Earth Is Not Random

The great filter is whatever obstacles stand in the way of simple dead matter eventually giving rise to a visibly expanding interstellar civilization. It is now confirmed that a non-trivial chuck of that filter is in planets having special orbits that let climates be stable over time:

Planetary anthropic selection, the idea that Earth has unusual properties since, otherwise, we would not be here to observe it, is a controversial idea. This paper … [compares] Earth to synthetic populations of Earth-like planets … [for] high (or low) rates of Milankovitch-driven climate change. Three separate tests are investigated: (1) Earth-Moon properties and their effect on obliquity; (2) Individual planet locations and their effect on eccentricity variation; (3) The overall structure of the Solar System and its effect on eccentricity variation. In all three cases, the actual Earth/Solar System has unusually low Milankovitch frequencies compared to similar alternative systems. All three results are statistically significant at the 5% or better level, and the probability of all three occurring by chance is less than 10^-5. It therefore appears that there has been anthropic selection for slow Milankovitch cycles. This implies possible selection for a stable climate, which, if true, undermines the Gaia hypothesis and also suggests that planets with Earth-like levels of biodiversity are likely to be very rare.

Regions of Earth that have stable temperatures (e.g., tropical rainforests) have high levels of biodiversity. The hypothesis that this link is direct and causal is reinforced by the observation that the deep ocean seafloor also has high biodiversity, even though the conditions are, stability excepted, poor and biological productivity therefore low. Further evidence of a link between rapid climate change and loss of species richness has been gleaned from studies of Earth’sglacial-interglacial cycles. The most recent ice ages have resulted in reduced biodiversity within the temperate zones where the greatest changes in climate occurred. There are, therefore, two independent lines of evidence that support the proposition that biodiversity is, in general, lower when climate change is significant. …

I concentrate on the climatic influence of Milankovitch cycles, that is, the periodic variations in Earth’s climate that are induced by changes in Earth’s orbit and orientation in space. The key factors here are axial precession (time varying axis orientation), orbital precession (time varying orbital orientation), and time variation in orbital eccentricity (circularity of the orbit). Note that changes in obliquity (the tilt of Earth’s axis relative to its orbit) are the consequence of interaction between axial precession and orbital precession, and this important factor is therefore included in the following analyses. The evidence that Milankovitch cycles affect Earth’s climate is secure. (more)