Snowball Earth transitions from Last Glacial Maximum conditions provide an independent upper limit on Earth's climate sensitivity

Abstract. Geological evidence of a snowball Earth state indicate persistent tropical sea ice cover during the Neoproterozoic (>635 million years ago). Current theory is that a strengthening of the positive surface albedo feedback with cooling temperatures, eventually exceeding the sum of all other feedbacks, leads to a global climate instability. Several recent high sensitivity climate models with strongly positive cloud feedbacks have not been able to simulate the much warmer Last Glacial Maximum (LGM) state, suggestive that they cool excessively in response to a modest decrease in atmospheric carbon dioxide levels and therefore enter the snowball instability by this mechanism. Using a coupled Earth system model, MPI-ESM1.2, we show that clouds accelerate the transition to a snowball Earth state and reduce the radiative forcing required to trigger the snowball instability. Positive cloud feedbacks over tropical oceans and ahead of the sea-ice edge act to cool down the oceans and promote sea ice formation. Regardless, when approached slowly, the snowball Earth transition appears to occur around a global mean temperature of zero degree Celsius, simultaneously with the sea ice edge advancing into the sub-tropics thereby strengthening the surface albedo feedback. This temperature threshold, if supported by several climate models, could be used as a novel and independent constraint on the upper bound of climate sensitivity by using the relationship of simulated LGM temperatures and the models' equilibrium climate sensitivity. The constraint depends only on the simple fact that Earth did not enter a snowball instability during the recent ice ages. Using the here estimated transition temperature, we find it is implausible that Earth's climate sensitivity exceeds 6.2 °C (3.9–8.4 °C, 5 %–95 % confidence). This upper bound estimate of climate sensitivity is only weakly sensitive to uncertainty in the transition temperature, approximately 0.3° per degree.