March 11, 2024Open Access

Crossing planetary boundaries could lead to Hothouse Earth

ECErik ChavezNew Mexico State University MGMichael GhilCentre National de la Recherche Scientifique JRJan Rombouts

Key Points

A potential tipping to a hothouse climate was observed, with temperatures rising about 10 °C beyond the present level.
The model effectively captured carbon fluxes in the atmosphere-land-ocean system, aligning well with observational data and advanced simulations.
Analysis using a simplified energy balance model and biogeochemical dynamics shows key regional mechanisms leading to this tipping point in the climate system, growing concern for future impacts as anthropogenic emissions increase. The findings propose that harnessing these understandings could foster meaningful discussions within climate model communities.

Abstract

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 crossing of planetary boundaries (Rockstrm, 2009) and 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., 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 two box-type models of global biogeochemical dynamics, does exhibit such an alternate stable hothouse climate with T higher by roughly 10 C than the present. This highly simplified biogeophysical (BGP) 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, which lead to the crossing of planetary boundaries that in turn 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 algal blooming (e.g., Williamson et al, 2020); 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 systems potential for global tipping in response to anthropogenic greenhouse gas emissions. References: Boers, N. et al. Envir. Res. Lett. 17, 113739 (2022). Hammond, W. M. et al. Nat. Commun. 13 (2022). Lenton, T. M. Livina, V. N. In Chavez, M., Ghil, M. Urrutia-Fucugauchi, J. (Eds.), American Geophysical Union - John Wiley and Sons, Inc., 214, 51-62, isbn: 978-1-119-15703-8 (2016). Lorenz, E. N. J. Atmos. Sci. 20, 130-141 (1963). Popp, M. et al. Nat. Commun. 7 (2016). Rockstrm, J. et al. Nature 461, 472-475 (2009). Steffen, W. et al. PNAS 115, 8252-8259 (2018). Stommel, H. Tellus 13, 224-230 (1961). Williamson, C. J. et al. PNAS 117, 5694-5705 (2020).

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