What We Publish

The science of geoengineering

Ken Caldeira, Govindasamy Bala, and Long Cao · Annual Review of Earth and Planetary Sciences 41, 231-256 · 2013

Key finding. Solar geoengineering and carbon dioxide removal face fundamentally different limiting constraints: because some solar geoengineering approaches are thought to be low in cost, the scale of deployment will depend primarily on considerations of risk, whereas the scale of carbon dioxide removal deployment will depend primarily on cost.

Four world maps in two rows. The left column shows annual mean temperature change in degrees Celsius on a scale from minus 1 to 7; the right column shows annual mean precipitation change in metres per year on a scale from minus 1.3 to 1.3. The top row is doubled carbon dioxide, with warming of roughly 2 to 3 degrees over most of the globe and more at the poles, and substantial precipitation changes concentrated in the tropics. The bottom row adds a 1.84 per cent reduction in solar insolation: temperature change is close to zero nearly everywhere, with small residual warming remaining at the poles, and the precipitation pattern is much weaker but not eliminated.
Reducing sunlight to offset doubled carbon dioxide removes most of the temperature change but leaves residual warming at the poles, and it does not restore the hydrological cycle — an overall decrease in precipitation remains. The two panels in the bottom row are the whole argument about what solar geoengineering can and cannot substitute for. Figure 4 from Caldeira, Bala, and Cao (2013), Annual Review of Earth and Planetary Sciences 41, 231-256, reproducing Caldeira and Wood (2008). Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

The term geoengineering covers two quite different proposals. One reflects sunlight back to space so that less energy is retained; the other removes carbon dioxide from the atmosphere so that less energy is retained in the first place.

This review asked what is actually known about each, and — more usefully — what determines how much of each might ever be deployed.

What did we find?

The review separates the two categories at the outset. Solar geoengineering, also called solar radiation management, reflects incoming sunlight; carbon dioxide removal takes CO2 back out of the atmosphere. They share a name and very little else.

Solar geoengineering is cheap enough that cost will not be what limits it. Some approaches are thought to be low in cost relative to their effect, so the binding constraint is how much risk is judged acceptable.

Because it is cheap and globally effective, solar geoengineering raises governance questions that do not arise for other climate measures — a single actor could in principle affect the whole planet.

Carbon dioxide removal raises a different set of issues, concerning scale, cost, effectiveness, and local environmental consequences, and its deployment will be limited primarily by what it costs.

Why does it matter?

The asymmetry is the review's most useful contribution. Debates that treat geoengineering as one subject repeatedly confuse the two, and arguments valid against one are frequently deployed against the other.

Identifying the binding constraint also identifies where useful work lies. For carbon dioxide removal that means reducing cost; for solar geoengineering it means understanding risk and building governance, since cost reduction would accomplish nothing.

Citation

Ken Caldeira, Govindasamy Bala, and Long Cao (2013). The science of geoengineering. Annual Review of Earth and Planetary Sciences 41, 231-256.

Related