Abstract
Abstract. Permafrost peatlands store large amounts of carbon potentially vulnerable to decomposition. However, the fate of that carbon in a changing climate remains uncertain in models due to complex interactions among hydrological, biogeochemical, microbial, and plant processes. In this study, we estimated effects of climate forcing biases present in global climate reanalysis products on carbon cycle predictions at a thawing permafrost peatland in subarctic Sweden. The analysis was conducted with a comprehensive biogeochemical model (ecosys) across a permafrost thaw gradient encompassing intact permafrost palsa with an ice core and a shallow active layer, partly thawed bog with a deeper active layer and a variable water table, and fen with a water table close to the surface, each with distinct vegetation and microbiota. Using in situ observations to correct local cold and wet biases found in the Global Soil Wetness Project Phase 3 (GSWP3) climate reanalysis forcing, we demonstrate good model performance by comparing predicted and observed carbon dioxide (CO2) and methane (CH4) exchanges, thaw depth, and water table depth. The simulations driven by the bias-corrected climate suggest that the three peatland types currently accumulate carbon from the atmosphere, although the bog and fen sites can have annual positive radiative forcing impacts due to their higher CH4 emissions. Our simulations indicate that projected precipitation increases could accelerate CH4 emissions from the palsa area, even without further degradation of palsa permafrost. The GSWP3 cold and wet biases for this site significantly alter simulation results and lead to erroneous active layer depth (ALD) and carbon budget estimates. Biases in simulated CO2 and CH4 exchanges from biased climate forcing are as large as those among the thaw stages themselves at a landscape scale across the examined permafrost thaw gradient. Future studies should thus not only focus on changes in carbon budget associated with morphological changes in thawing permafrost, but also recognize the effects of climate forcing uncertainty on carbon cycling.
Highlights
Confidence in future climate projections depends on the accuracy of terrestrial carbon budget estimates, which are presently very uncertain (Friedlingstein et al, 2014; Arneth et al, 2017)
(1) What are the biases embedded in the Global Soil Wetness Project Phase 3 (GSWP3) climate reanalysis dataset? (2) How do these biases affect model predictions of thaw depth, CO2 exchanges, and CH4 exchanges? (3) How does climate sensitivity vary across the stages of permafrost thaw? In addition to improving understanding of permafrost responses to climate, we identify ecosystem carbon prediction uncertainty induced by climate forcing uncertainty in general as the biases found in GSWP3 were consistent with other climate reanalysis datasets during the last decade (Sect. 3)
We found that the cold and wet biases in the GSWP3 climate reanalysis dataset significantly alter model simulations, leading to biases in simulated active layer depths, net carbon balance, and net greenhouse gas balance by up to 28.6 %, 38 g C m−2 yr−1, and 298 g CO2 eq m−2 yr−1, respectively
Summary
Confidence in future climate projections depends on the accuracy of terrestrial carbon budget estimates, which are presently very uncertain (Friedlingstein et al, 2014; Arneth et al, 2017). Increases in atmospheric carbon dioxide (CO2) concentrations can directly stimulate carbon sequestration from plant photosynthesis (Cox et al, 2000; Friedlingstein et al, 2006) and indirectly stimulate carbon emissions (e.g., from soil warming and resulting increased respiration), the predicted magnitudes of these exchanges strongly depend on model process representations (Zaehle et al, 2010; Grant, 2013, 2014; Ghimire et al, 2016; Chang et al, 2018). Lundin et al (2016) reported that it is plausible (71 % probability) for the subarctic landscapes to serve as a net carbon source to the atmosphere while its peatland components are atmospheric carbon sinks, which highlights the importance of spatial heterogeneity in high-latitude carbon budget estimation O’Donnell et al (2012) suggested that permafrost thaw would result in a net loss of soil organic carbon from the entire peat column because accumulation rates at the surface were insufficient to balance deep soil organic carbon losses upon thaw. Jones et al (2017) indicated that the loss of sporadic and discontinuous permafrost by 2100 could result in a release of up to 24 Pg of soil carbon from permafrost peatlands to the atmosphere. Lundin et al (2016) reported that it is plausible (71 % probability) for the subarctic landscapes to serve as a net carbon source to the atmosphere while its peatland components are atmospheric carbon sinks, which highlights the importance of spatial heterogeneity in high-latitude carbon budget estimation
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