Earth's lifespan extended by a billion years

  • 02 Jun 2009

The Earth could be habitable for another 2.3 billion years, extending previous estimates of life’s horizon by more than a billion years.

King Fai Li and his colleagues at Caltech hypothesise that Earth’s atmospheric pressure has always varied, and that it could fall in the distant future, keeping Earth from frying for far longer than previous research had shown.

If the new idea proves correct and can be extended to other planets with biospheres, it could increase the chances that earthly civilisation finds extraterrestrial life by doubling the percentage of time that planets could be inhabited.

“[T]he Earth will be identifiable as an inhabited planet for nearly half the total lifetime of the sun, an important point to consider in the search for life on extrasolar planets,” the authors write in the Proceedings of the National Academy of Sciences.

Over the next hundreds of millions of years, the sun will continue to get brighter until, eventually, Earth becomes too hot to inhabit. Previous calculations had pegged that time at about a billion years from now, but the new paper argues that earlier models had neglected the role of atmospheric pressure in regulating the temperature of the planet on astronomical timescales.

Atmospheric pressure is a key variable in the overall greenhouse-gas effect because it determines how much infrared radiation greenhouse gases absorb. Higher pressures mean more absorption and consequently, more heat. Lower pressures have the opposite effect.

Life itself would be the mechanism for these temperature changes. By “fixing” nitrogen, pulling it out of the air and eventually into the Earth’s deep ocean, microbes could be making the atmosphere lighter one atom at a time.

“I am glad that Li and colleagues have raised the issue of how overall variation in atmospheric pressure may have affected past and may affect future climate,” ecologist Ken Caldeira of Stanford University said in an e-mail. “This could be relevant for understanding climate change on the billion-year time scale.”

Despite this potentially important role, atmospheric pressure in the distant past has gone uninvestigated.

“We have a lack of data about the past history of the atmospheric pressure,” said Li.

Admittedly, that means that there is a lot of uncertainty in their calculations.

“I think more work needs to be done before we can say with any confidence how the total mass of the atmosphere has varied in the past and how it might vary in the future,” Caldeira said.

While there are implications for very long time scales, Li said the work was unlikely to have an impact on the models of anthropogenic climate change.

“We all know that the human activity which is influencing the atmosphere has a timescale of hundreds of years or thousands of years,” Li said. “Even if the biosphere is really controlling the climate, it’s not on that timescale.”

Li said that an important next step in the research field would come from astrobiologist Roger Buick’s lab at the University of Washington, which is attempting to measure atmospheric pressures deep in the past.

“The assumption that we’ve always had an atmosphere of the same pressure as now is widespread but there’s no justification for it,” Buick said. “The reason that everyone just assumes an atmosphere of roughly current pressure is that it is exceedingly difficult to measure in the past. The weight of the atmosphere doesn’t leave much record in geology.”

But Buick found a set of basalt rocks from 2.7 billion years ago in northwest Australia that he thinks bear the marks of that pressure. Gas bubbles trapped in the rocks could provide the answer to whether or not the atmospheric pressure was different way back then.

Using the size of gas bubbles to establish the altitude of an eruption is a well-known scientific practice &mdash; smaller bubbles mean lower, larger ones mean higher. Buick, though, knows from other evidence that the rocks were formed at sea level, so variation in the bubble size will be an indication of atmospheric pressure, not elevation.

“If you know that your basalt erupted exactly at sea level, you can use it as a paleobarometer rather than a paleoaltimeter,” he said.

Buick’s “got an open mind” about what he might find, but no matter what the data shows, he’s not confident that it’ll be well-received by the scientific establishment.

“If the number is substantially different, either greater or lesser from today, we’ll have quite a bit of explaining to do and I expect people will not believe it anyway,” he said.

Li’s group, though, is waiting on Buick’s data to let them know whether they’re on the right track with their speculations that Earth’s biosphere can actually change the weight of the atmosphere.

“If the atmospheric pressure is changing, the only effective mechanism we can think of is by the biosphere itself,” Li said.