Both land use changes and greenhouse gas (GHG) emissions have significantly modified regional climate over the last century. In the central United States, for example, observational data indicate that rainfall increased, surface air temperature decreased, and surface humidity increased during the summer over the course of the twentieth century concurrently with increases in both agricultural production and global GHG emissions. However, the relative contributions of each of these forcings to the observed regional changes remain unclear. Results of both regional climate model simulations and observational analyses suggest that much of the observed rainfall increase—as well as the decrease in temperature and increase in humidity—is attributable to agricultural intensification in the central United States, with natural variability and GHG emissions playing secondary roles. Thus, we conclude that twentieth century land use changes contributed more to forcing observed regional climate change during the summer in the central United States than increasing GHG emissions.

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The tropospheric westerly jet is a key feature of Southern Hemisphere climate. In recent decades the jet strengthened in austral summer (December–February [DJF]) and moved poleward owing to the Antarctic ozone hole. Future jet trends will be influenced by recovery of the Antarctic ozone hole and greenhouse gas (GHG) forcing. Here, we examine 21st century projections of ozone, temperature and winds in the sixth Coupled Model Intercomparison Project models with (CHEM) and without (NOCHEM) interactive chemistry. NOCHEM models use an ozone data set that was produced with GHG forcings inconsistent with those used by CHEM models, leading to less ozone recovery in the Antarctic springtime lower stratosphere. This propagates to different stratospheric temperature projections and DJF westerly winds: NOCHEM models project a 78 ± 52% stronger increase in DJF westerly wind speeds than CHEM models under the high GHG emissions scenario SSP585. Our results show the importance of simulating stratospheric ozone accurately for Southern Hemisphere climate change projections.

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