Natural Gas
However natural gas reserves have for example been quoted at as much as 240 years remaining use. Figure 2 indicates how gas reserves might be used based on various growth rates. It should be noted that the USA gas use increased annually between 1945 and 1970 at an average rate of 6.5%, although this has now reduced this is still greater than any of the estimated rates of consumption shown in the graph.
	Figure 2:Projected gas reserve time scales based on various rates of usage growth.
Most discussion has centred on the quantity of oil petroleum) that may or may not be available as this is the most widely used form of commercial energy. In addition to its value as a fuel, oil is the precursor for innumerable substances, such as plastics, fertilisers, medicines and many other products indispensable to the industrialised world. Some oil users contend that the known reserves are a poor measure of how much oil actually still exists. Particularly since the estimates of oil, coal and gas reserves have been revised upwards as more oil is found. However in general over the past two decades the new sources of fossil fuels have been smaller and of lower quality. Countries such as Venezuela who have reserves of "thick oil" known as Tar sands will be able to add to the oil resource but at what cost?. Eventually the cost of extraction or the charge that can be levied may cause oil to "run out" faster in terms of what we can afford than of what is physically available. In addition to concerns that we are quickly depleting our energy supplies is the issue of global warming i.e. can we use what resources we have left safely. The burning of all fossil fuels result in the production of the greenhouse gas CO2, countries who attended the Earth Summit in Rio in 1992 agreed to reduce emissions of this and other greenhouse gases. Thereby increasing the pressure still further to become more efficient in our current use of fossil fuels whilst actively developing alternative renewable energy sources such as wind, wave and solar technologies.