Guest Post by Willis Eschenbach
I got to thinking that the eruption of Mount Pinatubo should provide a good test case for my theory that changes in albedo help regulate the temperature and keep it within a narrow range. When a big volcano erupts, it throws both black and reflective particles and aerosols high into the stratosphere. As a result, the albedo (reflectivity) and the shortwave absorption of the stratosphere both increase, available solar energy decreases, and the planet cools. By that theory, available solar energy (after albedo) should show a dip after the eruption followed by a recovery to pre-eruption values.
I have demonstrated in the post cited above and elsewhere that cloud albedo varies with temperature in order to prevent excessive heating or cooling. As a result, if my hypothesis is correct, we should see a more complex and different reaction to the eruption. We should see a planetary albedo response to Pinatubo that restores the temperature. If there is a temperature governing mechanism in play, we would expect the change in albedo to provide extra energy to counteract the cooling effect of the eruption. So we’d expect the available energy to overshoot, providing more energy than normal, until the balance is restored.
So I plotted up the change in available solar energy, which I calculated as total solar irradiance TSI * (1 – albedo), for the period 1984-1998. I also plotted the global average temperature for the period, converted to a blackbody equivalent temperature in watts per square metre. The results are shown in Figure 1.
How do these observations support the idea that there is an albedo-based temperature regulating mechanism?
Look at what actually occurred after the eruption. The amount of solar energy began rising, and continued to rise in opposition to the falling temperatures. It remained high, significantly higher than most of the record, until the temperatures started to rise in 1994. Only after temperatures had returned to pre-eruption levels did the amount of solar energy drop back down.
This is precisely the response that would be expected from a governing mechanism involving the albedo. It would let more energy into the system to counteract the volcanic cooling, and would continue doing so until the temperature was restored.
So I hold that this provides clear visual evidence that an albedo-based temperature compensating mechanism does exist. I also say that this mechanism explains the poor performance of the GISS climate model, whose predictions of Pinatubo I discussed here and here. Because it does not include any albedo-based compensatory mechanism, the GISS model predicted a much larger temperature drop than actually occurred.
The albedo data is from Long-term global distribution of Earth’s shortwave radiation budget at the top of atmosphere (PDF), N. Hatzianastassiou et al., Figure 8(c). Temperature data is from the HadCRUT3 anomaly dataset and the HadCRUT absolute temperature dataset. Absolute temperatures plus anomalies were first converted to the equivalent blackbody temperature in watts per square metre using the Stefan-Boltzmann equation. Both datasets then had their monthly averages removed before graphing.