Volume 12, Issue 10 e2024EF005098

Research Article

Open Access

Wildfire Emissions Offset More Permafrost Ecosystem Carbon Sink in the 21st Century

Xingru Zhu

orcid.org/0000-0002-5005-1610

Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

University of Chinese Academy of Sciences, Beijing, China

Contribution: Data curation, Formal analysis, Writing - original draft, Writing - review & editing

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Gensuo Jia,

Gensuo Jia

orcid.org/0000-0001-5950-9555

Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

University of Chinese Academy of Sciences, Beijing, China

Contribution: Conceptualization, Supervision, Writing - review & editing

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Xiyan Xu,

Corresponding Author

Xiyan Xu

[email protected]

orcid.org/0000-0003-2732-1325

Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

Correspondence to:

X. Xu,

[email protected]

Contribution: Conceptualization, Supervision, Writing - review & editing, Funding acquisition

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Xingru Zhu,

Xingru Zhu

orcid.org/0000-0002-5005-1610

Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

University of Chinese Academy of Sciences, Beijing, China

Contribution: Data curation, Formal analysis, Writing - original draft, Writing - review & editing

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Gensuo Jia,

Gensuo Jia

orcid.org/0000-0001-5950-9555

Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

University of Chinese Academy of Sciences, Beijing, China

Contribution: Conceptualization, Supervision, Writing - review & editing

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Xiyan Xu,

Corresponding Author

Xiyan Xu

[email protected]

orcid.org/0000-0003-2732-1325

Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

Correspondence to:

X. Xu,

[email protected]

Contribution: Conceptualization, Supervision, Writing - review & editing, Funding acquisition

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First published: 22 October 2024

https://doi.org/10.1029/2024EF005098

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Abstract

Permafrost ecosystems in high-latitudes stock a large amount of carbon and are vulnerable to wildfires under climate warming. However, major knowledge gap remains in the effects of direct carbon loss from increasing wildfire biomass burning on permafrost ecosystem carbon sink. In this study, we used observation-derived data sets and Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations to investigate how carbon emissions from wildfire biomass burning offset permafrost ecosystem carbon sink under climate warming in the 21st century. We show that the fraction of permafrost ecosystem carbon sink offset by wildfire emissions was 14%–25% during the past two decades. The fraction is projected to be 28%–45% at the end of this century under different warming scenarios. The weakening carbon sink is caused by greater increase in wildfire emissions than net ecosystem production in permafrost regions under climate warming. The increased fraction of ecosystem carbon sink offset by wildfire carbon loss is especially pronounced in continuous permafrost region during the past two decades. Although uncertainties exist in simulations of wildfire emissions and ecosystem carbon budget, results from different models still show that wildfire emissions offset more permafrost ecosystem carbon sink in the 21st century. These findings highlight that carbon sink capacity of permafrost ecosystems is increasingly threatened by wildfires under the warming climate.

Key Points

Wildfire emissions are expected to further weaken ecosystem carbon sink in permafrost regions by the end of 21st century
Increased fraction of ecosystem carbon sink offset by wildfire has been observed in continuous permafrost in the past two decades
The weakening carbon sink is caused by greater increase in wildfire emissions than net ecosystem production under climate warming

Plain Language Summary

Permafrost ecosystems have been important carbon sink in Northern Hemisphere high-latitudes. However, some regions may shift from carbon sink to source under climate warming, as the abundant carbon stored in permafrost regions are vulnerable to wildfires. Wildfire carbon emissions further enhance the global warming, and may trigger a tipping point through the positive permafrost carbon-climate feedback. However, how wildfires affect permafrost ecosystem sink through direct carbon loss from increasing wildfire biomass burning is not clear. Our results show that wildfire emissions are expected to further weaken ecosystem carbon sink in permafrost regions during the 21st century due to a faster increase of wildfire emissions than net ecosystem production. The increased offset of wildfire emissions to ecosystem carbon sink has been observed in continuous permafrost regions during the past two decades. Although the simulations of wildfire emissions and ecosystem carbon budget are uncertain, different models still showed that wildfire emissions offset more permafrost ecosystem carbon sink in the 21st century. These results highlight that the growing wildfire emissions under warming are expected to further increase vulnerability of permafrost carbon stock, accelerate the permafrost carbon and climate feedback, weaken permafrost ecosystem carbon sink, and potentially alter the trajectory of global climate change.

