With a cap and trade system, the emission reductions can be achieved at the lowest possible cost to society.

The European Union Greenhouse Gas Emission Trading Scheme (EU ETS) now with 27 member countries and started in 2005, is so far the largest emission trading scheme world-wide. Similar schemes are now in the building process in the United States, Australia and other areas of the world. The Kyoto Protocol allows the trading of Carbon allowances between the countries.