Friday, June 08, 2007

A third look: The doctor is in

In medical school, students learn about diseases and syndromes defined by a list of symptoms and underlying causes. When medical students become doctors, however, they don't work with disease entities; they work with patients. Patients present symptoms, not abstract disease labels or underlying causes, and what doctors see is collections of symptoms. Each symptom typically can match a number of illnesses. But the whole list of symptoms will usually match only one or two diseases, and any remaining ambiguity is usually eliminated by tests. The more symptoms you know - and test results can be viewed as just more symptoms to add to the list - the more a doctor can narrow down the possible diagnosis. Using multiple symptoms to isolate a unique disease is called differential diagnosis.

The same idea can be applied to the Earth's climate. Any changes humans make to the climate take place in a context of pre-existing natural trends. How can we be sure that one climate symptom indicates, say, enhanced infrared (IR) opacity ("global warming" in the popular sense), or solar cycle changes, or internal climate cycles? Actually, you can't be sure with one symptom. But a whole list of symptoms narrows down the scope of possible causes. If many symptoms simultaneously match a hypothesized cause, you can be much more sure that you've identified that cause correctly.

If enhanced IR opacity increases were dominating climate variability, would there be a distinctive or "smoking gun" list of symptoms? The answer is yes, all linked by a single theme: A "globally warming" climate of this type would become somewhat more like the tropics. Overall, the surface air temperature and pressure would rise slightly; evaporation would be enhanced, leading to more clouds and precipitation. Plant growth would be enhanced.

But what's really interesting is that this "tropicalization" is not geographically uniform. As always, it's differences that drive climate and everyday weather, and a slightly more tropical world would see disparate and distinctive climates impacts. A sense of these changes is communicated best by making a simple latitudinal division of the Earth into tropics (±30o lat of the equator), temperates (+30-60o lat away from the equator), and polars (within 30o lat of the poles).*

The tropics would become slightly more tropical; the temperates would shift a little more toward tropical; the polars would shift noticeably more toward tropical. The temperates would experience longer growing seasons, but certain types of plants would be favored. All plants need CO2, water, and sunlight; they would get more of two but less of the third. Some plants like that, some don't, and the former would be favored relatively more.

What are tropics anyway? They're places that get more "vertical weather" (a steeper temperature lapse rate) and thus more locally generated storms, but less "horizontal weather" due to reduced horizontal differences in temperature and saturation water vapor pressure. Remember that "climate is driven by differences": fewer differences, less "weather." The tropics generally disfavor large, laterally moving storm systems and weather fronts. Storms and fronts are less frequent, less intense, and shorter-living. The temperates are a good comparison. These latitudes experience the most "horizontal weather" - storms and fronts - because they span the greatest differences in temperature, all the way from the semi-polar to the semi-tropical.

Simply put: Reduced horizontal temperature, pressure, and water vapor differences between the polar and equatorial regions; enhanced vertical temperature, pressure, and water vapor differences between the surface and the upper troposphere/lower stratosphere.

This overall qualitative picture is robust and consistent. The exact numbers don't matter as much as the correlated multiple trends of definite nature and direction - the whole symptom list: horizontal differences would decline and vertical differences would rise, even as temperatures would rise overall. That's an exact way of saying the Earth would become "more tropical." Since the tropics are already tropical, the change wouldn't be so big there. It would be bigger in the temperates and biggest in the polars.

Whatever the quantitative uncertainty in the numbers, the qualitative picture is rigorous and ultimately a result of the Second Law: the extra heat content in the lower atmosphere (heat capacity times temperature change + change in latent heat of water vapor) wouldn't be distributed evenly. The Second Law requires that it go more to where it's most "needed," namely the polars. Less of the increase would linger in the tropics. (The dominant exchange mechanism would be the poleward ocean currents.) Overall the Earth would shift closer to global thermal equilibrium (reduced spatial variation in temperature), even as the overall temperature distribution rises.

The atmosphere overall would still be a nonequilibrium system, so this idealized equilibrium would only apply to an instantaneous "snapshot" of climate. However, greater geographic uniformity of temperature still indicates the Second Law at work. Solar radiation is converted from its incoming narrow beam to isotropic outgoing terrestial IR radiation; this transformation from unidirectional to omnidirectional, along with the upward shift in radiation wavelength, represents a "randomization" of photon energy - an increase in entropy. The more uniform the Earth's geographic temperature distribution, the more complete the "randomization" of the outgoing radiation. An upcoming posting will introduce a neighboring planet - Venus - where this "hotter, but more uniformly hotter" principle holds to a far greater extreme.**
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* A more rigorous definition uses the tropical latitudes (±23.5o of the equator) and the polar circles (±23.5o of the poles), based on the obliquity or tilt of the Earth's axis with respect to its orbital plane (ecliptic). This figure determines which latitudes near the poles experience at least one full day of polar winter night and which latitudes near the equators experience at least one summer day with the Sun straight overhead at noon. A later posting will make it clear why I've used divisions based on 30o latitude intervals instead.

** At least it's not a spherical cow emitting milk equally in all directions.

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