Forecasts of the length of time for which each form of fossil fuel will last differ greatly. This depends on the assumptions used to predict the trend in aggregate supply and demand and on the proven reserves (i.e. technically and economically recoverable reserves) and on the resources (i.e. the fields/deposits whose existence is proved or assumed but which are not yet technically and/or economically recoverable). In this environment, the price level also plays a vital role, as is evidenced by the boom in shale gas and oil in the USA and Canada since the mid-2000s. Previously untapped, these reserves were able to be tapped by using the costly technique of hydraulic fracturing (fracking) once the oil price had durably exceeded the level of $ 50 per barrel. Assuming stable consumption of fossil fuels, the proven oil reserves would be able to meet global needs for over 53 years, natural gas reserves for about 55 years and coal reserves for more than 100 years, according to BP's statistics. But most of the competent specialists and organizations consider that global demand for fossil fuels is likely to continue to increase in the medium and long term, driven by countries that are not members of the OECD, particularly China and India. The sustainability of proven fossil-fuel reserves in terms of years of consumption is reduced as a result.
Apart from the limited duration of the reserves, the major drawback of fossil fuels is the pollutant emissions generated during their combustion. Moderating consumption of these energies and replacing them with renewable energies, mainly in order to reduce CO2 emissions, are therefore some of the major challenges of energy policy, economics and research.