1 Introduction

Permafrost occupies nearly 25% of the Northern Hemisphere land area, and stores about 50% of global soil organic carbon, more than twice the carbon stored in atmosphere (Tarnocai et al., 2009). Ecosystems in permafrost regions act as an important terrestrial carbon sink with annual carbon dioxide uptake through photosynthesis overwhelming carbon dioxide loss from ecosystem respiration (Koven et al., 2011; Virkkala et al., 2021; Watts et al., 2023). However, the large amounts of carbon stored in the frozen soils are vulnerable to the changing climate (Natali et al., 2021) and disturbances (e.g., wildfire) (Mekonnen et al., 2022), and may shift the ecosystems from being a carbon sink to source (Koven et al., 2011; Natali et al., 2021). Thawing permafrost under climate warming promotes decomposition of frozen organic matter, thereby releasing more carbon into the atmosphere (Schuur et al., 2009). The release of extra carbon from thawing permafrost amplifies global warming and further accelerates permafrost degradation (Schuur et al., 2015). Such positive permafrost carbon feedback will likely trigger amplification of anthropogenic warming (Miner et al., 2022). Therefore, the greenhouse gases released from the large frozen carbon pool to the atmosphere may gradually alter the magnitude and path of climate change, as one of the tipping points that may irreversibly push the planetary system to a new warmer state (Steffen et al., 2018).

Wildfire is a major disturbance in permafrost ecosystems (Nitze et al., 2018), and wildfire-induced carbon loss regulates northern ecosystem carbon balances (Chen et al., 2021; Walker et al., 2019). Wildfires cause carbon loss through a direct biomass burning (Mack et al., 2011), which can offset a large fraction of net ecosystem carbon uptake (Kurz et al., 2007; Walker et al., 2019). Wildfires can also alter ecosystem carbon balances through soil-vegetation-atmosphere interactions (Mekonnen et al., 2022). Wildfires eliminate surface litter and topsoil organic matters that insulate permafrost (Yi et al., 2007). Loss of the insulating organic layer through wildfire combustion alters thermal and hydrological process and exposes the underlying permafrost to substantial warming, which subsequently induces permafrost degradation, for example, the rapid deepening of the active layer and thermokarst development in ice-rich permafrost regions (McGuire et al., 2009). The permafrost degradation caused by wildfires puts large amounts of ancient carbon stores at risk of releasing to the atmosphere (Brown et al., 2015). Besides, wildfires can cause abrupt thaw of permafrost, which leads to rapid thermal, hydrologic and vegetation changes (Miner et al., 2022). Fire-induced abrupt thaw and the subsequent decomposition of previously frozen organic matters are expected to become a dominant source of high-latitude carbon emissions in the near future (Natali et al., 2021).

The Arctic mean annual surface temperature has been rising at more than twice the rate of global average since the 1980s (Smith et al., 2022), and the more rapid warming over Arctic permafrost regions than global average is expected to continue by the end of 21st century (Hu et al., 2022). Simulations of Earth system models (ESMs) have shown that ecosystem carbon sink in high-latitudes will likely increase in the future due to fast increase in temperature, CO₂ fertilization effect, and high sensitivities to the warming climate (Qiu et al., 2023). Meanwhile, wildfire-driven carbon emissions have increased rapidly in high latitudes over the past few decades, especially the boreal regions covered by permafrost (Descals et al., 2022; Scholten et al., 2021), and are projected to increase in future (Phillips et al., 2022). However, how the growing wildfire emissions under warmer climate affect permafrost ecosystem carbon budget has not been well understood. Here, we used observation-derived data sets and outputs of ESMs to explore how carbon emissions from biomass burning offset permafrost carbon sink in the 21st century.

2 Data and Methods

2.1 Study Region

We focus on contribution of wildfire carbon emissions to terrestrial ecosystem carbon budget in the permafrost regions in the northern high latitudes. The permafrost map derived from an equilibrium state model simulating temperatures at the top of the permafrost was used to identify the northern permafrost region (Obu et al., 2018). We analyzed ecosystem carbon uptake and wildfire emissions in permafrost regions in the Northern Hemisphere excluding plateau permafrost regions mainly in the Tibetan Plateau where the occurrence of wildfire is negligible compared to that in the high latitudes. The northern permafrost regions are then divided into four subregions according to the underlain permafrost coverage. Continuous, discontinuous, sporadic and isolated permafrost subregions are defined with 90%–100%, 50%–90%, 10%–50%, and 0%–10% permafrost coverage, respectively (Figure S1 in Supporting Information S1). Continuous, discontinuous, sporadic and isolated permafrost cover areas of about 9.64 × 10⁶ km², 2.87 × 10⁶ km², 3.35 × 10⁶ km² and 4.93 × 10⁶ km² in this study, respectively. We resampled the permafrost map from 1 km to 1-degree spatial resolution using bilinear interpolation method to accommodate the resolution of other data sets used in this study. We also compared non-permafrost regions with permafrost regions. The non-permafrost regions are defined as the regions in high latitudes (the north of 50°N) excluding permafrost regions.

2.2 Net Ecosystem Productivity and Wildfire Emissions From Observation-Derived Data Sets

This study focuses on net ecosystem productivity (NEP), net biome productivity (NBP) and ecosystem carbon sink offset by wildfire carbon emissions over the northern permafrost regions:

\text{NEP}=\text{GPP}\,-\,\text{Ra}\,-\,\text{Rh}

(1)

\text{NBP}=\text{NEP}\,-\,\text{FE}

(2)

\%\text{NEP}\,\text{combusted}=\frac{\text{FE}}{\text{NEP}}\times 100\%

(3)

where GPP is gross primary production, Ra is autotrophic respiration, Rh is heterotrophic respiration, FE is wildfire carbon emissions from direct biomass burning, %NEP combusted is the fraction of ecosystem carbon sink offset by wildfire carbon loss. If the ecosystem is a carbon source (NEP < 0), the negative %NEP combusted denotes an enhanced carbon source due to wildfire carbon emissions.

