What Goes Down Must Come Up

by Michael Tobis     Department: Earth

Okay, continuing from last time, we are looking at the energy flows into and out of the planet. Last time we took note of the balance at the top of the atmosphere: the amount of radiation leaving the earth by reflection (about 105 watts per square meter) plus the amount leaving by
some other means shown by a wiggly line (about 237 watts, I'll be leaving out the "per square meter" for convenience here) balance the sunlight coming in (342 watts).

A brief note on the reflection. The diagram tries to show three parts; reflection from the surface, reflection from clouds, and scattering from the air (it looks a little blue from above, too!) The sort of hard-right turn on the way to the cloud is maybe confusing, but don't pay it any mind. Most of the light bounces directly off a cloud. There is actually some amount that is scattered first and then bounced from the cloud, but that all "comes out in the wash". The main thing to understand in this part is that almost a third of the light energy "bounces back" and never really affects the climate at all.

It's that other part that we need to talk about to get a grip on what is going on. What does the wiggly line represent? In the jargon it is "OLR", which stands for "outgoing longwave radiation", but what does that mean?

Well, for one thing, it is mostly infrared radiation. Now, sometimes people call infrared "heat rays" but that is actually very misleading. It is true that if you have a camera or goggles that detect infrared, warmer objects appear brighter. You aren't detecting the heat directly.

It turns out that heat is a sort of energy content that any object has. Every object glows with its heat. Some things are very hot, so they glow in frequencies we can see. We sometimes call them "red-hot". The sun is hotter still and glows mostly yellow. (Though we tend to perceive sunlight as white, that's another story. Color perception. Messy business...)

Cooler objects, objects which have the sort of temperature we encounter every minute, outdoors temperatures, glow at frequencies our eyes do not detect. But still they glow. And it is that glow which provides the majority of the energy heading back out to space. That energy, for many practical purposes, perfectly balances what the sun is sending in minus what gets reflected back out.

Why this balance? Well suppose more energy were coming in than was going out over a long time. Then the world would get hotter and hotter, as more energy collected without escaping. Then, as it got hotter, it would emit more infrared energy. Eventually it would get back into balance, just at a warmer temperature. If less energy were coming in than was going out, the world would get cooler and cooler, eventually re-establishing the balance. How long does it take to establish the balance? That turns out to be a complicated matter. If the imbalance is short-lived enough that the ocean doesn't change temperature, the adjustment time of the atmosphere is only a few weeks. So the approximation is pretty good if 1) climate isn't undergoing long-term changes of a persistent warming or cooling, and 2) you average
over a few months. There is a seasonal cycle, but to a good approximation it's in balance. 

(If you've seen undergrad thermodynamics, it has the same flavor of quasi-equilibrium that most of your problem sets offer as a simplifying assumption.)

But what happens of there is a persistent warming or cooling? We'll discuss that later, after we examine the other parts of the diagram.

Tags: climate, earth

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