text: The economic impacts estimated in the report are conservative because they make a very favorable assumption in favor of ACESA, namely, that domestic industries would be able to exceed the cap by about 30% during 2012-2050 by purchasing international offsets (e.g. payments to preserve forests in development countries). Access to international offsets are especially important to cost-containment in ACESA's early phase, totalling 83% of the required emission "reductions" in 2015. "However, in light of the difficulties in measuring, verifying, and ensuring the permanence of these offsets, international negotiations have stressed domestic sources of emission reductions over international offsets." The Kyoto II treaty that will be negotiated in Copenhagen "might allow far fewer" offsets than ACESA would provide. "This would drive up costs substantially."