Nonlinearity of the cloud response postpones climate penalty of mitigating air pollution in polluted regions

Aerosol–cloud interactions contribute substantially to uncertainties in anthropogenic forcing, in which the sensitivity of cloud droplet number concentration (N_d) to aerosol plays a central role. Here we use satellite observations to show that the aerosol–N_d relation (in log–log space) is not linear as commonly assumed. Instead, the N_d sensitivity decreases at large aerosol concentrations due to the transition from aerosol-limited to updraft-limited regime, making the widely used linear method problematic. The similar nonlinear behavior is also observed in weekly cycles; specifically, polluted conditions exhibit a reduced amplitude of weekly cycles in N_d compared to clean conditions with similar aerosol perturbations.  A sigmoidal transition is shown to adequately fit the data. When using this revised relationship, the additional warming that arises from air pollution mitigation is delayed by two to three decades in heavily polluted locations, compared to the linear relationship. This cloud-mediated climate penalty will manifest markedly starting around 2025 in China and 2050 in India after applying the strongest air quality policy, underlining the urgency of mitigating greenhouse gas emissions.