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Geoengineering Earth's radiation balance to mitigate CO2-induced climate change

Bala Govindasamy and Ken Caldeira · Geophysical Research Letters 27, 2141-2144 · 2000

Key finding. In simulations reducing solar luminosity to balance the radiative forcing from doubled atmospheric CO2, geoengineering markedly diminished regional and seasonal climate change as well as global mean warming, despite the differing spatial patterns of the two forcings — though such schemes could still prove environmentally risky.

Two contour panels of latitude from 90 south to 90 north against month of the year. The left panel shows the change in net longwave radiative flux at the tropopause from doubling carbon dioxide, which is nearly uniform in green and yellow bands across all latitudes and seasons. The right panel shows the reduction in incoming solar radiation needed to compensate it, which is strongly red near the summer pole of each hemisphere and falls to zero in the winter darkness at both poles.
The two forcings do not look alike at all. Carbon dioxide's effect is nearly uniform through the year and across latitude; the sunlight reduction needed to offset it must be concentrated where and when the sun shines, and can do nothing at all through the polar winter. That the compensation works despite this mismatch is the paper's finding. Plate 1 from Govindasamy and Caldeira (2000), Geophysical Research Letters 27, 2141-2144. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Reducing the sunlight reaching Earth would offset the global mean warming from higher CO2. But sunlight and CO2 do not force the climate in the same geographic pattern — sunlight peaks in the tropics and in summer, while CO2 acts everywhere and at all times.

Because of that mismatch, it was widely assumed that cancelling the global mean would leave substantial regional and seasonal climate change behind. No study had tested the assumption directly. This paper did.

What did we find?

The experiment used the NCAR CCM3 atmospheric general circulation model, reducing solar luminosity by the amount needed to balance the increased radiative forcing from doubling atmospheric CO2.

The regional and seasonal residuals were much smaller than expected. Cancelling the global mean forcing largely cancelled the regional and seasonal climate change too.

This was not the anticipated result. The reasoning that different forcing patterns must produce different climate patterns turns out not to survive contact with a model that resolves the atmospheric circulation, because the circulation redistributes energy and smooths the difference.

The paper is explicit that the finding does not make the approach safe. Geoengineering schemes could still prove environmentally risky for reasons the temperature comparison does not capture.

Why does it matter?

This was the first three-dimensional climate model study of solar geoengineering, and it established the empirical basis for a debate that had until then been conducted on intuition about forcing patterns.

The result is a good example of a case where a plausible physical argument gives the wrong answer. The mismatch between solar and CO2 forcing patterns is real; the inference that it must produce large regional residuals is not.

Citation

Bala Govindasamy and Ken Caldeira (2000). Geoengineering Earth's radiation balance to mitigate CO2-induced climate change. Geophysical Research Letters 27, 2141-2144.

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