# Expert assessment of future vulnerability of the global peatland carbon sink

J. Loisel, A. Gallego-Sala, M. Amesbury, G. Magnan, G. Anshari, D. Beilman, J. Benavides, J. Blewett, P. Camill, D. Charman, et al.

► To cite this version:

J. Loisel, A. Gallego-Sala, M. Amesbury, G. Magnan, G. Anshari, et al. Expert assessment of future vulnerability of the global peatland carbon sink. Nature Climate Change, 2020, pp.70-77. 10.1038/s41558-020-00944-0. hal-03064857

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# Expert assessment of future vulnerability of the global peatland carbon sink

J. Loisel ID 1✉, A. V. Gallego-Sala ID 2✉, M. J. Amesbury ID 2,3, G. Magnan4, G. Anshari5, D. W. Beilman ID 6, J. C. Benavides7, J. Blewett ID 8, P. Camill9, D. J. Charman ID 2, S. Chawchai ID 10, A. Hedgpeth11, T. Kleinen ID 12, A. Korhola3, D. Large13, C. A. Mansilla ID 14, J. Müller ID 15, S. van Bellen16, J. B. West ID 17, Z. Yu18,19, J. L. Bubier20, M. Garneau4, T. Moore21, A. B. K. Sannel ID 22, S. Page23, M. Väliranta ID 3, M. Bechtold ID 24, V. Brovkin ID 12, L. E. S. Cole ID 25, J. P. Chanton26, T. R. Christensen ID 27, M. A. Davies ID 28, F. De Vleeschouwer29, S. A. Finkelstein28, S. Frolking30, M. Gałka31, L. Gandois ID 32, N. Girkin ID 33, L. I. Harris ID 34, A. Heinemeyer ID 35, A. M. Hoyt ID 36,37, M. C. Jones ID 38, F. Joos ID 15, S. Juutinen3, K. Kaiser39, T. Lacourse ID 40, M. Lamentowicz ID 41, T. Larmola ID 42, J. Leifeld ID 43, A. Lohila44, A. M. Milner ID 45, K. Minkkinen3,46, P. Moss ID 47, B. D. A. Naafs ID 8, J. Nichols ID 48, J. O'Donnell49, R. Payne50,59, M. Philben51, S. Piilo ID 3, A. Quillet52, A. S. Ratnayake ID 53, T. P. Roland2, S. Sjögersten54, O. Sonnentag ID 55, G. T. Swindles56, W. Swinnen24, J. Talbot ID 55, C. Treat ID 30, A. C. Valach ID 57 and J. Wu ID 58

The carbon balance of peatlands is predicted to shift from a sink to a source this century. However, peatland ecosystems are still omitted from the main Earth system models that are used for future climate change projections, and they are not considered in integrated assessment models that are used in impact and mitigation studies. By using evidence synthesized from the literature and an expert elicitation, we define and quantify the leading drivers of change that have impacted peatland carbon stocks during the Holocene and predict their effect during this century and in the far future. We also identify uncertainties and knowledge gaps in the scientific community and provide insight towards better integration of peatlands into modelling frameworks. Given the importance of the contribution by peatlands to the global carbon cycle, this study shows that peatland science is a critical research area and that we still have a long way to go to fully understand the peatland-carbon-climate nexus.

Peatlands are often regarded as stable systems, with limited influence on annual carbon (C) cycling dynamics at the global scale. To some extent, this is true: their net C exchange with the atmosphere (a sink of ~0.14 Gt yr⁻¹) (ref. 1) is equivalent to ~1% of human fossil fuel emissions, or 3–10% of the current net sink of natural terrestrial ecosystems². However, and despite occupying only 3% of the global land area³, peatlands contain about 25% (600 GtC) of the global soil C stock⁴, which is equivalent to twice the amount in the world’s forests⁵. This large and dense C store is the result of the slow process of belowground peat accumulation under saturated conditions that has been taking place over millennia, particularly following the Last Glacial Maximum (LGM), as peatlands spread across northern ice-free landscapes⁴. Given their ability to sequester C over long periods of time, peatlands acted as a cooling mechanism for Earth’s climate throughout most of the Holocene⁶,⁷. Should these old peat C stores rejoin today’s active C cycle, they would create a positive feedback on warming. However, the fate of the global peat C store remains disputed, mainly because of uncertainties that pertain to permafrost dynamics in the high latitudes as well as land-use and land-cover changes (LULCC) in the boreal, temperate and tropical regions⁸.

