Using an intermediate complexity climate model (Planet Simulator), we investigate the so-called Snowball Earth transition. For certain values of the solar constant, the climate system allows two different stable states: one of them is the Snowball Earth, covered by ice and snow, and the other one is today's climate. In our setup, we consider the case when the climate system starts from its warm attractor (the stable climate we experience today), and the solar constant is decreased continuously in finite time, according to a parameter drift scenario, to a state, where only the Snowball Earth's attractor remains stable. This induces an inevitable transition, or climate tipping from the warm climate. The reverse transition is also discussed. Increasing the solar constant back to its original value on individual simulations, we find that the system stays stuck in the Snowball state. However, using ensemble methods i.e., using an ensemble of climate realizations differing only slightly in their initial conditions we show that the transition from the Snowball Earth to the warm climate is also possible with a certain probability. From the point of view of dynamical systems theory, we can say that the system's snapshot attractor splits between the warm climate's and the Snowball Earth's attractor. 1 arXiv:1906.00952v1 [physics.ao-ph] 31 May 2019 Ever since its discovery, the Snowball Earth, i.e. when the Earth's surface is nearly entirely frozen, received much attention within the climate science community. Much of the details of the transition to the planet's frozen state are still unexplored. Here, instead of focusing on the true Snowball events of Earth's history, we investigate the transition in an intermediate complexity climate model (PlaSim), with a continuously drifting solar constant (a hypothetical climate change scenario), in which a full return to the original value occurs. Using an ensemble based method we obtain both of the possible stable states as possible outcomes. We also show that the process is probabilistic and the probabilities of the corresponding outcomes are given by the ensemble's distribution. In addition, the third, unstable state (referred to as the edge state) is also recovered.
I. INTRODUCTIONSnowball Earth refers to the planet's coldest possible global climate. In this state, the whole Earth, from the poles to the Equator, is covered in ice and snow. Since the thick ice covering the surface reflects much of the energy radiated by the Sun, the global average temperature is very low, around 220 K 1 .Modern findings suggest that during the Earth's history, there were periods, when such Snowball events occurred. For example, several traces of glacial activity point to the presence of glaciers along the so-called Paleoequator 2 .This suggests that also the current configuration of the Earth system may be bistable, the two stable states being the Snowball state and our current climate. To better understand the phenomenon, there are simple models available that only take the global energy balance...