Irreversible Land Carbon Losses under Idealized Overshoot Experiments

Terrestrial vegetation and carbon storage may exhibit irreversible responses under anthropogenic climate change and may shift from a carbon sink to a carbon source. Understanding the reversibility of this transition is critical for assessing future carbon-climate feedback. Idealized overshoot experiments provide a controlled framework to test the response of the terrestrial vegetation and its carbon pools to climate forcing. Non-linear responses and incomplete recovery indicate tipping-like behavior. Here, we quantify land carbon changes under idealized overshoot scenarios, following the Tipping Point Model Intercomparison Project (TIPMIP) protocol, employing the LPX Dynamic Global Vegetation Model. We find non-linear responses in land carbon during and after transient warming, as well as recovery behavior. We perform a transient experiment (T1A1 protocol) initialized from pre-industrial conditions (1850) and force LPX with temperature, precipitation, and cloud cover from TIPMIP simulations of the GFDL ESM2M. In the T1A1 protocol, surface air temperature increases linearly by 2 °C over 100 years, remains constant for 50 years, and then decreases by 2 °C over the next 100 years. To assess the long-term recovery in land carbon, we extend the experiment by 1,000 years beyond the T1A1. During the warming phase global total land carbon decreases by about 100 PgC (~5%), comprising losses from vegetation (35 PgC), soil (25 PgC), permafrost (25 PgC), and litter (15 PgC) carbon pools. During the cooling phase back to pre-industrial conditions, approximately 75% of the carbon loss (75 PgC) is restored. The remaining 25% of the deficit reflects a quasi-permanent loss of permafrost carbon associated with warming-induced thaw. Pronounced non-linear responses emerge in northern peatlands, that suggest tipping-like behavior. Ongoing analysis will further constrain the role of hydrology in shaping these responses and their limited reversibility.