The rapid melting of Arctic sea ice is the largest and clearest signal of anthropogenic climate change. Current projections indicate that the first ice-free Arctic summer will likely occur by mid-century, owing to increasing carbon dioxide concentrations in the atmosphere. However, other powerful greenhouse gases have also contributed to Arctic sea ice loss, notably ozone-depleting substances (ODSs). In the late 1980s, ODSs became strictly regulated by the Montreal Protocol, and their atmospheric concentrations have been declining since the mid-1990s. Here, analyzing new climate model simulations, we demonstrate that the Montreal Protocol, designed to protect the ozone layer, is delaying the first appearance of an ice-free Arctic summer, by up to 15 y, depending on future emissions. We also show that this important climate mitigation stems entirely from the reduced greenhouse gas warming from the regulated ODSs, with the avoided stratospheric ozone losses playing no role. Finally, we estimate that each Gg of averted ODS emissions results in approximately 7 km2 of avoided Arctic sea ice loss.

Abstract Previous studies have used coupled climate model simulations with perturbed sea ice covers to assess the impact of future Arctic sea ice loss. The results of these studies suggest that Arctic sea ice loss will cause substantial climate impacts in the Arctic and beyond. The approaches used in these simulations can be broadly categorized into three methods: adding a ghost flux to the sea ice module, nudging, and modifying the surface albedo. Here we show that all three methods ultimately add heat to the Arctic in order to melt the sea ice, and that this artificial heating causes a spurious warming signal that is added to the warming that occurs due to sea ice loss alone. We illustrate this using an idealized climate model, which provides a preliminary rough estimate of the effect. In this model, the annual-mean warming due to sea ice loss alone can be directly calculated. We compare this with the warming that would be attributed to sea ice loss using each of the three methods in the idealized model. The results suggest that each method substantially overestimates the warming due to sea ice loss alone, overestimating the surface warming throughout the Northern Hemisphere by a factor of 1.5–2 in the idealized model. Hence, these results suggest that previous coupled climate modeling studies have overestimated the climate response to sea ice loss.

Nearly half of the freshwater flux from the Antarctic Ice Sheet into the Southern Ocean occurs in the form of large tabular icebergs that calve off the continent's ice shelves. However, because of difficulties in adequately simulating their breakup, large Antarctic icebergs to date have either not been represented in models or represented but with no breakup scheme such that they consistently survive too long and travel too far compared with observations. Here, we introduce a representation of iceberg fracturing using a breakup scheme based on the "footloose mechanism." We optimize the parameters of this breakup scheme by forcing the iceberg model with an ocean state estimate and comparing the modeled iceberg trajectories and areas with the Antarctic Iceberg Tracking Database. We show that including large icebergs and a representation of their breakup substantially affects the iceberg meltwater distribution, with implications for the circulation and stratification of the Southern Ocean.