Subsea permafrost carbon stocks and climate change sensitivity estimated by expert assessment

The median estimate by the group of experts for the area of formerly subaerial permafrost inundated after the LGM was 3.5 million km² (2.5–4.4; range is the 90% confidence interval), which agrees closely with estimates from the literature (Lindgren et al 2016, Overduin et al 2019). This estimate suggests the subsea permafrost domain is ∼1/5 the size of the terrestrial permafrost domain, which includes ∼18 million km² in the Northern Hemisphere (Hugelius et al 2014, Schuur et al 2015). Experts estimated that the current extent of subsea permafrost was ∼2 million km² (1–2.7; figure 2(a)), indicating a 42% decrease in subsea permafrost extent since the LGM (figure 2(a)). When calculated for each expert individually, the median decline in permafrost extent since the LGM was 47% (figure S3(a)).

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Experts estimated that 500 gigatons carbon (GtC) (250–750) in soil organic matter (SOM) was stored in and on the continental shelves at the LGM (figure 2(b)). Expressed on an areal basis (i.e. 140 kg C m⁻²), these estimates of SOM from the continental shelves are similar to carbon densities in the continuous permafrost zone (70–200 kg C m⁻²), where SOM is often deposited many meters below the surface by periglacial processes (Hugelius et al 2014, Shmelev et al 2017, Lindgren et al 2018). Current SOM stocks were estimated at 460 GtC (150–540; figure 2(b)), indicating a median decrease of 10% of the SOM present at the LGM, based on the two unpaired distributions. However, when calculated for each expert individually, median SOM decreases were 33%–40%, suggesting substantial mineralization since the LGM (figure S3(b)). In their comments, experts attributed this decrease in SOM to microbial decomposition of OM to CH₄ and CO₂ after permafrost thaw (Thornton et al 2016a, Winkel et al 2018). Current stocks of OM in sediment deposited following the marine transgression were estimated at 100 GtC (23–200; figures 1(d) and 2(b)). Together, these estimates suggest that 560 GtC (170–740) of OM is currently stored in surface sediment and paleosols of the subsea permafrost domain (figure 1(d)).

Experts estimated a wide range of current CH₄ stocks with a median of 45 GtC (14–110; figure 2(c)). Based on expert comments, the main reason for this uncertainty was the extremely patchy spatial coverage of observations of CH₄ deposits, primarily hydrates, but also dissolved and free gas in sediment (figure 1(d)). Experts highlighted that estimating organic carbon content of surface sediments across the vast subsea permafrost domain is already challenging (Martens et al 2019), and quantifying CH₄ deposits requires more expensive and complicated drilling beneath the seafloor (Frederick and Buffett 2014, Ruppel and Kessler 2017). Experts mentioned that the scarcity of CH₄ observations is exacerbated by reluctance or legal prohibition of sharing data generated during research expeditions, energy exploration by private companies, and national security activities.

We asked experts to estimate potential changes in CO₂ and CH₄ flux for three climate scenarios from the IPCC Fifth Assessment Report (Moss et al 2010). The selected RCPs were RCP2.6, which has a peak concentration of ∼490 ppm CO₂-equivalent (CO₂e) reached before 2100, RCP4.5 with a peak of ∼650 ppm CO₂e at 2100, and RCP8.5 with a peak of ∼1400 ppm CO₂e at 2100 (Moss et al 2010, Koenigk et al 2013). There are many potential controls on greenhouse gas production and consumption in the subsea permafrost domain, including changes in temperature, microbial activity, sea level, altered chemistry in the Arctic Ocean, and changes in photosynthesis associated with loss of sea ice or changes in nutrient availability (supplementary information). Because of this complexity, we first asked experts to estimate the percentage of the subsea OM (including relict SOM and sediment deposited since the LGM) and CH₄ stocks that could be affected thermally or biogeochemically by any of the RCP scenarios.

