Roughly a billion years from now, the ever-increasing radiation from the
sun will have heated Earth into inhabitability; the carbon dioxide in
the atmosphere that serves as food for plant life will disappear, pulled
out by the weathering of rocks; the oceans will evaporate; and all
living things will disappear.
Or maybe not quite so soon, say researchers from the California
Institute of Technology (Caltech), who have come up with a mechanism
that doubles the future lifespan of the biosphere—while also increasing
the chance that advanced life will be found elsewhere in the universe.
A paper describing their hypothesis was published June 1 in the early
online edition of the Proceedings of the National Academy of
Earth maintains its surface temperatures through the greenhouse
effect. Although the planet’s greenhouse gases—chiefly water vapor,
carbon dioxide, and methane—have become the villain in global warming
scenarios, they’re crucial for a habitable world, because they act as an
insulating blanket in the atmosphere that absorbs and radiates thermal
radiation, keeping the surface comfortably warm.
As the sun has matured over the past 4.5 billion years, it has become
both brighter and hotter, increasing the amount of solar radiation
received by Earth, along with surface temperatures. Earth has coped by
reducing the amount of carbon dioxide in the atmosphere, thus reducing
the warming effect. (Despite current concerns about rising carbon
dioxide levels triggering detrimental climate change, the pressure of
carbon dioxide in the atmosphere has dropped some 2,000-fold over the
past 3.5 billion years; modern, man-made increases in atmospheric carbon
dioxide offset a fraction of this overall decrease.)
The problem, says Joseph L. Kirschvink, the Nico and Marilyn Van
Wingen Professor of Geobiology at Caltech and a coauthor of the PNAS
paper, is that "we’re nearing the point where there’s not enough carbon
dioxide left to regulate temperatures following the same procedures."
Kirschvink and his collaborators Yuk L. Yung, a Caltech professor of
planetary science, and graduate students King-Fai Li and Kaveh Pahlevan,
say that the solution is to reduce substantially the total pressure of
the atmosphere itself, by removing massive amounts of molecular
nitrogen, the largely nonreactive gas that makes up about 78 percent of
the atmosphere. This would regulate the surface temperatures and allow
carbon dioxide to remain in the atmosphere, to support life, and could
tack an additional 1.3 billion years onto Earth’s expected lifespan.
In the "blanket" analogy for greenhouse gases, carbon dioxide would
be represented by the cotton fibers making up the blanket. "The cotton
weave may have holes, which allow heat to leak out," explains Li, the
lead author of the paper.
"The size of the holes is controlled by pressure," Yung says.
"Squeeze the blanket," by increasing the atmospheric pressure, "and the
holes become smaller, so less heat can escape. With less pressure, the
holes become larger, and more heat can escape," he says, helping the
planet to shed the extra heat generated by a more luminous sun.
Strikingly, no external influence would be necessary to take nitrogen
out of the air, the scientists say. Instead, the biosphere itself would
accomplish this, because nitrogen is incorporated into the cells of
organisms as they grow, and is buried with them when they die.
In fact, "This reduction of nitrogen is something that may already be
happening," says Pahlevan, and that has occurred over the course of
Earth’s history. This suggests that Earth’s atmospheric pressure may be
lower now than it was earlier in the planet’s history.
Proof of this hypothesis may come from other research groups that are
examining the gas bubbles formed in ancient lavas to determine past
atmospheric pressure: the maximum size of a forming bubble is
constrained by the amount of atmospheric pressure, with higher pressures
producing smaller bubbles, and vice versa.
If true, the mechanism also would potentially occur on any extrasolar
planet with an atmosphere and a biosphere.
"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," Pahlevan says. "And if it turns out that older
planets tend to have thinner atmospheres, it would be an indication that
this process has some universality."
Adds Yung: "We can’t wait for the experiment to occur on Earth. It
would take too long. But if we study exoplanets, maybe we will see it.
Maybe the experiment has already been done."
Increasing the lifespan of our biosphere—from roughly 1 billion to
2.3 billion years—has intriguing implications for the search for life
elsewhere in the universe. The length of the existence of advanced life
is a variable in the Drake equation, astronomer Frank Drake’s famous
formula for estimating the number of intelligent extraterrestrial
civilizations in the galaxy. Doubling the duration of Earth’s biosphere
effectively doubles the odds that intelligent life will be found
elsewhere in the galaxy.
"It didn’t take very long to produce life on the planet, but it takes
a very long time to develop advanced life," says Yung. On Earth, this
process took four billion years. "Adding an additional billion years
gives us more time to develop, and more time to encounter advanced
civilizations, whose own existence might be prolonged by this mechanism.
It gives us a chance to meet."
Source : California Institute of Technology