Weakening greenhouse gas sink of pristine wetlands under warming

Pristine wetlands have high potential for mitigating climate change because of their large carbon stocks. However, whether and where wetlands will act as a greenhouse gas sink or source under warming is uncertain. Here we report the observations from 167 sites of the responses of carbon dioxide, methane and nitrous oxide emissions to experimental warming in northern wetlands between latitudes 30° N and 80° N during the period 1990–2022. We show that the 100-year global warming potential of wetlands increased by 57% in response to an average temperature increase of 1.5–2.0 °C. The difference in dominant plant functional types explains the uncertainties in emissions. Although warming increased the CO2 sink in vascular plant sites, it enhanced the CO2 source in cryptogam-dominated sites. As a net source of CH4 and N2O, the permafrost wetlands dominated by vascular plants positively responded to warming. Our results show that warming undermines the mitigation potential of pristine wetlands even for a limited temperature increase of 1.5–2.0 °C, the main goal of the Paris Agreement.