Green plant in a desert enivonment. Photo.

Protecting crops by reflecting the sunlight

Global warming is expected to add pressures on agricultural production and food security in the not-too-distant future. Some crops are more vulnerable to climate change, and as the planet becomes warmer local growth environments are likely to change and it may be harder to sustain yields.

Solar geoengineering is a set of suggested methods for reflecting sunlight to quickly cool down the earth. This is proposed as a climate intervention strategy to lessen warming and reduce some of the negative consequences of climate change. A variety of solar geoengineering and related techniques have been devised, such as adding aerosols to the high atmosphere, brightening marine skies with sea salts, and thinning the cirrus clouds.

Solar geoengineering or emission reduction?

In an article recently published in the journal Nature Food, researchers from multiple institutions used the Norwegian Earth System Model to investigate the impacts and mechanisms of the three solar geoengineering techniques mentioned above and emissions reduction on the yields of six major crops that produce food, fibre and bioenergy. The authors found that global yields would increase by approximately 10% under the three solar engineering schemes but decrease by 5% under emission reduction relative to the business-as-usual high emission scenario by the late twenty-first century.

The researchers isolated the effects of different climate variables including surface air temperature, direct and diffuse solar radiation, precipitation, and relative humidity under each modeled scenario on the yields of maize, soy, wheat, rice, sugarcane, and cotton. They also isolated the effect of reduced CO2 concentration on yields and potential uncertainties associated with land-use change for the emissions reduction scenario.

They found that all three methods all have similar cooling effects but distinct radiation and humidity effects on different crops.
Emissions reduction has strong cooling and humidity benefit but is counteracted by the reduced CO2 fertilization due to lowered CO2 concentration compared to the high emission scenario, leading to reduced global yield compared to solar geoengineering. Global South benefit consistently from all scenarios, particularly for maize, soy, and sugarcane, whereas wheat and rice are less sensitive to the cooling induced by solar geoengineering and emissions reduction in general.

Cooling and humidity

Overall, cooling is the primary driver of yield increases under both solar geoengineering and emissions reduction. Relative humidity is an important second driver that controls crop transpiration and productivity but has often been ignored. Relative humidity increases in a cooler world under multiple scenarios, except cirrus cloud thinning, which will alleviate water stress for rainfed crops and thus increase their yields. In contrast, none of the scenarios found noticeable effect of precipitation change on crop yields. This calls for a re-evaluation of previous assessments suggesting that less rainfall under solar geoengineering would harm crop yields.

This is the first study to offer an extensive overview of the potentials and implications of multiple solar geoengineering methods on global crop yields. Furthermore, this is the first paper to explicitly compare the effectiveness of conventional mitigation via emissions reduction and solar geoengineering techniques as alternative pathways to reduce climate change impacts on crops.

While the overwhelming assumption has been that emissions reduction is better than solar geoengineering for protecting agriculture, no quantitative assessment existed to approve or disapprove this assumption until this publication. Based on climate and CO2 analysis and additional land-use change analysis, the researchers show how solar geoengineering may be more effective than emissions reduction in protecting crops from climate change but note that their relative difference depends on the assumption that their use is not associated with other management or technological changes. The loss of yields due to reduced CO2 fertilization under emissions reduction could be compensated through other means of adaptation, such as increasing nitrogen fertilizer use, which strongly affects yields. Climate mitigation ultimately depends on cooperative efforts from all social-economic sectors, considering the advantages of multiple solutions and their potential risks.

We don't want readers to take away a message that emissions reductions are not necessary given the global drive to cut emissions. We compared the impacts and mechanisms of multiple hypothesized scenarios so that it sheds light on designing a climate mitigation strategy that could perhaps utilize the complementary strengths of multiple tools.

Yuanchao Fan, Harvard's Solar Geoengineering Research Program Fellow, Harvard University

Collaborating parties are Harvard John A. Paulson School of Engineering and Applied Sciences, the Norwegian Research Centre and the Bjerknes Centre for Climate Research, the Norwegian University of Science and Technology, the National Center for Atmospheric Research in Boulder, Seoul National University and the Chinese Academy of Sciences.