With the United Nations Framework Convention on Climate Change (UNFCCC) Agreements reached in Cancun, Mexico, in December 2010, the future role of international GHG offsets in climate action has become particularly uncertain. One of the more vexing questions is: how offsets will be accounted for in reporting and reviewing progress toward meeting countries’ emission-reduction pledges under the Cancun Agreements. This article quantifies the implications of double-counting international offsets by constructing and applying a model to analyse how potential offset supply and demand balances may evolve, based on specific assumptions about accounting rules, offset mechanisms, and country pledges for the year 2020. It is found that extensive use of international offsets, if counted both by the supplying (developing) and buying (developed) country, could effectively reduce the ambition of current pledges by up to 1.6 billion tonnes CO2e in 2020, suggesting that the current pledges could well fall even further shor...