Carbon dating is commonly used to estimate the age of ancient artifacts, exploiting the fact that a particular type of carbon disappears, or decays, at a fixed rate, so that the amount left behind clocks how much time has passed.
A recently adapted technique, developed by the Energy Research Center of the Netherlands, takes monthly samples of the carbon-14 in the smokestack of power plants.
It tallies that with the energy produced to estimate how much carbon dioxide in the flue gas came from burning ancient fossil fuels and how much from greener, younger fuels such as wood, crop waste and other so-called biomass.
The practice of co-firing biomass with coal is increasing as power plants try to pare carbon costs and earn green energy incentives.
"(This) is enabling easy and accurate differentiation between CO2 emissions created from fossil and biogenic, renewable fuels," said the UK's National Center for Biorenewable Energy, Fuels and Materials (NNFCC), a lobby group.
The technology is already used in Belgium and the Netherlands, the NNFCC said.
It may be a simpler way of disentangling green from coal-fired power than the alternative, which is to measure the amount of biomass and coal fuel fed into a furnace.
"Independent reports concluded that the 14C technique is based on mature and well understood technology," said Britain's energy watchdog Ofgem last Friday, explaining its approval.
"The reports also concluded that results from this technique would be at least as accurate as the existing sampling methods used."
From 2013 all fossil fuel power plants in western Europe will have to pay for every tonne of carbon dioxide emissions, under the European Union's emissions trading scheme.
By contrast, energy producers in Britain are rewarded for burning biomass. Burning wood and plant waste is viewed as contributing less to climate change because it only returns to the atmosphere the same CO2 that the plants incorporated while they were growing.
(Reporting by Gerard Wynn; editing by Jason Neely)