This is a re-post from Yale Climate Connections by Jeff Masters
Crops don’t like drought, extreme heat, extreme cold, flooding, and air pollution. While reducing the ill effects of extreme cold on agriculture in recent decades, the warming climate is increasing impacts of drought, extreme heat, and air pollution. These increased impacts are greatly concerning as the world envisions feeding an additional two billion people by 2050. Of particular concern are changes in the atmospheric circulation – which may have both natural and human-caused components – that have led to an increase in concurrent heat waves and droughts, such as occurred in the summer of 2022.
A 2019 report by the Global Commission on Adaptation indicates that without adaptation, climate change may depress global agriculture yields by 5 to 30% by 2050, at the same time that an expanding population and increased meat consumption causes a 50% increase in global food demand. This first part of a two-part series examines observations of how climate change has already affected crops. Part two is, The future of agriculture: Increased drought and heat from climate change pose huge challenges.
An observed concerning increase in drought
Drought is the great enemy of human civilization, depriving people of the two essentials of life – food and water. When the rains stop and the soil dries up, cities can die and civilizations collapse, as people abandon lands no longer able to sustain them. Drought has been identified as the primary or significant contributing factor in the collapse of a surprising number of great civilizations in the past. So no reasons for complacency about threats drought poses to modern civilization: particularly since a hotter planet is producing longer-lasting and more intense droughts, and “stuck” jet stream patterns producing intense droughts globally, as reported below, are on the increase.
Figure 1. Deadliest disasters since 1970, from the international global disaster database, EM-DAT. (Image credit: WMO)
Figure 2. The number of droughts globally since 1970 has been increasing, according to the international disaster database, EM-DAT. Its database includes all disasters that kill at least 10 people, and/or affect 100 or more people, and/or result in a call for international assistance or declaration of a state of emergency. (Image credit: United Nations, 2022, Global Assessment Report on Disaster Risk Reduction – Our World at Risk: Transforming Governance for a Resilient Future)
Figure 3. Global economic losses from drought from 1975-2021 (in 2021 USD). (Image credit: Aon 2021 annual report)
According to the American Meteorological Society’s State of the Climate in 2021 report, a sharp increase in global drought area began in mid-2019, reaching a peak in August 2021, with 32% of global land area experiencing moderate or worse drought, and declining slightly thereafter. This peak beat the previous record of 31.6% of the global land area in drought, set in November 2002. Global drought coverage has likely remained at near-record levels in 2022, though the statistics will not be updated until the year is complete, said drought researcher Jonathan Barichivich in an email.
Authors of a May 2022 United Nations drought report found that the number and duration of droughts have risen 29% globally since 2000. They added that, while droughts account for only 15% of natural disasters, they have taken the largest human toll (approximately 650,000 deaths over 1970-2019 and 10 million over the past century).
Four of the top 10 deadliest disasters since 1970 have been droughts (Figure 1). Droughts have deep, widespread, and underestimated impacts on societies, ecosystems, and economies, with only a portion of the actual losses accounted for. According to Aon, a British multinational financial services firm, drought has cost the global economy $1.1 trillion since 1975, and the costliest decade for drought has been the decades of the 2010s (Figure 3). So far in 2022, Aon has tabulated five multi-billion-dollar droughts worldwide (in the U.S., China, Italy, Brazil, and Horn of Africa). Desertification, which threatens one-fourth of Earth’s land area and a fifth of the population, is estimated to cost developing nations 4-8% of their GDP, according to a 2011 United Nations report, and drought has reduced India’s gross domestic product by 2-5%.
Extreme heat … bad news for crops
While an increase in drought generally is bad news for crops, heat impacts are more complicated. Crops have an optimal temperature for performance, and hotter temperatures typically result in a steep decline in yields. Yields increase with temperature up to 29 degrees Celsius (84°F) for corn, 30 degrees Celsius for soybeans, and 32 degrees Celsius for cotton, but temperatures above these thresholds cause steep declines in yields. For every degree Celsius increase in global mean temperature, yields are projected to decrease, on average, by 7.4% for corn, 6.0% for wheat, 3.2% for rice, and 3.1% for soybeans (these percentages are independent of other factors, such as an increase in drought causing additional losses, or an increase in CO2 benefiting some plant growth).
Authors of a 2021 study led by Ariel Ortiz-Bobea, Anthropogenic climate change has slowed global agricultural productivity growth, found that the optimum global temperature for growing crops occurred prior to 1961. Since that year, global agricultural productivity has declined by 21% as a result of climate change – the equivalent of losing the past seven years of advances in agricultural technology. The losses were highest for warm regions – such as Africa (-34.0%), the Near East and North Africa (-30.0%) and Latin American and the Caribbean (-25.9%) – than for cooler regions such as North America (-12.5%) and Europe and Central Asia (-7.1%) (Figure 5).
