We have read the headline a number of times now warning us that increasing temperatures are threatening global crop production. One need only to recall the drought and heat wave that hit the mid-western United States last summer, damaging corn and soybean production. Higher temperatures are certainly part of the problem, but a new study led by FSE associate director David Lobell finds its impacts in the U.S. are more indirect. Water stress may be the main culprit.

To validate this hypothesis and to help differentiate the different mechanisms impacting crop yields at higher temperatures, the research team used a model known as an Agricultural Production Systems Simulator (APSIM). High temperatures had a strong negative effect on corn yield response in the United States, in agreement with the data, but the predominate effect of heat in the model was via increased water stress.

As temperatures increase, plants transpire more water into the atmosphere, just as people sweat more on hotter days. With more hot days, the corn plant finds it harder to maintain growth rates, and at the same time loses more water, which sets up the risk of even more drought stress later in the season.

“APSIM computes daily water stress as the ratio of water supply to demand, and during the critical month of July this ratio is three times more responsive to 2 ºC warming than to a 20 percent precipitation reduction,” writes Lobell and co-authors in a new paper published in Nature Climate Change. “Water stress during July is particularly important for overall biomass growth and final yield, with July being the month with the most total biomass growth.”

Direct heat stress on the plant, such as happens on extremely hot days, played a more minor role in determining final yield. The study suggests that increased CO₂ may reduce crop sensitivity to extreme heat by increasing water use efficiency, but gains are likely to be no more than 25 percent.

“The APSIM model has been valuable in its ability to discriminate the importance of these factors,” said Lobell. “Models like these are useful for guiding efforts to develop crops with greater tolerance to increased temperatures, an important component of most adaptation strategies in agriculture, and helping to identify which processes are critical for modeling efforts to consider when projecting climate change impacts.”

The researchers project sensitivity to extreme heat will remain a severe constraint to crop production in the foreseeable future, especially as the region warms. They are now using the models to evaluate different strategies for developing new varieties of corn that can better handle the heat.

It’s no secret that China is faced with some of the world’s worst pollution. Until now, however, information on the magnitude, scope and impacts of a major contributor to that pollution – human-caused nitrogen emissions – was lacking.

A new study co-authored by Stanford Woods Institute Senior Fellow Peter Vitousek (Biology) reveals, among other findings, that amounts of nitrogen deposited on land and water in China by way of rain, dust and other carriers increased by 60 percent annually from the 1980s to the 2000s, with profound consequences for the country’s people and ecosystems. Xuejun Liu and Fusuo Zhang at China Agricultural University in Beijing led the study, which is part of an ongoing collaboration with Stanford aimed at reducing agricultural nutrient pollution while increasing food production in China – a collaboration that includes Vitousek and Pamela Matson, a Stanford Woods Institute senior fellow and dean of the School of Earth Sciences. The researchers analyzed all available data on bulk nitrogen deposition results from monitoring sites throughout China from 1980 to 2010.

During the past 30 years, China has become by far the largest creator and emitter of nitrogen globally. The country’s use of nitrogen as a fertilizer increased about threefold from the 1980s to 2000s, while livestock numbers and coal combustion increased about fourfold, and the number of automobiles about 20-fold. All of these activities release reactive nitrogen into the environment. Increased levels of nitrogen have led to a range of deleterious impacts, including decreased air quality, acidification of soil and water, increased greenhouse gas concentrations and reduced biological diversity.

“All these changes can be linked to a common driving factor: strong economic growth, which has led to continuous increases in agricultural and nonagricultural reactive nitrogen emissions and consequently increased nitrogen deposition,” the study’s authors write.

Researchers found highly significant increases in bulk nitrogen deposition since the 1980s in China’s industrialized north, southeast and southwest regions. Nitrogen levels on the North China Plain are much higher than those observed in any region in the U.S., and are comparable to the maximum values observed in the U.K. and the Netherlands when nitrogen deposition was at its peak in the 1980s.

China’s rapid industrialization and agricultural expansion have led to continuous increases in nitrogen emissions and nitrogen deposition. China’s production and use of nitrogen-based fertilizers is greater than that of the U.S. and the E.U. combined. Because of inefficiencies, more than half of that fertilizer is lost to the environment in gaseous or dissolved forms.

China’s nitrogen deposition problem could be brought under control, the study’s authors state, if the country’s environmental policy focused on improving nitrogen agricultural use efficiency and reducing nitrogen emissions from all sources, including industry and transit.