Emission Reduction Purchase Agreements, the Evolution of the Dialectic Relationship of Private Contracts and International Environmental Law at the Heart of the Global Climate Regime In May 2013, an instrument near the summit of Mauna Loa in Hawaii recorded a long-feared climate milestone: the amount of carbon dioxide in the atmosphere has now exceeded 400 parts per million for the first time in 55 years of measurement— and probably more than 3 million years of Earth history.