For OM in 2050, 2100, and 2300, experts estimated that 3% (1–10), 8% (2–20), and 28% (10–55), respectively (figure 3(a)), could be influenced by climate change. This suggests that a globally-relevant store of OM may experience changes in physical or biogeochemical state due to anthropogenic climate change by 2100 (3.3–240 GtC) and 2300 (18–420 GtC; figure 3(c)). Experts estimated that a smaller percentage of CH₄ stocks would be affected by anthropogenic climate change, with median estimates of 2.5% (1–6.5), 8% (2–15), and 18% (5–35) by 2050, 2100, and 2300 (figure 3(b)). When combined with current estimates of CH₄ stocks (figure 2(c)), these percentages translate into highly uncertain but still substantial quantities of affected CH₄ by 2100 (0.2–42 GtC) and 2300 (0.6–64 GtC; figure 3(d)). Together, these estimates suggest that CH₄ in the forms of hydrates, dissolved, and free gas may be less sensitive to climate change than OM stored in the continental shelves. Experts suggested this could be due to the depth of some hydrate deposits and the combined effects of water pressure, temperature, and hydrate composition, which together determine the zone of hydrate stability (Frederick and Buffett 2014, Ruppel and Kessler 2017). These findings of relatively insensitive CH₄ deposits support the growing evidence from paleoclimate studies that subsea hydrates contributed minimally to the abrupt climate change and correspondingly abrupt increase in CH₄ at the beginning of the Holocene (Fischer et al 2008, Sowers 2010, Petrenko et al 2017, Dyonisius et al 2020) and during previous paleoclimate perturbations (Jurikova et al 2020).

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The central estimates of present and future fluxes of CO₂ and CH₄ were highly dispersed, varying by a factor of 30, based on average range of lower and upper values for each expert (figures 2(d) and 4(a), (b)). For present conditions, the net CO₂ flux to the water column was estimated at 38 MtC yr⁻¹ (13–110), though two high estimates were two orders of magnitude above that (figure 2(d)). We only asked for CO₂ flux estimates to the water column to avoid complexities associated with marine primary production, which is already included in Earth system models (Laufkötter et al 2015). For the current net flux of CH₄ to the water column, experts estimated 18 MtC yr⁻¹ (2.3–34) and for the release to the atmosphere, they estimated 4.3 MtC yr⁻¹ (3–16), indicating that ∼75% of CH₄ released from sediment is oxidized before reaching the atmosphere (figure 2(d); Thornton et al 2016a). The experts projected that the amount of CH₄ flux to the atmosphere would have a net increase of 2% (0–3), 5% (1–8), and 4.5% (0.5–15) for RCP2.6 by 2050, 2100, and 2300, respectively; 5% (0.4–10), 10% (4.5–23), and 15% (5–60) for RCP4.5; and 10% (2.5–25), 25% (13–55), and 49% (16–80) for RCP8.5 on the same time steps (figure 4(b)). The estimates paired by expert indicated a 75% (72–79) decrease in added CO₂ emissions for RCP2.6 versus RCP8.5 and a 73% (70–89) decrease in added CH₄ emissions (figure S4).

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To compare the overall climate forcing from the subsea permafrost domain, we converted CH₄ emissions into CO₂e, using the 100 year conversion factor of 28 from the IPCC Fifth Assessment Report (Schuur et al 2013, Abbott et al 2016). After conversion (table S5), CH₄ contributed more than half of the overall climate forcing, accounting for a mean of 65%, 67%, and 72% of the cumulative CO₂e releases for RCP2.6, RCP4.5, and RCP8.5, respectively (figure 5). Experts estimated substantially different responses depending on warming scenario, with the smallest increases associated with RCP2.6 and the largest with RCP8.5 (figure 4(a)). By 2050, 2100, and 2300, experts projected a change of 2% (0.1–3), 5% (1–10), and 4.5% (2–8) for RCP2.6; a change of 5% (0.5–10), 10% (3.5–30), and 28% (10–50) for RCP4.5; and a change of 10% (2.5–20), 20% (11–45), and 53% (20–100) for RCP8.5(figure 4(a)).

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When summed to estimate total climate forcing in CO₂e, the cumulative greenhouse gas release from the subsea permafrost domain under RCP2.6 was 11 Gt CO₂e (4–30) by 2050, 35 (10–83) by 2100, and 130 (34–270) by 2300. Under RCP4.5, cumulative greenhouse gas emissions were similar to RCP2.6 for 2050 and 2100–12 (5–32) and 38 Gt CO₂e (14–96), respectively—but higher for 2300: 150 Gt CO₂e (49–380). For RCP8.5, emissions were substantially higher across 2050, 2100, and 2300, with 13 (5–36), 43 (14–110), and 190 Gt CO₂e (45–590) released, respectively. Experts estimated substantially elevated emissions under all scenarios relative to current rates, with 35%–45% higher emissions by 2050, 50%–80% higher emissions by 2100, and 60%–135% higher emissions by 2300 (figure 5). Together, these results highlight how the permafrost domain continues to respond to past warming (i.e. after the LGM) yet is still sensitive to the degree of anthropogenic warming in the future.

