By Puneet Kollipara, Science Now, January 22, 2014
A hotter sun may not rob Earth of its water, and thus its ability to support life, until later than thought. Credit: Detlev van Ravenswaay/Science Source)
Take a deep breath—Earth is not going to die as soon as scientists believed. Two new modeling studies find that the gradually brightening sun won’t vaporize our planet's water for at least another 1 billion to 1.5 billion years—hundreds of millions of years later than a slightly older model had forecast. The findings won’t change your retirement plans but could imply that habitable, Earth-like alien worlds are more common than scientists thought.
Humans are warming the planet by emitting heat-trapping gases like carbon dioxide. But behind the scenes, a far slower, deadlier warming process is unfolding. The sun is getting brighter and hotter over time. As it does, more water evaporates from Earth’s surface into the atmosphere, where it traps additional heat from the planet. This water-driven greenhouse effect will keep going long after people have stopped burning fossil fuels that now add CO2 to the atmosphere. Eventually, Earth’s greenhouse effect will spin out of control, vaporizing all of our planet’s water and ending life as we know it.
How long does Earth have? Climate modelers disagree. In one recent study, planetary scientist Ravi Kopparapu of Pennsylvania State University (Penn State), University Park, and colleagues used computers to model how Earth would respond to increasing solar radiation. Just 6% more sunlight was enough to send the greenhouse effect into overdrive and vaporize Earth’s water, the researchers found. At the current rate of solar brightening—just over 1% every 100 million years—Earth would suffer this “runaway greenhouse” in 600 million to 700 million years. Earth will suffer some preliminary effects leading up to that, too. After just 150 million years, the researchers found, the stratosphere will warm enough to let some water vapor reach high in the sky, where solar radiation will break it down into molecules that can escape to space. In this "moist greenhouse," the planet would be too hot for complex surface life, but a few hardy marine organisms and microbes could soldier on.
But not so fast, says Eric Wolf, a doctoral student at the University of Colorado, Boulder. Kopparapu’s model is pretty rudimentary, Wolf says: It analyzes what happens in one dimension—altitude. As a result, the model excludes clouds and wrongly assumes that climate factors like humidity are the same everywhere on Earth. Wolf and his Boulder colleague, Owen Brian Toon, simulated Earth’s future using a more realistic 3D climate model from the National Center for Atmospheric Research. Their model included clouds, and a host of other details such as regional differences in moisture, Wolf says. It also assumed that atmospheric CO2 levels would start at 500 parts per million—25% higher than today—and stay there indefinitely.
Then Wolf and Toon cranked up the sun. After they made our star 15.5% brighter than it is today, the simulated Earth had warmed from its current average of 15°C to 40°C. That’s hot, but not too hot for liquid water to survive. The oceans didn’t boil off. The stratosphere also didn't heat up, so no moist greenhouse occurred either. The upshot: Earth has at least 1.5 billion years left to support life, the researchers report this month in Geophysical Research Letters. If humans last that long, Earth would be generally uncomfortable for them, but livable in some areas just below the polar regions, Wolf suggests. Earth warms slower than in Kopparapu’s model, Wolf explains, because clouds and dry regions such as deserts, both of which the 1D study lacked, send a lot of heat back into space.
A similar 3D climate-modeling study, reported last month in Nature, found that a runaway greenhouse wouldn’t occur for at least 1 billion years. The leader of that study—Jérémy Leconte, an astrophysicist now at the University of Toronto in Canada—says his group’s earlier date for Earth’s demise than Wolf and Toon’s stems partly from differences in how the studies modeled clouds.
The 3D models shed new light on how long Earth could potentially support life, says planetary scientist James Kasting of Penn State University Park, who wasn’t involved in either study. Still, they may slightly underestimate how long Earth could support life, he suggests. The studies assume that CO2 levels stay the same, Kasting says, but they may actually fall as Earth warms. That’s because calcium carbonate rock formation and other natural carbon-sequestering processes might speed up in warmer conditions, pulling CO2 from the atmosphere and blunting warming at least temporarily. He also cautions that the studies don’t model life’s response, so they can’t assess how long life actually will survive.
Kasting says the studies may be most useful for refining estimates of sunlike stars’ habitable zones: the range of distances at which orbiting rocky planets can host abundant liquid water. Kopparapu’s 1D study, on which Kasting was a co-author, suggested that our solar system’s habitable zone starts at 0.97 to 0.99 astronomical units (AUs), just inside Earth’s average orbit of 1 AU. The new studies, if correct, suggest that Earth-like worlds orbiting sunlike stars have a bit more breathing room: Leconte’s group’s study pegs the inner limit at 0.95 AU, and Wolf and Toon’s at 0.93. This change could mean that our galaxy harbors 5% to 6% more habitable planets than previously thought, Leconte estimates.