[PaulHoule]

An airplane that flies at 70,000 feet near the equator could release a load of SO2 which would make an aerosol that reflects sunlight and gradually makes it way north until it precipitates over the arctic.

Alternately build a solar sail factory at a volatile-rich asteroid (say 24 Themis) that makes plastic films coated with aluminum that fly back on their own power and park near the Earth-Sun L1 point.

This one is almost practical: Use a boat to disperse iron sulfate at carefully selected spots in the ocean. Actually more than one boat because if you don't survey ocean conditions you might not deliver the iron efficiently. The iron causes an algae boom, some of the carbon is trapped in plankton that sinks to the deep ocean.

The UN Law of the Sea forbids this, but the United States is not a signatory from that treaty so nothing stops you from doing a Kickstarter and running the operation of the coast of the US.

[toomuchtodo]

How much iron sulfate and how careful does one have to select locations to dump it?