Evaluating how much subsea permafrost has degraded and how much SOM has been decomposed since sea levels rose ∼14 000 years ago (Church et al 2010) can provide perspective on the current permafrost climate feedback. If the expert range of estimates of 30%–50% decline in area and 33%–40% decline in OM stocks are reliable (figure S3(b)), this suggests that the subsea permafrost system responds relatively slowly (i.e. millennial timescales) to climate change, and that anthropogenically-driven changes may only substantially alter subsea permafrost dynamics several hundreds or thousands of years from now. However, if the expert estimates of current carbon fluxes are reliable, the subsea permafrost domain is already contributing regionally and globally relevant quantities of greenhouse gases to the Arctic Ocean and atmosphere in response to paleoclimate changes since the LGM. Our results suggest the ocean-atmosphere flux of CH₄ from the subsea permafrost domain equals 10%–40% of CH₄ release from the five-fold larger terrestrial permafrost zone (Mcguire et al 2009). This range is bracketed by low (Thornton et al 2016a) and high (Shakhova et al 2013, Thornton et al 2016b) field estimates from the East Siberian shelf. Furthermore, our estimates suggest that subsea CO₂ flux could already be offsetting 10%–20% of the terrestrial permafrost carbon sink (Mcguire et al 2009).

Considering future emissions scenarios, the net ecosystem carbon balance of the subsea permafrost domain was projected to be negative under all scenarios (i.e. net loss to the atmosphere) in our study. This contrasts with estimates of future carbon balance in the terrestrial permafrost zone, where both positive and negative projections are considered plausible (Mcguire et al 2018). Under RCP4.5, the multimodel median of terrestrial permafrost carbon balance projects net removal of 140 and 94 Gt CO₂ from the atmosphere by 2100 and 2300, respectively (Mcguire et al 2018). Our central estimates suggest that the subsea permafrost zone could offset 27% of that uptake by 2100 and 160% of that uptake by 2300, i.e. releasing substantially more greenhouse gas than the terrestrial permafrost zone removes. Under RCP8.5, the terrestrial permafrost zone is expected to release 16 GtC by 2100 and 220 GtC by 2300 (Mcguire et al 2018). Our central results suggest that subsea carbon release could augment this terrestrial release by 32% in 2100 and 8% in 2300. Considering the upper estimates from our study, the subsea permafrost domain could augment terrestrial release by 100% in 2100 and 34% in 2300. These simplified comparisons suggest that the subsea permafrost domain may play an outsized role in determining the overall carbon balance of high latitude ecosystems. More generally, the carbon stocks and current and future emissions from the subsea permafrost domain are large relative to the geographical size of this region: ∼0.4% of the Earth’s surface area but up to 2% of global CH₄ release and 31% of oceanic surface sediment carbon (Saunois et al 2016, Friedlingstein et al 2019). This suggests that the subsea permafrost domain is already a hot spot of carbon storage and greenhouse gas release, justifying increased ecological research and monitoring.

The expert estimates from this study suggest that contemporary CO₂ and CH₄ emissions from the subsea permafrost domain are sensitive to anthropogenic climate change on decadal timescales. However, compound uncertainties surrounding the terrestrial and subsea permafrost climate feedbacks mean that the relative importance of these environments in determining greenhouse gas release will remain unknown until better empirical and modeling estimates are available (Mcguire et al 2018, Overduin et al 2019, Turetsky et al 2020). We emphasize that because the subsea and terrestrial permafrost zones are fundamentally linked (Vonk et al 2012, Tesi et al 2016), understanding the fate of old and new OM on the continental shelves of the Arctic Ocean basin should be a research priority.

