Is there a temperature rise number that will result in a runaway greenhouse effect sending the Earth towards the fate of Venus?

-- Richard, New Boston NH

Once upon a time, around small yellow sun, there existed a world with a rocky surface and a molten core. It harbored water and may even have been hospitable to life.

Then the planet got hot -- really hot. Its atmosphere filled with heat-trapping gases. Water evaporated into its atmosphere and then was lost to space. Whatever mechanisms the planet may have had for balancing its climate were broken. Nothing, not even a robot, could survive there.

This is not a scenario from a science fiction novel about climate change (or the director’s cut of Wall-E). It’s what scientists say really happened to a world in our own solar system: Venus.

Human-driven climate change will never get quite as bad as that, said Giada Arney, a planetary scientist at NASA’s Goddard Space Flight Center. But the story of Venus’s transformation from potentially Earth-like planet to the “closest thing to hell in our solar system,” she said, holds important lessons for people navigating a warming world.

For the past 4.6 billion years, Earth has kept its climate in balance via a process known as the carbon cycle. Over time, most carbon in the atmosphere is taken up by the ocean, then locked into rocks in the form of calcium carbonate, or limestone. The movement of tectonic plates pulls those rocks into Earth’s interior, where carbon can be stored for millennia before it is belched back into the atmosphere by volcanoes.

The carbon cycle is Earth’s thermostat, keeping temperatures from swinging too far toward either extreme. When volcanoes are particularly active, carbon builds up in the atmosphere, trapping heat the way a greenhouse roof does (that’s where we get the term “greenhouse effect"). But rising temperatures can lead to more rain, which wears away rocks, which releases material for making calcium carbonate, which locks up carbon in the form of seashells, limestone and other rock and cools things down again.

Scientists suspect that Venus once had its own thermostat. Spacecraft sent to probe the planet’s atmosphere have found molecular remnants of water -- evidence that, at one point, the planet was able to keep its temperature under control. When the planet got too hot, water would evaporate and form clouds in the atmosphere that reflected sunlight back into space. If the planet had plate tectonics, as some researchers have speculated, that system would have helped modulate the carbon.

But the sun, like all yellow dwarf stars, grows brighter as it ages. This gradual brightening is much too slow to account for the climate change Earth has experienced in the last century, but it has made radiation from the sun about 40 percent more intense than it was 4 billion years ago.

At some point -- perhaps as recently as half a billion years ago -- Venus could no longer handle the heat. Its clouds got too thick and started to trap more radiation than they reflected. Conditions became so warm that all the planet’s water turned to vapor, which was then broken up by the sun’s radiation.

Losing its water might also have disrupted Venus’s tectonics (if it had any), because water is thought to be an important “lubricant” for shifting tectonic plates. Without this recycling mechanism, carbon in the atmosphere accumulated to suffocating extremes.

“At that point it’s game over,” said Paul Byrne, a planetary scientist North Carolina State University. “That planet has no means of ridding itself of the heat energy from the star.”

Now Venus is the poster child for the “runaway greenhouse effect," a testament to the way a planet can change when the cycles that balance its climate are broken. The temperature at its surface is more than 850 degrees Fahrenheit -- as hot as a self-cleaning oven. The crushing pressure of an atmosphere thick with sulfuric acid clouds is as intense as what you’d experience half a mile beneath the ocean on Earth. If that wasn’t enough to kill you, breathing air composed of 96 percent carbon dioxide would do the trick.

The same gradual brightening of the Sun that turned Venus into a furnace will one day do the same to Earth, Arney said -- but not for another couple billion years.

Climate change is a much more immediate concern. Much the way a warming sun broke Venus’s temperature control system, humans have disrupted Earth’s natural cycle by burning fossil fuels, scientists agree. The buried carbon of ancient organisms that would otherwise have stayed locked up beneath Earth’s surface is now being released release at least 60 times as fast as it would under as natural processes. The planet is heating up at a rate of about 0.3 degrees Fahrenheit per decade --faster than any known change over the past 4.6 billion years.

We’re unlikely to see our planet become Venus anytime soon, Byrne said, even if we burned every gallon of oil and ounce of coal currently in the ground. Earth’s average temperature would have to rise by dozens of degrees Fahrenheit to trigger a runaway greenhouse effect, and the worst climate change scenarios don’t project warming greater than 8.1 degrees by the end of the century.

“But I think it’s certainly worth us being humble,” Byrne said. “Planetary systems are kept in a very fine balance ... and it’s important for humans to realize it doesn’t take very much to tip the balance and really fundamentally change things.”