The CarbonTracker is a CO₂ measurement and modeling system developed by the National Oceanic and Atmospheric Administration (NOAA) to keep track of global CO₂ sources and sinks with inversion model (Jacobson et al., 2020). It includes fire-point modules to estimate wildfire emissions. We used monthly NEP and wildfire carbon emissions at 1-degree resolution from CarbonTracker 2019B (CT2019B) over 2000–2018 to calculate the fraction of NEP combusted by wildfires.

Monthly Fluxcom ensemble carbon flux products are developed based on MODIS remote sensing plus meteorological data (RS + METEO) (Jung et al., 2020). Machine learning methodology is applied in Fluxcom to merge carbon flux measurements from Fluxnet eddy covariance towers with remote sensing and meteorological data (Jung et al., 2019). The RS + METEO setup of 0.5° spatial resolution used daily meteorological data and mean seasonal cycles of satellite data to reduce uncertainty and consider potentially important information on meteorological conditions for biosphere-atmosphere fluxes, and allowed for estimating fluxes beyond the satellite era (Jung et al., 2019). We used the average NEP of two members (two meteorological data sets) from RS + METEO with extended time span over 2000–2017 instead of the average of all members (five meteorological data sets) during 2001–2010. The mean NEP of the two members is highly correlated with the mean NEP of all members during their overlapping period (R² = 0.79, p < 0.01) (Figure S2 in Supporting Information S1). The significance test conducted in this study is based on the standard two-tailed Student's t test.

By assimilating the Greenhouse Gases Observing Satellite (GOSAT) ACOS v9 XCO₂ product, the GCAS2021 used the Global Carbon Assimilation System, version 2 (GCASv2) to generate a 10-year (2010–2019) global monthly terrestrial NEP data set (Jiang, 2022). It includes 1-degree global NEP and wildfire carbon emissions. We used monthly NEP and wildfire emissions from GCAS2021 to compare with CT2019B and Fluxcom. We used wildfire carbon emissions from Global Fire Emissions Database for Fluxcom to analyze ecosystem carbon sink offset by wildfire emissions. GFEDv4.1s provides fire emissions since 1997 of 0.25-degree spatial resolution (van der Werf et al., 2017). The GFED monthly data after 2000 was produced based on the Moderate Resolution Imaging Spectroradiometer (MODIS) burned area product (MCD64A1). Monthly data from the Visible and Infrared Scanner (VIRS) aboard the Tropical Rainfall Measuring Mission (TRMM) or the Along Track Scanning Radiometers (ATSR) on board multiple platforms were used for the pre-MODIS era. The GFED has the longest period and a boosted small fires product, which makes it widely used to well represent global fire emissions (Liu et al., 2020).

We calculated annual NEP using monthly data from the CT2019B (2000–2018), Fluxcom (2000–2017) and GCAS2021 (2010–2019), and the annual wildfire carbon emissions using the monthly data from the CT2019B (2000–2018), GFED (2000–2017) and GCAS2021 (2010–2019). Data from Fluxcom and GFED were resampled to 1-degree using 2 × 2 0.5-degree grids and 4 × 4 0.25-degree grids, respectively, to be further analyzed in different permafrost subregions. Then we calculated the annual fraction of NEP combusted by wildfires in the continuous, discontinuous, sporadic and isolated permafrost regions using these three data sets. The CT2019B and Fluxcom have relatively long-term record and can represent NEP not affected by fossil fuel burning, land use and land cover change compared to other inversion-based data sets, for example, Jena and CAMS (Wu et al., 2023). Therefore, we further compared NEP combusted, its spatial variation patterns and relative changes between different periods (2010–2018 vs. 2000–2009 for CT2019B, and 2010–2017 vs. 2000–2009 for Fluxcom) and in different permafrost subregions according to the data availability during the last two decades. Besides, we calculated NEP combusted in non-permafrost regions in the north of 50°N and its relative changes between different periods for evaluating the NEP offset by wildfires in high latitude permafrost versus non-permafrost regions.

