What We Publish

Simultaneous stabilization of global temperature and precipitation through cocktail geoengineering

Long Cao, Lei Duan, Govindasamy Bala, and Ken Caldeira · Geophysical Research Letters 44, 7429-7437 · 2017

Key finding. Under abrupt quadrupled carbon dioxide, an appropriate combination of stratospheric sulfate aerosol increase and cirrus cloud thinning restores global mean — and land mean — temperature and precipitation simultaneously to preindustrial levels, though it does not markedly improve regional agreement with the preindustrial climate over aerosol alone.

Two scatter plots of change in precipitation against change in temperature, panel a for global means and panel b for land means, both relative to preindustrial control. Coloured dots mark abrupt quadrupled carbon dioxide at the top right, stratospheric aerosol and cirrus thinning cases along the way, and two mixed cases near the origin where both axes are close to zero. Lines through the points are annotated with hydrological sensitivities of 0.45, 1.88 and 2.56 per cent per kelvin globally, and 0.49, 2.37 and 2.56 over land.
The origin is the target: preindustrial temperature and preindustrial precipitation together. Neither aerosol nor cirrus thinning alone lands there, but the two mixed cases do — because the schemes have very different hydrological sensitivities, so combining them satisfies both constraints at once. Figure 1 from Cao et al. (2017), Geophysical Research Letters 44, 7429-7437. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Solar geoengineering faces a well-known trade-off: reduce sunlight enough to cancel the warming from carbon dioxide and you overshoot on rainfall, leaving the world drier than it started, because carbon dioxide and sunlight act on the hydrological cycle differently.

This paper asked whether combining two schemes that act on opposite ends of the spectrum could hit both targets at once — blocking shortwave radiation coming in, and letting longwave radiation out.

What did we find?

The two levers act on different radiation. Stratospheric aerosol deflects incoming sunlight; thinning cirrus cloud, simulated by increasing the fall speed of ice particles, lets more longwave radiation escape to space.

Because they load the energy budget differently, mixing them in the right proportion satisfies two constraints at once rather than trading one against the other.

Both the global mean and the land mean can be restored, which matters because land is where the precipitation change is felt.

The result is explicitly bounded. Compared with aerosol alone, the cocktail does not markedly improve how closely the geoengineered climate resembles the preindustrial one at regional scales — the global means match while the map still does not.

The authors suggest a spatially non-uniform mixture might do better regionally, and leave that open.

Why does it matter?

It shows the temperature-precipitation trade-off is not fundamental to solar geoengineering, only to doing it with one lever. Two independent instruments can meet two targets, which is a basic result in control rather than in climate.

The honest limitation is the important part. Restoring two global numbers is not restoring the climate, and the regional pattern — where anyone actually lives — is largely unimproved.

It also makes cirrus cloud thinning worth taking seriously as a complement rather than a competitor to aerosol schemes, since its value here comes precisely from acting on a different part of the spectrum.

Citation

Long Cao, Lei Duan, Govindasamy Bala, and Ken Caldeira (2017). Simultaneous stabilization of global temperature and precipitation through cocktail geoengineering. Geophysical Research Letters 44, 7429-7437.

Related