Since I’m tired/bored of the climate “debate” I thought I might write a quick post about something that’s related to climate change, but probably not particularly controversial. I came across a paper recently by Shields et al. (2014) called Spectrum-driven Planetary Deglaciation Due to Increases in Stellar Luminosity.
Essentially, they used a GCM to try and understand how planets around different types of stars might move into – and out of – snowball states. Stars are typically divided into different spectral types, the spectral sequence being O B A F G K M, with O stars being really hot and luminous, and M stars being cool and relatively dim.
As an aside, the original spectral classification ran from A-Z and was determined by the strength of a hydrogen spectral line in the visible. However, this particular spectral line is only present if the gas is hot enough for enough hydrogen atoms to be excited into the second energy level. If most are in the ground state, then the dominant spectral line is actually in the UV, and so wasn’t detected by early instruments. Additionally, if the gas is so hot that the hydrogen is fully – or almost fully – ionised, then this spectral line isn’t present either. Therefore the original classification (A-Z) went from stars with surface temperatures around 10000K (A stars) to stars that were both hotter (weaker spectral lines because hydrogen was becoming ionised) and cooler (weaker spectral lines because not enough hydrogen was in the n=2 state). When they realised this, they re-arranged it as a temperature sequence and we get the OBAFGKM series.
Anyway, back to the paper. They considered planets around M-stars (cooler than the Sun), G-stars (like the Sun), and F-stars (hotter than the Sun) and either started with an insolation similar to what we have today and reduced it to see when the planet would move into a snowball phase, or started from a snowball phase and increased the insolation. They used CO2 concentrations similar to today on the Earth, and CO2 similar to what we’d expect as we moved out of a snowball phase. The basic result is shown below. The hysteresis comes from the difference between what happens if you start in a warm phase and reduce the insolation, or start in a cool (snowball) phase and increase the insolation.
What they find is that the transition out of, or into, snowball phases is smoother for M-stars (cool) than for F- and G-stars (hotter). The idea, if I understood it properly, is that M-stars emit most of their energy in the infrared, which is absorbed more by ice than is shorter wavelength (visible and UV) radiation. Therefore, hotter stars require a bigger input of energy before the ice starts melting and when it starts, the transition tends to be faster than around M-stars.
As you can see from the figure above, for modern CO2 concentrations a snowball planet around a Sun-like star would require an insolation greater (more than 100%) than our insolation today. We moved out of our snowball phase because volcanic outgassing increased CO2 concentrations to levels much higher than today. They also considered this, and showed that M-stars would more easily move out of their snowball phase if CO2 increased, than F- and G-stars. The basic conclusion of the paper is that
Planets near the outer edge of the habitable zones of M-dwarf stars will become more hospitable for surface life earlier in their host stars’ evolutionary paths than their ice-covered counterparts orbiting brighter stars, although this may take a longer absolute time, as an M dwarf brightens more slowly than a G dwarf.
which is interesting, although we have yet to find another planet that we really think might be habitable. Having said that, planets around M-stars are our best bet at the moment.
This paper also reminds me of something else that I think is related. I recently saw a paper that was arguing that the oceans are poor emitters of infrared, unlike ice. This suggests that the reduction of Arctic sea ice can further enhance warming because the ocean surface will emit less infrared than would be emitted were it still covered in ice. If anyone remembers which paper this was, maybe they can point it out. Anyway, enough from me.