2.3 Simulated Historical and Future NEP and Fire Carbon Emissions

Outputs of 17 ESMs participating in the sixth phase of the Coupled Model Intercomparsion Project (CMIP6) (Eyring et al., 2016) with historical simulations of NEP and wildfire emissions were used to analyze spatio-temporal changes of NEP offset by wildfire carbon emissions (Table S1 in Supporting Information S1). We chose these 17 models of CMIP6 because simulations of both fire carbon emissions and NEP are available in the historical period (1850–2015). Among these 17 models, only seven of them simulated NEP and wildfire carbon emission projections over 2015–2100 under four Shared Socioeconomic Pathway (SSP) scenarios (SSP126, SSP245, SSP370 and SSP585) (O'Neill et al., 2016). Before we used these 7 model simulations to analyze the ecosystem carbon budget at the end of the 21st century, we compared the results of the 7 models with the results of the 17 models and demonstrated that during the overlapping period, the 7 models can capture consistent ecosystem carbon budget in permafrost regions as the 17 models (Figure S3 in Supporting Information S1). Therefore, we can use the 7 models to further analyze the ecosystem carbon budget at the end of 21st century. We used the results from the first ensemble (r1i1p1f1, r for realization, i for initialization, p for physics and f for forcing) of each model to enable better direct comparison of ESMs. The monthly NEP during 2000–2009 from 17 models and projections over 2090–2099 from 7 models were used to analyze the fraction of NEP combusted by wildfires in the first and final decade of the 21st century. We chose 2000–2009 and 2090-2099 decades because we aim to explore how the fire carbon emissions offset permafrost ecosystem carbon sink under different scenarios at the end of 21st century compared to the beginning of 21st century. Besides, 2000–2009 and 2090–2099 periods have been used as the representatives of contemporary and end-of-century, respectively (Krayenhoff et al., 2018; Olonscheck et al., 2021). We also analyzed the changes in GPP and ecosystem respiration from seven models to explain the changes of NEP at the end of this century.

We also used NEP and fire emission simulation outputs from an ensemble of 11 Dynamic Global Vegetation Models (DGVM) compiled by the TRENDY project (TRENDY-v11, CLASSIC, CLM5.0, IBSA-CTRIP, JSBACH, JULES, LPJwsl, LPX-Bern, ORCHIDEE, SDGVM, VISIT and VISIT-NIES) (Sitch et al., 2015). Both fire carbon emissions and NEP were simulated by these 11 models during historical period up to 2005. We used the outputs from simulation experiment S3, which was run with varying atmospheric CO₂, and changing land use and climate during the historical period (2000–2005, the overlapping period of different data sets) to compare with other data sets. Besides, we used NEP and fire emission data from four models (CLM4.5, LPJmL, ORCHIDEE, ORCHIDEE-DGVM) in the second simulation round of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP 2b) (Frieler et al., 2017). We used the historical data (2000–2005) to compare with other data sets. For future projections, Representative Concentration Pathway 2.6 (RCP26) and RCP85 were selected to compare with SSP126 and SSP585 from CMIP6 simulations, respectively.

All ESM outputs were regridded to 1° × 1° using the bilinear interpolation method. Then the regridded monthly values for each model were averaged as annual gridded means. We calculated fraction of NEP combusted, its relative change and spatial variation patterns under four SSP scenarios in different permafrost subregions in the first and final decade of 21st century.

3 Results

3.1 Ecosystem Carbon Sink and Wildfire Emissions in Permafrost Regions

The estimates from different data sets showed that permafrost region was a carbon sink during the first two decades in the 21st century (Figure 1a, Figures S5 and S6 in Supporting Information S1). The annual NEP was about 858.41 ± 113.47 TgC from CT2019B, 672.99 ± 36.42TgC from Fluxcom, 830.10 ± 107.04 from GCAS2021 and 358.29 ± 184.77 TgC from CMIP6 historical simulations (Table S3 in Supporting Information S1), indicating that CMIP6 ESMs tend to underestimate ecosystem carbon sink in permafrost regions. Continuous permafrost covering about 46% of the area showed a carbon sink of 317.12 ± 76.08, 195.07 ± 15.12, 287.53 ± 51.99 and 82.04 ± 57.62 TgC yr⁻¹ from CT2019B, Fluxcom, GCAS2021 and CMIP6 historical simulations, consisting of about 36.94%, 28.99%, 34.64% and 22.90% of whole permafrost carbon sink, respectively.

Details are in the caption following the image — **Figure 1**
Open in figure viewer PowerPoint

NEP and wildfires in permafrost regions. (a) Annual NEP in the whole northern permafrost regions from CT2019B, Fluxcom, GCAS2021 and CMIP6 historical simulations. (b) Annual wildfire carbon emissions in the northern permafrost regions from CT2019B, GFED, GCAS2021 and CMIP6. (c) Fraction of NEP combusted by wildfires in the continuous, discontinuous, sporadic and isolated permafrost subregions. The box plots in (c) represent the interquartile range from the first to the third quartile, the horizontal lines indicate the median values, the whiskers represent the 5% and 95% confidence interval.

