Warmer and More Volatile: Summertime Temperature Variance Increases Under Climate Change

The CMIP6 global climate models project that under higher atmospheric CO₂ concentrations, variance in monthly-averaged summertime temperatures will increase. This has serious implications for global agriculture due to the well documented deleterious impacts of warm temperatures on many staple crops. Increased variance in summertime temperatures implies a higher risk of unpredictable food shocks and underscores the need to develop food systems that are resilient to climate change. However, the CMIP6 models tend to over-estimate the magnitude of historical summertime temperature variance, so understanding if their projections are credible is extremely important for assessing the impact of climate change on agriculture. We have developed a theoretical model of the land surface that accurately reproduces the spatial pattern of both observed summertime temperature variance and the CMIP6 multi-model-mean projection of how summertime temperature variance will increase in the future. Our results lend credibility to the CMIP6 projections of increasing temperature variance in a warming world. In particular, Eastern Europe and the Midwestern U.S. stand out as regions that will likely be subject not only to warmer temperatures, but to a 30-50% increase in temperature variance by the end of the century in a worst-case climate change scenario. Thus, not only are average summertime temperatures increasing due to climate change, but the underlying probability distribution of monthly summertime temperatures is widening, further increasing the likelihood of temperature extremes unheard of in the contemporary climate. The primary mechanism responsible for increasing temperature variance is the impact of a warming mean temperature on atmospheric humidity. By analyzing output from the CMIP6 ensemble, we argue that uncertainty in the vegetation response to CO₂ is the dominant contributor to the model spread of increasing temperature variance under climate change. Given this, the impact of agriculture on local atmospheric moisture represents an important research area, both for understanding the coupling between agriculture and regional climate, and for assessing the impacts of climate change on global agriculture.