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How Much Fossil Fuel is in the Earth?

. 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.”
In other words, analysts extrapolate as much as they think they can into the future to determine what new sources of fuel may come online somewhere down the road.
From an environmental standpoint, all that raw data on the unrecovered barrels of crude oil, Btu of coal (a measure of heat content) or other fossil fuel metrics are pretty abstract. What matters is demand, and whether that demand will drive more production, which will in turn spur more exploration and more proven reserves, in the short term at least.
The International Energy Outlook expects a nearly 50 percent jump in global energy demand by 2035. During that same period, renewable energy is slated to increase to 14 percent from 10 percent of global supply.
“While that (renewable energy) growth is not insignificant, it does imply that renewables will still account for a lower share of the world’s energy mix than oil, natural gas and coal,” Doman told Discovery News.
Meaning that coal, oil and gas will be able to meet demand as companies continue to ramp up production.
But the EIA's perspective has met with staunch criticism; energy experts have looked at the history of coal production, for example, to show that reserves have a habit of declining a lot faster from their peak than people think. Data from the EIA tend to suggest that peak oil production is comfortably far off, but that may not be the case, since it's often not possible to tell what a reserve actually can offer up in terms of supply.
Nevertheless, government agencies such as the U.S. Geological Survey (USGS), seek out undiscovered, technically recoverable fossil fuel resources in the meantime that have the potential for mining and production but haven’t been tapped yet.
“Because the Earth is far from fully explored, our understanding of the geologic occurrence of fossil fuel is incomplete, and changes over time as we learn more about the Earth’s geology. Further, the geologic occurrence of fossil fuels isn’t necessarily an indication of a resource that can realistically contribute to the energy supply,” said Brenda Pierce, program coordinator of USGS Energy Resources Program.
If the USGS identifies a previously undiscovered coal bed, as Pierce explains, it might be too thin to be properly mined, or lagging technology could hinder production of resources.
“Thus, what constitutes a resource changes over time,” Pierce said, adding, “The key point here is to recognize the distinction between the geological occurrence of fossil fuels, and the complex synergy of technological, environmental, social, economic and political factors that, when considered along with geologic occurrence, collectively give rise to energy supply.”
/Discovery News

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