Some techniques are based on carbon sequestration that seek to reduce GHGs in the atmosphere. They also include ocean iron fertilization, solar radiation management etc. Ocean iron fertilization is at the final stages of research and so far no large-scale geo-engineering projects have been planned.
Ocean iron fertilization is introduction of iron to the oceans. Fertilization supports the growth of marine phytoplankton blooms by physically distributing microscopic iron particles in nutrient-rich, but iron-deficient, ocean waters. An increasing number of ocean labs, scientists and businesses are exploring it as a means to revive declining plankton populations, restore healthy levels of marine productivity and sequester billions of tons of CO2 to reduce global warming and ocean acidification. Since 1993, ten international research teams have completed ocean trials demonstrating the effect. However, controversy over its safety and efficacy remain. Whilst increasing plankton population in the oceans will solve one half of the problems associated with climate change, the problem of already acidic oceans cannot be solved through this method.
In order to neutralize the oceans, scientists now propose adding limestone to restore the oceans to their previous alkaline state. Limestone mining however is an energy intensive process. Adding iron to oceans can increase algal blooms. My head refuses to wrap itself around the avoidance of negative affects by adding stuff to over 70% of the Earth's environs or even how you can experimentally prove the same on a scale as large as this with so many parameters for control.
Other geoengineering projects in the pipe-line include brightening clouds in order to increase cloud reflectivity so the albedo of the Earth is altered. Whilst I applaud all the technology and acknowledge that in the times of little hope, this offers a glimper of something like it; I remain dubious on the side-effects - are we planning on jumping from the frying pan into the fire?