Figure 5. Country-level impacts of human-caused climate change on agricultural productivity from 1961-2015. (Image credit: Ortiz-Bobea, A., Ault, T.R., Carrillo, C.M. et al. Anthropogenic climate change has slowed global agricultural productivity growth. Nat. Clim. Chang. 11, 306–312 (2021). https://doi.org/10.1038/s41558-021-01000-1)
Despite an increasingly hostile climate for growing grains, however, world grain production has risen steadily since improvements in agricultural technology led to the “green revolution” that began in the 1960s (Figure 6).
Figure 6. Global grain production from 1961-2018. Even though the amount of land farmed has been roughly constant since 1990, grain harvests have steadily increased, thanks to advances in agricultural technology. (Image credit: World Bank)
Simultaneous heat waves increase in frequency, size, and intensity
The 2021 Intergovernmental Panel on Climate Change (IPPC) report concluded that it is virtually certain that “there has been increases in the intensity and duration of heatwaves and in the number of heatwave days at the global scale.” Authors of that report said it is, again, virtually certain that “human-induced greenhouse gas forcing is the main driver behind observed changes in hot and cold extremes on the global scale.” But until a 2021 study by Washington State University’s Cass Rogers and co-authors, Six-fold increase in historical Northern Hemisphere concurrent large heatwaves driven by warming and changing atmospheric circulations, a key unknown involved how often multiple large heatwaves were occurring simultaneously – the type of extreme weather capable of causing concurrent droughts that would threaten global food supplies. This occurred in the summer of 2022, with concurrent major heat waves and droughts affecting both Europe and China, driving down crop yields.
Their study documented a highly concerning rapid increase in the size, incidence, and intensity of multiple large heatwaves in the Northern Hemisphere during May-September from 1979-2019. The authors found a 46% increase in the spatial extent of concurrent heatwaves, a 17% increase in their maximum intensity, and a six-fold increase in their frequency.
These increases, the researchers found, resulted primarily from warming global temperatures caused by human-caused climate change. However, some of the increases were the result of changes in behavior of the jet stream.
In particular, the pattern potentially of most concern for global food supplies – when simultaneous heat waves occur over the grain-producing areas of the midwest U.S., China, and eastern Europe, a pattern that was responsible for about 8% of all the concurrent heat waves identified – showed the largest increase in frequency during the study period, jumping from near-zero days per year in the early 1980s to about 24 days per year by 2019. Almost half of this increase was attributed to changes in the jet stream circulation, which could be driven by natural variation or by human-caused climate change, or by some combination of both.
Figure 7. Top: Frequency of concurrent heatwave days (blue line), and the mean number of heatwaves per heatwave day in the May-June-July-August-September (MJJAS) season (orange line) from 1979-2019. Heatwave days are defined as the number of days per MJJAS season with one or more heatwave. Concurrent heatwave days are defined as the number of days per MJJAS season with two or more heatwaves. Bottom: change in the number of concurrent heatwave days per decade. Areas with no statistically significant change are stippled with grey dots. (Image credit: Dr. Cass Rogers)
Concerning trends in ‘stuck’ summertime jet stream patterns
Concurrent heat waves that struck the planet during the summer of 2018 were caused by an extreme jet stream pattern that remained stuck in place for an unusually long time. Similar stuck jet stream patterns been getting more common and more extreme in recent decades, representing a significant danger to global food security, according to a 2019 paper, Extreme weather events in early summer 2018 connected by a recurrent hemispheric wave-7 pattern, by climate scientist Kai Kornhuber of Columbia University and co-authors. A similar stuck jet stream pattern was observed also during the European heatwaves of 2003, 2006, and 2015, and that pattern recurred this year (see Tweet below).
A follow-up 2019 study, Amplified Rossby waves enhance risk of concurrent heatwaves in major breadbasket regions – also led by Dr. Kornhuber – found that stuck jet stream patterns as occurred in 2018 are prone to bringing simultaneous heat waves and associated drought conditions to numerous important grain-producing regions of the world. The authors wrote that these stuck jet stream patterns can cause “reductions of 4% in crop production in the affected regions, with regional decreases up to 11%. Given the importance of these regions for global food production, the identified teleconnections have the potential to fuel multiple harvest failures posing risks to global food security.” (A teleconnection is a causal connection or correlation between meteorological phenomena occurring at substantial distances from each other.)
In a press release that accompanied the most recent paper, Kornhuber said, “We found a 20-fold increase in the risk of simultaneous heat waves in major crop-producing regions when these global-scale wind patterns are in place. Until now, this was an under-explored vulnerability in the food system. During these events there actually is a global structure in the otherwise quite chaotic circulation. The bell can ring in multiple regions at once.”
