3 Field experiments
We need climate models like GCMs to predict how the climate will respond to forcing. But just as important a task is to test those models with experimental data. Those data may come from laboratory experiments, or from field experiments performed outside the laboratory.
As yet (in 2021), there have been no large-scale field experiments of geoengineering, i.e. testing how the climate responds to SRM or CDR; there have only been small-scale tests of the physical processes involved, which are too small and local to affect climate.