Earth contains a finite supply of fossil fuels –- the big three being oil, coal and natural gas. And although we know it's finite, we don't really know how long they will last.
Experts attempt to measure how many fossil fuels are left by what's called proven reserves, fuels in the ground that haven't been brought out yet, but could be. And that number, while admittedly slippery to nail down, hasn’t significantly declined over time, as one might expect.
According to the 2010 International Energy Outlook, “as of Jan. 1, 2010, proved world oil reserves, as reported by the Oil & Gas Journal, were estimated at 1,354 billion barrels—12 billion barrels (about 1 percent) higher than the estimate for 2009.”
Wait: How could this happen? The global economy guzzles over 80 million barrels of oil a day, and yet the amount of fuel left in the ground goes up?
The trick here is the term "proved (or proven) reserves." Remember, those figures refer to reservoirs of oil, coal seams, and natural gas deposits that companies are sure they can make a profit from, if they could bring them up using current extraction technologies.
Plenty of deposits around the world aren't accessible, or simply wouldn't be profitable to drill or mine. Take the many oil formations lying under a mile of water at the bottom of the Gulf of Mexico, for example. Until a few years ago, giant oil rigs like the Deepwater Horizon didn't exist, and so all of those oil fields were effectively off limits. They may as well have been on the moon, so they simply didn't go on the ledger as "proved" reserves.
Some reserves are just plain deceiving, like the major Gulf oil field called Thunder Horse that's currently vastly under-performing estimates.
And as for coal and natural gas (and tar sands, for that matter), if you can't get it out of the ground, or can't make money doing it, it doesn't matter how much is down there.
That said, drilling, mining and refining technologies are evolving with lightning-quick speed. So the idea of a proven reserve becomes time-dependent based on how quickly the technology can be developed to bring it into our homes or gas tanks.
In an effort to keep up with this moving target, the U.S. Energy Information Administration (EIA) publishes the International Energy Outlook every year, a detailed analysis of available energy resources from nonrenewable oil to biofuels around the world.
“We don’t believe that proven reserves alone are an appropriate measure for judging total resource availability in the long run,” Linda Doman, an international forecasting expert with the EIA said. “For example, despite continued production, global reserves haven’t declined historically (because of) exploration, discovery and reserve replacement.”