"Green collar jobs" could more than double in the next 10 years, making it the fastest growing employment sector, according to a new report by the US Conference of Mayors.

The report, US Metro Economies: Current and Potential Jobs in the US Economy, estimates that there are roughly 750,000 jobs related to the green sector in the US today. By 2018, it says that this could grow to 2,540,800. By 2038, the figure could reach 4.2 million jobs.

The report defines "green" jobs as those related to renewable or nuclear electricity, jobs in agricultural operations supplying corn or soy for transportation fuel, manufacturing jobs producing goods used in renewable power generation, and equipment dealers and wholesalers specialising in renewable energy or energy efficient products.

Also included are jobs in the construction and installation of energy and pollution management systems, government administration of environmental programmes, and supporting jobs in engineering legal, research and consulting.

Engineering, legal, research and consulting – which the report characterises as "indirect" jobs – currently make up the lion's share of green jobs, with 418,715 employed.

Renewable power generation is the next largest employer overall, and the largest direct employer. However, as the number of green jobs grows, the proportion of indirect jobs is likely to fall, the report claims "We do not expect that each marginal electricity generation job will require another environmental lawyer, for example, and not every retrofitting position will require commensurate growth in research or consulting," it says.

Both presidential candidates have emphasised the environment in their campaign platforms, but Barack Obama is the only one to have committed to an explicit green jobs target. The Illinois senator has pledged to create five million green collar jobs using a $150bn investment in clean energy over the next 10 years.