The green dream of a carbon-free energy system seems like it's getting closer every day. But to do so, we'll either need to clean up coal or replace it.

Coal provides 50% of America's electricity and effective carbon capture and storage (CCS) doesn't exist yet. The reality may well be that ridding ourselves of coal, at least in the short term, isn't an option.

Researchers from MIT suggest a compromise: partial capture. One of the authors calls full capture "a policy of inaction, a policy that won't move forward either new coal plants or the CCS technology." In other words, it's a non-starter.

Adding carbon capture equipment to a brand new power plant can increase the capital cost by 30-60% and decrease efficiency. The researchers found that capturing a ton of CO2 gets cheaper the more you capture..up to a point. Once you hit the 60% mark, though, the savings level off. So if you quit there, you get maximum smog for your buck.

As much as every enviro would like to kick coal yesterday, for the next decade or two, the key will be managing emissions while the truly cleantechnologies mature.