The voluntary market is centred around a handful of major standard-setting organizations, known as accreditation agencies or carbon standards. Each standard sets its own criteria and protocols to ensure all projects are meeting their targeted goals and that the GHG reductions are genuine. Once these criteria are met, the agencies issue tradeable "credit" certificates, each one of which entitles the buyer or owner to offset one metric ton of CO2e.

The most commonly seen standards are Verra, Plan Vivo, and The Gold Standard, along with the American Carbon Registry, Climate Action Reserve and ART TREES (Architecture for REDD Transactions) for terrestrial forest projects. Verra currently has the largest market share and is known for rigorous technical methodologies, while Plan Vivo is the oldest, and focuses on delivering small-scale community-based projects.

In addition to these major standard-setting organisations, there are a number of smaller agencies. They too can offer credit certificates, but since they do not always apply rigorous criteria and methodologies, these credits may not reflect the level of reliability provided by the principal standards.

Why choose verified carbon credits?

Credits offered for voluntary purchase can fall into one of three categories: accredited carbon offsets, unaccredited verified carbon offsets, and unaccredited and unverified carbon offsets.

Accredited carbon offsets.

These are produced by projects which accurately measure emissions reductions or carbon sequestration using a published technical methodology approved by an accreditation agency for use with their carbon standard.

Projects not only need to meet agency requirements but must also be verified by an independent third-party organisation before any credit certificates can be issued. This is commonly referred to as the accreditation process. Projects are also obliged to monitor their performance and are subject to further external assessment and verification at agreed intervals to confirm their emissions reductions/carbon storage claims remain valid.

Credit certificates are issued after each external assessment, and their issuance and current status are recorded in registries. Certified credits can be retained, traded, or used once by the owner to offset one ton of emissions, at which point they are considered retired or canceled.

Unaccredited verified carbon offsets 

Unaccredited verified offsets are produced by projects which have chosen not to go through the accreditation process. In some instances accreditation is not pursued because of the high costs involved; in others there may be no intent to trade the generated offsets to outside individuals or organizations. Some will still offer offsets for sale through private websites or direct to interested parties.

Projects which are unaccredited but verified may use a methodology accepted by any of the carbon standards, a compliance mechanism like the CDM, or implement their own custom science-based methods. Regardless of the methodology used, they have engaged a third-party organisation to verify their claims. Since unaccredited projects are not required to submit information to any registry, it is difficult to know how many there are.

Unaccredited, unverified carbon offsets

Unaccredited, unverified carbon offsets are produced by projects making emissions reductions or carbon removal claims that have not been accredited by a carbon standard, nor verified by any kind of third-party organisation. These projects may or may not use reliable measuring methodologies. Some hold themselves to high scientific and ethical standards, delivering excellent results, others are outright fraudulent or may be operating outside any legal framework.

Multiple websites offer unaccredited, unverified carbon offsets for sale to companies or to the general public without much, if any, regulation. As the buyer's capacity for carrying out due diligence on these projects may be limited, even major brands have been documented purchasing "credits" for non-existent offsets in good faith.

This approach is not endorsed by any emerging regulatory bodies and is often regarded as operating outside the voluntary carbon market.