Peatland C stocks and fluxes have yet to be incorporated into Earth system models (ESMs), although they are beginning to be implemented in global terrestrial models⁹,¹⁰. As these models are

moving towards the integration of permafrost dynamics, LULCC and other disturbances (such as fire), the absence of peatland C dynamics could lead to many problems in the next generation of models (Fig. 1a). For example, the omission of organically rich soils was a key contributor to the inaccurate estimates of organic soil mass, heterotrophic respiration and methane (CH₄) emissions in recent Climate Model Intercomparison Project Phase 5 (CMIP5) simulations¹¹. Likewise, the successful integration of permafrost dynamics into land surface models necessitates the inclusion of peatlands, as the latter occupy approximately 10% of the northern permafrost area and account for at least 20% of the permafrost C stocks¹², of which a sizable fraction is susceptible to wildfire¹³. LULCC scenarios must also account for temperate and tropical peatland degradation to derive better estimates of C fluxes¹⁴ and associated impacts on radiative forcing¹⁵. The inclusion of peatlands in ESMs should help address the complexity of the interacting, cross-scale drivers of change that control peat C dynamics and quantify their contribution to a positive C cycle feedback, now and in the future.

Peatland conversion and restoration are also not considered in integrated assessment models (IAMs), although there is growing anthropogenic pressure on peatland ecosystems worldwide¹⁶,¹⁷. Atmospheric carbon dioxide (CO₂) emissions associated with degraded peatlands account for 5–10% (0.5–1 GtC) of the global
<figure><img src="image_2.png" /></figure>

**Fig. 1** | Integrating peatland knowledge in climate change modelling frameworks. a,b, Conceptual structure of an Earth system model (a) and an integrated assessment model (b). The ESM emphasizes peatland carbon, energy, water and nutrient pools, and exchanges with the atmosphere, aquatic systems and the world's oceans. The IAM focuses on the importance of considering peatlands in policy options and land management decisions, as these carbon-rich ecosystems can contribute substantially to strategies for GHG emission reduction. ES, ecosystem services; GDP, gross domestic product; grey arrows, fluxes with important contribution from peatlands; white arrows, non-peatland fluxes.

annual anthropogenic CO₂ emissions¹⁸,¹⁹, despite their small geo-
graphic footprint (Fig. 1b). Although the preservation of pristine
peat deposits would be ideal, the restoration of degraded sites, par-
ticularly through rewetting, could prevent additional CO₂ release to
the atmosphere and reduce the risk of peat fires²⁰,²¹. Even if restora-
tion leads to C neutrality (that is, sites stop losing C but do not start
gaining it), their GHG-saving potential would be similar to that of
the most optimistic sequestration from biochar and cover cropping
from all agricultural soils combined¹⁹,²². As IAMs move towards the
integration of nature-based climate solutions to limit global tem-
perature rise, peatland restoration and conservation are poised to
gain in importance in those models, as well as in the international
political arena²³. In turn, the socio-economic scenarios developed
in IAMs could help inform the role of management interventions in
future peatland use and could guide policy options to best inform
the implementation of GHG emission control strategies for decision
makers. Ultimately, these model outputs will help predict the effect
of peatland management on the global C cycle.

Here, we review the main agents of change of peatland C stocks
and fluxes, including drivers that can induce rapid peatland C losses
(peat fire, land-use change (LUC), and permafrost thaw) and grad-
ual drivers that can lead to rapid, nonlinear responses in peatland
ecosystems (temperature increase, water table drawdown, sea-level
rise, and nutrient addition) (Fig. 2). We use an expert elicitation
to assess the perceived importance of these agents of change on C
stocks, and ask one question: ‘What is the relative role of each agent
of change in shifting the peatland C balance in the past, present
and future?’ Estimates are based on responses from 44 peat experts
(see Supplementary Information for details). Four time periods are
studied: post-LGM (21,000 yr BP to 1750 CE), Anthropocene (1750–
2020 CE), remainder of this century (2020–2100 CE) and far future
(2100–2300 CE). The confidence and expertise levels are tallied for
each of the experts’ responses (Supplementary Tables S6 and S9),
along with the sources that guided their estimates (Supplementary
Information Appendix 4). Arithmetic means and 80% central ranges
(10th to 90th percentiles) are presented (below and in Fig. 3), as well
<figure><img src="image_3.png" /><figcaption>**Fig. 2** | The main agents of change impacting the global peatland carbon balance globally. With expert elicitation combined with a literature review, the importance of each agent in the past, present and future is assessed semi-quantitatively in this study (see Supplementary Information Appendix 5 for a high-resolution image without text details and a brief review of each agent of change).</figcaption></figure>

as other measures of central tendencies (Supplementary Tables S4 and S5). Central values provide order-of-magnitude estimates that may be useful to the reader, but the strength of this elicitation is in its ability to identify where experts agree and disagree, and to recognize ranges of responses across experts. Therefore, the elicitation findings can inform how integrating peatlands into modelling frameworks such as ESMs and IAMs could advance peatland process understanding and further test hypotheses that emerge from different schools of thought.

