Kostas Tsigaridis

COVID‐19 pandemic responses affected atmospheric composition and climate. These effects depend on the background emissions, climate, and season in which they occur. Although using multiple scenarios is common in explorations of long‐term climate change, they are rarely used to explore atmospheric composition or climate changes in response to transient emission perturbations on the scale of COVID‐19 lockdowns. We used the ModelE Earth system model to evaluate how atmospheric and climate impacts depend on the decade and season in which lockdowns occurred. Global COVID‐19‐related anomalies in aerosols and trace gases differed by up to an order of magnitude or more when comparing lockdowns in 1980, 2008, 2020, and 2051. Regional atmospheric composition anomalies tended to be largest when emissions were near a historical peak: 1980 in Europe and temperate North America, 2008 or 2020 in eastern Asia, and 2051 in south Asia. Regional aerosol direct effect anomalies were almost always less than 0.1 W m −2 during the first pandemic year, but over 0.1 W m −2 in Europe and exceeded 0.2 W m −2 in Europe and temperate North America in 1980, generally changing in tandem with regional emissions. In contrast, direct effect anomalies in Asia were positive in 1980 and negative in 2008, suggesting they may be primarily determined by exogenous emission anomalies. Shifting COVID‐19 onset in 2020 by 3, 6, or 9 months also altered atmospheric composition on the order of 2%–25% globally. In all scenarios, changes in surface temperature or precipitation appeared unrelated to local atmospheric compositional changes.