Climate processes

Abstract:

Anthropogenic aerosols are a primary source of uncertainty in future climate projections. Changes to aerosol concentrations modify cloud radiative properties, radiative fluxes, and precipitation from the micro- to the global scale. Due to computational constraints, we have been unable to explicitly simulate cloud dynamics in global-scale simulations, leaving key processes, such as convective updrafts, parameterized. This has significantly limited our understanding of aerosol impacts on convective clouds and climate. However, new state-of-the-art climate models are capable of representing these scales. In this study, we used the kilometer-scale Icosahedral Nonhydrostatic (ICON) earth system model to explore the global-scale rapid response of clouds and precipitation to an idealized distribution of anthropogenic aerosol via aerosol-cloud interactions (ACI) and aerosol-radiation interactions (ARI). In our simulations over 30 days, we find that the aerosol impacts on clouds and precipitation exhibit strong regional dependence. The impact of ARI and ACI on clouds in isolation shows some consistent behavior, but the magnitude and additive nature of the effects are regionally dependent. Some regions are dominated by either ACI or ARI, whereas others behaved nonlinearly. This suggests that the findings of isolated case studies from regional simulations may not be globally representative; ARI and ACI cannot be considered independently and should both be interactively represented in modelling studies. We also observe pronounced diurnal cycles in the rapid response of cloud microphysical and radiative properties, which suggests the usefulness of using polar-orbiting satellites to quantify ACI and ARI may be more limited than presently assumed. The simulations highlight some limitations that need to be considered in future studies. Isolating kilometerscale aerosol responses from internal variability will require longer averaging periods or ensemble simulations. It would also be beneficial to use interactive aerosols and assess the sensitivity of the conclusions to the cloud microphysics scheme.

Abstract:

Anthropogenic emissions of black carbon (BC) aerosols are generally thought to warm the climate. However, the magnitude of this warming remains highly uncertain due to limited knowledge of BC sources; optical properties; and atmospheric processes such as transport, removal, and cloud interactions. Here, we assess and constrain estimates of the historical warming influence of BC using recent observations and emission inventories. Based on simulations from four climate models, we show that the current global mean surface temperature change from anthropogenic BC due to aerosol-radiation interaction spans a factor of three—from +0.02 ± 0.02 K to +0.06 ± 0.05 K. Rapid atmospheric adjustments reduce the instantaneous radiative forcing by nearly 50% (multi-model mean), substantially lowering the net warming. Yet, recent satellite constraints suggest a stronger effect, highlighting the need for a more comprehensive reassessment of BC’s climate influence.