I'm glad someone mentioned it above, because my favorite solution is still "advanced weathering" of rocks: http://www.innovationconcepts.eu/res/literatuurSchuiling/oli...

Just US $250 billion per year to offset ALL of humanity's carbon emissions. Yes I said ALL. It would cost less than the ongoing 'war on terror.' If I was rich I would be building autonomous mining robots so I could do it myself because I'm not confident governments will take action until it's too late.