Yesterday The Diane Rhem Show on NPR spent an hour discussing carbon in the atmosphere. Her panel was Thomas Armstrong of the Madison River Group, a science policy consulting firm, Jane Long, former associate director of energy and environment at Livermore National Laboratory, and Noah Diech of the Center for Carbon Renewal. For part of the time they were joined by David Keith, professor of applied physics at Harvard and president of Carbon Engineering, devoted to developing industrial-scale technologies for direct air capture of CO2.
First, a bit of praise for the recent Paris conference. It didn't say the world should keep temperature change below 2C as has long been stated as a goal, but to claim the goal of 1.5C. But, according to recent projections by the IPCC, to do that we can't just stop all carbon emissions, we must actively intervene and take carbon, specifically carbon dioxide, out of the atmosphere.
We can't simply cut emissions to zero (which isn't happening all that quickly anyway) because CO2 stays in the atmosphere a really long time. The CO2 already in the atmosphere will have an effect on warming, even if no more is added. Most of the rest of the hour was spent discussing how we might actively take carbon out.
First, the scale of the problem. We're talking gigatons of CO2. That's billions of tons. And if we're aiming for all the CO2 emitted since the start of the Industrial Revolution we're talking 2,000 gigatons, or 2 trillion tons (though we probably don't have to deal with all of it). So a big component of the solution must be what to do with that amount of stuff. In addition, much careful research must be done to make sure putting that amount of stuff somewhere doesn't cause other problems.
On to possible solutions...
Geologic carbon capture. We've been doing a lot of fracking lately to extract oil and gas from cracks in the rock. Perhaps we can reverse the process to store CO2. Or put CO2 in oil reservoirs, which are now underpressurized. But we already know about earthquakes caused by fracking. How will we avoid earthquakes with carbon capture? This takes careful study of possible sites.
Carbon capture directly at the power plant. At the moment it is frightfully expensive (but so was solar power when it began) and there is still the issue of what to do with the stuff once it isn't going up the smokestack.
Trees absorb CO2, so planting more trees helps (at a time when we're cutting down trees faster than we can plant them). But a tree tends to live only 40-60 years and when it decays it releases the carbon again.
There is known technology to capture CO2 from the air. Keith's company is working to combine that CO2 with hydrogen generated from solar power to create a fuel with a much smaller environmental impact than biofuels or fossil fuels. All of his research is attempting to use known technologies so that the process is inexpensive. It is a carbon fuel, but it is recycled carbon.
It is possible to turn CO2 into cements, plastics, and composite materials for use in building. Good idea, but it won't deal with billions of tons.
CO2 can be dissolved in sea water, and our globe has huge oceans. But higher CO2 in water makes it more acidic, another environmental problem. Even so, CO2 could be put into the deep ocean. Alas, the process would require as much limestone as we currently pull out of the ground for all other purposes.
Some good ideas out there, some can be used. But none of them will be the "silver bullet" able to handle the problem. A lot of ideas must be developed, tested, and tried and a lot of ideas will be a part of the solution.