Is climate change good or bad for agriculture? As recently as the 1990s, it was widely believed that the first few degrees of global warming would boost world average crop yields and food production. Higher temperatures were expected to lengthen growing seasons in temperate regions, while more carbon dioxide (CO2) in the atmosphere would act as a fertilizer, promoting plant growth.
The research of the last decade has led to a more ominous outlook for agriculture, as Elizabeth Stanton and I explain in a new paper. Three areas of recent research challenge the older, optimistic picture of climate change on the farm: field research has reduced estimates of the carbon fertilization effect; new analyses identify a strong effect of extreme temperatures on crop yields; and in many regions, changes in precipitation and the availability of irrigation will be the limiting factor for food production.
Carbon fertilization benefits are real but limited. Some plants, including maize, sugar cane, sorghum, and millet, use a distinct style of photosynthesis and experience almost no yield gains from increased atmospheric CO2. For one major crop, cassava, increased CO2 causes sharply reduced yields. Most other crops do have higher yields at elevated CO2 levels – but research with realistic simulations of actual growing conditions has led to modest estimates of the carbon fertilization effect. William Cline has projected that 550 parts per million (ppm) of CO2 in the atmosphere (about a 40% increase over current levels) would cause a worldwide average increase of 9% in crop yields.
Most research, however, examines the effects of CO2 in isolation. Fossil fuel combustion, the principal source of atmospheric CO2, also produces ground-level ozone, which reduces yields of many plants. The net effect of increases in CO2 and ozone is likely to be less than experimental estimates for carbon fertilization alone.
A second area of new research highlights the effects of extreme temperatures. Yields often depend more directly on the number of degree-days above a threshold, rather than average temperatures. For maize, soybeans, and cotton in the United States, yields rise slowly as temperatures increase up to thresholds of 29° – 32°C, then drop rapidly beyond that point. Replacing 24 hours of the growing season at 29°C with 24 hours at 40°C would cause a 7 percent decline in maize yields.
Similar temperature threshold effects have been found in crops in Africa, India and elsewhere. A mirror image of this pattern can be seen in some perennials such as fruit and nut trees, which require a certain amount of “chill time,” or annual hours below a low temperature threshold such as 7°C.
Since temperatures vary widely around the long-term average, a small change in the average can cause a large increase in the number of hours above a high-temperature threshold (or a large decrease in chill time). The threshold model therefore suggests that there will be significant temperature-related yield losses by the middle of this century, sooner than expected under models based on average temperatures.
A third area of research focuses on climate-related changes in precipitation. Many dry regions are expected to become even drier, with dire consequences for local agriculture. Even if annual totals remain constant, changes in the patterns of precipitation can have adverse effects. Indian monsoon rainfall has already become less frequent but more intense, reducing wet-season rice yields; similar changes in precipitation have been predicted for parts of northern China. In the western United States, a shift of winter precipitation from snow to rain may replace gradual snowmelt, providing water for agriculture throughout the spring and summer, with a surge of water too early in the year. Irrigated U.S. agriculture, in California and other western states, now relies on unsustainable exploitation of limited groundwater reserves; climate change will hasten the coming crisis of groundwater exhaustion.
The current pace of climate change will require our best efforts at adaptation, developing new heat-resistant and drought-resistant crops and cultivars. Adaptation, however, is necessary but not sufficient. If global warming continues unabated, it will, in a matter of decades, reach levels at which adaptation is no longer possible. Any long-run solution must involve rapid reduction of emissions, to limit the future extent of climate change. Among many other important benefits, this will help to sustain the production of food.
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