From Near-Synchronously Rotating Planets to Tidal Lock: A New Class of Habitable Planets Examined for Forest Habitability

Haberle et al. (1996) and Joshi, Haberle &Reynolds (1997) demonstrated that for a planet receiving Earth-level insolation, a 100 mb pure CO_{2} atmosphere would carry enough heat flux to the dark side of a synchronously rotating planet (SRP) to prevent the atmosphere freezing out there. For a 1,500 mb pure CO_{2} atmosphere and 0.8 Earth insolation, liquid water could survive over much of the planet. Such high partial pressures of CO_{2} may seem like special pleading, but in fact, they are not essential for habitable conditions. The effective grey optical depth of the present terrestrial atmosphere (with just 350 ppm CO_{2}, and H_{2}O as the principal greenhouse gas) is approximately 0.9, as against 1.0 for a 1,000 mb pure CO_{2} atmosphere, so the latter serves merely as a useful approximation (for a truly Earth-type atmosphere, temperatures would be a few degrees lower over the lit hemisphere). Heath et al. (1999) concluded that even forest-habitable conditions, suitable for Earth-like trees (Heath 1996) were not to be ruled out on the basis of present knowledge. Recent work by Joshi (2004) has modelled the range of climatic options for certain land-sea distributions, and these leave the door open for further investigations of combinations of geography and insolation compatible with forest-habitability.