Earth is only habitable because different wavelengths of light interact differently with the things they touch. Energy streams out from the hot Sun as a broad symphony of light waves, and our rocky planet intercepts a tiny fraction of the torrent. The energy carried by that tiny fraction is what keeps us warm. But if that was all there was to it, the Earth’s average surface temperature would be a frigid 0°F, rather than its current comfortable 57°F. What saves us from being permanently frozen is the Earth’s “greenhouse” effect. The way it works has to do with different wavelengths of light interacting with the atmosphere in different ways.
Imagine the view from a hillside on one of those cartoon-like days when the sky is mostly blue but there are a few puffy white clouds meandering along to add a bit of variety. If you’re looking out over flatter land, you can see green trees, grass, and dark earth. Sunlight illuminates the scene, apart from the shadows left by the clouds. But what’s reaching the ground in front of you is different from what left the incandescent Sun. The atmosphere has absorbed the long infrared wavelengths, and most of the shorter ultraviolet wavelengths, but the visible light has sailed through unaffected. The atmosphere has already selected the waves that reach the ground. It just so happens that they’re the ones we can see. At the visible wavelengths, the sky behaves as an “atmospheric window,” letting everything through. There’s another window for radio waves (that’s why radio telescopes can see the cosmos), but most of the other waves are blocked by the air.
The darker the land you can see, the more of those visible waves it’s absorbing. And the absorbed energy eventually ends up as heat. If you touch dark ground on a sunny day, you’ll feel that heat. The rest is reflected upward, back out through the atmospheric window. If any aliens are out there looking at us, that’s what they’ll see us with.
But now the ground has heated up. And just like the toaster heating element, it must give away light energy because of its temperature. It’s relatively cool, so we can’t see the glow. But in the longer-wavelength infrared, the warm ground is a lightbulb. And this is where the greenhouse effect comes into play. Most of the atmosphere will just let these infrared waves through. But some gases—water, carbon dioxide, methane, and ozone—punch above their weight. Even though they make up only a fraction of the total atmosphere, they absorb the infrared waves very strongly. They’re known as the greenhouse gases. As you look out across the landscape, you can see the visible light leaving the surface, but you can’t see the infrared. If you could, you’d see that it faded away as it got farther from the ground. The atmosphere is absorbing the infrared waves as they travel upward. It won’t be long before those molecules will give up their new energy, and send it out again as more infrared waves. But here’s the important bit. When the new waves are sent out, they’ll be sent in all directions equally. Only some will travel upward and out of the atmosphere. Some will travel back downward, and be reabsorbed by the ground. So some of the traveling energy is trapped in the atmosphere. That extra little bit of heating is what keeps our planet warmer than it should be, allowing liquid water to exist. A new balance has to be established; ultimately, the same amount of energy must both arrive and leave, otherwise we’d continually be getting hotter. So the Earth heats up until it can give away enough infrared waves to balance the books.
This is the “greenhouse effect.”?? Most of it is natural—there’s lots of water and carbon dioxide in our atmosphere, and everything is in balance when the average surface temperature is 57°F. But as fossil fuels burn, humans are adding carbon dioxide to the atmosphere, so that more of the infrared energy traveling upward is trapped. This shifts the balance. So the planet will heat up until a new balance is achieved. The amounts of carbon dioxide involved are very small: CO2 made up 313 parts per million of the atmosphere in 1960, and 400 parts per million in 2013. Compared with all the other molecules up there, it’s a tiny increase. But these molecules select certain waves to absorb. Methane will absorb even more infrared than carbon dioxide. So these gases matter. The greenhouse effect is what made our planet habitable, but it also has the potential to change the temperature significantly. It’s all happening with waves that we can’t see directly. But we can measure the consequences already.
There are all sorts of waves rippling around our world—giant radio waves, minuscule visible light waves, ocean waves, ponderous deep sound waves emitted by whales underwater, and the high-frequency echo-sounding beacons sent out by bats. Each type is zooming through and past the others, but has no effect on them. But we have one more question to answer. What happens when a wave meets another of exactly its own type? The answer is beautiful if you are holding an iridescent pearl, but something to be avoided if you’re trying to hold a mobile phone conversation.
Pinctada maxima can be found parked on the seabed, just a few yards below the surface of a turquoise sea near Tahiti and other South Pacific islands. When it’s feeding, the two halves of its shell part slightly and it sucks in sea water, gallons each day. The mollusk inside the shell quietly filters out any valuable specks of food and then expels the cleaned water to rejoin the ocean. You could swim right over the top of it and never notice—the outside of the shell is coarse and unremarkable, mottled in beige and brown. These vacuum cleaners of the ocean look the part: functional and unglamorous. The inside of an oyster was never meant to be seen. And yet Cleopatra, Marie Antoinette, Marilyn Monroe, and Elizabeth Taylor were all proud owners of what happened when an oyster’s innards made the best of a bad job: pearls. Pinctada maxima is the South Pacific pearl oyster.