The River of Energy

by Michael Tobis     Department: Earth

In our last three installments, we looked at the energy balance of the earth, at the ground, at the top of the atmosphere, and at the middle of the atmosphere. In each case we saw that what comes in is balanced by what goes out. This is not a coincidence. Energy is a "conserved quantity"; like silver or gold or oxygen or carbon, it is (for practical purposes on the earth) never created or destroyed in significant quantity. Unlike the elelments, though, it comes in from the sun and escapes by infrared readiation.

We saw that the amount of infrared radiation going out has to be set by the amount of solar radiation coming in (and not reflected immediately). But the amount of infrared radiation depends on the temperature of an object. This is why it is informally called "heat radiation", confusing generations of tech-savvy people trying to learn physics. (Heat is not actually a form of radiation, but light, radio, and infrared are.) The reason an infrared camera "sees" warm objects is because they radiate more brightly in the infrared.

On worlds with no atmosphere, like the moon, that's pretty much the whole story. What comes down goes up, and that balance sets the temperature.

In the case of a planet with an atmosphere, it's complicated somewhat. If the atmosphere can itself absorb and reradiate infrared (that is, if it contains gases that are opaque at those frequencies), then the atmosphere helps set the temperature of the surface. This is what we saw in our diagram, where more energy hit the surface from teh atmosphere than directlyf rom the sun. The temperature is determines not only by the sun, but by the transparency of the atmosphere. And this is where we are getting into trouble, because human activities affect the transparency of the atmosphere substantially, both in the visible (dust) and the infrared (mostly CO2 but also methane and some other stuff.)

So we cannot have the same climate if we keep doing this. In the 70s and 80s there was some controversy as to which was winning, the dust or the greenhouse gases, so some people were concerned about cooling the earth. It turns out that the reason the greenhouse gases win is because they accumulate: they stay in the atmosphere a long time.

(The right way to think about this isn't obvious, and the press sometimes get it wrong, but the important fact is that about a half of out current CO2 emissions stay in the atmosphere long enough to make a difference, and about tenth of our current CO2 emissions stay in the atmosphere for thousands of years.)

Energy is like water flowing down a mountain side. If you look at a patch of the slope, you will see that what flows in eventually balances what flows out. A piece of bare rock won't hold much water in the process, but if plants start growing there and making cracks and soil, your piece of mountainside will get wetter. The same amount will still pretty much go out as came in; the a mount in storage to make it wetter is tiny compared to the flows. The wetness is a property of how smoothly the water passes over the territory.

By adding to greenhouse gases we make the atmosphere more capable of holding energy, just as more soil makes a slope able to hold more water. To overcome that, the soil gets wetter until the system is back in balance. And the atmosphere gets warmer.

Could something else be happening? Maybe the plants opened up a special underground crevasse and the water is now rushing through more quickly? It's conceivable but I wouldn;t bet the farm on it. Similarly it is conceivable that something bizarre will happen and the world won't warm up. The important thing to understand is that NOT warming is the peculiar outcome. T

here really is no strong reason not to expect global warming.

Tags: global warmin