Nordhaus proposed, based on the social cost of CO2 emissions, that an optimal price for carbon be about $30 per tonne and that inflation should be increased. In a kind of compromise, a buyer of emission credits pays in cash the right to emit more than the CO2 assigned by the Kyoto Protocol, and the seller receives cash for the obligation to produce less CO2. To implement this agreement, both parties must sign an ERPA document. Yale University economics professor William Nordhaus argues that the price of carbon must be high enough to motivate changes in behaviour and changes in the economic production systems needed to effectively limit greenhouse gas emissions. There are also many companies that sell carbon credits to professional and individual customers who are interested in reducing their carbon footprint on a voluntary basis. These CO2 offsets buy credits from an investment fund or a carbon development company that has aggregated the credits from individual projects. Buyers and sellers can also use a trading platform, which is like an issue credit exchange. The quality of the credits is based in part on the validation process and the sophistication of the fund or development company, which was the promoter of the CO2 project. This is reflected in their price; Voluntary units generally have a lower value than units sold by the rigorously validated clean development mechanism.  In 2018, EU emission credits increased from $7.78 to $25.19, an average of $16.21 per tonne.  There is a general consensus that voluntary CO2 offset projects must demonstrate additionality in order to ensure the legitimacy of environmental liability requirements arising from the retirement of offsets. The introduction of such credits has been ratified by the Kyoto Protocol.
The Paris Agreement confirms the application of emission credits and sets out provisions for further facilitation of emission credit markets. The principle of additionality in the Kyoto Protocol means that internal emission reduction must be a priority before a country buys emission credits. However, it has also introduced the Clean Development Mechanism as a flexible mechanism, which allows capped companies to voluntarily develop measurable and sustainable emission reductions in sectors outside the ceiling. Many criticisms of emission credits are due to the conclusion that CO2-equivalent greenhouse gas emissions have actually been reduced requires a complex process. This process has evolved as the concept of a carbon project has been refined over the past 10 years. An ERPA usually involves two countries. But it can also happen between a country and a large company. Often, the seller has introduced a new technology or developed a new project that he expects to reduce his greenhouse gas emissions, so that the seller would not need so many emission credits and will be able to take advantage of them. A carbon reduction agreement (ERPA) is a legal contract between companies that buy and sell emission credits. A CO2 credit is an authorization or certificate allowing the holder to emit carbon dioxide (CO2) or other greenhouse gases (GHGs) into the atmosphere.
In return, these countries set emission quotas for facilities operated by local companies and other organizations commonly referred to as ”operators.” Countries manage it through their national registries, which must be validated by the UNFCCC and subject to compliance checks.  Each operator has a credit premium in which each unit grants the owner the right to emit one tonne of carbon dioxide or other equivalent greenhouse gas.