Large uncertainties in global hydroxyl projections tied to fate of reactive nitrogen and carbon

Significance Reaction with the hydroxyl radical (OH) is the dominant loss mechanism for many atmospheric gases of interest for air quality, climate change, and stratospheric ozone. Understanding how and why OH may change in the future is therefore paramount for predicting changes in the societal impacts associated with such changes. Future models’ projections strongly disagree in how OH responds to changing emissions and climate—even in the sign. Here, we demonstrate that intermodel differences in OH are best explained by disparate implementations of chemical and physical processes that affect reactive oxides of nitrogen and organic chemical species. Targeted observations can reduce uncertainty in the chemical budgets of these key species to increase confidence in future projections of composition and its impacts.