“Drill, baby, drill” has become a slogan of those who want to produce more oil and gas and who scoff at alternatives to petroleum. But rarely mentioned is the expense required to get that oil and gas—and still more rarely mentioned is the energy required to access those resources.

Charles Hall, an ecologist at the State University of New York College of Environmental Science and Forestry in Syracuse, has spent most of his long career trying to get fellow researchers and the public to take a serious look at the energy required to get the energy we use. He is given credit for creating a measure known as the energy return on investment, or EROI—the ratio of energy output over energy input. (With oil, for example, the energy output would be the crude oil produced, and the energy input would be all that required to find the oil reservoir, drill the well and pump the oil out of the ground.) EROI is a crucial metric, Hall argues, because it helps us see which energy sources are high quality and which are not.

Hall and his students did pioneering work in this area, including a 1984 paper on the cover of Science. For many years, however, interest in the topic languished. But recent soaring oil prices and increasing difficulty of accessing new supplies have helped create economic hardships, leading to resurgent interest in EROI. Scientific American asked Hall to explain the basis of the EROI and how it pertains to our economy.

[An edited transcript of the interview follows.]

You’re a self-described “nature boy” who became an ecologist. So how did you create the idea of energy return on investment (EROI)?

I had this unbelievable doctoral advisor, H. T. Odum of the University of North Carolina in Chapel Hill. He said, “Well, Charlie, I don't think anyone has thought about fish migration from a systems perspective.”

I went down to the coast of North Carolina, looking for a place where I could do this research. And I found one: in this freshwater environment, where fish weren't supposed to be migrating, they were migrating like crazy.

And you approached this migration mystery from an energy-use perspective. How did you do that?

I measured the ecosystem productivity by the free-water oxygen technique. I measured it at five different places, upstream and downstream, and found some very clear patterns. The energy available to the fish was much more concentrated as you went upstream, and I developed this theory that the fish would migrate to capitalize on the abundance of energy for the first year or two of the life, and then the young fish would migrate downstream into a more stable but less productive environment.

The study found that fish populations that migrated would return at least four calories for every calorie they invested in the process of migration by being able to exploit different ecosystems of different productivity at different stages of their life cycles.

So from studying fish migration, was it a big leap to think about people and fossil fuels?

No, probably because Howard Odum was evolving in his thought processes. He wrote a book Environment, Power and Society at about that time. An amazing thing working with Odum was, for him, there are just systems. It doesn't matter if it's a forested system or a stream system or an estuarine system, or whether people are there or not. It's just a system—and systems have many similar patterns and many similar processes of consumption and production, and they often even have similar controls on them.

So, it was not difficult for me, because I was trained that way from Howard Odum. Also, when I was a graduate student there were a lot of very exciting things going on. Ecologists were much more involved—not just in biodiversity, which is where much of the focus is today, but in dealing with important issues of the relation of humans to resources. Paul Ehrlich [author of The Population Bomb (1968)], Garrett Hardin [known for his 1968 Science paper “The Tragedy of the Commons”], George Woodwell [founder of the Woods Hole Research Center], many other people—these were very influential to me as a graduate student.

For society's energy sources, is it important to consider EROI?

Is there a lot of oil left in the ground? Absolutely. The question is, how much oil can we get out of the ground, at a significantly high EROI? And the answer to that is, hmmm, not nearly as much. So that's what we're struggling with as we go further and further offshore and have to do this fracking and horizontal drilling and all of this kind of stuff, especially when you get away from the sweet spots of shale formations. It gets tougher and tougher to get the next barrel of oil, so the EROI goes down, down, down.

Is there some minimum EROI we need to have?

Since everything we make depends on energy, you can't simply pay more and more and get enough to run society. At some energy return on investment—I'm guessing 5:1 or 6:1—it doesn't work anymore.

What happens when the EROI gets too low? What’s achievable at different EROIs?

If you've got an EROI of 1.1:1, you can pump the oil out of the ground and look at it. If you've got 1.2:1, you can refine it and look at it. At 1.3:1, you can move it to where you want it and look at it. We looked at the minimum EROI you need to drive a truck, and you need at least 3:1 at the wellhead. Now, if you want to put anything in the truck, like grain, you need to have an EROI of 5:1. And that includes the depreciation for the truck. But if you want to include the depreciation for the truck driver and the oil worker and the farmer, then you've got to support the families. And then you need an EROI of 7:1. And if you want education, you need 8:1 or 9:1. And if you want health care, you need 10:1 or 11:1.

Civilization requires a substantial energy return on investment. You can't do it on some kind of crummy fuel like corn-based ethanol [with an EROI of around 1:1].

A big problem we have facing the alternatives is they're all so low EROI. We'd all like to go toward renewable fuels, but it's not going to be easy at all. And it may be impossible. We may not be able to sustain our civilization on these alternative fuels. I hope we can, but we've got to deal with it realistically.

Do you think we're facing limits to growth now?

I think if you correct the U.S. GDP for debt—in other words, the debt is some kind of not-real growth—then I think the GDP hasn't grown at all since 2005. It's just grown through debt. I think clearly growth has declined; it's possible that growth has either stopped or may soon stop.

We know that the middle class has not increased its income now for 20 years. Behind that—not always the immediate cause, but looking over the shoulder of the causes—I find the decline in the availability of energy.

It's terrifying to people—politicians and economists—who base everything on growth. I think they won't talk about it because the concept is terrifying.

Most economists think economic growth can continue indefinitely, right?

It was easy to make economic theories that worked while we pumped more and more oil out of the ground, because whether you're a capitalist or a communist or a this-ist or a that-ist, they'd work—because there was more oil to make them work. We could afford all the corruption and inefficiencies in the past and still have quite a lot trickle down.

But now the pie is not getting that much bigger. Now, it's pretty clear that there's a lot of economic theories that aren't working very well.

How do these economic arguments relate to people’s day-to-day lives?

Doesn't it mean food on the table, a roof over your head, gas in your car—a car itself? So economics isn't really about money. It's about stuff. We've been toilet trained to think of economics as being about money, and to some degree it is. But fundamentally it's about stuff. And if it's about stuff, why are we studying it as a social science? Why are we not, at least equally, studying it as a biophysical science?

Hall recently co-authored a book on this biophysical perspective with economist Kent Klitgaard, Energy and the Wealth of Nations: Understanding the Biophysical Economy (Springer, 2011).