ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: to solar system icy satellites, it could still possess enough water to be an aquaplanet , the term aquaplanet refering here to a planet where water is abundant enough to flow efficiently on a planetary scale. In practice, in our simulations, this means that the surface acts as an infinite source of water. In this section, we thus make an attempt at quantifying how much is enough. Then, we discuss the various climate regimes available to an aquaplanet and point out the main differences with the dry case. 5.1. Transition from small to large water inventory The exact water inventory for which Proxima b would transition from a land planet (previous section) to an aquaplanet (this section) is difficult to define. In particular, the nature of the transition depends of the amount of greenhouse gas in the atmosphere. For a low atmospheric greenhouse effect (meaning that ice is stable somewhere on the planet), there is a bistability between two possible climate regimes around the transition. To understand this bistability, we introduce a thought experiment in which we consider a planet with two very different initial conditions: Case 1: warm- and/or water-rich-start. We assume that theplanet starts with a global ocean, possibly covered by sea icewherever cold enough. This experiment has been carried outby Yang et al. (2014) with a325 m deep ocean. They showed that in sucha configuration, winds carry sea ice toward hot regions and theocean carries heat toward cold regions, so that an equilibriumcan be found with less than 10 m of sea ice inthe coldest regions. They however acknowledge that the pres-ence of continents could significantly alter this value. Yanget al. (2014) only looked at a syn-chronous planet, but the argument remains qualitatively valid fora non-synchronous planet, even though numerical values willsurely change significantly. Therefore, water content as small as 10 m GEL (and maybe even lower) is sufficient to maintain aquaplanet conditions. If water were to be removed or the temperature decreased, at some point, the oceanic transport would shut down and the planet would transition to a dry, cold regime with glaciers. Case 2: cold- and/or water-poor-start. As discussed in the previous section, if the planet started cold or dry enough, all the water would be trapped in ice caps and glaciers. Now, we demonstrated earlier that in this state, more than m GEL of ice could be stored this way. If the water inventory and/or the temperature were to be increased, glaciers would progressively spill toward hotter regions. This state could resist until the oceanic transport becomes more efficient than the atmospheric transport. After that, an ocean would accumulate and, in turn, warm the cold regions, speeding up the transition to the aquaplanet regime. Therefore, between roughly and m GEL, two distinct climate regimes coexist, depending on the history of the planet. As both the water inventory and the amount of greenhouse gases play a role, it