Carbon offsets — reductions of greenhouse gas emissions from an activity in one place to compensate for emissions elsewhere — are a way to address climate change by paying someone else to reduce emissions. To be credible, an offset must be additional — it must reduce emissions below the quantity emitted in a business-as-usual scenario — among other criteria. Assessing credibility is inherently challenging because it is difficult to make business-as-usual projections. Outside the U.S., offsets may be purchased on compliance markets to meet requirements to reduce emissions. In the U.S., there are no federal requirements and offsets may be purchased in the voluntary market. (Source: GAO / Photo: Flickr)