Wednesday, May 07, 2014

A plausible solution to the Fermi paradox

Review of "Lucky Planet: Why Earth is Exceptional - and What that Means for Life in the Universe" by David Waltham.

It’s been fashionable and perhaps even comforting to believe in the essential unity, benevolence and even environmental-competence of life on Earth. The Gaia hypothesis makes us feel good, but hard-nosed evolutionary biologists and planetary scientists crunch the numbers and just can’t get it to work. Forget the galaxy of a billion friendly alien civilizations, perhaps there’s just one: ourselves. Perhaps we’re just very, very fortunate.   Here’s a much abbreviated summary of what David Waltham has to say in this lively and intelligent book.

Our very existence shows that the Earth has experienced life-friendly climatic conditions for billions of years. During this time the output of the sun has increased by 30% while early high levels of greenhouse gases such as methane, water vapour and carbon dioxide have been almost scrubbed from the atmosphere. These changes ought to have produced enormous and lethal climatic variation yet somehow, by some magic, the effects have largely cancelled out.

For some people, this shows that powerful negative feedback mechanisms are at work, stabilising the climate for life. Strange then, that such benign processes are so hard to pin down. The alternative view is that for most planets like the Earth, the climate did indeed transition to fire or ice, with the consequent destruction of any biosphere; the Earth is special and very, very lucky.

 Of course, the fact that we’re here at all to make such an observation indicates that for the Earth it could hardly have been otherwise. This is called the principle of Anthropic Selection - to be contrasted with the Principle of Mediocrity, that the Earth is not that special in the universe.

David Waltham systematically takes us through the unique features of the Earth.  Our star, the sun, is unusually large and bright – most long-lived stars are smaller and redder than ours. However, they are prone to stellar flares which are extremely harmful to the biosphere. The Earth has an astonishingly strong magnetic field which deflects the solar wind, which otherwise could split water vapour into hydrogen and oxygen allowing the former to escape into space – this is how a planet loses all its water.

Despite the early sun emitting only 70% of today’s output, the Earth remained suitable for life due to the immense greenhouse effect of the early atmosphere. As the sun heated up, the greenhouse effect reduced in tandem: carbon dioxide was washed out of the atmosphere by rain and locked up in sedimentary rocks, while methane was oxidised away as soon as early photosynthesis evolved.

The Earth did not experience a smooth, stabilised, homeostatic climate – there were episodes of great heat interspersed with at least four ‘snowball earth’ episodes where the entire planet became icebound. Thanks, however, to plate tectonics and volcanism, carbon dioxide was released back into the atmosphere to unfreeze the Earth and to allow early life to reboot.

Some people believe that this is an example of the Gaia principle – life stabilising its own environment. The author sees instead systems of climate dynamics that could so easily have sheared off into uncontrolled positive feedback or blundered into wild oscillations. In his opinion, this is exactly what happens to most planets like ours ‘out there’ - but as a consequence, they have no observers to later theorise about it.

Parenthetically, the author’s concerns about current anthropogenic global warming are consistent with his view of underlying instabilities. It’s not so much that increased carbon dioxide levels in the atmosphere directly warm the climate; it’s more that they catalyse changes in more potent greenhouse gases (water vapour, methane) and it’s not at all clear that there are negative feedback mechanisms which could dampen their effects. The climate models are very complex and who knows if they’re either comprehensive or correctly tracking all the mechanisms?

Another climate-changing influence is the Earth’s axial inclination (currently around 23 degrees) and its orbit around the sun. Under the impact of the other planets in the solar system, the shape and tilt of the Earth’s orbit is continually changing on long-period cycles (69,000 years for orbital tilt and tilt-direction, 400,000 years for orbital eccentricity with other influences clustering around 100,000 years). These affect solar heating and drive the ice ages. The Earth also precesses on its axis every 26,000 years. We’re very lucky that these numbers are rather different because if they converged we would experience orbital resonances, and the inclination of the Earth’s axis would become unstable and chaotic (of the order of a few million years). This would trash the climate, leading to the extinction of all complex forms of life. How did we come by that luck?

It’s somewhat well-known that our large moon ‘spin stabilises’ the inclination of the Earth’s axis. What is less well-known is that as the moon continues to spiral away, the precession rate will slowly decay and in 1.5 billion years time resonance will occur with the orbital periods discussed above. At that point, the Earth will have an unstable spin axis. This is of academic interest only, as for reasons concerned with the sun’s increasing output, the earth will become uninhabitable for multi-cellular life within the next 500 million years. But, if the moon’s radius had been just 10 km larger and the early Earth’s day just ten minutes longer, the Earth would have an unstable spin axis today. What are the chances?

So why does it pay to have a large moon? The author suggests that a moon almost large enough to eventually generate axial instability also stabilises the spin prior to that, and in doing so allows the planet to have relatively mild and infrequent ice ages - another case of fine-tuning for intelligent life.

The author concludes that the chances of all these things coming together to guarantee a four billion year life-benign climate are so remote that the Earth is possibly the only planet with intelligent life in the entire visible universe: we are quite alone. This solution to the Fermi Paradox might be considered depressing, but it should increase our caution – “We may just find out the hard way that planets with nasty climates are quite easy to produce.”

The reader may be left with another thought: although few planets may experience multi-billion year climate stability, this is hardly a pre-requisite for interstellar colonisation, and there’s a lot of unoccupied real estate out there.