Short version: If we work hard at recovering fossil fuels, huge amounts remain. We are probably at least decades and perhaps longer from real scarcity of fossil fuels, although with notable uncertainty. But, we may be close to the point at which fossil fuels are scarce enough to start causing problems.
Friendlier but longer version: Experts in the field generally separate fossil-fuel “reserves” from “resources” (and, they have additional technical terms that subdivide these big types). You might say that “reserves” are what you are (almost) sure you can use in the modern economy with modern technology, whereas “resources” are what you think you can have in the future.
For example, the US Energy Information Agency, in defining “proved reserves” for oil (also known as “proven reserves”; similar definitions apply for gas and coal), says that this includes “the estimated quantities of all liquids defined as crude oil, which geological and engineering data demonstrate with reasonable certainty to be recoverable in future years from known reservoirs under existing economic and operating conditions.
Their data indicate that, as of 2011 (the last year for which full data were available at the time this was being written; a partial update in 2016 didn’t change these numbers much), the world had 46 years of proved reserves at current rates of use. These numbers were higher for gas (150 years) and coal (120 years). With oil slightly more important than the others in the world economy, these would give most of a century of fossil fuels before we run out. However, use has been rising rapidly. If everyone in the world used fossil fuels at the same rate as in the US, total use would be more than 4 times faster, reducing the life of the proved reserves to perhaps 20-30 years.
The resource is likely much bigger. If you spend a little while looking at the figure "Where is the Carbon?” just below, you’ll see first that the authors from the US NOAA are discussing how much fossil fuel we have, and how much we’re burning, in units of gigatons of carbon. (1 gigaton of carbon = 1 Gt C. A gigaton is a billion tons. Fossil fuels contain some hydrogen and a little bit of other things, but focusing on the carbon is a useful way to calculate.) The authors estimate that we have already burned fossil fuels containing about 244 Gt C, from an original 3700 Gt C, leaving 3456 Gt C to be burned. The figure is a few years old, and the use rate of 6.4 Gt C per year that they show has increased to perhaps 9 Gt C per year. That would leave almost 400 years of fossil fuels at current use rate, or less than a century if everyone reached the US rate (and even less if population continues growing). Some estimates of the resource are even bigger, in the range of 4000 to 6000 Gt C, and even more if we figure out how to use clathrate hydrates, which might have about as much carbon as the other fossil fuels although probably notably less.
DR. RICHARD ALLEY: This wonderful plot from NOAA and the IPCC is about where carbon is in the Earth's system and where it's going. The numbers of how much is in a place are in gigatons of carbon-- billions of tons. And how fast it's going is in billions of tons per year of gigatons of carbon per year.
The black numbers are before humans started messing with it. And a number like this one up here is how much carbon was in the atmosphere before human influence. The red number up here is how much humans have added to the atmosphere. The numbers with arrows are how much was going naturally in a particular direction before humans were in, and then how much the human change has been. And so you can see fluxes of carbon, and you can see reservoirs of carbon.
Perhaps the key one, they have estimated that before humans got into the game, there were about 3,700 gigatons of carbon as fossil fuels, of which we've burned only 244. So there's lots more carbon that can be burned in the system. They show a flux here of 6.4. That's probably about 9 now.
But you'll notice that at that rate, there's actually something like 400 years of fossil fuels remaining. Now if everyone burned it at the same rate as we do in the United States, that would drop it down to 100 or less. And if population continues growing, it would be even less than that.
And if we start to get into trouble when half of it is burned, then most of your students actually will live to see the time when we're getting into economic difficulties because fossil fuels are starting to get scarce. Nonetheless, it's pretty clear that there is much more fossil fuel in the earth than we've burned so far, and so we can make a lot bigger change than we've done so far.
But, because of the heating needed to get oil from tar sands and oil shales, and the extra effort to drill deeper and frack rocks, some of the resources will be used up recovering the rest of it, increasing the rate of use. And, we don’t really know very well what the resource is; serious investors and regulators tend to rely on the proved reserves for good reasons.
You may also recall from earlier in the course that peak whale oil production from the US fleet was followed almost immediately by a tripling of the cost, even though oil production continued at fairly high levels during the following decades—impacts of scarcity are felt long before the resource is exhausted. People often assume that production of a resource follows a sort of bell curve, starting slow, then rising rapidly, peaking, declining and tailing off to almost nothing. If that model is accurate, then the peak—peak oil, or peak coal, or peak-whatever, occurs when half of the resource has been used. The whale-oil experience suggests that scarcity shows up and starts to restrain the economy just about then. If so, then we now may be much closer to problems from fossil-fuel shortages than we realize.
Another point worth considering is that some countries have a lot of fossil fuel, and others not much (see the "World's Proven Reserves" figure just below). And, who has fossil fuel and who uses it are not always the same. For oil, for example, for most recent years including 2016, the US was the third-largest producer, although the US was #1 by a small margin in 2015, based on data from the International Energy Agency. But, because the US is the largest user, being third in production isn’t enough, and the US is the largest importer.
DR. RICHARD ALLEY: This shows where we know that there are oil, and coal, and gas. So the oil in each case is the black one, and then the coal is this little sort of purple-y one, and the gas is blue one. This happens to be the Middle East right here, and the Middle East has more than half of the proven reserves of oil in the world. It's not the place you go for coal, but there's a whole lot of gas there.
The US uses a lot more oil than most places. We have lots of oil in the US, but it's only 14% of the proven reserves. There's a huge amount of gas that's been found, but it still only gives 6%, and then the coal is fairly high in the US. And you can sort of walk around here. Europe uses a lot of energy, and they are very poor in basically everything. They've got more of the coal than anything else, but they really don't have all that much. There's a whole lot of some of these up in Russia, and so on. So you can run down. And Asia here, and there's a whole lot of coal, Australia especially.
But you can notice that because the way this is distributed, some people use more than they have, and some people have more than they use. There's a lot of trade that goes in various directions, and this makes the international economy and the international politics fairly interesting.
Notice that the sorts of numbers here can be “spun” in many, many ways in the public discussion. Estimates of how much we have already discovered in the ground are at least somewhat uncertain; estimates of how much is in the ground that we haven’t discovered yet, or haven’t learned how to recover, are much more uncertain. Businesses and companies might have reason to report optimistic numbers, or pessimistic ones, depending on what they want to accomplish or sell or buy just now. How long the resource will last depends on how fast we burn. Should we estimate using modern rates of burning, or future ones, and if future, what will they be?
The rise of gas and oil fracking in the US has led to a rapid increase in reserves. You may hear people talking about a century of gas, although others use numbers as small as 25 years for the US reserve. But, at the start of fracking, gas was only about ¼ of the US energy use, so relying on gas as our main fuel could bump the low-end estimates down to only about 6 years. You could find some justification for bragging about a century or more of gas or warning that we may run out in a decade or less, by carefully choosing which estimates to adopt and how to use them. The module is to be very careful about the first numbers you hear, and think and compare before using a number to make decisions on, say, where to invest your retirement fund! (This applies to what you see in this class, too; no one can give you the absolute truth on this topic!)