## Drivers of peatland carbon stocks since the Last Glacial Maximum

For the post-LGM time period, experts consider temperature to be the most important long-term driver of peat accumulation in extra-tropical peatlands (arithmetic mean = 524 GtC; 10th to 90th percentiles = 60 GtC to 890 GtC) (Fig. 3). A positive moisture balance is deemed a necessary condition for peatland development, maintenance and C preservation (238 (10 to 570) GtC). Several respondents comment that it is difficult, if not impossible, to separate the respective roles of these two agents of change (Supplementary Information Appendix 3). This exemplifies the need to integrate peatlands in ESMs, as cross-scale interactions between agents of change on peatland C dynamics could then be evaluated further. Permafrost is also thought to be of importance owing to its capacity to inhibit peat decay in northern high-latitude peatlands (218 (−14 to +531) GtC). That said, experts note that permafrost also probably contributes to lower C accumulation rates (when compared to non-permafrost sites); permafrost also possibly contributes to peat erosion in regions where wind-drifted snow and ice crystals can abrade dry peat surfaces²⁴. The large range of values for permafrost (Extended Data Fig. 1) stems from the fact that some respondents attribute the entire permafrost peatland C pool to the presence of permafrost itself, whereas others attribute the C pool mainly to temperature and moisture, with permafrost aggradation playing the secondary role of protecting C stocks. Experts suggest

that, in the tropics, long-term peat C sequestration is driven mainly by moisture availability (268 (24 to 360) GtC), with wetter conditions slowing down peat decomposition. Temperature (43 (0 to 128) GtC) and sea level (7 (−13 to +52) GtC) are identified as secondary agents promoting peat formation and growth. Estimates for the net role of sea level on tropical C stocks are near zero because some of the high C accumulation rates following sea-level rise in certain regions are counterbalanced by C losses owing to continental shelf flooding and associated peat erosion or burial in other regions²⁵ (Fig. 3).

These results are largely corroborated by the literature review. On the basis of extensive paleo records, we know that peatlands have spread across vast landscapes following the LGM¹. As long as sufficient moisture conditions are maintained, warmer and longer growing seasons can contribute to increases in plant productivity and peat burial in many extra-tropical regions²⁶⁻²⁸, but can contribute to enhanced decomposition and carbon loss in the tropics²⁹,³⁰ where the growing season length and temperature are not limiting factors for photosynthesis¹,³¹. Indeed, water saturation is a key control on oxygen availability in peat and on plant community composition, and therefore an important determinant for CO₂ and CH₄ emissions and for net ecosystem C balance in both intact and drained peatlands worldwide³²⁻³⁴. Soil moisture excess is a necessary condition for long-term peat development; surface wetness must remain sufficient to minimize aerobic respiration losses and to provide conditions that inhibit the activity of phenol oxidase³⁵. In the tropical and mid-latitude regions, water table depth is recognized as the main agent driving long-term peat accumulation³⁶⁻³⁸. The literature review tells us that, at the regional scale, sea-level rise may lead to either net C losses³⁹ or net C gains⁴⁰. For example, sea-level decline in the tropics⁴¹ and land uplift following deglaciation in the North⁴² contributed to peat expansion over the past 5,000 years. Conversely, in the (sub-)tropics, sea-level rise can drive up groundwater levels regionally, which allows coastal peatlands to expand and accrete at greater rates⁴³,⁴⁴. This process, which took place during the previous
<figure><img src="image_4.png" /><figcaption>**Fig. 3** | Expert assessment of the global peatland carbon balance over time. Changes in carbon stocks are shown for the extra-tropical northern region (blue) and the (sub-)tropical region (yellow) for the post-LGM (21,000 yr BP to 1750 CE), Anthropocene (1750-2020 CE), near future and remainder of this century (2020-2100 CE), and far future (2100-2300 CE). Agents of change: temperature (T), moisture (M), sea level (SL), fire (F), land use (LU), permafrost (P), nitrogen deposition (N) and atmospheric pollution (AP). Coloured bars, arithmetic means; error bars, 80% central range. Positive values represent carbon sinks to the atmosphere (see Extended Data Fig. 1 for individual survey responses).</figcaption></figure>

