Future impacts from extreme weather are likely to be most pronounced in summer, as most ecological activity and agricultural production takes place in this season117. Though the uncertainties are large, changes in atmosphere dynamics have the potential to cause rapid transitions at a regional scale leading to surprises for society. In summer synergistic effects between thermodynamic and dynamic drivers of extreme weather could act in the same direction to cause very-extreme extremes17. Recent summers have seen such anomalous weather (Box 1) and these events are not well understood. This presents risks for society and in particular for global food production, given that the major breadbasket regions are located in the mid-latitudes with many crop types vulnerable to heat extremes118.

The current literature provides robust evidence that AA influences mid-latitude summer circulation substantially by weakening the storm tracks. The uncertainties to do with other dynamical aspects and with how dynamical changes ultimately affect regional weather conditions are admittedly large. Nevertheless, we identified several possible feedback mechanisms for how storm track weakening can lead to persistent and therefore extreme weather in the mid-latitudes. Several studies suggest that Northern Hemisphere summer weather is indeed already becoming more persistent23,24,66,119.

In summary, this review shows that AA is likely to have substantial impacts on mid-latitude summer circulation. The societal impacts can be severe due to tail risks arising from radiatively forced mean summer warming combined with local and remote processes that favor more persistent summer weather. A coordinated research agenda focusing on summer circulation, its drivers and extremes is needed to resolve the key knowledge gaps.

https://www.nature.com/articles/s41467-018-05256-8