It is becoming increasingly clear that even if all global emissions were flat-lined today at atmospheric concentrations for CO2 of 400 ppm we would still be in trouble. The ocean acidification is presently threatening the base of the marine food chain. We risk massive releases of methane from the Arctic which would catapult us into a much warmer world.
We cannot risk uncontrollable abrupt climate change that appears to have already been initiated. Thus, we must lower atmospheric CO2 concentrations down to the safer level of 350 ppm, and then perhaps even down to 300 ppm which is the highest concentration prior to humans over the last million years or more.
Simply preserving existing forests around the world, and undergoing enormous tree planting programs would help reduce CO2 levels in the atmosphere. Converting dead vegetation by pyrolysis in the absence of oxygen to create a charcoal like material called biochar, and putting it into soils to aid in plant growth is another possibility.
On the oceans, seeding regions essentially barren of marine life with limiting nutrients such as iron could cause phytoplankton blooms that would capture a substantial quantity of carbon from the atmosphere/ocean system. Grinding up minerals such as olivine and placing them on the land surface or in the ocean can artificially enhance the natural weathering process to remove CO2 from the atmosphere and/or ocean.
There are a dozen or more Venture Capital startups that are working to devise mechanical devices to remove CO2 from the atmosphere.