It's that most wonderful time of the year! Christmas decorations are going up all over America. Those colorful little strings of twinkling lights racing down eaves and winding artfully around tree trunks can do more than paint pretty holiday vistas. We can use them as an intuitive probe to look at our nearest neighbor in space and speculate on earth's distant, or not so distant, future.
A typical Christmas tree light puts out about one watt of heat and light energy. That's not a lot but it does the job. On that same scale, the incident solar radiation, or insolation, received by earth when all wavelengths are taken into account works to about 250 watts per square meter1 (usually referred to as just "watts") when averaged over the entire surface, light and dark, from poles to equator. Some of it is reflected back, the rest is absorbed.
If you take an introductory planetary astronomy course you'll hear the usual spiel that the earth is in the solar system's Goldilocks zone, not too hot like our sister planet Venus, not too cold like our smaller cousin Mars. It's actually more complicated than that. At 250 watts, the earth is a little too far away, a little too cold, for our liking. By the laws of simple thermodynamics our lovely blue-green planet should boast an average temperature well below the freezing point of water. And the sun is very slowly heating up, roughly 5 to 10 percent per billion years, so ancient insolation was even less long ago than it is now. The earth should have started out frozen solid right down to the deep ocean trenches and it would be a brilliant iceball hanging in space like a snow-white ornament to this day. What's kept that grim fate at bay for billions of years are greenhouses gases (GHGs).
If you compare GHGs like carbon dioxide (CO2), water vapor (H2O), and ozone (O3), to non GHGs in our atmosphere like oxygen (O2) or nitrogen (N2), one of the first things you notice is that the GHGs have more than two atoms. That's not a coincidence. The physical arrangement of atoms that make up GHGs allows them to absorb a lot more heat than the simpler compounds. They act on our planet just like windows in your car; they let light through but retain heat, so when you come back to your SUV in a sunny parking after holiday shopping, it's noticeably warmer inside.
The Greenhouse Effect saved earth from turning into a snowball and made our world a haven for life, but the same phenomenon pulled Venus into the fiery pit of hell. Billions of years ago, lovely Venus was true to her mythological name, hospitable, inviting, and probably much more like earth. Evidence suggests she had warm liquid oceans and everything else a heat loving anaerobic microbe could want. But proximity to the sun and relentless solar heating evaporated more and more water. Water vapor is a potent heat trapping gas. The temperature rose further evaporating more water, the process fed back, viciously, and eventually the oceans boiled off completely cloaking the planet in thick steam. High in the atmosphere, under the influence of harsh solar UV, hydrogen atoms escaped their watery embrace with oxygen and bled into space. The oxygen combined with left over nitrogen to form clouds of acid. Nearer the now broiling surface, carbon was baked out of the rock and combined with oxygen to form CO2, trapping even more heat. The picture of the surface below taken by a Russian probe (Venera 13) gives an idea of what Venus is like now (Click on any image to enlarge offsite). The horizon is off in the upper right corner.
It's every bit as hot as it looks. Today Venus is as dry as a bone under a smothering atmosphere ninety times denser than ours and the planet roasts at 850 F. So hot it snows metal, so hot you wouldn't need a light in a Venusian cave because the walls glow red. Some scientist speculate that if Venus had a robust microbiology early on, microbes might still eek out a living high in the clouds. But any life that resembled even the hardiest terrestrial thermophiles on or near the surface was charred to a cinder.
Planetary astronomers don't know for sure when this happened. But it most likely occurred at least 500 million years ago, because that's the last time the entire surface melted and re-solidified in the tortured Hadaean tableau we see now. It may have periodically melted, several times, which means the climate on Venus may have spun out of control over two or three billion years ago when the insolation was not much greater than it is on earth today
That brings up a chilling question, no pun intended: could the same thing happen to earth? Yes, and it's not a matter of if, it's a matter of when. The sun will continue to slowly heat up until our world is forced into a similar greenhouse loop. It may take hundreds of million of years, but it probably won't take much more than a billion. And with a little help, say if a huge source of greenhouses gases were suddenly released, it might happen more quickly than we could ever dream up in our worst nightmare.
Climate scientists have found that at the current solar luminosity and continental configuration, the earth is amazingly sensitive to small perturbations in climate. The difference between the warm period we're in now and enough cooling to trigger an ice age is one, tiny twinkling little Christmas tree bulb over each square meter of the earth's surface for several centuries. If the lack of a single miniature decorative light could trigger ice sheets marching down into Indiana, imagine the warming that might ensue if 5 or 10 of them were added? Next week we'll look at the most recent work from the man who is arguably the world's foremost expert in climate science. He worries that, under some of the grimmer assumptions, we may embark on a whirl-wind journey to join our sister planet, in more ways than size and mass, a lot earlier than we think.