Households depend upon food prices, incomes, and disease burdens that impact the ability to use consumed food. Climate change and extreme temperatures impact all of these factors. In this talk, David Lobell focuses on the impact of heat in growing regions that are important for food prices. He reviews recent research on heat impacts and discusses whether crop yields are becoming more or less sensitive to heat.

According to a new study by FSE's David Lobell, satellite data can play a critical role in understanding yield gaps and meeting future crop demand. Lobell's review appeared in a special issue in Field Crops Research dedicated to crop yield gap analysis.

To date, satellite data have played a relatively small role in understanding the magnitude and causes of yield gaps in most regions. However, the few examples that exist indicate that remote sensing can help to overcome some of the inherent spatial and temporal scaling issues associated with field-based approaches.

"Yield gap profiles, based on multiple years of satellite data, provide a useful measure of how persistent yield-controlling factors are through time," writes Lobell in his review. "Although the cost or availability of satellite data with sufficient spatial resolution to discriminate agricultural fields was an obstacle in the past, this barrier is rapidly diminishing."

Improved algorithms to pre-process remote sensing data and estimate yields, and the increased availability of new, large geospatial datasets on soils, management, and weather should also benefit future efforts in this area.

"Improved knowledge of yield gaps will play a critical role in meeting future crop demands at affordable prices and with minimal environmental impacts," concludes Lobell. "The use of satellite data can accelerate the pace of discovery, and as such it represents an important area for future work."

All papers in this special issue can be accessed free of charge.

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.

Human activities are currently estimated to produce around 40 billion tonnes of carbon-dioxide equivalent every year. Model results indicate that agricultural adaptation measures would prevent around 350 million tonnes of carbon-dioxide emissions annually – equivalent to around 1% of total global emissions.

Adapting to climate change or mitigating climate change – which would you choose to invest your cash in? Mitigation and adaptation are often viewed as separate activities, with the former aiming to reduce greenhouse-gas emissions and the latter helping adjust to expected increases in greenhouse gases. A new study shows that when it comes to agriculture, adaptation measures can also generate significant mitigation effects, making them a highly worthwhile investment.

Food production is big. If farmers fail to adapt to climate change we can expect to see more land being turned over to agriculture, in order to keep up with food demand. With this in mind, David Lobell, from Stanford University, US, and colleagues used a model of global agricultural trade to investigate the co-benefits of helping farmers adapt to climate change, thereby avoiding some of the emissions associated with land-use change.

Running their model to 2050, they show that an investment of $225 bn in agricultural adaptation measures can be expected to offset the negative yield impacts associated with predicted temperature and rainfall changes. But that’s not all – the model revealed that this investment would also save 61 million hectares from conversion to cropland, resulting in 15 Gtonnes carbon-dioxide equivalent fewer emissions by 2050.

"I don't think any of us expected the mitigation benefits to be as big as they were," said Lobell, whose findings are published in Environmental Research Letters (ERL). "We had a hunch that they would be big enough to be an important co-benefit, but the fact they were often big enough to rival other mitigation activities was surprising."

Click here to read the full article.

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.