In the spirit of Nickel Famine and Ironed Out, which speculated about the causes of the the great oxygenation event about 2.5 billion years ago, consider the life span of the biosphere.

It is a given of astrophysics that stars like the sun get brighter over time. So in the Earth’s early history, three billion years back and more, the sun must have been distinctly fainter than it is today – so much fainter that, were the Earth then to have had an atmosphere anything like today’s, the planet’s surface would have frozen solid. . .

One of the ideas spurred by this observation was that the Earth could have a rough and ready sort of thermostat.

In the presence of water, the rocks of the Earth’s crust will react with the carbon dioxide continuously pumped out of the planet’s innards to form carbonate ions that end up incorporated into more rocks. The warmer the planet, the more vigorous this reaction is. But the more vigorous the reaction is, the more of the atmosphere’s carbon dioxide it uses up. And the more carbon dioxide is used up, the cooler things get. This is what is known as a negative feedback – negative in the sense that it damps down fluctuations rather than amplifying them. And this feedback system offers the climate a way to respond to the gradual heating up of the sun. A high level of carbon dioxide in the original atmosphere would have warmed the planet in the age of the faint young sun, but slowly have become incorporated into rocks as the sun warmed up; the thermostat would have kept the temperature much the same throughout the process. . .

In a paper called “Life span of the biosphere,” Lovelock and his friend Michael Whitfield pointed out that the ever-warming sun would eventually create a world warm enough that all carbon dioxide would be immediately sucked up by the rocks, and there would no longer be enough for the plants that need carbon dioxide for photosynthesis. No photosynthesis meant a massively pauperized biosphere bereft of oxygen and energy, the merest shadow of its former self.

The idea that the ever-warming sun would make the planet uninhabitable was well established, but it had always been assumed that it would do so directly, through its heat, not indirectly, through changing the composition of the atmosphere so far that life was no longer able to hold onto it. Finding that Gaia could “die” like this fitted into the way that Lovelock’s thinking was developing at the time towards a sense that Gaian self-regulation was limited in various ways, and that when those limits were reached the system might swap from one state to another. . .

Further work on the lifespan of the biosphere extended it from the 100 million years Lovelock and Whitfield had calculated to something more like a billion; but that still put the end far closer than solar physics alone would suggest. This week a clever paper in the Proceedings of the National Academy of Sciences suggests a way to extend the lifespan further still. Coming from the lab of Joe Kirschvink, an Earth scientist with a penchant for outrageous hypotheses, the paper by King-Fai Li and his colleagues deals with a rarely considered way life might modify the climate – not by removing carbon dioxide, which plants need, but by removing most of the rest of the atmosphere, which is in some ways surplus to requirements.

The amount of infra-red radiation absorbed by carbon dioxide depends on the ambient pressure. Decrease the pressure and the absorption decreases too. If atmospheric pressure started to drop quite severely in a billion years or so the greenhouse effect could be reduced enough for a modicum of carbon dioxide to persist. Thus photosynthesis might persist much longer than other studies have suggested – two billion years or more. . .

The paper by Li et al does not provide a compelling mechanism for the pressure drop; but it notes that most of the atmosphere, by bulk, is nitrogen, and that living creatures are capable of fixing the free nitrogen of the atmosphere into other compounds, some of which can be buried away in the crust of the earth. . .

This brings to mind work that a young scientist called Colin Goldblatt presented last December at the American Geophysical Union’s annual fall meeting in San Francisco. Goldblatt’s focus was Sagan’s original problem: the faint young sun paradox. Recently, thinking about how to keep the world warm under a faint sun has tended to stress a swaddling blanket of carbon dioxide and methane, but there may be some holes in this theory; the warming power of methane under such conditions has been overstated, and there is some evidence that the atmosphere didn’t actually contain the amount of carbon dioxide needed.

Goldblatt argued in San Francisco that this greenhouse deficit could be offset by increased atmospheric pressure – specifically, a nitrogen level more or less twice what it is today. When life appeared it would have sucked this nitrogen into sediments, but plate tectonics, by reprocessing the sediments, would have liberated it again, keeping levels high. At some point, though, the gradual cooling of the Earth’s interior (stars may heat up, but planets cool down) would have slowed that tectonic recycling, and the amount of nitrogen stored deep in the Earth would have started to rise.

Recent evidence suggest that the solar wind has been stripping away the earth's atmosphere at a greater rate than that for other planets that have weak magnetospheres. This is a surprise since it had been thought that the magnetosphere protected the earth somewhat from the solar wind. Instead, it seems, it makes it worse by interacting strongly with the charged particles of the wind and heating the upper atmosphere, causing it to expand and be blown away.

A deeper atmosphere in the past might have been even more affected than it is now, causing a higher loss rate. Fluctuations in the magnetosphere, which are common, may have had some role as well. Those fluctuations may be a product of changes in ocean circulation, which have been theorized to have a causal effect in creating the magnetosphere. Plate tectonics as well as episodes of freezing over seem like they would alter circulation.

Humans have been affecting the nitrogen cycle by converting abundant but unreactive nitrogen gas into forms more useful to plants. Many enviro-whingers complain of this but I wonder if it isn't fitting given that humans have also been raising carbon dioxide concentrations. Plants are happy about both of these alterations. As CO2 continues to rise, perhaps at an increasing rate due to feedbacks, increasing useable nitrogen could have mitigating effects. It seems improbable that this could reduce air pressure, and so the heating effects of GHGs, unless it continued for a very long time, but in this age of geoengineering speculation it seems proper to be imaginative and entertain outrageous hypotheses until they can be verified or proven to be flawed.