A runaway greenhouse effect – where a planet's atmosphere traps so much heat that temperatures rise to life-snuffing levels – may be easier to achieve than previously believed. And there may be more than one way to drive the increase.
Those are the implications of two recent studies looking at what planetary scientists describe as one of the fundamental processes that can render a planet uninhabitable.
In the sun's neighborhood, Venus is the textbook example. It is thought to have had oceans on its surface early in its history, but the planet's proximity to the sun and the relatively high concentration of heat-trapping carbon dioxide in its atmosphere combined to evaporate the oceans, triggering runaway warming that drove surface temperatures to levels that can melt lead.
The most recent of the two studies, published Monday in the journal Nature Geoscience, found that the amount of energy needed to shift a planet's climate into thermal overdrive at Earth's distance from the sun was about 10 percent less than estimates many scientists have been using for more than two decades.
The research suggests that from a standpoint of Earth's climate, it would likely take another 1.5 billion years, even accounting for the pace at which human activities are pumping greenhouse gases into the air, for a runaway greenhouse effect to take over, says Colin Goldblatt, an assistant professor at the University of Victoria in British Columbia who studies the evolution of Earth's climate.
The results also imply that a star's habitable zone – where a planet could capture enough warmth from its sun to allow liquid water to remain stable on the surface – may be smaller than previously estimated. If the results hold up, this would reduce the number of extrasolar planets deemed potentially habitable.
The study serves as a useful reminder that scientists can't determine habitability only from estimates of how much radiation reaches a planet, says Larry Esposito, a researcher who studies planetary atmospheres at the University of Colorado at Boulder. A planet's current climate and the history of that climate play key roles, too.
The atmospheric model used in looking at the greenhouse effect on Earth represents "a first pass at doing the problem again," says Dr. Goldblatt. It doesn't account for clouds, which would be crucial to determining the mount of sunlight reaching Earth's surface. Instead, the model operates assuming clear skies.
"You start off with simple models. You try to understand the answers. Then you go on to more complex models," he says.
Over the past 25 years, researchers have developed more-detailed measurements of water vapor and how it interacts with the infrared radiation the Earth's surface sends skyward. These improvements prompted the team to try to take another crack at measuring the energy needed to trigger a runaway greenhouse effect.
Water vapor and other greenhouse gases absorb most of that radiation and re-radiate it in all directions, including back toward Earth's surface. But radiation in a narrow band of wavelengths can escape, allowing some of that heat to head back toward space.
As the atmosphere warms, more water evaporates, and the atmosphere's ability to hold moisture increases. Runaway heating can occur when warming temperatures push enough water vapor into the air to in effect slam the infrared window shut, Goldblatt explains.
Nor is sunlight alone in determining the surface temperature. A study published earlier this year in the journal Astrobiology described how tidal heating – the friction created within a planet as it is tugged by a star's gravity – could produce enough heat at the planet's surface to push an otherwise stable climate into runaway greenhouse warming.
Runaway heating from these tidal forces would be limited to planets orbiting dim, low-mass red-dwarf stars along highly elliptical paths. Those paths might take the planet into and out of the star's habitable zone. While the planet might eventually stabilize in a circular orbit within a habitable zone, it would be bone-dry.
The team, led by Rory Barnes, a research scientist at the University of Washington in Seattle, dubbed these runaway-heating victims "tidal Venuses." [Editor's note: The original version of this story misspelled the name of Rory Barnes.]
For the more familiar Venus, the modeling Goldblatt and colleagues undertook imply that the planet may never have had oceans to begin with – unless the levels of nitrogen in its atmosphere were comparable to the relatively high levels seen today, Dr. Espositio suggests. Nitrogen is effective at scattering visible light and so would tend to be a cooling agent if it was present in sufficient amounts.
Though the study would seem to rule out any immanent runaway greenhouse effect on Earth, Goldblatt underscores the importance of reining in global warming.
"There is this thing known as a runaway greenhouse effect. It is easier than we thought to cause it. But it's not something that's likely to happen in the context of anthropogenic global change," he says. "But the flip side of that is that we really do need to still worry about anthropogenic global change. It's still a really big deal."