A stable hothouse triggered by a tipping mechanism

The climate system is nonlinear and affected by both natural variability and several types of forcing. The impact of anthropogenic forcing and environmental change on several of the system's nonlinear processes has led to considerable concern about the tipping of regional subsystems (e.g. Lenton, 2016), due to their potentially irreversible consequences. On the global level, these nonlinear effects have been shown to give rise to bistability (Stommel, 1961} and chaotic behavior (Lorenz, 1963) in the system's past (e.g., Boers et al, 2022), as well as having been proposed conceptually as due to occur in its future, too (e.g., Steffen et al, 2018). However, specific mechanisms for a sudden tipping to an alternate stable “hothouse”, several degrees warmer than the present climate, have not been explored so far to a satisfactory extent with ESM-based studies using aqua planets (e.g., Ferreira et al 2011, Popp et al, 2016).

   Here we show that a highly simplified energy balance model (EBM) of globally averaged temperature T representing the radiative budget, coupled with a three box-type model of global carbon dynamics, does exhibit such an alternate stable hothouse climate with T higher by roughly 10 °C than the present. This TCV model also captures quite accurately the fluxes of carbon between the separate reservoirs of the coupled atmosphere-land-ocean system, when compared with observations and with simulations by high-end models. The model includes two regional mechanisms, that trigger a global tipping to such a hothouse. The two regional mechanisms are (i) the decrease of terrestrial albedo due to the darkening of ice sheets by pervasive glacial micro algal growth (e.g., Williamson et al, 2020) not included in ESMs to date; and (ii) the limits of vegetation adapting to increased environmental stress and, hence, the reduction of its carbon absorbtion (e.g., Hammond, 2022).

    These findings and the mechanistic understanding of the processes leading to a global tipping can contribute to a fruitful dialogue between the conceptual-model and ESM communities. Such a dialogue can greatly enhance our understanding of the climate system’s potential for global tipping in response to anthropogenic greenhouse gas emissions.