Northern peatlands have accumulated large stocks of organic carbon (C) and nitrogen (N), but their spatial distribution and vulnerability to climate warming remain uncertain. Here, we used machine-learning techniques with extensive peat core data (n > 7,000) to create observation-based maps of northern peatland C and N stocks, and to assess their response to warming and permafrost thaw. We estimate that northern peatlands cover 3.7 ± 0.5 million km2 and store 415 ± 150 Pg C and 10 ± 7 Pg N. Nearly half of the peatland area and peat C stocks are permafrost affected. Using modeled global warming stabilization scenarios (from 1.5 to 6 °C warming), we project that the current sink of atmospheric C (0.10 ± 0.02 Pg C⋅y−1) in northern peatlands will shift to a C source as 0.8 to 1.9 million km2 of permafrost-affected peatlands thaw. The projected thaw would cause peatland greenhouse gas emissions equal to ∼1% of anthropogenic radiative forcing in this century. The main forcing is from methane emissions (0.7 to 3 Pg cumulative CH4-C) with smaller carbon dioxide forcing (1 to 2 Pg CO2-C) and minor nitrous oxide losses. We project that initial CO2-C losses reverse after ∼200 y, as warming strengthens peatland C-sinks. We project substantial, but highly uncertain, additional losses of peat into fluvial systems of 10 to 30 Pg C and 0.4 to 0.9 Pg N. The combined gaseous and fluvial peatland C loss estimated here adds 30 to 50% onto previous estimates of permafrost-thaw C losses, with southern permafrost regions being the most vulnerable.

Permafrost, which covers 15 million km 2 of the land surface, is one of the components of the Earth system that is most sensitive to warming 1,2 . Loss of permafrost would radically change high-latitude hydrology and biogeochemical cycling, and could therefore provide very significant feedbacks on climate change [3][4][5][6][7][8] . The latest climate models all predict warming of high-latitude soils and thus thawing of permafrost under future climate change, but with widely varying magnitudes of permafrost thaw 9,10 . Here we show that in each of the models, their present-day spatial distribution of permafrost and air temperature can be used to infer the sensitivity of permafrost to future global warming. Using the same approach for the observed permafrost distribution and air temperature, we estimate a sensitivity of permafrost area loss to global mean warming at stabilization of 4.0 +1.0 −1.1 million km 2 • C −1 (1σ confidence), which is around 20% higher than previous studies 9 . Our method facilitates an assessment for COP21 climate change targets 11 : if the climate is stabilized at 2 • C above pre-industrial levels, we estimate that the permafrost area would eventually be reduced by over 40%. Stabilizing at 1.5 • C rather than 2 • C would save approximately 2 million km 2 of permafrost.Permafrost, defined as ground that remains at or below 0 • C for two or more consecutive years, underlies 24% of the land in the Northern Hemisphere 12 . Under recent climate warming, permafrost has begun to thaw, causing changes in ecosystems and impacting northern communities, for example through collapse of roads and buildings as the ground becomes unstable 13 . Large quantities of carbon are stored in organic matter in permafrost soils 14 , which starts to decompose when the permafrost thaws, resulting in the emission of greenhouse gases such as carbon dioxide and methane. In the future, carbon release from permafrost thaw may have a significant impact on the Earth's climate 6 . Due to its global importance, numerous modelling studies have assessed the rate of permafrost thaw under future climate warming 9,10,15,16 . However, despite progress in process-based modelling on local and regional scales, for example, ref. 17, a lack of data availability and model limitations mean that permafrost is still poorly simulated in global climate models, where the historical simulations show a present-day permafrost area anywhere between 0.1 and 1.8 times the size of that observed 9 . Models often have shallow soil columns, a limited representation of soil properties, inadequate snow thermal and physical dynamics and other missing processes 9 . Here we present a projection of large-scale permafrost thaw that is based on observations, avoiding model bias, and accounting for observational uncertainty.Our approach is based on using the relationship between mean annual air temperature (MAAT) and permafrost occurrence

Scenarios that limit global warming to below 2 °C by 2100 assume significant land-use change to support large-scale carbon dioxide (CO2) removal from the atmosphere by afforestation/reforestation, avoided deforestation, and Biomass Energy with Carbon Capture and Storage (BECCS). The more ambitious mitigation scenarios require even greater land area for mitigation and/or earlier adoption of CO2 removal strategies. Here we show that additional land-use change to meet a 1.5 °C climate change target could result in net losses of carbon from the land. The effectiveness of BECCS strongly depends on several assumptions related to the choice of biomass, the fate of initial above ground biomass, and the fossil-fuel emissions offset in the energy system. Depending on these factors, carbon removed from the atmosphere through BECCS could easily be offset by losses due to land-use change. If BECCS involves replacing high-carbon content ecosystems with crops, then forest-based mitigation could be more efficient for atmospheric CO2 removal than BECCS.