At the end of this century, permafrost regions are expected to remain a carbon sink of 340.23 ± 64.97, 506.72 ± 88.08, 673.06 ± 110.16 and 768.98 ± 66.46 TgC yr⁻¹ under SSP126, SSP245, SSP370 and SSP585 scenarios, respectively, according to CMIP6 projections. Carbon sink of continuous permafrost region consists of about 20.59%, 25.21%, 27.11% and 32.24% for SSP126, SSP245, SSP370 and SSP585 of the whole permafrost regions during 2090–2099, indicating that continuous permafrost region contributes to a greater carbon sink in the whole permafrost regions under higher than lower emission scenarios. The non-permafrost regions in the high latitudes are projected to be a carbon sink of 359.79 ± 57.74, 448.54 ± 88.94, 562.53 ± 74.49 and 591.4 ± 77.97 TgC yr⁻¹ under SSP126, SSP245, SSP370 and SSP585, respectively, lower than that of permafrost regions. Therefore, CMIP6 model outputs suggest permafrost regions, especially continuous permafrost regions, are expected to be the major ecosystem carbon sink over high latitudes under high emission scenarios at the end of this century.

Annual wildfire carbon emissions were about 125TgC over the whole permafrost regions in the first two decades of 21st century according to observation-derived data sets. The annual wildfire carbon emissions from continuous permafrost regions accounted for about 26.19%, 25.7%, 31.17% and 23.53% of the whole permafrost regions in the four observation-derived data sets (Table S3 in Supporting Information S1). The increasing trend of wildfire emissions over continuous permafrost regions (Figures 3b and 3d), and decreasing trends of wildfire emissions over non-permafrost regions in the last two decades (Figure S8b in Supporting Information S1) indicate that continuous permafrost is the major contribution to the increase in wildfire emissions in high latitudes. By the end of this century, wildfire emissions in permafrost regions are 156.33 ± 13.64, 183.51 ± 24.46, 212.02 ± 29.09 and 225.83 ± 13.77 TgC yr⁻¹ under SSP126, SSP245, SSP370 and SSP585, respectively. Among them, wildfire emissions from continuous permafrost regions account for about 21.18%, 20.95%, 17.15% and 21.14% of the emissions from whole permafrost regions under SSP126, SSP245, SSP370 and SSP585, respectively.

3.2 Ecosystem Carbon Loss From Wildfires

In the first two decades of this century, annual fractions of NEP combusted by wildfires were between 14% and 25% over permafrost regions (14.67 ± 6.25%, 18.65 ± 8.66%, 22.09 ± 5.37% and 24.94 ± 4.43% from CT2019B, Fluxcom, GCAS2021 and CMIP6 historical simulations, respectively, Table S3 in Supporting Information S1) corresponding to 2.87 ∼ 8.82 gC m⁻² decade⁻¹ carbon loss from biomass burning. The continuous permafrost showed the lowest fraction of NEP combusted among different permafrost subregions (Figure 1c and Figure S7 in Supporting Information S1). The wildfire emissions over non-permafrost regions in high latitudes were between 3.98 and 5.05 gC m⁻² decade⁻¹ with different data sets. Therefore, the annual fractions of NEP combusted over non-permafrost regions in high latitudes were much lower than those of permafrost regions, 8.86 ± 5.47%, 12.28 ± 3.85%, 8.04 ± 3.84% and 9.18 ± 2.12% from CT2019B, Fluxcom, GCAS2021 and CMIP6 historical simulations, respectively. Therefore, permafrost ecosystem carbon sink is more vulnerable to wildfire disturbance than that of non-permafrost. The annual carbon emissions from wildfire biomass burning are projected between 75.20 and 108.62 gC m⁻² decade⁻¹ over the entire permafrost regions at the end of this century. The fractions of NEP combusted are about 45.95 ± 8.00%, 36.21 ± 10.08%, 31.50 ± 14.73% and 28.69 ± 14.09% under SSP126, SSP245, SSP370 and SSP585, respectively (Figure 4b).

Growing fraction of NEP combusted in continuous permafrost regions has been observed within the last two decades (Figures 2 and 3, Figure S9 in Supporting Information S1). The fraction of NEP offset by wildfires increased from 9.94% during 2000–2009 to 10.84% during 2010–2018 according to CT2019B, and from 14.09% during 2000–2009 to 20.06% during 2010–2018 according to Fluxcom, while in other permafrost and non-permafrost regions the fraction of NEP combusted either decreased (Figure 3a) or increased less than that in continuous permafrost regions (Figure 3c, Figure S8a in Supporting Information S1). These results reveal that continuous permafrost regions have been experiencing more ecosystem carbon loss through wildfires, and the carbon loss from biomass burning increased by 8.21 ∼ 9.13 gC m⁻² decade⁻¹ in the last two decades. The increased fraction of NEP combusted in continuous permafrost regions was caused by a more increase of wildfire carbon emissions and a less increase or decrease of NEP (Figures 3b and 3d). By contrast, the fraction of NEP combusted in the southern isolated permafrost declined due to an obvious decrease in wildfire carbon emission of 7.13 ∼ 15.19 gC m⁻² decade⁻¹ (Figure 2).