interglacial²⁵ and other past warm climates, is likely to be most pro-
nounced in the large coastal peatlands of the (sub-)tropics. Tectonic
subsidence can lead to vast accumulations of lignite over millions
of years⁴⁵,⁴⁶, but its conjunction with rapid sea-level rise, rapid sub-
sidence or peat surface collapse due to water abstraction or LUC
can lead to peatland loss⁴⁷,⁴⁸. In general, sea-level rise has been sug-
gested to be a threat for coastal peatlands⁴⁹,⁵⁰, as these systems have
limited capacity to move inland because of topography or human
development.

**Drivers of peatland carbon stocks during the**
**Anthropocene**

During the Anthropocene, short-term peat C losses across the northern high latitudes are linked to LUC (-7 (-23 to 0) GtC) and fire (-3 (-8 to 0) GtC) by the experts (Fig. 3). As for permafrost dynamics, small C gains (2 (0 to 10) GtC) are suggested, though many experts warn that large and rapid losses of old C have only recently begun and are expected to increase in the future (Supplementary Information Appendix 3). Peat drainage for agriculture, forestry, industrial-scale peat extraction and grazing

were identified as the main sources of anthropogenic pressure on
these peatlands (Fig. 3). The loss of peat C to human activity must
have been considerable during the pre-industrial time and the start
of the industrial era across Europe, but historical reports are too
few to provide a reliable estimate¹⁸. In this case, LULCC simulations
from IAMs could reduce this uncertainty or provide several scenar-
ios. The loss of C to fire is attributed to an increase in both natural
and anthropogenic burning. Similarly, the main suggested causes of
peat C losses in the tropics are LUC (-8 (-14 to -2) GtC) and fire
(-4 (-10 to 0) GtC). Despite these losses, the trend suggests that
northern high-latitude peatlands have persisted as C sinks through-
out the Anthropocene. Experts attribute the net C gain across the
northern high latitudes primarily to greater peat accumulation
rates that are induced by longer and warmer growing conditions
from climate warming (16 (0 to 38) GtC). An increase in moisture
from greater precipitation is suggested as an additional agent that
leads to C gain in the Arctic, although several experts mention C
losses due to drought across the boreal and mid-latitude regions; an
overall increase of 11 (-1 to +31) GtC from moisture is suggested
by the survey respondents. Finally, nitrogen (N) deposition and
other atmospheric pollution are thought to have a negligible impact
(<1 (-1 to +1) GtC) on the peatland C sink capacity worldwide.

The importance of permafrost and fire revealed in the expert elicitation is reflected in the main findings from the literature review. For instance, across the northern high-latitude regions, increasing air temperatures and winter precipitation have been linked to a greater than 50% reduction in palsa or peat plateau area since the late 1950s⁵¹⁻⁵³, although this varies by region⁵⁴. In general, thermokarst landforms such as ponds or collapse-scar wetlands with saturated soils form when ice-rich peat thaws and collapses. These mainly anaerobic environments are characterized by high CH₄ emissions⁵⁵⁻⁵⁷; mass-balance accounting for C stocks indicates as much as 25-60% of 'old' permafrost C is lost in the years to decades that follow thaw⁵⁸⁻⁶⁰. Over time, increased C sequestration and renewed peat accumulation occur in drained thermokarst lake basins⁶¹,⁶² and collapse-scar wetlands, but it can take decades to centuries and sometimes millennia for collapse-scar wetlands to transition from having a positive (warming) to a negative (cooling) net radiative forcing⁵⁹,⁶³. Moreover, the combustion of peat layers has led to direct losses of plant and peat C. Fire-derived emissions can be substantial, exceeding biological emissions from peat decomposition in some years⁶⁴. The highest emissions are observed from drained tropical peatlands in extremely dry years such as the 1997 El Niño period (810-2570 TgC yr⁻¹) (ref. ⁶⁵) and the 2015 fire season (380 TgC yr⁻¹) (ref. ⁶⁶) in Indonesia. However, as a result of drainage, peat fires are observed even in wet years⁶⁷. Although peat C losses from northern peat fires are smaller than those from tropical peat fires (for example, 5 TgCyr⁻¹ from Alaskan wetlands)⁶⁸, there is a need to consider wildfires in permafrost thaw dynamics because of their effects on soil temperature regime⁶⁹. Peatland surface drying, as a result of both droughts and human activity, has been shown to increase the frequency and extent of peat fires¹³,⁷⁰, which could lead to deeper burns and hindered recovery⁷¹ as well as peat water repellency⁷². In terms of LUC, it is well accepted that widespread peatland conversion, drainage and mining across the temperate and tropical regions have led to large C losses⁷³⁻⁷⁶, in addition to immediate ecosystem damage and land subsidence⁴⁷,⁷⁷. Most peatland management practices result in decreased CH₄ emissions owing to drainage³², but peatland inundation or rewetting can lead to episodic CH₄ releases⁷⁸,⁷⁹. Finally, the structure and function of peatlands are now threatened by increased N availability and atmospheric phosphorus (P) deposition⁸⁰ from anthropogenic emissions⁸¹. For example, *Sphagnum* moss cover dies off after a few years of sustained N loading⁸²⁻⁸⁴, and changes in climate can exacerbate these negative effects⁸⁵. Changes in microbial communities and litter quality associated with N deposition can also contribute to
increased decomposition⁸⁶,⁸⁷ by lowering the peatland surface⁸⁸ and
causing a rise in the water table and CH₄ emission⁸⁹. Conversely, a
study reported a net C gain with modest N deposition in a Swedish
peatland, which was driven by a greater increase in plant production
than in decomposition⁹⁰; this illustrates differences, and perhaps a
threshold response, in C balance response to N deposition.

