From the early 1970s, the United Nations (UN) has provided the main forum for international negotiations and agreements on environmental policies and objectives. The 1972 Stockholm conference was the first international conference on environmental issues and was followed by the United Nations Conference on Environment and Development (UNCED) summits in Rio de Janeiro in 1992 and in Johannesburg in 2002. The UN also hosted special conferences on climate change, such as those of 1996 in Kyoto and 2009 in Copenhagen.

Those conferences and summits responded to the global character of some of the most-challenging environmental problems, which would require international cooperation to solve. Those conferences were effective in setting an international agenda for regional and national environmental policy making that resulted in treaties and protocols, also known as “hard law,” and in nonbinding resolutions, statements, and declarations, or “soft law.” Whereas the 1992 Rio conference agreement was a soft law, the Kyoto Protocol was a hard law, with clear-cut reduction targets of greenhouse gas emissions for regions and countries. Nation-states, in their efforts to meet the targets, could make use of three so-called flexibility mechanisms designed to lower the costs of compliance.

Joint implementation, the first mechanism, allowed countries to invest in lowering emissions in other countries that had ratified the Kyoto Protocol and, thus, had a reduction target to meet. For industrialized, developed countries that had already invested in emission reductions in their own economies, it was cheaper to invest in emission reductions in other countries with economies in transition, where the same investment would lead to greater reductions. In other words, the investing country could get credit for helping a country with an economy in transition to lower its emissions.

Clean development, the second mechanism, allowed industrialized countries that have ratified the protocol to meet their targets in any country where it is cheapest to invest—that is, in developing countries—even if that country did not ratify the protocol. That mechanism is not undisputed, since it involves questions of intervention in the economies of developing countries, which may have an impact on the economic development of those countries. To prevent industrialized countries from not reducing their own emissions, the mechanism can only be used in supplement to domestic reductions, but no definition of such supplemental action was given, which led some countries to achieve 50 percent of their reduction target through that mechanism.

The third mechanism, carbon-emission trading (which is also known as “cap and trade”), is a market-based instrument and can be applied in the form of voluntary markets or in a mandatory framework. Most trading schemes are based on a cap-and-trade model. A central authority puts a cap on the overall carbon emissions allowed in a country or region. Within that cap, emission rights are allocated to the polluters, and emissions produced beyond those rights are penalized. The idea is that polluters choose between investing in emission reductions or emission permits. By lowering the cap over time, total emission reduction can be achieved. The trade of permits will ensure that emissions reduction is achieved at the lowest costs.

The instrument of tradable permits has been applied to other emissions. The first emission-trading schemes date back to 1974, when the United States experimented with emissions trading as part of the Clean Air Act.