I was poking through a couple of people's denial of global warming, and it astounds me that they seem to have little grasp of physics. Lets lay it out in the simplest possible terms:
All energy eventually ends up as heat. That's basic thermodynamics, and with the exception of the zero-point energy crackpots, this is indisputable.
When an object is warmer than its surroundings, heat energy is transferred from warmer to cooler. This transfer occurs by one of three methods: conduction (direct transfer from particle to particle), convection (transfer by the movement of warm particles), and radiation (transfer through electromagnetic radiation). That's it; there's no other way to move heat energy around. The neat thing is that the greater the difference between the warm object and the cool surroundings, the faster the transfer of heat by all three methods.
Space is a vacuum; I think we can all agree on that. In a vacuum, only conduction and radiation are possible--there being no fluid to carry heat via convection. Since the planet isn't regularly in contact with other spatial object, we'll take a little leap here and say that only radiation can carry heat away in any significant quantities.
So let's do a little thought experiment: take a planet, any planet, and make it geologically dead and lifeless. Energy (in the form of radiation from the star it orbits) goes in, warming it. Energy goes out (thermal radiation from the planet), cooling it. What happens?
Thermal equilibrium, that's what happens. If energy in is more than energy out, the planet warms. The more it warms, the faster the rate of energy out. Eventually, energy in equals energy out, and the planet's temperature stabilizes.
Now let's add an internal source of energy; say, by making the planet geologically active due to internally generated heat from radioactive decay at the planet's core. What happens? This internal heat energy adds to the energy in side of the equation. The planet warms until a new equilibrium point is reached, which is higher than the previous point. Why? More energy in, that's why.
Now let's take the last step: Add life to our little planet. A life that has a penchant for taking the complex chemicals that are found on or near the surface and releasing the stored chemical energy to do work. What happens?
Eventually this energy ends up as heat. Heat that adds to the energy in side of the equation. The planet warms until a new equilibrium point is reached, which is higher than the previous point. Bingo. Human-caused global warming.
Now, I realize that this is an overly-simplistic example. It ignores niceties like living organisms turning solar radiation into chemical potential energy. It ignores the exact rates, and the complexities of an active atmosphere. It also ignores the complexities of how atmospheric composition changes the rate of energy out (i.e., greenhouse gases alone change the thermal equilibrium point by reducing the effectiveness of thermal radiation, resulting in a lower energy out and a higher thermal equilibrium even with no additional energy in; or greater cloud cover reducing the energy in by reflecting solar radiation back into space).
But the basic fact is true: energy in > energy out => increasing temperature.
I should note that this holds true for any source of energy we use except three. We can improve things by not mucking with the energy out side of the equation (reducing greenhouse gas emissions, for example), but so long as we're adding to the energy in side of the equation, warming is inevitable--we can only play with the rate.
The "except three" above? Solar, geothermal and tidal energy (we didn't discuss this above, but tidal forces are part of the energy in side, adding some energy from friction). These three source of energy are already active in the energy in side of the thermal equilibrium equation. If we can use some of the energy coming in to do the work we want, then there's no alteration to the energy in side, and thermal equilibrium can be maintained. Note that solar energy also means tapping any solar-driven cycle: biodiesel (carbon + sun -> plant matter -> fuel -> carbon), wind (remember convection? That's wind), water currents (convection again), and hydro (sun + surface water -> evaporation -> condensation + gravity -> rain -> surface water).
The only other ways out of this basic physical fact is to figure out how to decrease the energy in or increase the energy out part of the equation. But while it's possible to approach global warming like this--in a science fiction kind of way--it's not very practical: when people object to windmills in their neighborhoods, how do you think they'd feel about a multi-tens-of-miles-high radiator or a huge reflective sun shield overhead?
And it really is as simple as that.
(As an aside, space-based power stations--that mainstay of science fiction--would actually contribute to global warming. Why? Because they increase the amount of solar energy being received by this planet, adding to the energy in side of the equation. To keep our current thermal equilibrium, we have to work with the existing energy in value, not increase it.)
Posted by cerebus at August 26, 2006 11:04 AM