Tuesday, March 27, 2007

Climate as heat engine

It's helpful to conceptualize the climate, like any thermodynamic system, as if it were an engine driven by the second law. Heat flows from higher temperatures to lower - it's disorganized energy that seeks increasingly disorganized forms, sometimes doing organized work along the way. The ultimate source of the energy flow is the Sun. But the immediate reservoirs for the captured solar energy lie on the Earth: the tropical oceans and the Earth's surface. These two heat baths drive two distinct engines.

This double heat engine has two parts, a vertical one in the air, and a horizontal one in the oceans. The vertical engine has a water evaporation-condensation heat subengine inside. Since the climate is an open system, the heat is not absorbed and stored all at one time. Some parts are really good at absorbing and storing heat - water above all. Others pose "bottlenecks" that make heat flow difficult, and the heat flow gets "backed up." We saw earlier that big reservoirs with a lot of heat capacity (water in all its forms, mainly) tend to moderate temperature changes. Heat "backing up" tends to raise "upstream" temperatures above what they would have been otherwise.

Recall the three basic heat transport mechanisms: convection, radiation, and conduction. It's helpful to break out water evaporation and condensation as a separate heat flow mechanism, even though efficiently moving evaporated water up in the atmosphere requires vertical convection, together with diffusion. Heat flows in the most efficient way possible, if that's available. If it's not, heat will flow in the next most efficient way, etc. The most efficient method is dominant, but that doesn't mean the less efficient mechanisms are not also at work - just that they aren't dominant.

The poleward flow of ocean heat. In the oceans, heat flows from the tropics, around the equator, towards the colder poles. The organized work is manifested as ocean currents. This flow is "horizontal" (actually, parallel to the Earth's curved surface) and does little work against gravity.

This ocean heat transport is a combination of conduction and convection. Water is denser and more conductive than air, so a lot of heat does spread by diffusion in the oceans and other bodies of water. But convection, where it happens, is more efficient, and that (in the form of well-defined currents like the Gulf Stream) is the dominant way ocean heat is transported.

Air currents also transport heat from equator to poles, but the oceanic heat transport is responsible for the bulk of the transfer.*

The upward flow of surface heat. In the atmosphere, heat flows upward from the surface as radiation and as turbulent, convected air holding evaporated water. Some of the radiation heats the lower atmosphere by absorption; the water vapor, upon condensation, deposits its heat there as well. The remaining flow heats the upper atmosphere. This flow is "vertical" (actually, outward from the Earth's surface). The radiation is essentially unaffected by gravity, but the upwards convecting moist air does have to do work against gravity. Its vertical flow represents another type of organized work.

Vertical heat transport in the atmosphere breaks down into these mechanisms, in ranked order:
  • Radiation
  • Evaporation/condensation (when available, which is many times and places)
  • Convection (when available, which is most times and places)
with the second working together with the third. The lower atmosphere (troposphere) is dominated by radiation, but convection, in combination with evaporation, is still significant, representing about 1/5 of the upward heat flow. The tropospheric convection is turbulent and inefficient, at least in the clear air. (It's a lot more efficient in clouds.) As parcels of air are convected upwards, they expand and cool, sometimes enough for water vapor to condense as clouds. The condensation, when and where it happens, mitigates the cooling and makes the temperature-altitude profile less steep than it would be for unstaturated air.

The junction of the troposphere and the stratosphere (the tropopause) is a critical layer, where upward convection stops. In the upper atmosphere (above the tropopause), only radiation matters. There's no convection, and the air is too thin and dry for conduction or condensation to be important. Because the stratospheric temperature is, at first, constant and then rising with altitude (we'll learn why in a later posting), no matter-based heat transport would be possible anyway: heat cannot flow from colder to hotter temperatures. But the radiative heat transport encounters little obstacle in the upper atmosphere and just proceeds on its merry way. The stratosphere acts as a "lid" on the boiling, convective troposphere below.

An interesting, if extreme, case is the polar regions during polar night. In that situation, neither evaporation nor convection is available. Vertical convection of air requires heating from below, which typically comes from the Earth's absorption of light from the Sun and conversion of that into heat. For six months at the poles, this source is out of sight, and the lower atmosphere at that time and place moves heat upward by radiation only. It's as if the sky had fallen down to the ground - the stratosphere resides at the surface.
* Conduction and convection are also important in the solid interior of the Earth. The source of heat deep down is the decay of long-lived but unstable radioactive elements left over from the Earth's formation. That heat diffuses out slowly by conduction and, here and there, by much faster convective plumes that sometimes burst forth on the surface as volcanoes. This core heat raises the Earth's surface temperature slightly beyond the solar heating-alone value and drives plate tectonic motions. Volcanoes also change the chemical composition of the Earth's atmosphere when they erupt.

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