In letter to Nature, James Lovelock (of Gaia hypothesis) and Chris Rapley suggest using:
Ocean pipes could help the Earth to cure itself
Article on Nat Geog site says they’ve done laboratory experiments showing this may be feasible, and now have sponsor to start small scale trial.
But, plenty of uncertainties – the pipes may exhale CO2 as process begins, then may disrupt ocean cycles and impact marine life.
Giant Ocean Tubes Proposed as Global Warming Fix