Quantification of future peatland stocks

Experts anticipate that, during the remainder of this century (2020–2100 CE) and in the far future (2100–2300 CE), the C loss mechanisms presented above will be amplified (Fig. 3). In the northern high latitudes, whereas C gains are still linked to shifts in temperature and precipitation (17 (–16 to +47) and 3 (–37 to +32) GtC, respectively), C losses to fire are expected (–7 (–10 to 0) GtC). Many respondents suggest that better fire management could mitigate this. These losses are predicted to be accompanied by additional ones from permafrost degradation (–30 (–102 to +12) GtC), sea-level rise that would inundate coastal peatlands (–3 (–9 to +1) GtC), and LUC (–14 (–38 to +3) GtC). The latter, and primarily drainage for agriculture, are expected to cause substantial peatland C losses, although many experts anticipate the rate to decrease with increasing conservation and restoration efforts. Regional drought-induced C losses are also suggested for the mid-latitude regions. For the tropics, experts generally agree that every agent of change will negatively impact C stocks. Net peat C losses are predicted owing to higher temperatures (–22 (–14 to +4) GtC; the mean is skewed outside 10th–90th percentile range by an outlier), fires (–23 (–54 to –2) GtC), negative moisture balance (–9 (–31 to +3) GtC) and sea-level rise (–3 (–5 to 0) GtC). Of particular importance is the evolution of the El Niño–Southern Oscillation, as El Niño droughts may lead to substantial C losses to the atmosphere. LUC (–13 (–44 to +3) GtC) is also predicted to play a key role in the future, as it could lead to the drainage of large peat basins such as the Amazon and Congo.

The confidence of experts in their predictions declines for
the far future (Supplementary Tables S6 and S7, and Extended Data
Fig. 2), in part because of the lack of models capable of simulating
the effect of agents of change on peatland C stocks, but also because
policy and land management decisions will influence the future of
peatlands. In this area, the integration of peatlands into IAMs would
allow the generation of pertinent scenarios to help inform the sci-
ence as well as policy options and land management decisions. A
growing world population may put additional pressure on peatlands
as farming becomes possible at higher latitudes, and further defor-
estation may occur in the tropics, but the need to conserve peat
resources may eventually outweigh these pressures. In this case, the
adoption of policies designed to protect peatlands would greatly
limit C losses. Likewise, the pricing of C could change the way peat-
lands are perceived, valued and managed. These diverging opinions
are all included in our assessment (Supplementary Information
Appendix 3), but explicit IAM simulations would allow exploration
of different policies and socio-economic scenarios. Noteworthy is
that extra-tropical peatlands could play an important role, second
only to the oceans, in reducing the global atmospheric CO₂ con-
centration if cumulative anthropogenic emissions are kept below
1,000 GtC (refs. ⁹¹,⁹²). Mitigation is therefore highly important in
counterbalancing the climate impact of peatland C loss⁹³.

