Over the past few years, I've heard several people in the natural gas industry estimate that the United States is sitting on 100 years worth of natural gas. Every time I've heard the 100-year estimate batted around, it's been presented as a positive thing, a shorthand way of saying, "We've got tons of home-grown energy, people! We don't need to worry about the future of energy at all!"
It's an interesting example of the fundamental disconnect between short-term and long-term thinkers.
All things considered, 100 years is not really a very long time. Especially given the fact that estimates like this are based on current natural gas usage rates, but are presented with an implication that we should be using more natural gas than we currently do. I don't think that a 100-year-supply of something as critical as energy represents a time of plenty. I think it represents a ticking clock. At best, what you've got there is a transitional energy source—something with the potential to be cleaner and less politically complicated than coal and oil, that you can use while you build up an energy infrastructure based on something other than fossil fuels.
But the critique of that "100 years of gas" estimate goes even deeper. That's because any estimate of fossil fuel reserves is made under the limitations of corporate secrecy. Different well owners estimate reserves in different ways, so you can't just add up everybody's estimates and compare apples to apples. There's no way for independent sources to check estimates. And there's not really any independent bodies reviewing the state of fossil fuel reserve estimation science. (The closest you'll get to that is a review of oil reserves done in 2009 by a small group of researchers in the US and UK.)
Meanwhile, as energy analyst Chris Nelder points out on Slate.com, if you take a close look at the information about the estimates that is available, you'll find that "100 years of natural gas" doesn't necessarily mean 100 years of natural gas.
But what is that estimate based upon? Those details haven’t been made freely available to the public, but their summary breaks it down as follows here and in the graph below: 273 tcf are "proved reserves," meaning that it is believed to exist, and to be commercially producible at a 10 percent discount rate. That conforms with the data of the U.S. Energy Information Administration. An additional 536.6 tcf are classified as "probable" from existing fields, meaning that they have some expectation that the gas exists in known formations, but it has not been proven to exist and is not certain to be technically recoverable. An additional 687.7 tcf is "possible" from new fields, meaning that the gas might exist in new fields that have not yet been discovered. A further 518.3 tcf are "speculative," which means exactly that. A final 176 tcf are claimed for coalbed gas, which is gas trapped in coal formations.
I'd recommend reading Nelder's full article. It's a nice summary of why estimates of fossil fuel reserves need to be approached skeptically, why the job of measuring this stuff is difficult to begin with, and why we don't really have enough information to declare a "Golden Age" of natural gas.