13 January 2014

Earth could remain habitable far longer than previously thought

Posted by Nanci Bompey

Artist's concept of lightning on Venus

An artist’s concept of Venus, where a thick atmosphere creates temperatures of more than 470 degrees Celsius (880 degrees Fahrenheit), according to NASA. Conditions on Earth are expected to become like those of Venus as the Sun’s intensity increases, but a new study says the planet could remain habitable for the next 1.5 billion years and possibly longer.
Credit: ESA

Our distant descendants may one day need to flee to other planets to escape fatally hot temperatures and boiling seas from a long-predicted and inexorable intensification of our Sun’s heat. A grim prospect.

But, some good news, indicated by a study published Jan. 10, is that habitable places could remain on Earth for much longer than scientists had previously thought. That might mean more time for humanity to adapt to Earth’s hellish end game — and possibly to find a new place to live before the last survivable spots disappear.

Our planet will remain habitable for at least 1.5 billion years from now and likely even longer, according to recent climate simulations conducted by scientists at the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder (CU-Boulder).  The computer model used to carry out the simulations takes into account all three dimensions of Earth including its atmosphere, ocean and land surface.

That time span is far longer than scientists previously calculated using a more simplistic, “one-dimensional” Earth model in which climate factors, like temperature, varied with altitude alone. It’s also longer than recent predictions from another relatively realistic 3D model similar to the one used by the CU-Boulder group.

“Undeniably the human race wants to survive in the universe long term, and at some point in the next billion or two billion years, we will have to leave the planet Earth and go someplace else if we want to survive,” said Eric Wolf, a doctoral student in CU-Boulder’s atmospheric and oceanic sciences department. “The future of the human race depends on it.”

Wolf is first author of the new study published in Geophysical Research Letters, a journal of the American Geophysical Union.

He foresees an eventual human exodus from Earth because our sun is slowly but surely radiating more heat into space. That natural amplification of the brightness of the sun, by about 1 percent every 110 million years, is gradually cranking up Earth’s thermostat. Based on studies dating back to the 1980s, scientists expect this steady heat increase to cause Earth eventually to become much like Venus is today – too hot for water to remain a liquid on its surface, and therefore uninhabitable.

The new study doesn’t address the other kind of not-so-natural global warming taking place as a consequence of human emissions of carbon dioxide and other greenhouse gases into Earth’s atmosphere. The temperature increase caused by humans burning fossil fuels will pose major problems for the planet in decades to come but even worst-case scenarios get nowhere near the temperature increase caused by the ultimate intensification of the sun, Wolf said.

One-dimensional models previously predicted that the ultimate doomsday would come in 650 million years.  In December, French scientists published a paper in Nature predicting the Earth would be able to support liquid water for another billion years based on a 3-D model.

The CU study, posted online only days after the French group published its results, gives the longest timeframe for Earth’s habitability yet. Liquid water will endure on the planet’s surface even with a 15.5 percent increase in the sun’s luminosity – a measure of the amount of solar radiation received by Earth’s atmosphere, according to the paper.

That increase above the current solar luminosity would cause the temperature on Earth to increase to a global average of about 40 degrees Celsius (104 degrees Fahrenheit) over the next 1.5 billion years. That’s hot, but it is well short of temperatures needed to make the whole planet uninhabitable, Wolf said.

Earth’s average global surface temperature in 2013 was about 15 degrees Celsius (58 degrees Fahrenheit), according to the National Oceanic and Atmospheric Administration.  Worst-case scenarios for global warming from greenhouse gases predict increases of about 8 to 16 degrees Celsius above current levels, Wolf said.

The Earth would be an unpleasant place to live and the changes in temperature would pose an extreme challenge to humanity. Temperatures in areas just below the Arctic Circle would resemble today’s tropics, and there would be a lot more rain as the oceans evaporated, according to Wolf and Brian Toon, a professor at CU-Boulder and co-author of the paper

“There would be twice as much rainfall everywhere, a lot more floods and things like that,” Toon said. “It will be like a really unpleasant day in the Sahara Desert, but rainy.”

The CU-Boulder and French teams are the first to calculate the future habitability of the Earth using 3-D models. The model used by CU researchers was originally developed by the National Center for Atmospheric Research, also in Boulder. Unlike 1-D models, 3-D models take into account dynamic processes such as heat transport between the poles and cooling by clouds and air not fully saturated with water vapor. Those processes contribute to the planet remaining habitable longer than predicted by simpler 1-D models, Wolf said.

The results generated by the 3-D model also mean Earth’s fortunate place in the solar system for sustaining life is not as improbable as previously thought. They suggest that the planet is not teetering on the inner edge of its habitable zone, but that water could have survived if the Earth was even closer to the sun.

That not only makes our planet’s existence less of a long shot, but it also means that livable planets in other solar systems could be orbiting closer to their suns than previously estimated. That widens the hunt for scientists searching for those distant worlds, Wolf said.

“They can look at little bit closer to the stars to find habitable planets,” he said.

— Nanci Bompey, AGU Public Information Specialist – Writer