Based on expert comments, the primary contributor to uncertainty in the subsea permafrost domain is insufficient field observations. Almost every expert mentioned this conspicuous knowledge gap. The lack of data reduces the reliability of estimates of carbon pools and fluxes as well as the thermal and hydrological conditions of submerged permafrost. Experts pointed out that our ignorance of terrestrial and marine permafrost linkages does not simply create uncertainty in current estimates, it limits our ability to anticipate thresholds and unexpected linkages. For example, the subsea permafrost climate feedback could follow qualitatively different trajectories than identified here, if changes in Arctic runoff, sediment balance, and sea ice alter organic carbon inputs or the location of the CH₄-hydrate stability zone (Lenton et al 2008, Wrona et al 2016, Bamber et al 2018, Trusel et al 2018). Specific questions raised by experts that cannot currently be answered with satisfactory certainty include: what were rates of sedimentation and OM burial at the sea bottom during the last several thousand years (Martens et al 2019); what was and is the vertical and lateral distribution of carbon (OM and CH₄) in paleosols and marine sediments on the continental shelves (Lindgren et al 2018); how much local variability was there in climate and ecosystem type before the LGM (Huang et al 2017); what was the effect of marine transgression on the OM stocks of shelf sediments and their resulting re-distribution (Winterfeld et al 2011, Günther et al 2013); and where are different kinds of microbial metabolism active in the subsea permafrost domain (Overduin et al 2015, Winkel et al 2018)?

One unexpected finding of this research was that the dearth of data on the subsea permafrost domain is partially due to divisions in the subsea permafrost research community. While previous expert assessments on other topics have always involved strong opinions and evidence-based disagreements (Schuur et al 2013, Abbott et al 2016), we found that many invited experts declined to participate or at least expressed serious concerns because of political and territorial considerations, including perceived or real threat of retribution or negative professional consequences. These rifts between research groups and culture of antagonistic competition long precede this paper, as evidenced by unsuccessful synthesis efforts in the past and frequent rebuttals and conflict surrounding published and presented research products (e.g. Thornton et al (2019)). We hope that this exercise, which involved permafrost researchers from many research groups, institutions, career stages, and cultural backgrounds, can contribute to a détente and improvement of collaborative research. At the least, we trust that these initial and uncertain estimates will encourage the publication of expansions and rebuttals. Indeed, new (or newly published) observations of the physical state (e.g. subsurface geologic structure, temperature, pressure), chemical state (e.g. pore-water chemistry, pH, redox conditions, hydrate composition), and biological state (rates of aerobic and anaerobic metabolisms (Koch et al 2009, Overduin et al 2015, Winkel et al 2018)) of the subsea permafrost domain are desperately needed to reduce the uncertainty around the estimates presented here and in recent work (Vonk et al 2014, Shakhova et al 2017, Lindgren et al 2018). These data could better constrain models of present and future biogeochemistry as well as reveal past behavior of subsea permafrost and OM, generating fundamental insights into biological dynamics at millennial timescales. Because of the logistical challenges of collecting data from the continental shelves, which ultimately requires deep scientific drilling within the exclusive economic zones of various countries, international collaborations as well as private-public partnerships will be required to meet these goals.

Although there is evidence that subsea permafrost could be a major greenhouse gas source, it has not been quantitatively considered in any major climate change reports (table S1). This is attributable to the lack of data and published estimates of policy-relevant information. For example, in the few reports that do mention subsea permafrost, there is no detail on the extent and magnitude of subsea carbon stocks or potential greenhouse gas release (table S1). The readers of these reports, which are mainly written for public and policymakers, cannot therefore have an accurate understanding of the potential influence of this ecosystem feedback on the climate system.

While policymaking is inherently based on values, it should be informed by the best available scientific evidence (Joly et al 2010, Sutherland and Burgman 2015). To integrate the latest science into policymaking and to direct scientific inquiry to address societally relevant problems, it is important to ensure the timely communication of relevant information between policymakers and scientists (figure 6). Despite the necessity of the relationship between science and policy, this link is often indirect or weak (Brownell and Roberto 2015, Cherubini et al 2016). The uncertainty and complexity involved in both science and policymaking can make it difficult for representatives from one world to appreciate the implications of uncertainties presented by the other (Bradshaw and Borchers 2000, Maxim and van der Sluijs 2011). The communication of these compound uncertainties between science, policy, and public circles is inherently challenging, potentially causing frustration and even undermining science and policy objectives (Morgan 2014, Sutherland and Burgman 2015). Consequently, for many environmental issues, which always involve high levels of complexity and uncertainty, much of the latest science is not considered by policymakers and much of the best policy knowledge is unknown by researchers (Cortner 2000, Liu et al 2008). In the case of the permafrost climate feedback (both terrestrial and subsea), we suggest that expert assessment should be more regularly implemented to quantify uncertainties and identify research priorities. As more data and simulations become available, repeat assessments (e.g. every 5 or 10 years) could reveal whether estimates are converging and ensure that the most up-to-date information is available to policymakers.

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