No, President Joe Biden did not engineer Hurricane Milton. No, the ice-crystal contrails left behind high-altitude aircraft are not “chemtrails” sprayed by governments to control the weather or manipulate populations. No, state-run cloud seeding did not cause the record floods in Dubai.

Yet claims like these are widespread online and increasingly find their way into political rhetoric. This blurs the distinction between conspiracy theories about weather manipulation and the very real scientific debate over solar geoengineering.

Solar geoengineering, also known as solar radiation modification (SRM), would try to temporarily cool the climate by altering the amount of energy entering or leaving the Earth system. Outdoor field experiments are particularly contentious, with researchers and other experts divided over whether they should proceed at all and, if so, under what conditions.

The best-known proposal, stratospheric aerosol injection, involves dispersing sulphur dioxide high in the atmosphere, where it can form reflective particles known as aerosols. These can remain in the stratosphere for long periods, broadly mimicking the cooling effect of major volcanic eruptions.

Stratospheric aerosol injection explained

The other main proposals are marine cloud brightening, which would add sea-salt particles to low clouds over the sea to make them more reflective, and cirrus cloud thinning, which would aim to reduce the net warming effect of high-altitude clouds.

What could outdoor experiments actually tell us?

In our recent study, colleagues and I examined what outdoor SRM experiments might look like in practice. We identified plausible experiments across the three approaches and examined how........

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