The fraction of NEP combusted by wildfires is projected to increase in the whole permafrost regions by the end of this century (Figure 4 and Figure S10 in Supporting Information S1). The carbon loss from biomass burning is projected to increase by 6.64 ∼ 28.98 gC m⁻² decade⁻¹ at the end of the 21st century compared to the beginning of this century. The relative changes in fraction of NEP combusted over permafrost regions are 74.20%, 37.30%, 19.43% and 8.79% under SSP126, SSP245, SSP370 and SSP585, respectively, indicating more wildfire-induced carbon loss under high emission scenarios, that is, SSP370 and SSP558 (Figure 4c). The increased fraction of NEP combusted is relatively low because of greater enhancement of GPP than ecosystem respiration in response to the warming under high emission scenarios resulting in relatively high NEP (Figure S11 in Supporting Information S1). The continuous permafrost regions show little decrease in fraction of NEP combusted under SSP370 and SSP585 due to more increase of NEP than wildfire emissions. The less increase of wildfire emissions in continuous permafrost regions is probably because ESMs tend to underestimate them. In particular, the simulations cannot reproduce the satellite retrieved very high wildfire emissions from continuous permafrost regions in recent years. As shown in Figure 4a, under low emission scenario, that is, SSP126, the greatest increase in the fraction of NEP combusted occurs in the northern part of permafrost regions. By contrast, the greatest increase occurs in the southern part of permafrost regions under high emission scenarios (SSP370 and SSP585).

4 Discussion

Whether permafrost will act as a carbon source or sink under climate warming is still under debate (Liu et al., 2022). Permafrost ecosystem carbon budget not only depends on the tradeoff of simultaneous enhancement of carbon uptake through photosynthesis and carbon loss through ecosystem respiration under warming (Qiu et al., 2023), but also wildfire carbon emissions (Zheng et al., 2023). Wildfire carbon emissions increase uncertainties of ecosystem carbon budget, and they may shift permafrost ecosystem from being a carbon sink to a source (Natali et al., 2021). In this study, we found that permafrost region is a terrestrial ecosystem carbon sink during the first two decades of the 21st century, and the carbon sink remains by the end of this century according to ESM simulations.

We found annual fraction of NEP combusted by wildfires is between 14% and 25% over permafrost regions during the first two decades of the 21st century. In boreal and Arctic ecosystems, continuous carbon loss was observed in areas with severe burning during the first decade of 21st century, and the large carbon source remained for years due to post-fire vegetation canopy and GPP reductions, and enhanced litter decomposition and soil respiration (Yi et al., 2013). The carbon source caused by wildfires in boreal forests can even last for decades until the ecosystems transition from a carbon source to a sink (Goulden et al., 2011; Ueyama et al., 2019). Based on a mass balance model, an average of about 10 ± 30% of annual NPP has been consumed by wildfires over the past 6,500 years in boreal upland forests (Harden et al., 2000). According to an available estimate, about 21% total NEP sink was reduced by additional emissions of CO₂ and CH₄ from open water aquatic bodies and wildfires in high latitude ecosystems, which even shifted many far northern tundra landscapes and some boreal forests from a net carbon sink to a net source (Watts et al., 2023). Our results show that wildfires have caused more ecosystem carbon loss over continuous permafrost regions where ecosystem carbon sink is reduced by the growing wildfires (Figures 2 and 3). With the highest content of topsoil carbon (<30 cm deep) underlain by continuous permafrost (Pellegrini et al., 2022), the ecosystems therein are the most sensitive to wildfire disturbance (Mekonnen et al., 2022).

We found the growing fraction of NEP offset by wildfires over continuous permafrost regions is mainly caused by more rapid increase of wildfire emissions than NEP. Wildfires are rapidly growing in high latitudes during recent decades (Kasischke & Turetsky, 2006; Zhao et al., 2021), especially in Siberian Arctic and boreal regions covered by large area of continuous permafrost (Descals et al., 2022; Xu et al., 2022; Zhu et al., 2023). These increasing boreal wildfires are largely facilitated by the drying fuel due to climatic water deficit caused by rising temperature (Descals et al., 2022; Ellis et al., 2022; Zheng et al., 2023) and the blocking events connected to large scale atmospheric circulations (Scholten et al., 2022; Zhu et al., 2021), which results in record-high wildfire carbon emissions from boreal regions in recent years (Zheng et al., 2021, 2023). Continuous permafrost regions with higher soil carbon storage are more sensitive to the drying fuel (Zhu et al., 2024). When soil moisture decreases, dryer fuel loading and more aerobic conditions facilitate wildfire burn and spread (Walker et al., 2018). Besides, permafrost thaw and degradation can transform below-ground organic carbon into flammable fuel for wildfires (McCarty et al., 2020). Although GPP (McGuire et al., 2016) and NEP (Qiu et al., 2023) are increasing in high-latitude regions under warming, the faster increase of wildfire emissions than NEP leads to weakening carbon sink in continuous permafrost regions (Figures 3b and 3d).

