Wednesday, August 15, 2007

Day, night, and the turn of seasons: Variations

What happens to these daily and annual variations, their symmetries and subtle differences, if the infrared (IR) opacity of the atmosphere is increased?

The key to answering this question is the earlier conclusion about the heightening and reduction of spatial temperature differences: more IR opacity means overall enhanced vertical temperature differences (lapse rate) and diminished horizontal temperature differences. The daily and annual variations are modulated by the basic latitudinal variation of insolation: the equator gets the most, the higher latitudes less, progressively up to the poles. Standing at one point in space on the surface or in the atmosphere, we should expect reduced daily and annual variations. But the effect is not symmetric between day and night, or between summer and winter. Nighttime temperatures are increased, if we compare temperatures from one night to another at the same time and at the same spatial point; wintertime temperatures are increased, if we compare temperatures from one winter to another at the same day of year and at the same spatial point. OTOH, daytime temperatures aren't changed as much, compared from one day to another at the same hour and at the same spatial point; summertime temperatures aren't changed as much either, compared from one summer to another at the same day of year and at the same spatial point.

These comparisons are made across time, without moving in space and being sure to compare like with like: the same hour of the day or the same day of the year. The former comparisons is done between points in space (actually, different latitudes) without regard to time. These properties of climate under increased IR opacity allow us to extend the "differential diagnosis" technique outlined earlier. The earlier conclusions were comparisons in space irrespective of time. Let's now expand the list by systematically examining temperature differences measured over and over at the same point in space.

1. The daily range of temperatures decreases. The daytime temperatures change only a little; the diminution of range is due to increased nighttime temperatures. The day-night gap narrows, even as the overall 24-hour-averaged temperature rises.

2. The seasonal range of temperatures decreases. The summertime temperatures change only a little; the diminution of range is due to increased wintertime temperatures. The summer-winter gap narrows, even as the overall 365-day-averaged temperature rises.

Simultaneously, temperature variation by latitude is diminished. We now have to imagine the whole temperature distribution in space and time - four dimensions!* Fasten your seatbelt.

1. Overall, temperatures go up.

2. The temperature contrast by latitude is weakened.

3. The temperature contrast by altitude is enhanced.

4. The temperature contrast by day and by season is redcued, with the colder hours and seasons experiencing the main increase and the warmest hours and seasons experiencing the least increase. These conclusions merge in the polar regions, where night=winter and day=summer.

All in all, a climate dominated by increasing IR opacity (due to increased atmospheric concentrations of IR-active gases like water vapor and carbon dioxide) has a highly characteristic profile of change. If the Earth's climate exhibited all these changes together over years and decades, then we would know: "global warming" in its popular sense would be happening. If the Earth's climate were moving in the opposite direction, then "global cooling" (reduced IR opacity). If the Earth's climate shows a mixed picture (as it actually is), then IR opacity might be going up - but the climate is dominated by other causal factors, and the "global warming" one (if it is happening at all) is not dominant.
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* Actually, only three - we're ignoring variation by longitude, since that's (almost) equivalent to variation by hour, as the Earth rotates on its axis.

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