The data were drawn from published studies utilising one of four analytical techniques: global grid-based simulations and local point-based simulations; statistical regression models – in which historical data on temperature changes and crop yield changes in a particular region are used to predict the temperature-yield relationship; and field-warming experiments – in which crops are artificially exposed to different temperatures in an otherwise normal field setting, and the yield changes in response to differences in temperature directly measured.
Based on the data drawn from studies employing these four methods, the study aims to quantify the impact on crop yield of a 1 degree C increase in temperature at both a global and at a regional level in the five top-producing countries for each crop. For their values of future temperature changes, the authors use climate modelling under four different emissions scenarios. As the authors are seeking to isolate the direct effect of temperature increase on crop yield, they do not account for indirect temperature effects (e.g. on weather patterns), effects of elevated CO2, or any mitigation or adaptation measures. By assuming a linear relationship between temperature increases and crop yield impact, and using their calculated crop yield impact of a 1 degree increase, the authors were able to estimate the crop yield effects of future projected temperature increases (assuming no adaptation, genetic modification, or other measures taken improve crop yield).
The key findings of the multimethod analysis were:
- At a global scale, the highest impact of warming was shown to be for maize, with a 7.4 ± 4.5% decrease in yield per degree C increase. The impact on maize at the country level was fairly consistently bad in four of the five main maize-producing countries (US, China, Brazil and India) but less bad (or even positive) in the fifth (France).
- For wheat the average global yield loss per degree C was 6.0 ± 2.9%. In the five top-producing countries the most severe per degree C yield losses were found to be in India (9.1 ± 5.4%) and Russia (7.8 ± 6.3%); the estimated per degree yield losses for the US (5.5 ± 4.4%) and France (6.0 ± 4.2%) were similar to the global average; and China has the least severe estimated yield loss (2.6 ± 3.1% - note that this uncertainty means that some analyses estimate a slight yield increase).
- For rice the average global yield loss per degree C was 3.2 ± 3.7%. For rice there is a much larger disagreement between the different analytical methods than for wheat and maize, with the grid- and point-based simulations and field warming experiments indicating a much more severe impact than the historical regression modelling which suggests almost no impact of warming on rice yields. Across four of the five main rice-producing countries (China, followed distantly by Indonesia, Bangladesh and Vietnam) these disagreements between the methods lead to a prediction of little effect of warming on yields; however, the methods agree with respect to India, where it is estimated that a degree of warming would lead to a 6.6 ± 3.8% yield loss.
- Fewer data were available for soybean, leading to a non-statistically-significant global yield loss estimate of 3.1% (with large uncertainties). Out of the five main soy producers (US, Brazil, Argentina, Paraguay and China), the largest estimated yield reduction is 6.8 ± 7.1% per degree in the US, the biggest producer. The results for China do not indicate a statistically significant effect of temperature on soybean yield.
- The authors discuss some of the methodological reasons for agreements or disagreements between the different analytical approaches used.
- Assuming a linear relationship between temperature increase and yield change (i.e. assuming that a one degree increase from 18 to 19 degrees would have the same percentage effect on yield as a change from 24 to 25 degrees, which the authors acknowledge is unlikely to accurately reflect the real-life relationship), and based on four emissions scenarios, the authors estimated the global yield losses (due to temperature alone) for the combined crops by the end of the 21st century, as 5.6% under the lowest emissions scenario, up to 18.2% under the highest emissions scenario.
The authors argue that it is important to isolate the effects of individual climate change factors on future crop yields as changes in different factors will require different adaptation strategies. They conclude that the direct and indirect effects of temperature on crop yields have already started to be observed and that they will continue to worsen in the future. They advocate for a “reinvigoration” of national research programs to develop crop- and region-specific adaptation strategies to offset the future crop yield impacts of climate change.
Wheat, rice, maize, and soybean provide two-thirds of human caloric intake. Assessing the impact of global temperature increase on production of these crops is therefore critical to maintaining global food supply, but different studies have yielded different results. Here, we investigated the impacts of temperature on yields of the four crops by compiling extensive published results from four analytical methods: global grid-based and local point-based models, statistical regressions, and field-warming experiments. Results from the different methods consistently showed negative temperature impacts on crop yield at the global scale, generally underpinned by similar impacts at country and site scales. Without CO2 fertilization, effective adaptation, and genetic improvement, each degree-Celsius increase in global mean temperature would, on average, reduce global yields of wheat by 6.0%, rice by 3.2%, maize by 7.4%, and soybean by 3.1%. Results are highly heterogeneous across crops and geographical areas, with some positive impact estimates. Multimethod analyses improved the confidence in assessments of future climate impacts on global major crops and suggest crop- and region-specific adaptation strategies to ensure food security for an increasing world population.
Zhao, C., Liu, B., Piao, S., Wang, X., Lobell, D. B., Huang, Y., ... & Durand, J. L. (2017). Temperature increase reduces global yields of major crops in four independent estimates. Proceedings of the National Academy of Sciences, 201701762.
Read the full paper here.