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Carbon budgets for 1.5 and 2 °C targets lowered by natural wetland and permafrost feedbacks | Nature Geoscience
Comwyn-Platt et al. 2018: Global methane emissions from natural wetlands and carbon release from permafrost thaw have a positive feedback on climate, yet are not represented in most state-of-the-art climate models. Furthermore, a fraction of the thawed permafrost carbon is released as methane, enhancing the combined feedback strength. We present simulations with an inverted intermediate complexity climate model, which follows prescribed global warming pathways to stabilization at 1.5 or 2.0 °C above pre-industrial levels by the year 2100, and which incorporates a state-of-the-art global land surface model with updated descriptions of wetland and permafrost carbon release. We demonstrate that the climate feedbacks from those two processes are substantial. Specifically, permissible anthropogenic fossil fuel CO2 emission budgets are reduced by 9–15% (25–38 GtC) for stabilization at 1.5 °C, and 6–10% (33–52 GtC) for 2.0 °C stabilization. In our simulations these feedback processes respond more quickly at temperatures below 1.5 °C, and the differences between the 1.5 and 2 °C targets are disproportionately small. This key finding holds for transient emission pathways to 2100 and does not account for longer-term implications of these feedback processes. We conclude that natural feedback processes from wetlands and permafrost must be considered in assessments of transient emission pathways to limit global warming.
Climate_Science_study  methane  wetlands  PermafrostThaw  climate_modeling  1.5  feedbackloop  CarbonBudget 
may 2019 by huntercutting
Permafrost and wetland emissions could cut 1.5C carbon budget ‘by five years’
Carbon Brief on Comyn-Platt, E. et al. (2018) Carbon budgets for 1.5 and 2C targets lowered by natural wetland and permafrost feedbacks, Nature Geoscience. Many of these studies use global climate models to make their estimates. However, there are some processes which affect the climate that are not yet incorporated into these models.

The new study, published in Nature Geoscience, aims to fill that gap. It focuses on two processes on the land surface: thawing permafrost and natural wetlands.

These are both positive feedbacks for the climate because, as they respond to rising temperatures, they cause the release of more greenhouse gases into the atmosphere.

In natural wetlands, plants absorb CO2 from the atmosphere as they grow. But, because they are waterlogged, when the plants decompose they release methane, rather than CO2. This makes wetlands “effectively engines for turning CO2 into methane”, says Jones.

In the Amazon, for example, flooded forests actually release methane through the trees themselves.

Wetlands respond to a warming climate in three ways, Jones explains:

“As [the climate] warms, the local decomposition rate increases. Also, as rainfall changes, we may see increased or decreased areas of wetlands. This varies regionally and, again, is a large uncertainty across climate models. Finally, a direct effect of increased CO2 in the atmosphere is to increase the growth rate of the vegetation and, hence, increase the amount of CO2 being turned into methane.”

The study is the first to bring all these factors together,

When the additional CO2 and methane emissions are incorporated, the available carbon budget shrinks substantially – falling to 533bn-753bn tonnes of CO2 for 1.5C, or 14-20 years of emissions
methane  wetlands  climate_modeling  temperatures  CarbonBudget 
may 2019 by huntercutting

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