2020NatAs...4...58Y

Tidally locked terrestrial planets around low-mass stars are the prime targets for future atmospheric characterizations of potentially habitable systems1, especially the three nearby ones-Proxima b (ref. 2), TRAPPIST-1e (ref. 3) and LHS 1140b (ref. 4). Previous studies suggest that if these planets had surface oceans they would be in an eyeball-like climate state5-10: ice free in the vicinity of the substellar point and ice covered in the remaining regions. However, an important component of the climate system-sea-ice dynamics-has not been fully considered in previous studies. A fundamental question is whether an open ocean is stable against a globally ice-covered snowball state. Here we show that sea-ice drift cools the ocean's surface when the ice flows towards the warmer substellar region and melts through absorbing heat from the ocean and the overlying air. As a result, the open ocean shrinks and can even disappear when atmospheric greenhouse gases are not much more abundant than on Earth, turning the planet into a snowball state. This occurs for both synchronous rotation and spin-orbit resonances (such as 3:2). These results suggest that sea-ice drift strongly reduces the open-ocean area and can significantly impact the habitability of tidally locked planets.