by Al Gore
By analogy, if you pull the drain in a bathtub filled with water, the water rushing down the drain does not come just from the part of the tub directly over the drain, it comes from the whole tub. In the same way, the great basins of water vapor in the sky are funneled to the “drains” opened above the land by rainstorms and snowstorms. When these basins are filled with much more water vapor than in the past, the downpours are more intense. The bigger downpours lead to bigger floods. The floods rush across the land, eroding the soil. And less of the water seeps down through the soil to recharge the underground aquifers.
Climate change is also driving desertification by altering atmospheric circulation patterns and drying out the land and vegetation. The same extra heat that evaporates more water vapor from the oceans also speeds up the evaporation of soil moisture—leading to longer, deeper, and more widespread droughts. Since the refilling of the atmospheric “basins” of moisture still takes a lot of time, many areas of the world are experiencing longer periods without rain in between the intense downpours. These longer periods of hotter temperatures in between precipitation events lead to more widespread and even deeper droughts. Once it is devoid of vegetation, the surface begins to absorb more heat. When the soil moisture is gone, the ground is baked, local temperatures rise higher still, and the topsoil becomes more vulnerable to wind erosion.
The parching and desiccation of the most highly productive agricultural breadbaskets of the world portend a food crisis in the future that could have humanitarian and political consequences too horrific to imagine. A top official with the International Maize and Wheat Improvement Center in Mexico, Marianne Bänziger, said, “There’s just such a tremendous disconnect, with people not understanding the highly dangerous situation we are in.”
The consequences for food production and water availability are already extremely harsh. In 2012, largely because of climate-related events that reduced crop yields, the world experienced a record one-month price increase for food, with additional record price hikes predicted for 2013. More than 65 percent of the U.S. suffered from drought conditions in 2012. In addition to the impacts on industrial agriculture in North America, Russia, Ukraine, Australia, and Argentina, subsistence agriculture has been hit hard in many tropical and subtropical countries by large alterations in the timing, duration, and magnitude of precipitation patterns due to global warming’s disruption of the hydrological cycle. As a rice farmer in northeastern India, Ram Khatri Yadav, told Justin Gillis of The New York Times, “It will not rain in the rainy season, but it will rain in the non-rainy season. The cold season is also shrinking.”
Along with the impacts discussed in Chapter 4—including the depletion of topsoil and groundwater and the competition that farmers face for land and water from fast-growing cities, industry, and biofuels production—the rising temperatures threaten many food crops with catastrophic yield reductions from heat stress alone. Stanford researcher David Lobell, who recently completed a study of the impact of temperature increases on crop yields with Columbia researcher Wolfram Schlenker, said recently, “I think there’s been an under-recognition of just how sensitive crops are to heat, and how fast heat exposure is increasing.”
In the last three years, new scientific research has overturned the long-held view by agricultural experts that, in the absence of drought, food crops would be relatively unharmed by rising temperatures. Many had thought that the higher CO2 levels might fertilize plant growth by enough to counterbalance any yield decreases due to heat stress. But unfortunately, intensive research designed to confirm that hypothesis now shows that food crop yields are likely to decline much more rapidly with higher temperatures than previously believed, and that the CO2 fertilization effect is much smaller than predicted. Moreover, weeds appear to benefit from extra CO2 much more than food crops.
As temperatures continue to increase, corn (maize)—the most widely grown crop in the world—appears to be the most vulnerable to heat stress. Corn yields start to decrease at a range of temperatures the Earth is already experiencing regularly in summer months. Every day during the growing season (roughly from the beginning of March to the end of August) that temperatures climb above a threshold of 84 degrees F (29 degrees C), corn yields drop by 0.7 percent.
As temperatures grow hotter than 84 degrees F, the yield declines plummet further with every degree added. If temperatures in the United States are allowed to rise as much as is now projected as a result of global warming, by the end of this century corn yields could fall by as much as a third from heat stress alone, with the impact of worsening droughts and the disruption of precipitation patterns taking a larger toll still. Soybeans have a higher threshold for heat stress than corn (86 F/30 degrees C), but the same accelerated drops in yields begin when temperatures reach and exceed that level.
