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Abstract
Abstract. Peatlands are a major terrestrial carbon store and a persistent natural carbon sink during the Holocene, but there is considerable uncertainty over the fate of peatland carbon in a changing climate. It is generally assumed that higher temperatures will increase peat decay, causing a positive feedback to climate warming and contributing to the global positive carbon cycle feedback. Here we use a new extensive database of peat profiles across northern high latitudes to examine spatial and temporal patterns of carbon accumulation over the past millennium. Opposite to expectations, our results indicate a small negative carbon cycle feedback from past changes in the long-term accumulation rates of northern peatlands. Total carbon accumulated over the last 1000 yr is linearly related to contemporary growing season length and photosynthetically active radiation, suggesting that variability in net primary productivity is more important than decomposition in determining long-term carbon accumulation. Furthermore, northern peatland carbon sequestration rate declined over the climate transition from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA), probably because of lower LIA temperatures combined with increased cloudiness suppressing net primary productivity. Other factors including changing moisture status, peatland distribution, fire, nitrogen deposition, permafrost thaw and methane emissions will also influence future peatland carbon cycle feedbacks, but our data suggest that the carbon sequestration rate could increase over many areas of northern peatlands in a warmer future.
Highlights
Peatlands contain around 600 gigatonnes of carbon (Gt C) that has accumulated since the last glacial maximum in northern mid–high latitudes, tropical regions and temperate areas of the Southern Hemisphere, and the steady accumulation of carbon has been a small but persistent sink for atmospheric CO2 throughout the Holocene (Yu, 2011)
The results suggest that spatial variability in peatland carbon accumulation over the last 1000 yr is primarily controlled by spatial variation in net primary productivity (NPP), which in turn is driven by growing season length and growing season PAR
That the Medieval Climate Anomaly (MCA) was warmer than the Little Ice Age (LIA) is not contested (Jansen et al, 2007), but we suggest from our sensitivity analysis that temperature alone cannot explain the magnitude of the change in observed carbon accumulation over the last millennium
Summary
Peatlands contain around 600 gigatonnes of carbon (Gt C) that has accumulated since the last glacial maximum in northern mid–high latitudes, tropical regions and temperate areas of the Southern Hemisphere, and the steady accumulation of carbon has been a small but persistent sink for atmospheric CO2 throughout the Holocene (Yu, 2011). The relationship between climate change and the rate of carbon sequestration is important for understanding the past and future global carbon cycle, and it has generally been assumed that because temperature drives increasing decay (Ise et al, 2008; Dorrepaal et al, 2009), peatlands could be part of the positive feedback from the global carbon cycle (Friedlingstein et al, 2006). 7–10 ppmv decline in atmospheric CO2 concentration (Ahn et al, 2012) This pattern supports the existence of a positive global climate–carbon cycle feedback, as suggested by coupled climate–carbon cycle models (Friedlingstein et al, 2006; Denman et al, 2007). The models do not take into account possible climate-related variations in the rate of peatland carbon sequestration
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