Carbon Credits create a market for reducing greenhouse emissions by giving a monetary value to the cost of polluting the air. This means that carbon becomes a cost of business and is seen like other inputs such as raw materials or labor.

By way of example, assume a factory produces 100,000 tonnes of greenhouse emissions in a year. The government then enacts a law that limits the maximum emissions a business can have. So the factory is given a quota of say 80,000 tonnes. The factory either reduces its emissions to 80,000 tonnes or is required to purchase carbon credits to offset the excess.

A business would buy the carbon credits on an open market from organizations that have been approved as being able to sell legitimate carbon credits. One seller might be a company that will plant so many trees for every carbon credit you buy from them. So, for this factory it might pollute a tonne, but is essentially now paying another group to go out and plant trees which will, say, draw a tonne of carbon dioxide from the atmosphere.

As emission levels are predicted to keep rising over time, it is envisioned that the number of companies wanting/needing to buy more credits will increase, which will push the market price up and encourage more groups to undertake environmentally friendly activities that create for them carbon credits to sell. Another model is that companies that use below their quota can sell their excess as 'carbon credits.' The possibilities are endless hence making it an open market.

Managing emissions is one of the fastest-growing segments in financial services in the City of London's financial district with a market now worth about $30 billion, but which could grow to $1 trillion within a decade. (The New York Times)