The warm season is longer; spring is arriving about a week earlier (and fall about a week later) in both the northern and southern hemispheres. Moreover, the decreasing size of mountain snowpacks and glaciers is adding to the worsening shortages of water for agriculture in several important regions, bringing bigger spring floods earlier in the year and depriving these regions of water during the hot summer months when it is most needed. And while the focus is normally on daytime high temperatures, nighttime temperatures are at least as important. Both the computer models and consistent observations confirm that global warming increases nighttime temperatures more than daytime temperatures.
According to some studies, each degree increase in nighttime temperatures corresponds with a linear decrease in wheat yields. A large global review of the impact of climate change on crop yields between 1980 and 2010 showed that worldwide wheat production fell due to climate-related factors by 5.5 percent. A researcher at the International Rice Institute in the Philippines, Shaobing Peng, published findings in the Proceedings of the National Academy of Sciences showing that yields of rice declined by 10 percent with each one degree Celsius increase in nighttime temperatures during the dry part of the growing season, even though there were no significant drops in yield associated with increasing maximum temperatures during the daytime.
Crop diseases and pests are also increasing with global warming. Higher temperatures are leading to a dramatic expansion in the range of insects harmful to food crops, sending them farther north in the northern hemisphere and farther south in the southern hemisphere, and into higher altitudes. A team of crop scientists publishing in Environmental Research Letters wrote, “These range expansions could have substantial economic impacts through increased seed and insecticide costs, decreased yields, and the downstream effects of changes in crop yield variability.”
Other scientists have determined that higher levels of CO2 also stimulate insect populations. Evan DeLucia, a plant biologist working with a team of entomologists at the University of Illinois, tested the impact of higher carbon dioxide levels on soybeans and found that aphids and Japanese beetles flocked to the soybeans grown in higher CO2 environments, ate more of the plants, lived longer, and produced more eggs. “That means crop losses may go up in the future,” DeLucia said.
Other scientists on DeLucia’s team found that higher carbon dioxide levels caused soybeans to deactivate genes that are crucial to the production of chemicals that help to defend them against insects by blocking enzymes in the stomachs of beetles that digest soybean plants, and by deactivating other genes used by soybeans to lure the natural enemies of the beetles. As a result, according to team member Clare Casteel, the soybeans grown in higher levels of CO2 “appear to be helpless against herbivores.”
Higher temperatures are having the same effect in boosting pest populations in most areas of the world. One of the leaders of an Asian international agricultural research group, Pramod K. Agrawal, said, “Warmer conditions and longer dry seasons linked to climate change could prove to be the perfect catalyst for outbreaks of pests and diseases. They are already formidable enemies affecting food crops.” A team of Indian scientists noted that, because insec
ts are cold-blooded, “Temperature is probably the single most important environmental factor influencing insect behavior, distribution, development, survival, and reproduction.… It has been estimated that with a 2 degree C temperature increase, insects might experience one to five additional lifecycles per season.”
Scientists at the International Center for Tropical Agriculture, for example, found that the cassava crop in Southeast Asia—worth an estimated $1.5 billion each year—is seriously threatened by pests and plant diseases that expand with warmer temperatures. According to cassava entomologist Tony Bellotti, “The cassava pest situation in Asia is pretty serious as it is. But according to our studies, rising temperatures could make things a whole lot worse.” Bellotti adds, “One outbreak of an invasive species is bad enough, but our results show that climate change could trigger multiple, combined outbreaks across Southeast Asia, Southern China and the cassava-growing areas of Southern India.”
Microbes that cause human diseases—and the species that carry them—are also expanding their range. In the highly populated temperate zones of the world, the prevailing climate conditions in which civilization developed were unfavorable to the survival of many disease-causing organisms. But now that warmer climate bands are moving poleward, some of these pathogens are moving with them.
