There's a new market growing in the United States, but unlike others trading in tangible commodities, this one trades in the absence of something that no one wants: greenhouse gases in the atmosphere. According to industry figures, the global markets for both voluntary offsets and those sold to meet compliance requirements are growing rapidly, and U.S. offset sales will likely accelerate further under the Obama administration. However, mounting evidence shows that offsets often don't deliver a measurable, genuine reduction in net emissions. Although a number of independent market-based standards have emerged to impose a measure of honesty and integrity on the business of offsets, many energy and Earth science experts urge policy makers not to bank too heavily on offsets as a carbon mitigation strategy.