Congressional Research Service (CRS) environmental policy analyst Jonathan Ramseur has written The Role of Offsets in a Greenhouse Gas Emissions Cap-and-Trade Program: Potential Benefits and Concerns. Ramseur notes:

If Congress establishes a greenhouse gas (GHG) emissions reduction program (e.g., cap-and-trade system), the treatment of GHG emission offsets would likely be a critical design element. If allowed as part of an emissions program, offsets could provide cost savings and other benefits. However, offsets have generated concern. … The main concern with offset projects is whether or not they represent real emission reductions. … Another concern is whether the inclusion of offsets would send the appropriate price signal to encourage the development of long-term mitigation technologies. … Some object to the use of international offsets due to concerns of fairness: the low-cost options would be unavailable to developing nations if and when they establish GHG emission targets. However, some offset projects may promote sustainable development. On the other hand, international offsets may serve as a disincentive for developing nations to enact laws or regulations controlling GHG emissions because many projects would no longer qualify as offsets. Several tables are included that compare offset treatment programs in Congressional legislative proposals, in emissions reduction initiatives of state governments and in international emissions trading programs.