"Nobody knows or can know how much oil exists under the earth's surface or how much it will be possible to produce in the future."

That's how BP starts its website page defining oil reserves, and it's why we should take statistics like "53.3 years of oil reserves" with a pinch of salt.

But currently, that number is BP's informed estimate of how much oil we have left, given known global reserves and estimated production rates.

Illustrating just how malleable such figures are, that's actually a 1.1 percent improvement over last year's estimate.

Numbers fluctuate with time, as scientists and oil companies find new reserves, and new, more cost-efficient ways of extracting oil from known reserves. Reserve estimates have been rising for several years now, and are likely to do so for some time.

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It's those new methods of extraction that pose a much greater problem than the half-century of remaining reserves.

As Autoblog notes, shale extraction is one such method, one becoming popular in the U.S. as huge reserves like those in the Permian Basin of west Texas are discovered.

Environmental concerns for shale oil extraction include increased erosion, introduction of harmful metals into surface and groundwater, and large volumes of waste material--before the high energy input of extraction is even considered.

Such methods are currently popular as they reduce the need to search for oil from--or buy it in from--overseas.

MORE: U.S. Oil Future: Energy Independent By 2030, Bigger Than Saudis In 2020

To illustrate, reserves like those under Texas could add an additional 75 billion barrels to the United States' existing 44.2 billion barrels estimates. Globally, the total reached 1,687.9 billion barrels at the end of 2013, from when BP's statistics are drawn.

Whether that will be enough to satiate increasingly industrial countries like China is another matter--BP's figures reveal Asia-Pacific reserves sit at just 14 years at current production rates.

That means buying it in from elsewhere to meet demand--or increasingly damaging methods used to extract oil from ever-more-difficult sources.


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