Universal McCann will be working on the TV, press, radio, cinema and sponsorship areas, while Carat will work on the online and outdoor elements.
The Carbon Trust appointed a media buying roster in January, which also comprised Manning Gottlieb OMD and Brilliant Media. A final appointment was made after a further two-stage procurement process took place.
The contracts, worth a combined value of £12 million over the year, aim to raise the Trust's profile and promote its services, and also feature an option to extend for a further 12 months.
Earlier this month, VCCP was appointed the lead creative agency for the Carbon Trust after beating the roster shops Rainey Kelly Campbell Roalfe/Y&R and WCRS.
This week, the Carbon Trust launched its carbon reduction label across a number of major brands, including Walkers, Boots and Innocent. Companies earn the label for their commitment to reduce carbon emissions.
The appointments come as carbon neutrality rises up the business agenda. Earlier this year, Marks & Spencer announced that it was to become carbon neutral.