The principle of Supplementary within the Kyoto Protocol means that internal abatement of emissions should take precedence before a country buys in carbon credits. However it also established the Clean Development Mechanism as a Flexible Mechanism by which capped entities could develop real, measurable, permanent emissions reductions voluntarily in sectors outside the cap. Many criticisms of carbon credits stem from the fact that establishing that an emission of CO2-equivalent greenhouse gas has truly been reduced involves a complex process. This process has evolved as the concept of a carbon project has been refined over the past 10 years.

The first step in determining whether or not a carbon project has legitimately led to the reduction of real, measurable, permanent emissions understands the CDM methodology process. This is the process by which project sponsors submit, through a Designated Operational Entity (DOE), their concepts for emissions reduction creation. The CDM Executive Board, with the CDM Methodology Panel and their expert advisors, review each project and decide how and if they do indeed result in reductions that are additional.

Additionality:

It is also important for any carbon credit (offset) to prove a concept called “Additionality”. Additionality is a term used by Kyoto's Clean Development Mechanism to describe the fact that a carbon dioxide reduction project (carbon project) would not have occurred had it not been for concern for the mitigation of climate change. More succinctly, a project that has proven Additionality is a beyond-business-as-usual project.

It is generally agreed that voluntary carbon offset projects must also prove Additionality in order to ensure the legitimacy of the environmental stewardship claims resulting from the retirement of the carbon credit (offset).

Green House Gases (GHG) emission trading programs operate by capping the emissions of a fixed number of individual facilities or sources. Under these programs, tradable 'offset credits' are issued for project-based GHG reductions that occur at sources not covered by the program. Each offset credit allows facilities whose emissions are capped to emit more, in direct proportion to the GHG reductions represented by the credit. The idea is to achieve a zero net increase in GHG emissions, because each ton of increased emissions is 'offset' by project-based GHG reductions. The difficulty is that many projects that reduce GHG emissions (relative to historical levels) would happen regardless of the existence of a GHG program and without any concern for climate change mitigation. If a project 'would have happened anyway,' then issuing offset credits for its GHG reductions will actually allow a positive net increase in GHG emissions, undermining the emissions target of the GHG program. Additionality is thus critical to the success and integrity of GHG programs that recognize project-based GHG reductions."

How buying carbon credits can reduce emissions?

Carbon credits create a market for reducing greenhouse emissions by giving a monetary value to the cost of polluting the air. Emissions become an internal cost of doing business and are visible on the balance sheet alongside raw materials and other liabilities or assets.

By way of example, consider a business that owns a factory putting out 100,000 tonnes of greenhouse gas emissions in a year. Its government is an Annex I country that enacts a law to limit the emissions that the business can produce. So the factory is given a quota of say 80,000 tons per year. The factory either reduces its emissions to 80,000 tons or is required to purchase carbon credits to offset the excess. After costing up alternatives the business may decide that it is uneconomical or infeasible to invest in new machinery for that year. Instead it may choose to buy carbon credits on the open market from organizations that have been approved as being able to sell legitimate carbon credits.

We should consider the impact of manufacturing alternative energy sources. For example, the energy consumed and the Carbon emitted in the manufacture and transportation of a large wind turbine would prohibit a credit being issued for a predetermined period of time.

One seller might be a company that will offer to offset emissions through a project in the developing world, such as recovering methane from a swine farm to feed a power station that previously would use fossil fuel. So although the factory continues to emit gases, it would pay another group to reduce the equivalent of 20,000 tons of carbon dioxide emissions from the atmosphere for that year.
Another seller may have already invested in new low-emission machinery and have a surplus of allowances as a result. The factory could make up for its emissions by buying 20,000 tons of allowances from them. The cost of the seller's new machinery would be subsidized by the sale of allowances. Both the buyer and the seller would submit accounts for their emissions to prove that their allowances were met correctly.

Carbon Credits versus Taxes:

Carbon credits and carbon taxes each have their advantages. Credits were chosen by the signatories to the Kyoto Protocol as an alternative to Carbon taxes. A criticism of tax-raising schemes is that they are frequently not hypothecated, and so some or all of the taxation raised by a government may be applied inefficiently or not used to benefit the environment.

By treating emissions as a market commodity it becomes easier for business to understand and manage their activities, while economists and traders can attempt to predict future pricing using well understood market theories. Thus the main advantages of a tradable carbon credit over a carbon tax are: