Last week we discussed the ancient Paleocene-Eocene Thermal Maximum and what might be learned from it in the context of present-day anthropogenic climate change. This week, we look near the horizon for more clues on what Earth’s future may look like: Venus.
The planet Venus is named after the Roman goddess of love and beauty. To ancient stargazers it appeared alluring. Next to the sun and moon it is the brightest object in the sky—so bright it can, under ideal conditions, cast a faint but visible shadow. But under its shiny yellow-white clouds, Venus is anything but lovely. Like a page right out of Dante’s Inferno, the surface is a tortured, rocky hellscape hot enough to melt lead, where it rains molten metal from high, thick clouds of concentrated sulphuric acid and super-heated steam. And one day the Earth will probably fall into the same runaway greenhouse trap. The big question is when that might happen.
Planetary astronomers now believe that long ago, when the solar system was young, Venus may have been hospitable and wet. It formed at the same time our world did, from more or less the same stuff, and it’s almost exactly the same size so it probably had plenty of water and air similar to our own primeval atmosphere 4 billion years ago. In fact, Venus may have been even more hospitable than the Earth was—for a while, at least.
The problem for Venus was the sun and its proximity. Stars like our sun slowly heat up after they form, and astrophysicists estimate they get around 5 to 10 percent hotter every billion years or so. That means the sun is significantly brighter today than it was early on. So the sun delivers about 20 to 40 percent as much heat and light to Venus as it did when the planets first formed more than 4 billion years ago.
If you think this is only a problem for Venus, think again. The same forces causing the sun to get bigger and brighter are still at work. Old sol has billions of years of normal life left in it. But the Earth will run out of time well before the sun finally blows up into a red giant. It’s only a question of when.
Picture Venus 3 billion years ago. It’s a watery, steamy place. Volcanic snouts poke through the water, with island smokestacks going full blast like planetary locomotives. It's a true twin to the ancient Earth in many respects, sans the oversized moon. But it’s warmer of course, much warmer. Warm enough that vast amounts of water vapor are being held in the atmosphere. That water vapor is a potent greenhouse gas.
Planetary astronomers believe that as the temperature rose and the sun grew warmer, more and more water evaporated, accelerating the greenhouse process, causing more water vapor to form, and the cycle ran away with itself. Eventually the oceans boiled and water vapor drifting high into the atmosphere was teased apart by harsh sunlight into oxygen and hydrogen. The hydrogen floated away, the oxygen combined with anything and everything, and the result is the hellish version of Venus we see today.
The Earth is a lot like Venus may have been in its infancy. And there will come a time when the sun will grow as hot and bright in our sky as it once shone in the ancient skies of Venus. We have a robust biological cycle that has, so far, provided a degree of thermal homeostasis, in effect blunting the worst of this process, cooling the planet when it heats up and warming it when it freezes over. And unlike Venus, our planet rotates every day, giving rise to a magnetic field that offers additional protection from some of the more virulent rays blasting into the upper atmosphere of our sister world. But our time will surely come. The question is when.
There are climatologists who worry that global warming could bring about this dire result sooner than it would otherwise happen. In the book Storms of my Grandchildren, retired NASA climatologist James Hansen wrote he has come to believe that if we extract and burn all the available fossil fuels we can get our hands on, a Venus-like syndrome becomes a real possibility.
But what’s important to understand is that this extreme scenario—the Earth as a slightly cooler, dead, antiseptic version of modern day Venus—is probably unlikely in the near future without a really huge kick in the pants to get it going. Climate changes can probably come and go for whatever reason for hundreds of millions more years without kickstarting an extreme runaway greenhouse effect like we see on Venus. Use healthy skepticism when someone claims the planet will be destroyed: the planet will get along fine. Venus, after all, is still there, and the changes on its surface undoubtedly took many millions of years to go from sauna to blast furnace.
The immediate threat from human-induced climate change is to the Earth’s surface, and mostly limited to the stability of the current industrial and natural systems that now exist on it. But we really don’t know for sure if the Venus scenario is too far-fetched in the foreseeable future. Regardless, the danger that climate change poses to our modern, global civilization and our species is not only very real—it’s happening right now.