Will Global Warming Affect Crop Yields?

A recent study raised the question of whether global warming trends will affect crops during their reproductive time and hence the yields of four major food sources for the world. They are maize (corn), rice, wheat, and soy. The study by Stanford’s Sharon Gourdji states that “Crop breeders need to think carefully about how to incorporate heat tolerance, particularly during the flowering period, into wheat, maize and rice.” She succinctly presents the issue and the potential impacts in this short video.
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The warming climate affects crop production in several ways. Photosynthesis is speeded up due to higher temperatures and CO2 levels. Rainfall patterns are changed leading to changes in farming due to reduced or enhanced precipitation. Gourdji’s teammate David Lobell and others showed that crop production growth worldwide has been negatively impacted by these climate changes in the last 30 years.

This study did not discuss how these climate changes might affect future worldwide food availability. To do that kind of modeling requires much more data and mathematical complexity. Instead, the researchers focused on a more narrow question. Other scientists have published data for the critical high temperatures beyond which a crop is damaged during the flowering stage. For wheat the threshold is 34°C, maize is 35°C, rice 36°C and soybean 39°C. FYI…human body temperature is normally 37°C.

“We focused on extreme heat during flowering,” Sharon said. “This is one aspect of global environmental change that could be particularly risky for crops regardless of other more gradual changes that are taking place simultaneously. We identified a 30-day window between sowing and harvest that should include flowering, Then we simply counted the number of days over the critical high temperature threshold during the 30-day window for each crop, location and year in the study.

If crop yields are significantly reduced by excessive heat damage during flowering, the global crop production of four main food crops may cause widespread and increasi-ing challenges to feeding the people of the world.

Past and Future Trends

This chart shows the percentage of harvested acreage globally with at least 5 days exceeding the critical temperature threshold during the 30 day flowering reproductive interval. Past data includes 1980-2011 plotted points. Red points use the critical high temperatures noted above. For example, the maize red trend line begins at 12% using a 35˚ critical temperature. The orange points and trend line assume a hybrid of maize is developed and grown which has a critical high temperature of 36˚. It would be more tolerant to heat. The orange trend line starts at 7%, significantly lower. The amount of harvested maize exposed to damaging heat effects is reduced. Would this maize yield better? Probably.

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One point of interest is how maize is relatively more affected by excess heat than soy. Also note, the future trends show a marked non-linear increase in the acreage exposed to excess heat due to global warming projections out to 2050. Dotted lines show a range of results from various other models.

While this study doesn’t make any links to the amount of each crop finally harvested, its findings underline the need for farmers to prepare to adapt. “National-level governments, particularly in tropical countries that grow a lot of wheat, maize and rice, need to think proactively about how to help farmers adapt to expected climate conditions in the next few decades.”

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