On a site next to a sawmill in Waverly, Virginia, a startup takes sawdust and offcuts from the mill and heats it up to turn it into biochar, a material that can store carbon for hundreds or thousands of years. The project, which recently began operating, will capture a little more than 10,000 metric tons of CO2 each year. Microsoft will buy the carbon removal credits as part of its plan to become a carbon negative company.