that statistically significant drought conditions occurred twice as frequently during the recent two decades than over the century preceding.  They find that warmer temperatures are an important factor in the recent twenty years in which droughts occur.  Simultaneous occurrence of lower than normal precipitation, significant changes in standardized drought indices and warm temperatures has been more frequent in the last two decades than in the preceding century.

The authors compared regional climate model simulations that omitted man-made greenhouse gas contributions to warming with simulations that added the greenhouse gases, beginning about 1885.  The models show with high statistical significance that since 1976 inclusion of man-made greenhouse gases produces a warming trend considered alone, as well as simultaneous occurrence of  decreased precipitation and a warming trend.  These simulations “suggest[] that human [activity causing atmospheric warming] has caused the observed increase in probability that moderately dry precipitation years are also warm”, leading to drought conditions.

The authors extended the model simulations to project behavior up to 2070, assuming no significant measures to reduce emissions of man-made greenhouse gases.  They find with high significance that the probabilities for occurrence of years that are both very warm and very dry are exceptionally high, approaching 100%. 

The authors conclude by pointing out that focusing on low precipitation without considering warming “misses a critical contributor to drought risk….[T]he risk of severe drought in California has already increased due to extremely warm conditions induced by [man-made] global warming”.

Severe droughts are projected for the United States in this century.  Benjamin Cook and coworkers foresee “[u]nprecedented 21st century drought risk in the American Southwest and Central Plains” (Sci. Adv. 1, e1400082 (2015); published electronically 12 February 2015 ).  The authors studied measurements of climate in the United States based on tree ring variations going back to the year 1000 CE.  This permits characterization of periods of drought throughout this interval.   They also performed regional projections for drought through the 21st century using an ensemble of climate models and the assumption that worldwide emissions of greenhouse gases will continue growing without significant constraints.  The models were calibrated by applying them to generate the drought characteristics for the historical record from 1931 to 1990.  Three characteristics were evaluated, a standardized index of drought, as well as evaluations of soil moisture content to depths of 30 cm (11.8 in) and 2 m (6.6 ft).  This calibration reproduced the historical tree ring record with a high level of statistical significance, giving confidence that the simulations would provide valid results when projected into the future.

The graphic below presents the authors’ results.
Historical tree ring record of climate from 1000 to the present (brown), smoothed with a 50-year window.  Modeled results for summer season moisture balance from 1850-2099 give the climate moisture index (red), and the 30 cm (pale green) and 2 m (dark green) simulations for soil moisture.  (Gray lines show excursion in the simulations from individual climate models.)
The results shown in the graphic are remarkable for the severity of the drought conditions projected through this century (although it should be noted that the projections have not been smoothed over a 50-year window as the historical record was).  Recalling that the models assumed no significant constraints on future greenhouse gas emissions, it is seen that both regions, the Southwest and the Central Plains, are forecast to have extreme and apparently unrelenting drought conditions after 2050, to extents not seen in the 1000 year historical record up to the present.  The drought in the Southwest is projected to be more severe than in the Central Plains. 
The risk of an 11-year drought in the 2050-2099 period is about 84 to 98% in the Central Plains, and about 92-97% in the Southwest.  The risk of a 35-year drought is about 69 to 88%, and 84-89% for the two regions, respectively.  The authors state that the severe drought conditions are “driven primarily by the [earth system’s energy] response to increased greenhouse gas concentrations …, rather than by any fundamental shift in ocean-atmosphere dynamics” (i.e., rather than by ongoing naturally occurring earth system processes).  The authors note that these projections are characterized by greater certainty than expressed in recent work by others.
In conclusion, the authors project “a remarkably drier future that falls far outside the contemporary experience of natural and human systems in Western North America, conditions that may present a substantial challenge to adaptation….[T]he loss of groundwater and higher temperatures will likely exacerbate the impacts of future droughts, presenting a major adaptation challenge….”
© 2015 Henry Auer

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