Only in the last 100-200 years have people had global impacts. Also has many ancillary benefits including improved soil quality, watershed hydrology, increased plant growth. Need a lot of soil samples in order to see that increase. If we pay farmers for what they actually sequester instead of by practice, it would be a much more efficient policy. But U.S. farmers have to realize that they will be competing against many others that can store C at a low price.
Sinks require resolution.
Easy in concept, difficult in practice. Two approaches are possible, a project by project assessment or sector benchmarks. Project assessments have been the dominant use to date. To answer additionality question, must always ask what would have happened without the project. For example, a forest is preserved, but another forest is cut down in its place.