Crossing planetary boundaries could lead to Hothouse Earth

Abstract The climate system is nonlinear and affected by both natural variability and several types of forcing(1). 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(2) and the tipping of regional subsystems(3–5), due to their potentially irreversible consequences. On the global level, these nonlinear effects have been shown to have given rise to bistability(6) and chaotic behavior(7) in the system’s past(8–10) and could do so in its future(11). However, specific mechanisms for a sudden tipping to an alternate stable “hothouse,”(12) several degrees warmer than the present climate, have not been explored so far to a satisfactory extent(13,14). Here we show that a highly simplified energy balance model of globally averaged temperature T representing the radiative budget, coupled with global carbon dioxide dynamics, does exhibit such an alternate stable hothouse climate with T higher by roughly 10 C than the present. The model captures two biogeophysical 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(15–19); and (ii) the limits of vegetation adapting to increased environmental stress and, hence, the reduction of its carbon absorbtion(20–22).