Insights from the expert elicitation and their limits

Expert assessment is critical for informing decisions that require
judgements that go beyond established knowledge and model
simulations⁹⁴. For this reason, expert opinion is often used in envi-
ronmental assessments, either as a means to assess confidence levels
or rank potential outputs⁷ or as data points that offer estimates that
could not be provided otherwise⁹⁵,⁹⁶. This expert assessment also
highlights key knowledge gaps and uncertainties, for example in the
impact of permafrost aggradation and degradation on the future

peatland C balance (see Supplementary Information and Extended
Data Fig. 1). Our dataset reflects two main schools of thought that
are anchored in conflicting evidence from the literature: rapid C
loss from deep peats and slow recovery of the peatlands following
permafrost thaw⁵⁹,⁶⁰, and net C gain from rapidly recovering plant
production owing to warm and moist conditions following thaw¹,²⁸.

Our results indicate low to medium confidence in future C flux
estimates. Confidence levels are highest for the post-LGM and
Anthropocene time periods, in part reflecting the majority of paleo
researchers among the survey respondents, and in part because of
compounded uncertainties that pertain to future levels of GHG
emissions from the energy and land systems, patterns of LUC and so
on, as these emissions are affected by social, economic, political and
policy drivers (Supplementary Information Appendix 3). The overall
confidence level for the post-LGM and Anthropocene is medium (a
value of 3 on a scale of 1 to 5); even respondents who rate themselves
highly as experts (score of 4 or 5) give low to medium confidence
to some of their answers, which could suggest great uncertainty
based on current literature (Supplementary Tables S6 and S7, and
Extended Data Figs. 2 and 3). For the remainder of this century and
the far future, confidence drops to low (a value of 2), which probably
reflects the low confidence in our projection of human-based deci-
sions (Supplementary Information Appendix 3 and Extended Data
Fig. 2). Areas of research for which expertise is lowest include LUC,
N deposition and atmospheric pollution (Supplementary Tables S8
and S9, and Extended Data Fig. 2), which may have contributed to
some of the low confidence levels mentioned above. Here again,
sources from the expert elicitation provide a unique opportunity to
generate pertinent socio-economic scenarios that will help inform
our science, policy options and land management decisions.

Although the present assessment may be used as a bridge towards
policy (as decisions need to be made even when uncertainty is high
and confidence is low), we are not interested in offering 'consensus
statements' on peatland C storage. Rather, our intent is to contribute
a novel perspective that identifies the central tendencies, commu-
nicates uncertainties and highlights contradictions; we anticipate
this will improve understanding of the peat C process and press
the community to add organic soils and peatland plant functional
types in ESMs and IAMs (see Supplementary Information for fur-
ther discussion). Overall, results from the expert elicitation can help
determine which ecosystem mechanisms and properties should
be prioritized and integrated into ESMs; in turn, those model out-
puts will help constrain the peat-carbon-climate feedback, inform
future data collection strategies and advance understanding by fur-
ther testing different hypotheses. As such, the inclusion of peatland
process understanding in models, and particularly better attribu-
tion of the role of each agent of change in peatland C dynamics,
would help increase confidence in C flux predictions. Modelling
efforts that include peatland dynamics would improve ESM and
IAM outputs and benefit the peatland and climate research com-
munities in a positive feedback loop.

Online content

Any methods, additional references, Nature Research report-
ing summaries, source data, extended data, supplementary infor-
mation, acknowledgements, peer review information; details of
author contributions and competing interests; and statements of
data and code availability are available at https://doi.org/10.1038/
s41558-020-00944-0.

Received: 9 March 2020; Accepted: 1 October 2020;
Published online: 07 December 2020

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© The Author(s), under exclusive licence to Springer Nature Limited 2020

<footnote n="1">¹Department of Geography, Texas A&M University, College Station, TX, USA. ²Department of Geography, University of Exeter, Exeter, UK. ³Ecosystems and Environment Research Programme, University of Helsinki, Helsinki, Finland. ⁴Department of Geography and Geotop Research Center, University of Quebec at Montreal, Montreal, Quebec, Canada. ⁵Magister of Environment and Soil Science Department, Tanjungpura University, Pontianak, Indonesia. ⁶Department of Geography and Environment, University of Hawaii at Manoa, Honolulu, HI, USA. ⁷Department of Ecology and Territory, Pontifical Xavierian University, Bogota, Colombia. ⁸Organic Geochemistry Unit, School of Chemistry, and School of Earth Sciences, University of Bristol, Bristol, UK.</footnote>

