The response of sea ice and climate to non-CO2 forcing

Abstract

The climate system response to forcing presents an opportunity to learn about the system and has important implications for the climate we experience. Large volcanic eruptions are an example of a phenomenon other than increased atmospheric CO2 with a substantial impact on global climate and for which we have direct observations. In particular, we investigate the response to the eruption of Mt Pinatubo in June 1991 (the largest eruption for which we have direct observations). We quantify its impact on surface temperature and sea ice using large ensembles of climate model simulations that have recently become available. We show that the response is asymmetric between the hemispheres despite the symmetric nature of the forcing due to the eruption. In the most recent Intergovernmental Panel on Climate Change Assessment Report the global-scale cooling following the eruption of Mt Pinatubo was used as a constraint on the Earth's equilibrium climate sensitivity (ECS). However, when we repeat the analysis that led to this conclusion with a new expanded dataset of model large ensembles, we show that the response to the eruption does not constrain the Earth's ECS due to the low signal-to-noise ratio and differing radiative feedbacks from those found in response to increased CO2. The value of Earth's climate sensitivity has important implications for understanding how climate will change in the future in response to increased CO2, so an erroneous constraint could lead to poor policy decisions, including inadequate or unnecessary mitigation plans or a rush to deploy a geoengineering solution. Finally, we investigate the outcome from one localized non-CO2 forcing in the Arctic: increased flooding of the snow layer on top of sea ice. By simulating a proposed geoengineering scheme to thicken Arctic sea ice by artificially flooding the snow layer, we also gain insight into future natural changes in the Arctic, as the fraction of liquid precipitation is projected to increase. We find that the effect of this flooding is modest and that the geoengineering proposal is unlikely to provide any substantial benefit to the climate.