From 1981 to 2002, fields of wheat, corn and barley throughout the world have produced a combined 40 million metric tons less per year because of increasing temperatures caused by human activities.
'There is clearly a negative response of global yields to increased temperatures,' said David Lobell, a Lawrence Livermore National Laboratory researcher and lead author of the study that appears today in 'Environmental Research Letters' online.
'Though the impacts are relatively small compared to the technological yield gains over the same period, the results demonstrate that negative impacts of climate trends on crop yields at the global scale are already occurring,' said Lobell.
Annual global temperatures increased by about 0.7 degrees Fahrenheit between 1980 and 2002, with even larger changes observed in several regions.
'Most people tend to think of climate change as something that will impact the future, but this study shows that warming over the past two decades already has had real effects on global food supply,' said Christopher Field, co-author on the study and director of Carnegie Institution’s Department of Global Ecology in Stanford, California.
Lobell and Field studied climate effects on the six most widely grown crops in the world – wheat, rice, soybeans, barley, maize or corn, and sorghum, a genus of about 30 species of grasses raised for grain. Production of these crops accounts for more than 40 percent of global cropland area, 55 percent of non-meat calories and more than 70 percent of animal feed.
Using global yield figures for 1961-2002 from the United Nations Food and Agriculture Organization, Lobell and Field compared yields with average temperatures and precipitation over the major growing regions.
They found that, on average, global crop yields respond negatively to warmer temperatures for several of the crops.
Lobell and Field then used these relationships to estimate the effect of observed warming trends.
'To do this, we assumed that farmers have not yet adapted to climate change, for example by selecting new crop varieties to deal with climate change,' Lobell said. 'If they have been adapting – something that is very difficult to measure – then the effects of warming may have been lower.'
Most experts believe that adaptation would lag several years behind climate trends, because of the difficulty of distinguishing climate trends from natural variability.
The importance of this study, the authors say, is that it demonstrates a clear and simple relationship at the global scale, with yields dropping by approximately three to five percent for a one-degree Fahrenheit increase.
'A key moving forward is how well cropping systems can adapt to a warmer world,' Lobell said. 'Investments in this area could potentially save billions of dollars and millions of lives.'
Agriculture's vulnerability to global warming was revealed late last year when the Consultative Group on International Agricultural Research, CGIAR, a network of 15 of the world's top crop research centers, issued an estimate of the impact of climate change on a single cereal crop, wheat, in one of the world's breadbasket regions.
Researchers using computer models to simulate the weather patterns likely to exist around 2050 found that the best wheat growing land in an arc from Pakistan through Northern India and Nepal to Bangladesh would be destroyed. Much of the area would become too hot and dry for growing wheat, placing 200 million people at greater risk of hunger.
CGIAR also warned in December that famines lie ahead unless new crop strains adapted to a warmer future are developed. To avert famine, CGIAR announced plans in December to accelerate efforts aimed at developing new strains of staple cereal crops including maize, wheat, rice and sorghum.
'The impacts of climate change on agriculture will add significantly to the development challenges of reducing poverty and ensuring sufficient food production for a growing population,' said Dr. Robert Zeigler, director general of the International Rice Research Institute, a CGIAR supported research center.
'The livelihoods of billions of people in developing countries, particularly those in the tropics, will be severely challenged as crop yields decline due to shorter growing seasons,' Zeigler said.
CGIAR researchers are focused on a comprehensive climate change agenda that is already generating climate-resilient innovations, including crops bred to withstand heat, salt, submergence or waterlogging, and drought, and more efficient farming techniques to help poor farmers better use increasingly scarce water and fragile soil.
Researchers are also focusing on boosting agriculture’s role in reducing climate-altering greenhouse gases.
'Anticipating and planning for climate change is imperative if farmers in poor countries are to avert forecast declines in yields of the world’s most important food crops,” said Dr. Louis Verchot, a climate change scientist with the CGIAR supported World Agroforestry Centre in Kenya. 'Yet, adaptation is not a substitute for reducing new and removing existing greenhouse gases from the atmosphere - our only long-term option.'
As a result of rising temperatures, the climatic conditions best suited to wheat growing will shift away from the tropics, where most of the world’s poorest countries are situated, toward the poles and to higher elevations.
CGIAR expects that North American wheat growers will be able to farm new lands as far as 65 degrees north, 10 degrees beyond their current planting limit. Wheat growing would extend from its current limit – from Ketchikan, Alaska in the West to Cape Harrison, Labrador in the East – to less than two degrees beneath the Arctic Circle.
In Eurasia, much of Siberia would become farmland.
While poor tropical countries' capacity for food production will diminish, developed countries, most of which are located far from the equator, will experience an increase in productive capacity as land that was previously frost-bound opens to cultivation.
Said Dr. Verchot, 'Developing countries, which are already home to most of the world's poor and malnourished people and have contributed relatively little to the causes of global warming, are going to bear the brunt of climate change and suffer most from its negative consequences.'