Comparison of the fast and slow climate response to three radiation management geoengineering schemes
Key finding. Offsetting the warming from doubled carbon dioxide requires different amounts of forcing from each scheme: stratospheric aerosol increase has a radiative forcing efficacy about 15% smaller than carbon dioxide, while marine cloud brightening and cirrus cloud thinning are each about 10% larger — and the apparent hydrological sensitivity differs substantially between all four forcings.
What question did this research address?
Solar geoengineering is often discussed as though the method were interchangeable: reduce the energy retained by some means and the warming is offset. But the schemes act on different parts of the spectrum, at different altitudes, over different surfaces.
This paper asked whether a watt of forcing from each scheme does the same work as a watt from carbon dioxide, and what each leaves behind in temperature contrast and rainfall once the global mean is balanced.
What did we find?
Slab-ocean and prescribed sea surface temperature simulations in the NCAR Community Earth System Model compare three schemes — stratospheric aerosol increase, marine cloud brightening and cirrus cloud thinning — against carbon dioxide forcing.
With forcing defined as top-of-atmosphere imbalance in prescribed-sea-surface-temperature runs with land temperature adjustment, efficacy is about 15% below carbon dioxide for stratospheric aerosol and about 10% above it for the two cloud schemes.
Those differences trace to different feedback processes acting on each forcing agent, not to the size of the forcing.
The schemes also produce different land-ocean temperature change contrasts, so a global mean that has been restored can still leave continents and oceans differently displaced.
Apparent hydrological sensitivity — the change in global mean precipitation per degree of temperature change — differs substantially between carbon dioxide and all three schemes, mainly because their fast precipitation adjustments differ.
Once the fast adjustment is removed, the northward movement of the intertropical convergence zone under each forcing agent is tightly related to changes in interhemispheric energy exchange and the hemispheric temperature gradient.
Why does it matter?
It makes clear that "solar geoengineering" is not one intervention. The schemes differ by tens of per cent in how much cooling a unit of forcing buys, so estimates built on one cannot be carried over to another.
Separating fast adjustment from slow response is what makes the hydrological differences legible. Most of the divergence in precipitation is established before the surface has warmed or cooled, which is why it cannot be tuned away by adjusting the amount of intervention.
The intertropical convergence zone result gives a single organising principle: where the rain band sits follows from the interhemispheric energy balance, whatever the agent that disturbed it.
Citation
Lei Duan, Long Cao, Govindasamy Bala, and Ken Caldeira (2018). Comparison of the fast and slow climate response to three radiation management geoengineering schemes. Journal of Geophysical Research: Atmospheres 123.
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