CGD Special Seminar- The role of anthropogenic emissions in historic large-scale atmospheric flow changes

The large-scale atmospheric flow plays a central role in shaping Earth's regional weather and climate. On daily to multi-decadal timescales, the tropical and extra-tropical flows transport heat, momentum, and moisture across latitudes and longitudes, thereby affecting the distribution of precipitation, temperature, and winds. It is thus critical to assess the impacts of anthropogenic emissions on the large-scale flow. Yet, while observed thermodynamic changes have been attributed to human emissions with high confidence, currently, there is a large uncertainty in recent circulation changes. This uncertainty precludes assessing the impacts of human emissions on the circulation and informing our confidence in climate projections. Here, I will present two examples of how the large-scale flow has changed in recent decades and assess the relative role of anthropogenic emissions in these changes. First, using sea-level pressure measurements we constrain the historic changes in the Hadley cell strength and show that the Hadley cell has weakened in recent decades, in agreement with climate models. The weakening of the circulation is further attributed to anthropogenic emissions, which allow us to constrain the projected weakening of the flow by the end of this century and the associated precipitation changes. Second, we show that unlike the previous generation of climate models, the latest climate models are able to capture the historic weakening of the Northern Hemisphere summer storm tracks. Using these models, we reveal that anthropogenic emissions are responsible for the weakening of the storm tracks by warming the high latitudes more than the low latitudes in recent decades.