The increase of ecosystem carbon loss due to wildfires is projected to continue by the end of this century over permafrost regions (Figure 4). Under SSP585 scenario, carbon loss from soil carbon decomposition can be canceled out by enhanced vegetation carbon accumulation, leading to a strong carbon sink in the 21st century in the northern Asian permafrost regions (Liu et al., 2022). However, when taking wildfire disturbance into consideration, effects of wildfires on the interactions between vegetation and soil carbon stocks will accelerate high-latitude soil carbon loss (Mekonnen et al., 2022). Our results from CMIP6 simulations showed that faster increase of wildfire emissions than NEP is expected to cause more ecosystem carbon loss at the end of this century over permafrost regions. The wildfire-induced abrupt thaw, ground collapse and subsequent permafrost degradation may increase permafrost carbon loss and offset potential carbon sinks (Brown et al., 2015; Holloway et al., 2020). However, the indirect effects of wildfires on post-fire permafrost thaw and carbon loss are not well understood with observation-derived data sets or represented in ESMs. There are no process-based global models integrating abrupt thaw, combustion of soil organic matter, or the impact of wildfires on permafrost vulnerability (Natali et al., 2021). Meanwhile, none of the CMIP6 models are able to fully represent fire-permafrost interactions (Canadell et al., 2021). The lack of post-fire carbon process simulation may cause underestimation of soil carbon loss across the northern high latitudes (Varney et al., 2022). Therefore, projections in wildfire-induced weakening permafrost carbon sink may be conservative.

We found that the Fluxcom NEE driven by different meteorological forcing data ensemble shows fairly consistent result in the permafrost regions (Figure S2 in Supporting Information S1). However, it has been found that Fluxcom tends to overestimate global land carbon sink due to systematic bias and uncertainties in scaling up flux tower estimates and bias in the sampling distribution of the flux towers, or missing components that involve the release of carbon from ecosystems to the atmosphere (e.g., crop harvest, volatile organic compound emissions) (Jung et al., 2019). Besides, the abnormally high carbon flux from Fluxcom has been reported for wetlands in northwestern Canada due to presence of snow and water at the subpixel scale (Cheng et al., 2022). Although large uncertainties in flux estimates were found in different data sets developed by different models, Fluxcom has advantages in capturing the strongest ecosystem carbon sinks in boreal biomes relative to other regions (Virkkala et al., 2021).

ESMs are essential for modeling ecosystem carbon budget in high latitudes (Chen et al., 2021), however, great uncertainties in simulations remain in these regions (Qiu et al., 2023). After a fire in high latitude regions, it can even take several decades for boreal forests to recover (Mack et al., 2011; Mekonnen et al., 2019). According to ground observations with eddy covariance towers, it took 11–12 years for the boreal forest ecosystem to transition from carbon source to sink after burning and then NEP remained decreased in the following decades related to carbon loss from live biomass due to tree mortality and continued carbon decomposition from forest floor (Goulden et al., 2011). Although the ecosystems can finally return to a carbon sink following NEP recovery, the recovered NEP was still lower than its loss by fire (Goulden et al., 2011). These recovery processes are not well characterized in models in which carbon from combusted biomass is distributed into the atmosphere and the litter pool (Li et al., 2012). Some ESMs showed high-latitude ecosystem as a carbon sink while others indicated a carbon source (Friedlingstein et al., 2014; Qian et al., 2010). The inconsistent results arisen from different model structures, external variables, and parameterizations compromise the confidence in model simulations (Bradford et al., 2016; Luo et al., 2015). Compared with estimates from inversion models, the process-based models tend to underestimate terrestrial ecosystems carbon sink according to the Global Carbon Budget 2022 (Friedlingstein et al., 2022). Our results also show that CMIP6 ESMs underestimate ecosystem carbon sink in permafrost regions (Figure 1a).

Meanwhile, we found that the simulated wildfire emissions from ESMs, especially CESM2, EC-Earth3, NorCPM1 and NorESM2-LM, were much lower than that derived from satellite observations in continuous permafrost regions (Figure 1b, Figure S4 in Supporting Information S1). The CLM, land model of CESM underestimates wildfire emissions in the northern permafrost regions, especially during June-August over the northern part of Eurasian continuous permafrost (Li et al., 2013) due to the lack of combustion parameters (e.g., fuel availability, fuel combustibility and combustion completeness) from field measurements in Siberia (van Leeuwen et al., 2014). Fuel availability in some ESMs is specified without change to follow the growing productivity and biomass in high emission scenarios (Li et al., 2012), which may lead to underestimated fuel availability under warming. Moreover, the dynamic scheme of combustion completeness based on fire characteristics and the moisture content of different fuel classes is only used in few models (Table S2 in Supporting Information S1) to represent fire spread and intensity (van Marle et al., 2017). Thus, the growing fire risk under high emission scenarios may not lead to high wildfire carbon emissions from CMIP6 models, and may cause underestimation of NEP offset by wildfire under high emission scenarios compared to low emission scenarios (Figure 4).

