Now that I have your attention, I should probably make clear that this post is not about the Earth. I’m just back from a meeting where one of the speakers was Ian Boutle, lead author of a paper in which they Explor[ed] the climate of Proxima B with the Met Office Unified Model (pre-print available here).
Proxima Centauri B is a recently discovered Earth-sized planet in an 11-day orbit around Proxima Centauri, the closest star to the Sun. There are a couple of aspects of this system that may influence the planet’s climate sensitivity. One is that the star is much cooler than the Sun, and so emits most of its radiation at longer wavelengths. The other is that the planet is probably tidally locked – its rotation period will match its orbital period so that one side always faces its host star.
What Boutle et als. model indicates is that the above factors appear to result in a climate sensitivity that is quite a bit lower than that of the Earth (about two-thirds). One reason is that the albedo of ice decreases with increasing wavelength. Since the host star to Proxima Centauri B emits mainly at longer wavelengths (compared to the Sun) the ice albedo feedback is significantly reduced. Also (and this is the bit I wasn’t quite clear on) the changes in cloud cover appear to mainly occur on the night side, and so have little impact on climate sensitivity. There also appears to be global-scale circulations that also suppress the temperature on the day side, due to the efficient cooling of the night side of the planet.
The above has some potentially interesting implications for habitability. To be clear, we don’t really know what is required for a planet to be habitable, or not, so – in this context – it simply refers to the possibility of there being liquid water on the surface. However, if Proxima Centauri does have a smaller climate sensitivity than the Earth, then this implies that it is less sensitive to changes in stellar flux and, hence, that there is a greater range of parameter space over which it could support liquid water on its surface.
Of course, this is all based on models, so we don’t know even if Proxima Centauri B actually has an atmosphere and, if it does, if it can actually support liquid water on its surface. However, future space missions (such as the James Webb Space Telescope) and future ground-based telescopes (such as the European Extremely Large Telescope) might be able to make observations that could tell us something about Proxima Centauri B’s atmosphere, so we may have some idea about this in the not too distant future.