Centuries of thermal sea-level rise due to anthropogenic emissions of short-lived greenhouse gases

Abstract

Mitigation of anthropogenic greenhouse gases with short lifetimes (order of a year to decades) can contribute to limiting warming, but less attention has been paid to their impacts on longer-term sea-level rise. We show that short-lived greenhouse gases contribute to sealevel rise through thermal expansion (TSLR) over much longer time scales than their atmospheric lifetimes. For example, at least half of the TSLR due to increases in methane is expected to remain present for more than 200 y, even if anthropogenic emissions cease altogether, despite the 10-y atmospheric lifetime of this gas. Chlorofluorocarbons and hydrochlorofluorocarbons have already been phased out under the Montreal Protocol due to concerns about ozone depletion and provide an illustration of how emission reductions avoid multiple centuries of future TSLR. We examine the "world avoided" by the Montreal Protocol by showing that if these gases had instead been eliminated in 2050, additional TSLR of up to about 14 cm would be expected in the 21st century, with continuing contributions lasting more than 500 y. Emissions of the hydrofluorocarbon substitutes in the next half-century would also contribute to centuries of future TSLR. Consideration of the time scales of reversibility of TSLR due to short-lived substances provides insights into physical processes: sea-level rise is often assumed to follow air temperature, but this assumption holds only for TSLR when temperatures are increasing. We present a more complete formulation that is accurate even when atmospheric temperatures are stable or decreasing due to reductions in short-lived gases or net radiative forcing. climate change | sea-level rise | greenhouse gases | reversibility | Montreal ProtocolA tmospheric concentrations of a range of greenhouse gases (GHGs), including carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), and halocarbons (HCs), have increased since the beginning of the industrial revolution and have been the main drivers of warming global air and ocean temperatures as well as rising sea levels (1). The goal of this paper is to assess whether the sea-level rise through thermal expansion (TSLR) induced by anthropogenic increases in GHGs that are short-lived (i.e., those with atmospheric residence times of years to decades) is reversible, and on what time scales (2, 3). We also discuss the TSLR due to long-lived gases (defined here as those with residence times of several centuries or longer) and show how the comparison of responses of TSLR to different gases elucidates the climate-system processes that control TSLR reversibility. Understanding how emissions of different gases each affect the Earth's climate over time is central to policy evaluation (e.g., when tradeoffs between mitigation options for different GHGs are considered).A fundamental goal of the United Nations Framework Convention on Climate Change (UNFCCC) and its Paris agreement is the stabilization of GHGs at a level that would avoid dangerous anthropogenic interference with the climate system. Future...

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