What Determines Temperature?

April 9th, 2013 by Roy W. Spencer, Ph. D.

I continue to get blog comments and e-mails from well-intentioned folks who still don’t understand what determines temperature.

More specifically, I’m talking about those who claim that the atmosphere cannot influence the temperature of the surface because the atmosphere is (usually) colder than the surface. You know who you are. 😉

Their argument goes like this…since net heat flow must be from warmer to colder temperatures (the 2nd Law of Thermodynamics), the presence of the cold atmosphere cannot cause “heating” of the surface. I understand the source of this confusion, and it’s partly a matter of semantics: rather than saying that the “atmosphere heats the surface”, it would be less confusing to say that the “atmosphere reduces the ability of the surface to cool”.

To examine the issue, I’m going to keep the discussion as simple as I possibly can without sacrificing accuracy. Let’s return to one of my favorite examples, an open pot of warm water on the stove. Let’s assume the stove is set on low, and the water has reached a rather warm temperature.

Now, we all know from personal experience that if you put a lid on the pot, you can cause the water’s temperature to rise.

But how can that be, if the lid is colder than the water?

It’s because temperature is determined by both the rates of energy gain AND energy loss, and the lid reduces the water’s ability to cool to its surroundings. It doesn’t matter what the specific mechanism of energy loss is: conductive, convective, evaporative, or radiative.

When we put a lid on the pot, we reduce the rate of evaporative and convective heat loss, as well as radiative loss from the water surface, and the water’s temperature rises until the pot once again reaches a state of energy equilibrium. Convective and radiative energy losses increase with the water’s increasing temperature compared to its surroundings. In a sense, the lid further insulates the warm water from its cooler surroundings, where “insulates” means reducing heat flow in a general sense.

The same is true of the atmosphere. Greenhouse gases in the atmosphere represent a sort of “radiative lid”, reducing the rate at which the Earth’s surface cools to outer space.

One of the major points I am making is that you cannot determine equilibrium temperature based upon the rate of energy input alone: it’s a function of rates of energy gain AND energy loss.

An extreme example is the Sun. At the core of the Sun, “weak nuclear force” reactions produce energy (so I am told) at a rate even less what the human body produces…yet temperatures in the core reach an estimated 15,000,000 deg. C. The reason why the temperature reaches such extreme values is that energy LOSS outward from the Sun’s core is so inefficient.

The everyday examples of the presence of cooler objects keeping things warmer than they would otherwise be are everywhere. For example, coffee in a cold Styrofoam cup. Stack a second cup with the first, and the temperature of the coffee will stay warmer than it would otherwise be.

In fact, everything I can think of that has a heated warm core has its equilibrium temperature controlled by cooler materials surrounding that core. A blanket over your body, etc.

No doubt my detractors will claim I am making absurd comparisons, between a pot of water and the climate system. No, the basic principles of heat flow are the same. If you pump energy into an object, no matter what it is, its temperature will increase until it’s mechanisms of energy LOSS increase to the point where they equal the rate of energy gain. The temperature will then stabilize.

But those mechanisms of energy loss routinely involve materials with cooler temperatures than the warm object itself, materials which reduce the rate of energy loss.

I’ve purposely stayed away from arguments over the specific ways in which infrared radiation courses through the atmosphere so that I can make the more general point.

This issue is so basic I cannot fathom how seemingly intelligent people refuse to accept it, and are so militant in their attempts to refute it. I sometimes wonder whether they are funded by global warming alarmists to waste my time. 😉

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