What climate engineering means

Climate engineering here means solar radiation modification (SRM): deliberately reflecting a small fraction of incoming sunlight to reduce warming. The main proposals are stratospheric aerosol injection, which mimics the cooling that follows large volcanic eruptions, and marine cloud brightening, which increases the reflectivity of low marine clouds. Carbon removal is a different intervention with a different risk profile and is covered in Part 4.

The distinction that matters most: SRM treats a symptom. It reduces temperature without reducing atmospheric CO2, so ocean acidification proceeds regardless. It is not, and is not claimed by serious researchers to be, a solution to the carbon problem.

Where the science stands

Established The physical basis is sound and observationally grounded: large volcanic eruptions inject sulphate aerosols into the stratosphere and measurably cool the planet for a year or two afterwards. Nobody disputes that reflecting sunlight reduces temperature.

Established Two properties make SRM unlike everything else in this programme. It is fast — effects within years rather than decades — and it is cheap relative to mitigation, cheap enough that a single moderately resourced state, or conceivably a private actor, could attempt it. Every governance problem in Part 6 follows from those two facts.

Frontier Modelling suggests SRM deployed as a complement to emissions reduction might lower climate risk globally while raising it regionally, with changes to precipitation patterns and monsoon behaviour among the principal concerns. Research capacity has broadened deliberately: an initiative supporting SRM research in developing countries has funded over 170 researchers across 22 countries, producing the first modelling studies of SRM impacts for South America, the Caribbean, Africa, the Middle East and Southeast Asia — regions that would bear consequences and were absent from the early literature.

Established Termination shock is well characterised in the modelling literature: if sustained SRM stops abruptly while greenhouse gas concentrations remain elevated, warming returns rapidly — faster than under the no-SRM baseline, and fast enough that ecosystems and infrastructure would have less time to adapt than they would have had otherwise. Deployment is therefore a multi-decade commitment from the moment it begins.

Frontier Field research is now real rather than hypothetical. The UK committed £56.8 million across 21 projects in 2025, five of which involve outdoor experiments including Arctic sea-ice thickening and marine cloud brightening. Total global SRM research funding from 2007 to 2025 was around $340 million, roughly half public — small for a field of this consequence. Speculative Whether outdoor experiments can produce information that models cannot is itself contested, and it is the crux of the research-governance dispute.

The binding bottleneck

Frontier The constraint is irreducible uncertainty about regional effects. Global mean temperature is the easy variable; precipitation, monsoon timing and regional extremes are the ones people live under, and models disagree. Established Second is commitment: once begun, stopping is worse than not starting, which makes the decision unusually irreversible for something so cheap. Frontier Third is attribution — distinguishing an SRM effect from natural variability after deployment may be impossible for years, which undermines any liability regime. Speculative Fourth is that the system cannot be tested at scale without deploying it, which is the argument that SRM is untestable in principle.

Ethics and governance

Frontier Moral hazard is the central objection and it is structurally serious: a cheap temperature fix reduces pressure to do the expensive carbon work in Parts 1 through 4. Frontier The distributional objection is equally serious — a global intervention with regionally uneven effects means some populations bear costs from a decision they had no part in. Established Intergenerational commitment is unavoidable: beginning SRM binds successors to continue it. Frontier Against all of that sits an argument that also deserves weight: if warming causes severe harm and mitigation continues to fall short, refusing even to study the option is itself a choice with consequences. The governance of that dispute is Part 6.

Timelines

  • 10 yr: Frontier expanded modelling and limited outdoor experiments; sustained governance dispute; no deployment expected.
  • 25 yr: Speculative deployment plausible if warming outpaces mitigation, most likely announced unilaterally or by a small coalition.
  • 50 yr: Speculative if begun, an established multi-decade commitment with an institutional apparatus to sustain it.
  • 100 / 250+ yr: Speculative termination becomes possible only once concentrations have fallen, which returns the problem to Part 4.