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The Moist Greenhouse Effect Could Be the One That Destroys Our Atmosphere

The greenhouse effect risks drying out the planet, but a moist greenhouse effect could be equally devastating.
The surface of Venus, which has succumbed to the Greenhouse Effect. via

While the planet itself can survive a lot (like the asteroid impact that wiped out the dinosaurs), the environment that keeps us alive is comparatively tenuous. Climate scientists have warned us about the greenhouse effect that could see our planet heating up and drying out. But there’s another lingering threat, the moist greenhouse effect, which needs far lower surface temperatures to wreak havoc on the planet.

The greenhouse effect is something we hear about a lot. And if you’ve ever gotten into a car that’s been sitting in the sun all afternoon you’ve experienced it on a small scale. It happens when thermal radiation from a planet’s surface is absorbed by atmospheric greenhouse gases which send that thermal radiation bouncing around in all directions. This heats the planet’s surface, and if the surface gets too hot all the water will boil off and evaporate.

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If the surface temperature passes a critical point (740 degrees Fahrenheit for pure H2O), all the water, right down to the bottom of the oceans, will evaporate. That’s when you have a runaway greenhouse effect on your hands, and what you get is a planet like Venus: a hot, arid world devoid of life.

A graph of the rising global temperature. via

Carbon and fossil fuel emissions contribute to the greenhouse effect by adding greenhouse gases into the atmosphere, the very gases that trap and re-radiate thermal radiation. The good thing is that the Earth isn’t on the cusp of falling into a runaway greenhouse effect just yet. The bad news is that the Earth’s temperature has been rising steadily over the last century, so we’re getting closer.

But there’s a more immediate threat in terms of dangers from planetary heating: a moist greenhouse effect.

A moist greenhouse requires a lower overall global temperature, just 152 degrees Fahrenheit compared to the 740 degrees Fahrenheit needed for a runaway greenhouse. This increase in temperature can have the effect of raising the tropopause—the point where the lowest level of the atmosphere, the troposphere, thins enough and gives way to the stratosphere (it’s currently slightly above the top of Mt. Everest). This in turn increases the concentration of water vapour in the stratosphere, making the planet’s whole atmosphere wetter higher up. That water can be lost through photodissociation, which leads to ocean loss via hydrogen escape to space

Water vapor is a greenhouse gas; NASA recently recognized it as a major player in climate change. And that it comes into play at lower temperatures mean that water’s role in developing a moist greenhouse effect is a more immediate threat than other greenhouse gases causing a runaway greenhouse effect.

Recent research suggests that a moist greenhouse could be triggered by an 11-fold increase in atmospheric carbon dioxide. With as little as a 4-fold increase in CO2, our environment could face fatal heat stress. The kind that could kill off humans.

Of course, the idea that the Earth will succumb to any kind of runaway greenhouse effect in the near future is speculation. There are a lot of factors to consider with climate change, and a lot of other ways our environment might be destroyed. Like with a meteor strike massive enough to wipe out all life. Still, it’s good to know the other risks that are out there. With a moist greenhouse, it takes a lot less damage on our part to get worse effects.