REVIEW SUMMARY ◥SOCIAL SCIENCES

Social and economic impacts of climateTamma A. Carleton* and Solomon M. Hsiang*†

Science p. 1112 9 SEPTEMBER 2016 VOL 353 ISSUE 6304

…Economic impacts: Agricultural yields

Study of the direct effect of climate on economic outcomes began in agriculture, where the importanceof climatic factors is clearest. Despite centuries of agricultural experience, a surprising recent finding is the importanceof temperature, often dominating rainfall, in the production of staple crops. Highly nonlinear yield losses on the hottestdays drive much of this effect, a relationship recovered in the United States, Africa, Europe, Southeast Asia), and India.Crops are most sensitive to temperatures during specific phases of the growth cycle. Although temperature impactsgenerally outweigh those of rainfall, low and very high total seasonal rainfall levels do damage yields in many contexts,an effect that is partially attenuated when water storage and irrigation are widely available. Similarly, within a singlegrowing season, farms that experience a small number of extremely rainy days suffer damaged yields, relative to thesame quantity of rain distributed evenly across growing days. These various dose-response functions have beenrecovered and replicated for major global crops like maize, rice, soy, and wheat, but less is known about effects onregional crops like millet and cassava, which can be critical in poor rural regions and specialty crops like fruits andvegetables, with some notable exceptions. A body of research in dairy science suggests that both temperature andhumidity non-linearly affect milk yields, while linearly lowering cattle pregnancy rates, but little is known outside of highlymanaged livestock operations in industrialized countries.

Effective adaptation to climate in agriculture appears modest, as dose-response functionschange little across time and space, even when warming effects are gradual. Furthermore, large but temporaryclimate events, like the U.S. Dust Bowl, have had persistent multi-decadal impacts on farm values. Thesefindings contrast with historical narratives of farmer adaptability, such as the 200-year-long spread ofagriculture into previously non-arable land and adjustment of cultivars in response to drought . These twoviews of agriculture adaptability remain unreconciled, and identifying obstacles to adaptation, such as poorincentives or high adaptation costs, are a critical area for future research.…

The article mentions a third route to nitrogen-Independent crops, a route that may be more practical: inserting N-fixinggenes into bacteria that associate with,but do not enter, roots. A modified bacterium,Pseudomonas protegens Pf-5 X940, “fixednitrogen and released it to the roots at a rate 1000 times higher than other nitrogen-fixing bacteria.”