According to a study in Science by Princeton University researcher Andrew Dobson and others, global warming is causing the spread of bacteria, viruses, and fungi that cause human diseases into areas that were formerly hostile to them. “Climate change is disrupting natural ecosystems in a way that is making life better for infectious diseases,” said Dobson. “The accumulation of evidence has us extremely worried.” Another coauthor of the study, Richard S. Ostfeld, said, “We’re alarmed because in reviewing the research on a variety of different organisms, we are seeing strikingly similar patterns of increases in disease spread or incidence with climate warming.”
Although the prevalence of international travel has increased dramatically and some disease-carrying insects have been unwittingly transported from the mid-latitudes to other regions, the shifting climatic conditions are contributing to the spread of diseases like dengue fever, West Nile virus, and others. The Union of Concerned Scientists wrote that, “Climate change affects the occurrence and spread of disease by impacting the population size and range of hosts and pathogens, the length of the transmission season, and the timing and intensity of outbreaks.”
They also noted, “Extreme weather events such as heavy rainfall or droughts often trigger disease outbreaks, especially in poorer regions where treatment and prevention measures may be inadequate. Mosquitoes in particular are highly sensitive to temperature.” Improvements in public health systems are crucial to control the spread of these migrating diseases, but many lower-income countries are pressed to find the resources needed for hiring and training more doctors, nurses, and epidemiologists. They also warned that in many of the areas to which these pathogens and their hosts spread with warmer temperatures, “The affected populations will have little or no immunity, so that epidemics could be characterized by high levels of sickness and death.”
In the summer of 2012, the United States experienced the worst outbreak of West Nile virus since it first arrived on the Eastern Shore of Maryland in 1999 and spread rapidly to all fifty states in only four years, during a period of unusually warm temperatures. Dallas, Texas, was the first to declare a public health emergency and began aerial spraying of the city for the first time since 1966. As concern peaked, public safety officials issued an appeal for people to stop calling 911 when they were bitten by mosquitoes. The disease eventually spread by the end of 2012 to forty-eight of the fifty states, killing at least 234 people.
The late Paul Epstein, a professor at Harvard Medical School and a close friend, wrote in 2001 about the relationship between West Nile virus and the climate crisis. More recently, he said, “We have good evidence that the conditions that amplify the lifecycle of the disease are mild winters coupled with prolonged droughts and heat waves—the long-term extreme weather phenomena associated with climate change.”
According to Christie Wilcox with Scientific American:
They have been predicting the effects of climate change on West Nile for over a decade. If they’re right, the US is only headed for worse epidemics.… Studies have found that mosquitos pick up the virus more readily in higher temperatures. Higher temperatures also increase the likelihood of transmission, so the hotter it is outside, the more likely a mosquito that bites an infected bird will carry the virus and the more likely it will pass it along to an unwitting human host. In the United States, epicenters of transmission have been linked closely to above-average summer temperatures. In particular, the strain of West Nile in the US spreads better during heat waves, and the spread of West Nile westward was correlated with unseasonable warmth. High temperatures are also to blame for the virus jumping from one species of mosquito to a much more urban-loving one, leading to outbreaks across the US.… Record-breaking incidences of West Nile are strongly linked to global climate patterns and the direct effects of carbon dioxide emissions.
In 2010, the world experienced the hottest year since records have been kept, and ended the hottest decade ever measured. Last year, 2012, broke even more high temperature records. October 2012 was the 332nd month in a row when global temperatures were above the twentieth-century average. The worst drought since the Dust Bowl of the 1930s ravaged crops and dried up water supplies in many communities. Many farmers have already been forced to adjust to the drying of soil. The lack of water has caused a buildup of toxins in corn and other crops unable to process nitrogen fertilizer.
