Evidence of Strong Contributions from Mixed-Phase Clouds to Arctic Amplification

Earth’s Arctic is warming at a faster rate than the global average. This accelerated warming, referred to as Arctic amplification, has been shown by various climate models to be caused by climate feedbacks involving the atmospheric lapse rate, surface albedo, temperature, clouds, water vapour and heat transport. However, recent studies have revealed that climate models tend to underestimate the proportion of supercooled liquid in mixed-phase clouds containing both supercooled liquid droplets and ice crystals compared to observations. Here we show that constraining cloud supercooled liquid fraction in a climate model to improve comparisons with satellite observations by modifying the microphysical processes that affect it can lead to a ~20% change in Arctic amplification following a doubling of atmospheric CO₂ concentration. The ice-containing portion of mixed-phase clouds tends to be replaced by longer-lived liquid droplets with warming, which subsequently contributes to an enhanced positive lapse rate feedback and thus enhanced Arctic warming. Correcting for the low bias in cloud supercooled liquid weakens these warming-induced phase changes and therefore reduces Arctic amplification. Our results emphasize the importance of realistic representations of not only cloud thermodynamic phase in and of itself for climate change simulations, but also the sensitivity of cloud thermodynamic phase to the cloud microphysical processes that influence it, particularly in the Arctic.