Damage of Land Biosphere due to Intense Warming by 1000-Fold Rapid Increase in Atmospheric Methane: Estimation with a Climate–Carbon Cycle Model

Abstract

ecadal-time-scale responses of climate and the global carbon cycle to warming associated with rapid increases in atmospheric methane from a massive methane release from marine sedimentary methane hydrates are investigated with a coupled climate?carbon cycle model. A 1000-fold methane increase (from <1 to 1000 ppmv) causes surface air temperatures to increase with a global warming of >6°C within 80 yr. The amount of carbon stored in the land biosphere decreases by >25%. This is mostly due to a large decrease in tropical net primary production during the first few years (~?40%), which is caused by a decrease in photosynthesis and an increase in plant maintenance respiration with the early warming of ~3°C, leading to tropical forest dieback (>20%) and the largest decrease in vegetation carbon of >50% (~80% of the decrease in global vegetation carbon). The decrease in global land carbon is also partly due to forest diebacks (mainly boreal forest dieback by heat stress) at northern middle latitudes. In contrast, vegetation increases by >50% at northern high latitudes because of the amelioration to warm and wet conditions. Sensitivity experiments show that the warming of >6°C consists mainly of >5°C by the 1000-fold atmospheric methane and an additional increase of 1°C by the atmospheric CO2 increase due to the land CO2 release and that the CO2 fertilization of land plants prevents further warming of 1°C by limiting the atmospheric CO2 increase. The large decrease in land biomass estimated in this study suggests a critical situation for the land ecosystem or agricultural production, especially in the tropics. Because global methane content of marine methane hydrates is estimated at ~10 000 Gt, more intense warming leading to greater damage to the land biomass than the authors? experiment (~2000 Gt) is possible in the future methane release event that would be caused by the ongoing anthropogenic warming.