A primer on the science of global warming


Talks between the US and China last month broached the topic of how the world's two greenhouse gas-producing behemoths might deal with the problem, but got nowhere concrete.

The Heartland Institute, meanwhile, released what it calls "an 880-page, 1-1/2 inch-thick, six-pound rebuttal of global-warming alarmism": "Climate Change Reconsidered: the 2009 report of the Nongovernmental International Panel on Climate Change (NIPCC)."

Two years ago, Newsweek's Sharon Begley said – somewhat controversially – that this sort of stuff was the product of a global warming "denial machine."

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For my part, I thought I'd review the basics of global warming science – how and why carbon dioxide molecules absorb heat. Thankfully, it has nothing to do with whether you drive an SUV or a hybrid, whom you vote for, or even whether you think global warming is nonsense. It has to do with physics.

Carbon dioxide absorbs infrared radiation (aka heat) because of its molecular size, and how it vibrates. That may seem weird – and quantum mechanics can be quite weird – but that's simply how it works. The CO2 molecule jibes perfectly with infrared wavelengths – it's in tune with them – so it absorbs their energy.

Other gases in the atmosphere, such as oxygen (O2) and nitrogen (N2), don't "harmonize" with infrared. They're "invisible" to that wavelength. (They do "scatter" the blue wavelengths, however, giving us blue skies.)

Here's a graph of what molecules absorb what wavelengths.

So let's begin at the beginning – the sun. The sun emits photons. Photons have no mass, but they have energy and a wavelength. The sun emits most photons in the "visible light" spectrum – the wavelengths of a rainbow. They hit earth's atmosphere. Ultraviolet frequencies are the first to go. Why? Again, it's about harmonization. The ozone molecule, a trio of oxygen atoms, jibes perfectly with UV wavelength, and catches it. (That's also how your sunblock works.)

Photons traveling at visible light frequencies keep going through the atmosphere. About half make it to Earth's surface. About 4 percent are immediately reflected back the way they came. We wouldn't see much if they weren't. But we're stuck with the energy of the rest. That means lots of excited and vibrating molecules.

These molecules re-emit photons not in the visible light spectrum, but at longer wavelengths – what we experience as heat.

Here's a slide show of how it all works.

CO2 molecules in the atmosphere absorb those outgoing infrared photons, reradiating them in all directions. (Here's an animation.) Imagine those infrared photons as balls bouncing around a pinball game. Eventually they exit, but the more obstacles there are, the longer the journey.

Likewise, the more CO2 in the atmosphere, the longer heat – photons traveling at infrared frequencies – takes to escape back to space. And if you're sitting in the middle of it, you experience that extended journey as raised temperatures. That's global warming in a nutshell.

To illustrate this point, look at our planetary neighbor, Venus. It's one stop closer to the sun, so it receives more of the sun's energy. But constant clouds reflect about 75 percent of that energy back into space. Still, Venus' surface temperature is around 860 degrees F., hot enough to boil mercury and melt lead. Why? Its atmosphere is 97 percent carbon dioxide.

Now look at Mercury, the planet closest to the sun. It receives even more energy than Venus, and daytime temperatures also surpass 800 degrees F. But nighttime temperatures sink below -300 F. Why? The planet has little atmosphere. There's nothing to keep the sun's energy from immediately escaping back into space.

Back to Earth. Life as we understand it is possible here only because, among many other things, we have a nice concentration of heat-absorbing gas in the atmosphere. Not enough to vaporize our thermometers, but not so little that we freeze solid every night.

So here's the question: If you increase the amount of those gases in the atmosphere, will it get a little toastier? The physics says yes.

Earth's climate system is, of course, much more complex than the picture I've painted here. And certainly there's much uncertainty surrounding what extra energy in the climate system might do: Will more water vapor, a potent greenhouse gas, further warm the atmosphere? Or will more clouds cool it? Will warmer oceans cause more hurricanes, or will changes in the atmospheric temperature gradient snuff them out?

All good questions, but as to the fundamental physics, there's no doubt. CO2 absorbs and reradiates infrared radiation. More of it means more heat.

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