Simulations with dynamic vegetation models also show that the offset of wildfire emissions on NEP is enhanced in the 21st century (Figure 5) although studies reported that ESMs even integrated with dynamic vegetation models can hardly capture the interannual changes of wildfires (van Marle et al., 2017) as well as the severe fire years (Kantzas et al., 2013) in high-latitude regions. Dynamic vegetation parametrization tends to overestimate wildfire emissions thus higher fractions of NEP combusted than models in CMIP6 and the observation-derived results (CT2019B and Fluxcom). Although uncertainties exist in the simulations of ecosystem carbon budget, results from different models still showed that wildfire emissions would offset more permafrost ecosystem carbon sink in the 21st century. The underestimated wildfire emissions from ESMs of CMIP6 over permafrost regions and the other data-driven projections of rapidly increasing wildfire emissions in the northern high-latitudes (Descals et al., 2022; Hanes et al., 2019; Veraverbeke et al., 2017) indicate that the fractions of ecosystem carbon sink combusted by wildfires will be greater than the estimations from CMIP6 simulations.

5 Conclusion

In this study, we show that wildfire emissions are expected to reduce more ecosystem carbon sink in permafrost regions during the 21st century, and the increasing fractions of ecosystem carbon sink offset by wildfires have been observed in continuous permafrost regions during the first two decades. A faster increase of wildfire emissions than NEP leads to higher fraction of NEP combusted over continuous permafrost. According to the results from CMIP6 ESM simulations, the fraction of NEP consumed by wildfires are expected to further increase in permafrost regions by the end of the 21st century. Although simulations of wildfire emissions and ecosystem carbon budget are uncertain, results from different models still showed that wildfire emissions are expected to offset more permafrost ecosystem carbon sink in the 21st century. The growing wildfire emissions followed by further warming in permafrost regions will increase the vulnerability of the permafrost carbon reservoir and reduce more permafrost ecosystem carbon sink, which potentially accelerate the permafrost–carbon–climate feedback and alter the trajectory of global climate change.

Acknowledgments

This study is funded by National Key R&D Program of China (2022YFF0801904) and China Postdoctoral Science Foundation (#2023T160633). We acknowledge Key Laboratory of Earth System Numerical Modeling and Application of Chinese Academy of Sciences, and the World Climate Research Programme through its Working Group on coupled modeling and coordinating of CMIP6. We thank the climate modeling groups for producing and making available their model outputs, the Earth System Grid Federation (ESGF) for archiving the data and providing access, and the multiple funding agencies who support CMIP6 and ESGF, as well as the efforts of all involved modeling centers. We thank all the modelers that contributed to the TRENDY project. We also thank the ISIMIP cross sectoral science team for their roles in producing, coordinating, and making available the ISIMIP 2b data.

Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

Open Research

Data Availability Statement

The data on which this article is based are available in Obu et al. (2018), Jacobson et al. (2020) and Jiang et al. (2022). Fluxcom data is available at http://fluxcom.org/CF-Download/ Global Fire Emissions Database, Version 4.1s (GFED4.1s) is available at https://www.geo.vu.nl/~gwerf/GFED/GFED4/ CMIP6 data is available at https://esgf-node.llnl.gov/search/cmip6/ ISIMIP 2b data is available at https://esg.pikpotsdam.de/search/isimip/.

Supporting Information

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Volume12, Issue10

October 2024

e2024EF005098

Wildfire Emissions Offset More Permafrost Ecosystem Carbon Sink in the 21st Century

Abstract

Key Points

Plain Language Summary

1 Introduction

2 Data and Methods

2.1 Study Region

2.2 Net Ecosystem Productivity and Wildfire Emissions From Observation-Derived Data Sets

2.3 Simulated Historical and Future NEP and Fire Carbon Emissions

3 Results

3.1 Ecosystem Carbon Sink and Wildfire Emissions in Permafrost Regions

3.2 Ecosystem Carbon Loss From Wildfires

4 Discussion

5 Conclusion

Acknowledgments

Conflict of Interest

Open Research

Data Availability Statement

Supporting Information

References

References From the Supporting Information

Figures

References

Information

RESOURCES

PUBLICATION INFO

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Wildfire Emissions Offset More Permafrost Ecosystem Carbon Sink in the 21st Century

Abstract

Key Points

Plain Language Summary

1 Introduction

2 Data and Methods

2.1 Study Region

2.2 Net Ecosystem Productivity and Wildfire Emissions From Observation-Derived Data Sets

2.3 Simulated Historical and Future NEP and Fire Carbon Emissions

3 Results

3.1 Ecosystem Carbon Sink and Wildfire Emissions in Permafrost Regions

3.2 Ecosystem Carbon Loss From Wildfires

4 Discussion

5 Conclusion

Acknowledgments

Conflict of Interest

Open Research

Data Availability Statement

Supporting Information

References

References From the Supporting Information

Figures

References

Related

Information

RESOURCES

PUBLICATION INFO