<footnote n="9">⁹Environmental Studies Program and Earth and Oceanographic Science Department, Bowdoin College, Brunswick, ME, USA. ¹⁰Department of Geology, Chulalongkorn University, Bangkok, Thailand. ¹¹Department of Geography, University of California, Los Angeles, Los Angeles, CA, USA. ¹²Max Planck Institute for Meteorology, Hamburg, Germany. ¹³Faculty of Engineering, Chemical and Environmental Engineering, University of Nottingham, Nottingham, UK. ¹⁴Centro de Investigación GAIA Antártica, University of Magallanes, Punta Arenas, Chile. ¹⁵Climate and Environmental Physics, Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland. ¹⁶Consortium Érudit, Université de Montréal, Montreal, Quebec, Canada. ¹⁷Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX, USA. ¹⁸Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA, USA. ¹⁹Institute for Peat and Mire Research, School of Geographical Sciences, Northeast Normal University, Changchun, China. ²⁰Department of Environmental Studies, Mount Holyoke College, South Hadley, MA, USA. ²¹Department of Geography, McGill University, Montreal, Quebec, Canada. ²²Department of Physical Geography, Stockholm University, Stockholm, Sweden. ²³School of Geography, Geology and the Environment, University of Leicester, Leicester, UK. ²⁴Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium. ²⁵School of Geography & Sustainable Development, University of St Andrews, St Andrews, UK. ²⁶Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, FL, USA. ²⁷Department of Bioscience, Aarhus University, Roskilde, Denmark. ²⁸Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada. ²⁹Instituto Franco-Argentino para el Estudio del Clima y sus Impactos, Buenos Aires, Argentina. ³⁰Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA. ³¹Department of Geobotany and Plant Ecology, University of Lodz, Lodz, Poland.</footnote>

<footnote n="32">³²Laboratoire d'Ecologie Fonctionnelle et Environnement, UMR 5245, CNRS-UPS-INPT, Toulouse, France. ³³Cranfield Soil and Agrifood Institute, Cranfield University, Cranfield, UK. ³⁴Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada. ³⁵Stockholm Environment Institute, University of York, York, UK. ³⁶Max Planck Institute for Biogeochemistry, Jena, Germany. ³⁷Lawrence Berkeley National Laboratory, Berkeley, CA, USA.</footnote>

<footnote n="38">³⁸Florence Bascom Geoscience Center, United States Geological Survey, Reston, VA, USA. ³⁹Department of Marine and Coastal Environmental Science, Texas A&M University at Galveston, Galveston, TX, USA. ⁴⁰Department of Biology, University of Victoria, British Columbia, Canada. ⁴¹Faculty of Geographical and Geological Sciences, Climate Change Ecology Research Unit, Adam Mickiewicz University, Poznań, Poland. ⁴²Natural Resources</footnote>
Institute Finland (Luke), Helsinki, Finland. ⁴³Agroscope, Zurich, Switzerland. ⁴⁴Institute for Atmospheric and Earth System Research, University of Helsinki, Helsinki, Finland. ⁴⁵Department of Geography, Royal Holloway, University of London, Egham, UK. ⁴⁶Department of Forest Sciences, University of Helsinki, Helsinki, Finland. ⁴⁷School of Earth and Environmental Sciences, University of Queensland, Brisbane, Queensland, Australia. ⁴⁸Lamont-Doherty Earth Observatory, Palisades, NY, USA. ⁴⁹National Park Service, Washington DC, WA, USA. ⁵⁰Department of Environment & Geography, University of York, York, UK. ⁵¹Department of Chemistry, and Department of Geological and Environmental Science, Hope College, Holland, MI, USA. ⁵²Department of Geography and Environmental Science, University of Reading, Reading, UK. ⁵³Department of Applied Earth Sciences, Uva Wellassa University, Badulla, Sri Lanka. ⁵⁴School of Biosciences, University of Nottingham, Nottingham, UK. ⁵⁵Département de Géographie, Université de Montréal, Montréal, Québec, Canada. ⁵⁶Geography, School of Natural and Built Environment, Queen's University Belfast, Belfast, UK. ⁵⁷Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA, USA. ⁵⁸Department of Environment and Sustainability, Grenfell Campus, Memorial University, Corner Brook, Newfoundland, Canada. ⁵⁹Deceased: R. Payne. e-mail: julieloisel@tamu.edu; a.gallego-sala@exeter.ac.uk
## Data availability

Data supporting the findings of this study, as well as references used to generate the maps, are available within the supplementary information files. All anonymized survey data generated and analysed during this study are available from the corresponding authors upon request.

