This is a guest post by David Waltham, Reader in Mathematical Geology at Royal Holloway, University of London. David’s new book, Lucky Planet, is out in April 2014. Visit his ‘Strange Worlds Catalogue‘ for more exoplanet oddities.
The issue of manmade global-warming seems far removed from questions of exoplanet habitability but there is a close link. A planet whose climate is highly sensitive to greenhouse-gas changes is also a planet that responds strongly to increasing heat from its aging star; and it’s hard for such a world to remain habitable for long. The Earth seems to be one such world (that’s why global warming is such a threat) but it has never-the-less remained habitable for billions of years. How it managed pull off this trick is an intriguing, but not particularly new, mystery.
In 1972 Carl Sagan and George Mullen recognized that, since our Sun produced 30% less heat when she was young, surface temperatures on the early Earth should have been far below freezing. However, geological evidence showed running water when our world was just a few hundred million years old. Sagan and Mullen called this the faint young Sun paradox and, forty years later, there is still no consensus on how to resolve it. However the concept of climate sensitivity, an idea refined over the last thirty years by climate scientists interested in anthropogenic global-warming, now gives us a clear framework for discussing the issues.
Climate sensitivity tells us how much warmer a planet becomes for a given increase in the heat it receives. It’s a bit like going from gas-mark 5 to gas-mark 6; how much hotter does this make an oven? At gas-mark 6 more gas is being burnt and temperature rises but, in a badly insulated oven for example, the increase would be less than expected. Similarly, different planets warm up by different amounts for a given increase in heating and this difference in climate sensitivity depends upon the relative strengths of positive and negative feedbacks in the climate system. As I’ll show below, the faint young Sun paradox occurs because Earth’s high climate sensitivity is incompatible with the flowing of liquid water on her surface when she was young.
Climate sensitivity is usually expressed by how much warmer the Earth becomes if carbon dioxide concentrations are doubled. Doubling of CO2 is expected by the end of the current century and so this is a very concrete way of expressing the expected impact. The best guess is that climate sensitivity is in the range 1.5-4.5 °C . This range is largely based upon computer models of the present-day climate system but it is backed up by simulations of Earth’s past climate which only match observations when similar climate sensitivities are used . If anything, these geological studies suggest that the computer estimates are too low but let’s be conservative and stick with the computer models. What does a climate sensitivity of 3 °C predict concerning temperature changes over the life time of our planet?
To calculate this we need to re-express climate sensitivity in a slightly different way. Doubling CO2 increases heating at the Earth’s surface by 3.7 Wm-2 but, to produce an equivalent amount of heating at ground level, solar radiation must go up by 5.3 Wm-2 because some is reflected back into space. Thus, temperatures go up 3 °C if solar heating increases by 5.3 Wm-2. Earth’s climate sensitivity is therefore 0.6 °C per Wm-2. Heat from the Sun has actually gone up 90 Wm-2 over the last 4 billion years and so temperatures should have risen more than 50 °C. This implies a young Earth that endured average temperatures near -40 °C and that is inconsistent with liquid water anywhere on our planet’s surface.
An obvious objection to this analysis is that the ancient climate system was very different to that of the modern Earth and so the present-day climate sensitivity may not be relevant. That’s a fair point but we can get around it by concentrating instead on the Phanerozoic Eon (i.e. the last 542 million years) when there is no reason to think that climate sensitivity would have been massively different to today. Solar heating has increased 15 Wm-2 over this time and so temperatures should have risen by about 10 °C but there is no evidence whatsoever for such a rise. Analysis of oxygen isotopes in ancient marine organisms suggest that Phanerozoic temperatures have fluctuated around a steady mean or perhaps even dropped a little. Thus, whether we look at the whole of Earth’s history or just the last half-billion years, there is no evidence for the expected overall warming despite the steadily increasing luminosity of our Sun. What’s going on?
The missing part of the puzzle is that Earth itself has evolved, both geologically and biologically, during its long history. For example, the slow growth of the continents and the biological evolution of more effective rock-fragmenters (e.g. lichens and trees) has steadily increased the efficiency with which CO2 is removed from the atmosphere by the chemical reaction of acid-rain on volcanic rock. Another greenhouse gas, methane, has also greatly declined through time as oxygen levels have grown following the evolution of photosynthesis. Furthermore, land, especially plant-covered land, is more reflective than sea and so, as the continents grew and as they became colonized by life, more of the Sun’s heat has been reflected into space. These processes, and perhaps others, cooled our planet as the Sun tried to warm it.
Two opposing forces therefore fought for dominance of climate trends and, coincidentally, roughly cancelled out. But what produced this coincidence? Some would ascribe it to the Gaia hypothesis that a sufficiently complex bio-geochemical system will inherently produce environmental stability. However there’s no credible mechanism for this and, in any case, Gaia may have confused cause and effect: Earth’s complex biosphere didn’t produce a stable climate; rather a stable climate was a necessary precondition for a complex biosphere. If this is right, then biospheres whose complexity and beauty rival that of the Earth will be rare in the Universe. On the majority of those few worlds where life arises, it will all-too-soon be frozen by bio-geochemistry or roasted by its sun. However a few worlds will, purely by chance, walk the fine line between these fates long enough for intelligent life to arise. We live on one of those rare, lucky planets.