A new study from the University of Victoria is suggesting that our atmosphere is far more susceptible to a runaway greenhouse effect than previously assumed.
The culprit here is the sun and its ever-increasing level of activity. Our planet has attained a delicate state of equilibrium in which it releases or blocks as much incoming energy from the sun as it can absorb.
But eventually, once the sun enters into its late stages, the increase in its brightness and thermal activity will result in untenable levels of trapped radiation. This will result in a runaway greenhouse effect and cause the Earth to overheat. Our oceans will boil, filling the atmosphere with steam — creating a suffocating blanket that will likely turn it into a Venus-like world. Water vapor is a particularly nasty greenhouse gas, as it absorbs thermal radiation from sun-warmed surfaces — thus making the effect even worse.
This isn’t a new revelation, of course. But what is new is the suggestion that it’s relatively easy for a planet to induce a runaway greenhouse effect.
A Narrower Habitable Zone
By using computer models, researchers Tyler Robinson and Colin Goldblatt discovered a lower thermal radiation threshold for the runaway greenhouse process. Their model took new measurements into account of how water and carbon dioxide absorb different wavelengths of light.
Their finding means that planetary atmospheres are far more fragile than we thought, and that the habitable zones of solar systems are narrower than assumed (a habitable zone is the band around a star within which planets can maintain liquid water). And in fact, the new study has deep implications for astronomers and astrobiologists; they may have to reset their notions of exoplanet habitability. What’s more, some extrasolar planets declared as ‘potentially habitable’ may need to have their status revoked.
“The habitable zone becomes much narrower, in the sense that you can no longer get as close to the star as we thought before going into a runaway greenhouse,” noted Robinson through a statement.
But as the researchers admit, their simulation wasn’t perfect. It was done in a “single-column, clear-sky model,” or a one-dimensional measure averaged around a planetary sphere — which doesn’t account for the atmospheric effect of clouds. Future work will address these shortcomings.
As for Earth, the runaway greenhouse effect could start to occur in about 0.5 to 1 billion years from now. Previous estimates had it at about 1.1 to 2 billion years from now.
The Good News
But there is some good news. According to Robinson and Goldblatt, it’s likely impossible for human activity to induce a runaway greenhouse effect. To do so, we’d have to raise the concentration of CO2 in the atmosphere to around 30,000 parts per million. This is considerably more that could be attained even if we burned all the available fossil fuel reserves on the planet — 10 times more to be exact.
As for the situation a billion years from now, it’s also conceivable that future humans — or whatever form intelligence might take at that point — could deal with the situation. By that stage, posthumans may have already moved on. Or, in an effort to preserve life on Earth, they could initiate a stellar engineering project. Some futurists speculate that it might be possible to prolong the life of a star, what’s called star uplifting, by rejuvenating it through a number of techniques (e.g. resupplying it with nuclear fuel).
Alternatively, future posthumans could move the Earth away from the sun. As Goldblatt himself admits — and he once worked for NASA — it may be possible to slingshot an asteroid into an orbit around the Earth, where it would absorb some of the Earth’s orbit energy, moving the planet away from the sun. If done every 3,000 years, the Earth would gradually migrate outwards to balance the rate the sun is getting hotter.
Read the entire study at Nature Geoscience: “Low simulated radiation limit for runaway greenhouse climates.”
Images: Ron Miller.