As with any purchase, buyers need to choose their offsets carefully, particularly as the voluntary offset market is largely unregulated.

One issue to consider is the offset project type. For example, although quite popular, offsets from tree-planting projects are problematic for a number of reasons, including their lack of permanence and the fact that these projects do not address our dependence on fossil fuels. Similarly, offset projects involving the destruction of halocarbon gases such as HFC-23 have sustained numerous criticisms, including the fact that they actually result in a perverse incentive (due to the sheer volume of offsets - and profits - that they generate) for more of the ozone-depleting gas to be created. The price of offsets from these projects is also so low (due to the very high global warming potential of the gas) that they tend to flood the market and squeeze out more sustainable offset projects, like solar and wind.

Another important issue to consider when purchasing offsets is 'additionality'. An offset project is considered additional if it isn't business as usual. Typically this means that the project wouldn't have happened without the extra funding from the sale of offsets. Additionality is extremely important, as the entire concept of offsetting - i.e. purchasing greenhouse gas reduction credits from a project elsewhere to neutralize one's own emissions - is based on the premise that those reductions wouldn't have happened otherwise. Only by buying offsets that have met additionality criteria can you be assured that your purchase is resulting in a net benefit for the climate.

Other criteria of high quality carbon offsets include: validation and verification of the project by reputable third-parties; steps by the project developer to ensure that each offset is only sold once (e.g. by listing the offsets on a public registry); and systems in place to control 'leakage', where the creation of a GHG reduction in one region causes an unintended increase in GHG emissions somewhere else (for example, protecting a forest in one location could simply shift logging to a forested area in a new location).

A number of organizations have published comparisons of offset vendors; these can be found in the Resources section below. For example, Clean Air Cool Planet has published a Consumer's Guide to Carbon Offsets for Carbon Neutrality that lists some some questions that potential buyers can ask of offset vendors:

1) Do your offsets result from specific projects?
2) Do you use an objective standard to ensure the additionality and quality of the offsets you sell?
3) How do you demonstrate that the projects in your portfolio would not have happened without the greenhouse gas offset market?
4) ave your offsets been validated against a third-party standard by a credible source?
5) Do you sell offsets that will actually accrue in the future? If so, how long into the future, and can you explain why you need to 'forward sell' the offsets?
6) Can you demonstrate that your offsets are not sold to multiple buyers?
7) What are you doing to educate your buyers about climate change and the need for climate change policy?

Because it can be difficult for offset buyers to get clear answers to each of the above questions, a good way to ensure that your offset purchase is making a positive contribution to the climate is to purchase offsets that meet recognized standards. Just as consumers can feel confident when purchasing food products that meet strict third-party standards for organic agriculture, standards for carbon offsets provide assurance that certain criteria are met when the offset is developed and sold.

A number of standards exist for carbon offsets, including the VCS, Green-e, and The Gold Standard. More standards are being announced regularly, and WWF recently published a comparison of the most common offset standards. Each of these standards differs in key ways, with some being more rigorous than others.

The Gold Standard is widely considered to be the highest standard in the world for carbon offsets. It ensures that key environmental criteria have been met by offset projects that carry its label. Significantly, only offsets from energy efficiency and renewable energy projects qualify for the Gold Standard, as these projects encourage a shift away from fossil fuel use and carry inherently low environmental risks. Tree planting projects are explicitly excluded by The Gold Standard.

First, Gold Standard projects must meet very high additionality criteria to ensure that they contribute to the adoption of additional sustainable energy projects, rather than simply funding existing projects. The Gold Standard also includes social and environmental indicators to ensure the offset project contributes to sustainable development goals in the country where the project is based. Finally, all Gold Standard projects have been independently verified by a third party to ensure integrity.

Currently, The Gold Standard is restricted to offset projects in countries that don't have emission reduction targets under the Kyoto Protocol, which are primarily developing countries. Supporting offset projects that meet The Gold Standard therefore helps these countries 'leapfrog' us technologically so they don't go down the same fossil fuel path as developed countries - which would be disastrous for the climate.

The Gold Standard is supported by over fifty non-governmental organizations worldwide, including WWF International, and Greenpeace International.