Increased CO2 emissions surpass reductions of non-CO2 emissions more under higher experimental warming in an alpine meadow.

It is well documented that warming can accelerate greenhouse gas (GHG) emissions, further inducing a positive feedback and reinforcing future climate warming. However, how different kinds of GHGs respond to various warming magnitudes remains largely unclear, especially in the cold regions that are more sensitive to climate warming. Here, we concurrently measured carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes and their total balance in an alpine meadow in response to three levels of warming (ambient, +1.5 °C, +3.0 °C). We found warming-induced increases in CH4 uptake, decreases in N2O emissions and increases in CO2 emissions at the annual basis. Expressed as CO2-equivalents with a global warming potential of 100 years (GWP100), the enhancement of CH4 uptake and reduction of N2O emissions offset only 9% of the warming-induced increase in CO2 emissions for 1.5 °C warming, and only 7% for 3.0 °C warming. CO2 emissions were strongly stimulated, leading to a significantly positive feedback to climate system, for 3.0 °C warming but less for 1.5 °C warming. The warming with 3.0 °C altered the total GHG balance mainly by stimulating CO2 emissions in the non-growing season due to warmer soil temperatures, longer unfrozen period, and increased soil water content. The findings provide an empirical evidence that warming beyond global 2 °C target can trigger a positive GHG-climate feedback and highlight the contribution from non-growing season to this positive feedback loop in cold ecosystems.