The global COVID-19 response created an unprecedented real-world perturbation to human activity, enabling direct evaluation of the climate forcing efficiency of short-lived pollutants. Using a satellite-constrained chemical reanalysis and climate simulations, we quantify the radiative impacts driven by the abrupt reduction in anthropogenic NOx emissions. We show that NOx declines reduced tropospheric ozone by 4.07 ± 0.55 Tg, producing immediate regional cooling. However, the associated suppression of OH increased methane lifetime, leading to a delayed rise in methane and secondary ozone production that offset the initial cooling. Together, these chemical adjustments produce a net global radiative forcing of +17.56 ± 0.39 mW m⁻². Our results clarify how rapid changes in NOx emissions propagate through the coupled NOx–O3–OH–CH4 system, revealing that the short-lived cooling effect of reduced ozone is outweighed by longer-lived methane-driven warming and illustrating the climate trade-offs inherent to NOx mitigation.