It is unknown how much of the increase in these stuck jet stream patterns might be from human-caused climate change, but multiple studies have proposed such linkages, including Mann et al. (2018), Projected changes in persistent extreme summer weather events: The role of quasi-resonant amplification, Mann et al. (2017), Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events, and Lehmann et al. (2015), Increased record-breaking precipitation events under global warming. In an October 2018 realclimate.org post climate scientist Michael Mann, of Penn State, gave a good summary of these studies, which have generally found that human-caused climate change may be to blame for this highly concerning change in jet stream behavior. Mann predicted that the future climate is likely to bring a significant increase in stuck summertime jet stream patterns capable of bringing a rise in extreme destructive weather events.
Multiple extreme weather disasters can amplify ripple effect in global economy
When multiple extreme weather disasters such as heat waves, floods, and hurricanes affect the globe in a short period of time, the impacts on the global economy can be much larger than if the events were spread out over a longer time period, according to Kilian Kuhla (Potsdam Institute for Climate Impact Research) and co-authors in an October 2021 study, Ripple resonance amplifies economic welfare loss from weather extremes.
“Ripple resonance, as we call it, might become key in assessing economic climate impacts especially in the future,” Kuhla said in a press release. “The effect of weather extremes in our globalized economy yield losses in some regions that face supply shortages and gains in others that see increased demand and thereby higher prices. But when extremes overlap, economic losses in the entire global supply network are on average 20 percent higher. This is what we see in our simulations of heat stress events, river floodings, and tropical cyclones; and it is a most worrying insight.” The authors of that study found that richer countries were most strongly affected, with China showing an above-average effect of more than 27% of extra losses when extreme events overlapped rather than hitting separately from each other.
Is there a ‘disaster gap’ for a record-shattering midwestern U.S. heat wave?
The U.S. Midwest holds a key position in the global supply of grain, accounting for 40% of global corn production and 50% of global corn exports. Corn, accounting for approximately one-third of global grain production and trade, is the most heavily traded grain in international markets.
According to a 2021 paper by Erich Fischer and co-authors (Switzerland Institute for Atmospheric and Climate Science), Increasing probability of record-shattering climate extremes, the U.S. Midwest may be experiencing a “disaster gap.” The trend in hot extremes has been small there in recent decades, possibly because of natural variability, and the authors say this trend may increase the probability that the region could suffer a record-shattering heat wave far beyond historical precedent.
“There is still a tendency in society to respond and adapt to the maximum event experienced during one’s lifetime (as measured in the observational record or documented in historical archives) but not more,” the authors wrote. The result of a record-shattering heat wave in the Midwest, combined with the lack of experience with dealing with such an extreme event, would be a significant challenge to global food supplies.
The 2012 U.S. heat wave and drought in the Midwest reduced the U.S. corn crop by 13% from 2011 levels. With the current record-high global food prices in 2022, the globe badly needed a good harvest in the U.S. this year: A repeat of 2012’s weather conditions would have been a serious blow. According to an article today in the Washington Post, the U.S. corn crop had its worst year since 2012, because of drought. Fortunately, the latest outlook from the USDA calls for a decent U.S. harvest of wheat and soybeans in 2022.
Figure 8. A severe drought in 2012 caused this Iowa corn crop to fail. (Image credit: USDA/Dave Kosling)
A success story: reduced air pollution has helped U.S. crops
In addition to drought and extreme heat, air pollution is also a great enemy of plants. In the U.S., ground-level ozone pollution caused an estimated 16% reduction in corn yields and 10% in soybean yields in 1980. Small particle pollution (primarily from burning coal) caused an estimated 19% reduction in corn yields and 33% in soybean yields. Ozone pollution forms when precursor human-emitted hydrocarbons and nitrogen oxides react in the presence of sunlight. The reactions that create ozone are more efficient in warmer temperatures, so global warming will cause more ozone pollution in the absence of reduced emissions.
However, air pollution reductions brought about by the Clean Air Act of 1970 have significantly helped crops in the U.S. in recent decades. In 2019, ground-level ozone pollution caused only an estimated 4% reduction in corn yields and 2% in soybean yields; pollution from small particles caused an estimated 8% reduction in corn yields and 18% in soybean yields. Fine particulate pollution is expected to continue to fall in coming decades as a result of reduced fossil fuel burning, further benefiting crops. However, ozone pollution will not decrease as rapidly because the globe’s atmosphere will be hotter.
The story is different in East Asia, where increasing ozone pollution has caused more crop damage in recent decades. In a 2022 paper by Feng et al., Ozone pollution threatens the production of major staple crops in East Asia, the authors found that ozone pollution currently causes yield losses of 33%, 23%, and 9% for wheat, rice and corn, respectively in China. However, China declared a “war on pollution” in 2014, and the situation is turning around there. According to a 2022 study by the Becker Friedman Institute for Economics at the University of Chicago, five years after China’s peak pollution levels in 2013, national-level fine particle (PM2.5) levels declined by about 40%, and SO2 and CO concentrations fell by 65% and 33%, respectively. Pollution levels in China are still far in excess of the global standards for healthy air, but if China progresses in cleaning up its air, Chinese agriculture is expected to benefit significantly.