Permafrost at high latitudes and altitudes is potentially one of the largest Earth system feedbacks to climate change. Over thousands of years, these vast regions have accumulated trillions of tons of organic carbon in soils and vegetation, far exceeding the carbon stocks in the atmosphere and all living things . Though permafrost has been a persistent carbon sink for millennia, human-caused climate change, which is unprecedented in the Earth’s history, could trigger widespread permafrost thaw and greenhouse gas release. Permafrost degradation has acute effects on local human communities and permafrost ecosystems, which are some of the last remaining wilderness on Earth. At its coarsest level, the permafrost zone can be split into terrestrial and subsea components. On the terrestrial side, there remains disagreement about whether vegetation will accelerate or offset greenhouse gas release from soils (McGuire et al 2018, Abbott et al 2016). Likewise, the role of disturbances such as permafrost collapse, drought, nutrient limitation, pests, and wildfire remain poorly understood (Turetsky et al 2020, Abbott et al 2021). There are even more severe knowledge gaps about the response of subsea permafrost to climate change. While there is increasing consideration of the terrestrial permafrost feedback, subsea permafrost remains virtually absent from the policy sphere (Sayedi et al 2020).

In this talk, we will describe the current understanding of the terrestrial and subsea permafrost climate feedbacks. Drawing on the rapidly expanding body of literature on the permafrost zone, we will discuss the research and policy challenges of improving understanding and reducing human emissions to decrease the severity of these feedbacks. We will pay particular attention to the subsea permafrost feedback, which has received much less coverage. Because the subsea permafrost system is unfamiliar to many, we provide some background below.

During the last ice age, the unglaciated continental shelves of the Arctic ocean and surrounding seas accumulated significant amounts of organic carbon in their soil profiles (Osterkamp and Harrison 1982). As ice sheets and glaciers melted after the last glacial maximum, sea level rose more than 130 m, inundating millions of square kilometers of tundra and taiga (Lindgren et al 2016). The submerged permafrost began thawing immediately, but because of the immense thermal inertia of this system, it is still degrading today (Shakhova et al 2009). In a recent study, we used expert assessment—a method that has been long used for making decisions under uncertainty and limited knowledge—to generate first-order estimates about carbon stocks and sensitivity in the subsea permafrost domain (Sayedi et al 2020). Drawing on published and unpublished observations and simulations, the 25 experts in our study estimated that the subsea permafrost domain contains ~560 gigatons carbon (GtC) in organic matter and 45 GtC in CH4. Current fluxes of CH4 and CO2 to the water column were estimated at 18 and 38 megatons C yr-1, respectively. Under Representative Concentration Pathway RCP8.5, the subsea permafrost domain could release 43 Gt CO2-equivalent by 2100 and 194 Gt CO2-equivalent by 2300, with ~30% fewer emissions under RCP2.6. All of these estimates had wide ranges of uncertainty, demonstrating a serious knowledge gap but providing initial estimates of the magnitude and timing of the subsea permafrost climate feedback. Experts emphasized that the lack of field data creates high uncertainty regarding carbon stocks and emissions. Additionally, ignoring the hydrochemical links between the subsea and terrestrial permafrost limits our ability to anticipate thresholds in both systems.

Ultimately, for both terrestrial and subsea permafrost, these feedbacks are likely to amplify human-caused warming. However, given that primary emissions from humans are two orders of magnitude higher than the likely range of the permafrost climate feedback, accelerating the decarbonization of the global economy should be the policy focus.