Thursday, June 07, 2007

A second look: Global warming

To continue from the first look at CO2 in the atmosphere: what would doubling the 1950 value of the atmospheric CO2 concentration (300 ppm) do to the climate? First, the bottom line up front: "best estimate" of shift in surface temperature = +0.3 - 0.4 oC, a little more than a third of a degree Celsius.

The primary cause is an increase in the infrared (IR)-active gas concentration, which then triggers a lengthy list of secondary causes, some enhancing the temperature, some reducing it, until the final net result is reached. The specific number is an estimate. But the qualitative consequences are rigorous: the IR opacity increases; water evaporation, condensation, and precipitation increase; cloudiness and convection increase. The type and direction of these changes is not in doubt; the uncertainty is quantitative: different mechanisms of very roughly the same size pull the temperature in opposite directions.

We can assemble various partial estimates, although these are misleading if mistaken for complete estimates. For example, the clear-sky-only estimate with just the opacity increase from CO2 would be +0.95 oC, nearly one degree Celsius. Adding the effect of the existing clouds reduces that to +0.65 oC. They generally have a strong cooling effect:
  • Clouds reflect incoming solar radiation back into space.
  • Clouds efficiently convect heat upward.
  • Clouds efficiently radiate heat into space.
But a full atmospheric estimate requires the full list of changes discussed here over the last few months:*
  • Increased CO2 concentration (doubled)
  • Enhanced evaporation, condensation, precipitation of H2O (rather more than 1%)
  • Enhanced H2O vapor concentration (same)
  • Enhanced cloudiness (shift from 0.54 to about 0.55)
  • Enhanced convection from a steeper temperature lapse rate
These changes result in both temperature enhancements:
  • Opacity increase due to increased CO2 concentration
  • Opacity increase due to increased H2O concentration
  • Enhanced latent heat release from H2O vapor condensation
and temperature anti-enhancements:
  • Enhanced cloudiness
  • Enhanced convection
The enhancement of cloud cover is the significant anti-enhancement; the enhancement of (clear-air) convection is small.

What happened to convection? Convection is present in the CO2-enhanced atmosphere, as it is in ours. But the increase in (clear-air) convection stimulated by the increase in IR opacity is small. Convection is larger in clouds and more enhanced there. Being much more efficient, cloud convection is very important in moving heat to the cloudtops, which are effective upward radiators.

And CO2 so far? This is effect of doubling the CO2 in the atmosphere. The increase from 1950 to 2006 has been 75/300 = 1/4 of that, so even the most extreme estimate (clear-sky only) is about +0.24 oC, one-quarter of a degree Celsius - half or less of the larger, pre-exiting variations in climate due to multiannual and multidecadal climate oscillations (NAO and El NiƱo) and solar luminosity variations. In reality, the half-century (1950-2006) trend is more like +0.075 to +0.10 oC, less than a tenth of a degree Celsius, smaller than the uncertainties inherent in the calculation, and definitely too small to detect against a background of non-anthropogenic variations.

Feedback on the CO2 concentration itself. Still missing are two important feedback effects that further moderate the effect. Both remove some of the CO2 injected from its sources (manmade, volcanic). These mechanisms reduce the atmospheric CO2 level:
  • Geophysical: diffusion of CO2 into the ocean and subsequent fixing of C in sinking, dissolved carbonates.
  • Biological: more plant growth (existing plants growing faster, new plants), since CO2 is plant air.
For the most part, we'll stick with atmospheric-only estimate. The important thing to notice at this point is that the estimated temperature enhancement is small. Although modern thermometer and satellite measurements are precise enough to detect such small changes locally, what they add up to globally is not a well-defined question. The pre-existing natural changes are larger, and it's hard to see how such a small effect in the current era could be identified.
* The final "best estimate" also includes the slight loss of polar ice and snow and the consequent reduction in Earth's reflectivity (albedo). The mechanism is similar to the enhancement of evaporation, but the effect is like reducing the cloud cover slightly.

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