The Science of Climate Change: What the Physics Actually Says

Geoengineering is the set of technologies proposed for deliberately intervening in the climate system to counteract global warming. It's one of those topics where scientific discussion and public discourse are so completely disconnected that it's almost hard to know where to start.

The basic categories:

**Carbon Dioxide Removal (CDR)** — pulling CO₂ out of the atmosphere and storing it. This ranges from natural approaches (afforestation, soil carbon sequestration, ocean iron fertilization) to engineered approaches (Direct Air Capture, Bioenergy with Carbon Capture and Storage). The important thing about CDR is that it addresses the actual cause of warming — CO₂ accumulation — rather than the symptoms.

**Solar Radiation Management (SRM)** — reflecting more sunlight away from Earth to counteract warming. The most discussed approach is Stratospheric Aerosol Injection (SAI): injecting sulfate aerosols into the stratosphere to scatter incoming sunlight, mimicking the cooling effect of large volcanic eruptions. Mount Pinatubo's 1991 eruption injected about 20 million tonnes of SO₂ into the stratosphere and caused about 0.5°C of global cooling for roughly two years.

SAI is fast, relatively cheap, and well-understood in terms of radiative effect. It is also deeply problematic in several respects.

First, it doesn't address CO₂ accumulation. It masks the warming while CO₂ continues to build up. If you start SAI and then stop it for any reason — political instability, loss of funding, international disagreement — you get a rapid "termination shock" as the masked warming reasserts itself, potentially at a pace far faster than gradual warming. Termination shock could be more damaging than gradual warming would have been.

Second, SAI would change precipitation patterns globally. The stratospheric aerosols affect the distribution of solar energy reaching the surface, which affects atmospheric circulation and rainfall. There's no reason to expect the changes to be geographically equitable — some regions might benefit from reduced temperatures while others experience drought. The beneficiaries and losers might be very different countries. This creates obvious governance problems.

Third, there's no global governance framework for authorizing deployment. Who decides whether to inject aerosols into the stratosphere? Who compensates countries that experience agricultural losses from altered monsoons? The technology could technically be deployed by a single moderately wealthy nation or non-state actor, which makes multilateral governance essential and also very difficult.

Direct Air Capture (DAC) is less controversial in principle — removing CO₂ from the atmosphere is straightforwardly good — but faces cost and energy challenges. Current DAC costs are around $300-1000 per tonne of CO₂ captured. At current volumes, we're removing thousands of tonnes per year and need to remove billions to make a dent. Costs need to fall by one or two orders of magnitude to be deployable at meaningful scale.

The research community is in an odd position: studying geoengineering options seems necessary because the deployment trajectory of emissions suggests that some form of intervention may be needed. But studying it openly risks creating a "moral hazard" where the possibility of geoengineering reduces political pressure to cut emissions. There's no clean resolution to this dilemma, which is why geoengineering research governance is almost as contentious as geoengineering deployment would be.

Geoengineering — The Options, the Risks, and the Governance Problem

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