It seems like the only things that can
1. work at a planetary scale
2. mitigate our multi-year (and growing) emission problem
3. not on an energy flywheel (consume a significant amount of energy, mined resources or themselves emit significant waste heat)
have to rely directly on solar energy (not intermediated by PV cell). To this end azolla ferns [1] and olivine weathering seem to be the most promising.