WORLD FEVER
In order to pinpoint the difference between global warming and natural variability, Dr. James Hansen, the single most influential climate expert in the scientific community, produced with two of his colleagues, Makiko Sato and Reto Ruedy, a groundbreaking statistical analysis of extreme temperatures all over the world from the years 1951 through 2010 that compared the more normal baseline period of 1951 through 1980 to more recent decades, 1981 through 2010, and especially the last several years when the impacts of global warming have been more prominently manifested, 1981 through 2010.
By breaking down the surface temperatures of almost the entire world into blocks of 150 square miles each, Hansen was able to calculate the frequency of extremely high temperatures (and all other temperatures) during the last sixty years. The results—which do not rely on climate models, climate science, or any theories of causation—demonstrate clearly that there has been up to a 100-fold increase in extreme high temperatures in recent years compared to earlier decades. The statistical analysis shows that in the last several years, extreme temperatures have been occurring regularly on approximately 10 percent of the Earth’s surface, while during the earlier decades such events occurred on only 0.1 to 0.2 percent of the Earth’s surface.
Hansen’s chosen metaphor to explain the difference consists of two dice, each with the requisite six sides. The first die, which shows the range of temperatures over the years between 1951 and 1980, has two sides representing “normal” seasons, two other sides representing “warmer than normal” seasons, and the final two sides representing “cooler than normal” seasons. That used to be the “normal” distribution of temperatures. The second die, however, showing the range of temperatures in more recent years, has only one side representing a normal season and only one side representing a cooler than normal season, but three sides representing warmer than normal seasons and the remaining side now representing extremely hot seasons—seasons that are way outside the boundary of the statistical range that used to prevail.
In the language of statisticians, a standard deviation quantifies how far the range, or spread, of a particular set of phenomena differs from the average spread. Extreme—in this case, either unusually hot or unusually cold—seasons naturally occur far less frequently than average or near-average seasons. Because se
asons with extreme temperatures used to be so much less frequent, they nevertheless often surprised us, even though they fell within the normally expected range. Seasons that are three standard deviations from the average were exceedingly rare, but still did occur from time to time as part of the normal range.
The average temperature is warmer overall even though extremely cold events still continue to occur, though rarely. In other words, the entire distribution of temperatures has moved to much warmer values, and the bell curve of distribution has widened and flattened slightly, so that there is much more temperature variability than used to occur. But the most significant finding is that the frequency of extremely hot temperatures has gone up dramatically.
Hansen infers that the cause is global warming—and indeed, these results turn out to be perfectly consistent with what global warming science has long predicted. (In voluminous other studies, Hansen and climate scientists around the world have proven causality to a degree judged “unequivocal” and “indisputable” by virtually all of the world’s scientific community.) But the results themselves are based on observations of real temperatures in the real world. They cannot be argued with, and the implications are powerfully clear.
As the old saying has it in Tennessee, if you see a turtle on top of a fence post, it is highly likely that it didn’t get there by itself.* And now we are seeing turtles on every tenth fence post in every field in the world. They didn’t get there on their own. It is now abundantly obvious that all the extreme temperatures and the extreme weather events associated with them are like turtles on a fence post. They didn’t happen without human interference in the climate.
In 2012, new World Bank president Jim Yong Kim released a study showing temperatures will likely rise by 4 degrees C (7.2 degrees F) without bolder steps to reduce CO2, and that there is “no certainty that adaptation to a 4 degree world is possible.” Gerald Meehl, of the National Center for Atmospheric Research, uses a different metaphor to explain what is happening: if a baseball player who takes steroids hits a home run, it’s possible that he might have hit the home run even without the steroids. But the fact that he took the illegal performance-enhancing drug makes it much more likely that he will hit a home run in his next at bat. Within Meehl’s metaphor, the 90 million tons of global warming pollution that we are putting into the atmosphere every twenty-four hours are like steroids for the climate. An innovative 2012 study of the previous decade’s climate predictions showed that the “worst case” future projections are the ones most likely to occur.
