If we can find a way to double the lifespan of Earth’s biosphere, we’ll have changed the odds for finding extraterrestrial civilizations. After all, the amount of time an advanced culture can exist is one of the variables in the famous Drake equation, which estimates how many intelligent civilizations there are in the Milky Way. Lengthen potential habitability and you give any civilization that much more chance to spread into the cosmos.

Thus recent work out of Caltech intrigues us in several directions. Joseph Kirschvink and colleagues look at effects that could add a billion years on to our planet’s projected habitability. Consider: Earth took some four billion years to develop intelligent life, leaving us about a billion before our planet becomes uninhabitable. That result would be caused by a brighter and hotter Sun, the loss of carbon dioxide in the atmosphere through the weathering of rocks, and the eventual evaporation of water from the oceans, leaving nothing alive.

Reducing the amount of carbon dioxide in the atmosphere helps to ease the warming effect, and indeed, according to this Caltech news release, the pressure of carbon dioxide in the atmosphere has dropped 2,000-fold over the past 3.5 billion years, with man’s industrial activities serving to offset only a fraction of the decrease. Kirschvink believes “…we’re nearing the point where there’s not enough carbon dioxide left to regulate temperatures following the same procedures.” That points to a hot and uninhabitable future in a billion years or so.

Reducing atmospheric pressure could be a way around this outcome. The scientists say that removing massive amounts of molecular nitrogen, which already makes up 78 percent of the atmosphere, would allow us to regulate surface temperatures, ensuring that carbon dioxide will remain in the atmosphere and adding 1.3 billion years to Earth’s habitable life. It sounds like a science fictional solution relying on some vast future technology to re-tune the atmosphere, but Kirschvink’s team thinks the process may actually happen naturally.

After all, nitrogen is incorporated into the cells of growing organisms and gradually removed from the atmosphere as they die. Nitrogen reduction may be an ongoing process, suggesting that Earth’s atmospheric pressure may be lower now than in earlier epochs. One way to examine this, say the scientists, would be to study gas bubbles in ancient lavas, which could provide a reading on the change in atmospheric pressure over time.

Could exoplanetary studies give us clues to our own future? Grad student Kaveh Pahlevan, who worked with Kirschvink on this paper, sees a strong possibility:

“Hopefully, in the future we will not only detect earth-like planets around other stars but learn something about their atmospheres and the ambient pressures. And if it turns out that older planets tend to have thinner atmospheres, it would be an indication that this process has some universality.”

What would we do with an additional billion or so years of life? If technological civilizations are not in fact self-destructive, longer lifespans should give them a much greater chance of finding each other. The paper is Li et al., “Atmospheric Pressure as a Natural Regulator of the Climate of a Terrestrial Planet with Biosphere,” Proceedings of the National Academy of Sciences, published online June 1, 2009 (abstract).