## Acknowledgements

This work developed from the PAGES (Past Global Changes) C-PEAT (Carbon in Peat on EArth through Time) working group; we acknowledge support from PAGES funding by the American and Swiss National Science Foundations. We also thank the International Union for Quaternary Research (INQUA) and the Department of Geography at Texas A&M University for workshop support. We thank P. Campbell for creating the peatland infographic, as well as D. McGuire for his comments on a previous version of the manuscript. We also acknowledge research support from Universidad Javeriana (J.C.B.); the National Science Foundation of the United States under grant nos. 1802838 (J. Loisel), 1019523 (J.L.B.) and 1802825 (C.T. and S.F.); the National Environment Research Council of the United Kingdom under grant nos. NE/1012915 and NE/S001166/1 (A.V.G.-S. and D.J.C.); the Attraction and Insertion of Advanced Human Capital Program of the National Commission for Scientific and Technological Research of Chile and the NEXER-UMAG project under grant no. 7718002 (C.A.M.); the Geological Survey Land Resources Research and Development program of the United States (M.C.J.); the Natural Sciences and Engineering Research Council of Canada (M. Garneau, S.F., T. Lacourse and J.W.); the National Science Centre of Poland under grant no. 2015/17/B/ST10/01656 (M.L.); the Academy of Finland projects 286731 and 319262 (T. Larmola); the Belgian National Fund for Scientific Research under grant no. 1167019N (W.S.); the Office of International Affairs and Global Network at Chulalongkorn University (S.C.); the Alexander von Humboldt Foundation of

Germany (M.B.); the Accelerating Higher Education Expansion and Development and Development Oriented Research programs of the World Bank (A.S.R.); and the Swiss National Science Foundation under grant no. 200020_172476 (F.J. and J.M.).

## Author contributions

J. Loisel, A.V.G.-S., M.J.A. and G.M. performed most of the analyses and wrote most of the manuscript. D.B., J.C.B., J. Blewett, P.C., D.J.C., S.C., A.V.G.-S., A. Hedgpeth, T.K., A.K., D.L., J. Loisel, C.A.M., J.M., S.v.B., J.B.W. and Z.Y. formulated the research goals and ideas during the 2018 C-PEAT workshop in Texas, United States. J.L.B., M. Garneau, T.M., A.B.K.S., S. Page, M.V., A.M.H., S.J., T. Larmola, A.L., K.M. and C.T. wrote parts of the review section. Other co-authors (A. Heinemeyer, S.P., T. Lacourse and M. Gałka) contributed with unpublished data or completed the expert opinion survey. All co-authors contributed to data analysis and writing of the manuscript.

## Competing interests

The authors declare no competing interests.

## Additional information

**Extended data** is available for this paper at https://doi.org/10.1038/s41558-020-00944-0.

**Supplementary information** is available for this paper at https://doi.org/10.1038/s41558-020-00944-0.

**Correspondence and requests for materials** should be addressed to J.L. or A.V.G.-S.

**Peer review information** *Nature Climate Change* thanks Katharine Mach and Matthew Warren for their contribution to the peer review of this work.

**Reprints and permissions information** is available at www.nature.com/reprints.
<figure><img src="image_5.png" /></figure>

**Extended Data Fig. 1** | All survey results (individual data points). Each individual response is shown as a spot. Positive values represent peatland sinks, negative values represent peatland sources to the atmosphere. When a range of values was given, the midpoint is used. Codes for drivers: T = temperature, M = moisture balance, SL = sea level, F = fire, LU = land use, P = permafrost, N = nitrogen deposition, AP = atmospheric pollution.
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

**Extended Data Fig. 2** | All self-reported confidence and expertise levels, organized by time period and peatland region. Blue (yellow) bars represent high-latitude (tropical) peatlands. Confidence and expertise values specified in the survey were 1 = very low, 2 = low, 3 = medium, 4 = high, 5 = very high.
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

**Extended Data Fig. 3** | Comparison of survey results from all respondents vs. those from highly self-rated experts. Data shown as mean and 10th - 90th percentiles. High-latitude peatland results shown in blue (dark = all data, light = E>2). Tropical peatland data shown in yellow (dark yellow = all data, light beige = E>2). Positive values represent peatland sinks, negative values represent peatland sources to the atmosphere. Codes for drivers: T = temperature, M = moisture balance, SL = sea level, F = fire, LU = land use, P = permafrost, N = nitrogen deposition, AP = atmospheric pollution.