In contrast, proven oil and gas reserves are equivalent to about 50 and 52 years respectively at current production levels.

Coal reserves are available in almost every country worldwide, with recoverable reserves in about 70 countries. The biggest reserves are in the US, Russia, China and India.

“Falls in the coal reserves-to-production ratios over recent years have prompted questions over whether we have reached 'peak coal',” according to WCA.

“Peak coal is the point in time at which the maximum global coal production rate is reached after which the rate of production will enter irreversible decline. However, falls in the RP ratio can be attributed to the lack of incentives to prove up reserves, rather than a lack of coal resources.”

The WCA said exploration activity was typically carried out by mining companies with short planning horizons rather than state-funded geological surveys.

“There is no economic need for companies to prove long-term reserves,” it said.

“All fossil fuels will eventually run out and it is essential that we use them as efficiently as possible.”