THEGREENGROK

Warmer Climate May Depress Crop Yields


by Bill Chameides | September 22nd, 2009
posted by Erica Rowell (Editor)

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Warmer temperatures can increase crop yields but only up to a point. When temperatures rise above a certain level — for instance, 86 degrees Fahrenheit for soybeans — yields tend to drop precipitously. (NREL)

Will corn and soybeans wilt as temperatures rise due to global warming?

Trying to figure out how the world will respond to rising global temperatures and the resultant climatic disruptions is not a slam-dunk. For one, we are not sure exactly how the climate will change — or how fast; and, because we are headed into uncharted climate territory, we have little data to infer what the climate change impacts will be.

Of all the responses to climate change, perhaps the most critical is crop yields. We all gotta eat, and with world population projected to top 9 billion by mid-century, we are clearly going to need a whole lot of food. If climate change enhances crop yields, the increased food supply will help us meet the challenge. If crop yields drop, we may be headed for some serious geopolitical unrest as hungry populations compete for dwindling foodstuffs.

CO2 Fertilization

Climate change will affect crop yields through a number of mechanisms. On the plus side, increases in carbon dioxide (CO2) will probably enhance crop growth. Green plants use CO2 in photosynthesis and so adding CO2 to the atmosphere can effectively fertilize faster growth. However there are some caveats:

(i) the magnitude of the enhancement may be less than previously thought (for example, some data suggest that CO2 fertilization will be of limited effect because plant growth will be limited by the availability of other nutrients like nitrogen);
(ii) the increased growth from enhanced CO2 does not necessarily go into production of the part of the plant that is used for food (i.e., the grains);
(iii) the reduced uptake of nitrogen lowers the overall nutritional quality of some plants and at the same time increases plant toxicity.

Temperature Effect on Crops

The effect of increased temperature on crops has been studied for many years using historical crop yield and meteorological data to infer a relationship that could then be extrapolated into the future. Typically these studies use averaged temperature data. However, such an approach fails to take into account temperature extremes that occur during the day and that over a growing season can have a significant cumulative effect on the amount of growth.

Wolfram Schlenker of Columbia University and Michael Roberts of North Carolina State University wondered if daily temperature extremes could significantly impact crop yields. A new paper they published in the Proceedings of the National Academy of Sciences indicates that the answer is yes.

The authors used county-level data for the yields of corn, soybeans, and cotton from the U.S. Department of Agriculture’s National Agricultural Statistical Service. These yields as a function of location were then correlated with the amount of time the crop at that location was exposed to a given temperature over the course of the growing season.

The results are pretty striking. After accounting for confounding variables like precipitation, they found that as temperatures increase, there is a slow rise in crop yields until an optimal temperature is reached. That optimal temperature for:

  • corn is 84.2 degrees Fahrenheit,
  • soybeans 86 degrees Fahrenheit, and
  • cotton 89.6 degrees Fahrenheit.

Once temperatures go above those optimum levels, all three crop yields drop precipitously.

The reason for this varying dependence on temperatures is not well understood. It may relate to water availability — higher temperatures can cause water to evaporate more quickly, desiccating the plant. However, the analysis of Schlenker and Roberts suggests that the changes in water availability could not explain all of the temperature dependence they found. Another possible reason relates to the biochemistry of the photosynthetic process itself. When temperatures become extremely high, the chemical reactions that lead to the production of sugars during photosynthesis can be short-circuited by a reaction of the sugar precursors with oxygen. Whatever the reason, the effect Schlenker and Roberts uncovered may not bode well for our globally warming world.

The Climate Bottom Line

So what does this mean for the future? To answer this, the authors used the four warming scenarios from the Hadley III climate model to develop yield predictions for the 21st century. From this analysis, they predict that by the end of the century, crop yields will decrease by:

  • 30–46 percent (for the slowest warming scenario) and
  • 63–82 percent (for the most rapid warming scenario).

Yield losses in the 2020–2050 time frame are projected at about 18 to 28 percent.

Crop yield losses of this magnitude are hardly consistent with aspirations to alleviate world poverty, while populations increase by some 50 percent. One potential solution is to get busy developing new strains of crops that are better adapted to temperature extremes. But what if those new strains are developed using genetic modification? Will the world accept GMOs instead of starving? I’m thinking yes.

filed under: agriculture, climate change, faculty, global warming, temperatures
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1 Comment

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  1. David Peppers
    Nov 27, 2009

    Technology will only be part of the solution if applied with wisdom and compassion, otherwise it will create more problems. Technology, after all, has played a large role in creating this problem. Genetic engineering has done little to improve yields or nutrition so far. Large and relatively easy improvements have already been made. Additional progress will be more difficult and come more slowly. There are so many confounding factors. The free market won’t save us. The corporations which control Genetically Modified Organisms are not in the business of feeding the poor, they do not have enough money to be a market. Until our culture changes, technological progress will most likely be a leverage for increasing the riches of the rich rather than feeding the poor and hungry. Technology has done a great job in enabling increased consumption, and overall wastefulness with little impact on overall happiness in the west since the 1950s.

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