New Climate Modeling Of Venus May Hold Clues To Earth's Future

February 18, 1999

New computer models that indicate the climate of Venus has wavered radically in its relatively recent past may prove valuable to scientists tracking Earth's changing climate, according to two University of Colorado at Boulder researchers.

Despite the recent intrigue in Mars shown by the public, Venus actually is more Earth-like because it's the only other planet with a complex, evolving climate, said Mark Bullock and David Grinspoon of CU-Boulder's Laboratory for Atmospheric and Space Physics. Given the dynamic, rapid climate change that has occurred in Earth's history and the current impact of human activity, such computer models of the changing Venusian climate system may hold clues to Earth's future.

"Our model shows Venus has changed dynamically in the recent past," said Bullock. "Since Venus and Earth have a number of similarities, there are implications here for our own future." An article by Bullock and Grinspoon regarding global change on Venus appears in the March issue of Scientific American.

Although Venus and Earth were virtual twins early in their histories, they took vastly divergent evolutionary paths. Today Venus is hot enough -- 900 F -- to make rocks glow and has a noxious carbon dioxide atmosphere encased by a dense cloud deck of sulfuric acid. But the clouds may have waxed and waned over time.

Computer modeling by Bullock and Grinspoon indicates that volcanic activity some 800 million years ago "repaved" Venus with lava, as evidenced by a dearth of impact craters on its surface. Bullock and Grinspoon estimate enough lava erupted from the surface to cover the planet with a layer up to six miles thick.

Their model suggests the massive volcanism would have increased the abundance of water vapor in the atmosphere by 10 times and the amount of sulfur dioxide by 100 times. Since both are greenhouse gases, the atmosphere would have begun to heat, they hypothesized.

But the gases also would have produced droplets that thickened the clouds over the planet, causing more light to reflect back to Venus, according to the model. "Eventually the clouds could have cut off the amount of sunlight required to power the greenhouse, cooling the planet," said Bullock. "We think the clouds initially won out."

The thick cloud deck may have cooled Venus by about 200 degrees F, they theorized. As it cooled, the water vapor likely rose higher in the atmosphere and was disassociated by ultraviolet radiation, allowing a large amount of hydrogen to escape into space.

Much of the sulfur dioxide in the atmosphere may have been taken up by carbonates on the surface of Venus, speculated Grinspoon. As the clouds thinned, more solar energy reached the planet's surface and heated it. Eventually the hotter surface temperatures may have evaporated the clouds from below, causing the immense cloud deck to disappear entirely by about 200 million years ago, according to the model.

For well over 100 million years, Venus may have been unshrouded, the researchers said. "But we calculated that volcanism must have been active within the past 30 million years to support the thick clouds observed today," said Grinspoon.

The volcanoes likely are still active, said Bullock. In 1984, LASP colleague Larry Esposito used data from NASA's Pioneer Venus satellite to determine that concentrations of sulfur dioxide in the high clouds declined dramatically from 1978 to 1983, indicating a massive volcanic eruption occurred a decade before.

"Venus is the only chance for studying an evolving climate system like Earth in our own solar system," said Grinspoon. He pointed out that researchers determined that chlorine reduced the levels of free oxygen above Venusian clouds, shedding light on the destruction of ozone by chlorine in Earth's atmosphere.

The model indicates "the climates of Earth-like planets can undergo abrupt transitions because of interactions among planetary-scale processes." Studies of Earth's ice cores show temperatures can rise nearly 20 F in less than a decade.

While the effects of climate tinkering on Earth by humans remain unknown, "In the long term Earth's fate is sealed," they wrote. "As the sun ages, it brightens. In about a billion years, the oceans will begin to evaporate rapidly and the climate will succumb to a runaway greenhouse. Earth and Venus, having started as nearly identical twins and diverged, may one day look alike."

University of Colorado at Boulder

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