Temperatures at the earth's surface have increased by between 0.2 and 0.4 degrees C in the past 30 years. The vast majority of scientists attribute this warming trend to higher concentrations of greenhouse gases – CO2, methane, CFCs, and others – which warm both the earth's surface and lower atmosphere by holding heat in.
But one of the seeming paradoxes of more greenhouse gases is that while they seem to warm the earth's surface, they also seem to be cooling the higher layers of the atmosphere: Surface temperatures have gone up in recent decades, but they've declined to varying degrees in the stratosphere (above 20 km), the mesosphere (above 50 km), and the thermosphere (above 90 km).
In the lower and middle mesosphere, for example, temperatures have fallen by between 5 and 10 degrees C during the past three decades. And the outermost part of the atmosphere, around 350 km high — the so-called thermosphere — has, as would be expected by cooling, contracted.
(Here's a review of these observed changes in Science, the journal of the American Association for the Advancement of Science.)
The science behind the observed stratospheric cooling is complex, but important to understand.
Some people cite this cooling as evidence that greenhouse gases aren't warming and that human-induced climate change isn't happening. But the conclusion, it seems, should be the opposite.
In 1989, scientists predicted that more greenhouse gases would cool the stratosphere.
Indeed, Venus, which many say has a "runaway" greenhouse effect — its atmosphere is 97 percent carbon dioxide and temperatures at its surface can reach 900 F. — has a stratosphere that's four to five times cooler than ours.
It's also worth remembering that Earth supports life as we know it only because of a greenhouse effect. Without some heat-trapping ability, Earth's surface temperature should be, on average, around -0.4 F. Instead, it's a nice 57 F.
So why is our stratosphere cooling?
As Dr. Elmar Uherek of the Max Planck Institute explains, human activity affects the stratosphere in two ways:
1. By ozone depletion.
2. By increasing carbon dioxide.
Cooling by ozone depletion is the simpler of the two mechanisms. Stratospheric ozone absorbs ultraviolet radiation emitted by the sun as it enters Earth's atmosphere. Once absorbed, the radiation has, in effect, transferred its energy to the ozone molecule and warmed it.
By inadvertently depleting this ozone layer with CFCs, we've lessened its ability to absorb that energy. It now passes on to lower layers of the atmosphere, or the surface of the Earth itself, and is absorbed there instead. (Here's a nice graphic of what wavelength is absorbed where in Earth's atmosphere )
The second mechanism is slightly more complicated, and underlines how trace gases act differently at different pressures and densities in the atmosphere.
Earth's atmosphere is made almost entirely oxygen (21 percent) and nitrogen (78 percent). Both gases are largely invisible to infrared radiation emitted by Earth, or by other greenhouse gases, such water vapor, methane, or carbon dioxide.
In the troposphere, greenhouse gases slow the dissipation, eventually to space, of energy emitted by Earth as infrared radiation. They do so by intercepting the outgoing heat radiation, and re-emitting it back down to the earth's surface.
But in higher, thinner layers of the atmosphere, the increased carbon dioxide has a cooling effect by improving these layers' ability to emit heat radiation into the void of space.
In the stratosphere, heat is transferred between molecules mostly by radiation or conduction. Conduction means molecules exchanging energy by slamming into each other, and radiation means they exchange energy by emitting and absorbing radiation.
Just as in lower atmospheric layers, carbon dioxide molecules here can release energy they absorb from jostling as radiation.
But at these heights, photons released like this — traveling at infrared wavelengths — have a good chance of escaping directly back into space. There's not much around to absorb them. Thus, the cooling ability of these higher layers is enhanced by increased carbon dioxide.
One important facet, say some, of the observed stratospheric cooling is the following: It seems to debunk the notion that the sun is behind the warming of the earth's surface during the past 30 years. That's a point made by Real Climate's Gavin Schmidt.
If increased solar activity were warming Earth, we'd expect it to warm not just the troposphere, but Earth's stratosphere as well.
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