Nobel Laureate Robert B. Laughlin exposes the consequences and limitations of biofuels from manure and corn ethanol to switchgrass and algae.

It had to happen eventually. Even the most desolate desert has an oasis, the most exhausted mine a fresh seam of gold. The gloom of global warming guilt had to break momentarily and let a ray of hope shine through. When it did, armies of frustrated journalists rose to the occasion: Pig Poo to Help Solve Energy Crisis! Manure Power Goes Live in Texas! It’s Clean, It’s Green, It’s Cow Manure Power! Cow Dung Lights Up L’Oreal Factory! Dairy Cow Manure to Power Children’s Train Ride! Tons of Turkey Dung to Help Fuel Power Plant in Minnesota! Dutch Harvest Chicken Poo to Power Ninety Thousand Homes! Plenty O ‘Poultry Power! Put a Chicken in Your Tank!

Saving the world with manure is such a humor gold mine that it’s delightful to think about day or night, but it’s also serious, in that microbe-generated fuel is likely to become important two hundred years from now when the coal runs out. People probably won’t be driving to work using gas made from cow pies because there will never be enough cow pies, but they are likely to be driving on gas with a cow pie component. At the very least, sky-high prices for carbon-based fuels will induce farmers to digest their manure and market the gas it generates rather than simply letting the manure decompose. The world may or may not go nuclear, and it may or may not have sound government, but it will always have plenty of poo.

There’s a lot of energy in manure. This isn’t surprising, for there’s a lot of energy in the forage, silage, and grain the animals eat before they poop. If collected, dried in the sun, and burned, the world’s agricultural manure would supply about one-third as much energy as all the coal presently consumed. If anaerobically digested first, it would render up about one-third of this energy as methane, enough to supplant one-fifth of the world’s present natural gas consumption. What digestion left behind, about half the original energy content, would normally become fertilizer, but it could equally well be dried and burned to produce, on average, a trillion watts, or about half the present electric power consumption of the world. Although manure power is funny, it definitely ain’t hay.

The immensity of poop’s energy content is more than merely impressive, however, for it also tells us that agriculture has the capacity to supply the carbon part of the world’s energy budget once the coal, oil, and natural gas run out. Manure itself would provide only 12 percent, even if we could collect it all (which we can’t), but manure is just a waste product sloughed off by an industry dedicated to food production. We could easily raise the fraction if we made energy production our top priority. For example, we could double it just by removing farm animals from the picture and burning the crops directly. We wouldn’t want to do that, of course, because we need the food. The price structure of present-day animal methane, the stuff generating all the hilarious headlines, also tells us that agriculturally produced fuels will be reasonably priced. They’ll be substantially more expensive than present-day fuels, but probably not ten times more expensive.

However, there’s no getting around the fact that agricultural fuel production competes with food production for resources. This occurs to some extent even when we burn manure, because doing so reduces the amount of nitrogen-rich fertilizer available for spreading under food crops. But the earth can only render up so much energy from the good land it has, and the amount of this energy we would have to divert to power civilization is extremely large. For example, suppose we dried the entire U.S. corn crop, including stems and leaves, and burned it to make electricity instead of feeding it to livestock. This is an admittedly brutish way of getting the energy out, but it provides the cleanest accounting because all other extraction methods discard some of the energy in processing. This desperately reckless act, which would zero out the economy of the entire Midwest, would get us in return only about one-fourth the energy that the United States presently consumes in oil every year.

This is obviously a bad idea.

We might imagine avoiding competition with food by growing special energy plants with no food value on lands presently too poor to farm. That is a bad idea too, as it turns out, because there are no such plants. Food energy and wood energy are the same thing as far as a plant’s concerned, so a woody or weedy plant won’t produce well on marginal lands for the same reason a food plant won’t. Thus we don’t gain anything by planting, for example, tail prairie switchgrass, a perennial bioenergy favorite, as a fuel crop. It likes the same kind of land corn does, being native to the corn belt, and thus displaces corn when it grows. It also produces about the same amount of energy per unit of land that corn does – or perhaps slightly less because corn is an extraordinarily efficient plant. Tall prairie switchgrass will indeed grow in the desert with lots of help from people, but then so will corn – as any farmer west of the Rockies will happily explain to you.

Accordingly, although the agricultural practices that supply food two hundred years from now are likely to be the same as today’s (they haven’t changed fundamentally in centuries), the practices that supply carbon for fuel are likely to be radically different. The reason is simply that if they aren’t, they’ll compete for resources with the food industry, a political fight they cannot possibly win. We can expect some marginal contributions from conventional agricultural waste products such as sawdust or sewer gas because they’re nuisances we have to dispose of anyway, and the temptation to exploit high fuel prices will be irresistible, but none of these things can scale up sufficiently to handle the stupendous masses of carbon required to keep the world economy humming.

Given how thinly stretched the world’s freshwater supplies presently are, it’s difficult to imagine any version of this parallel energy agriculture system not based on saltwater cultivation. That narrows the field a bit, for there just aren’t that many crops that will grow cheaply in saltwater.

The plants most likely to be exploited in the long term as an industrial carbon crop are saltwater microalgae. This prediction is somewhat of a stretch, because there is no saltwater agriculture industry at the moment from which you can make crop comparisons. However, microalgae grow, and thus fix carbon from the atmosphere, faster than any other plants in the world, so they’re very strong candidates. Their near-term production cost per unit mass is probably between five and ten times coal’s. This makes algae agriculture badly uncompetitive as an industrial carbon source while coal is plentiful but a potentially acceptable one when coal is exhausted. Other saltwater plants would have to beat this cost to win in the marketplace, and none of them can do so at the moment.

Microalgae also have an immense public relations advantage over other potential energy crops in being quintessentially green. They’re not just natural things compatible with the environment; they’re the fundamental food source for the ocean’s entire ecosystem. These free-floating one-celled plants are visible at most ocean beaches as a green or greenish-blue tint of the light passing through the peaking surf. The powder blue color we find in open sea or in warm tropical waters indicates that the nutrients necessary for aggressive algae growth, chiefly nitrates and phosphates, are in short supply. But given a flow of nutrient-rich coastal runoff or a deep water upwelling, as we find on the Grand Banks of Newfoundland or Peru’s Humboldt Current, the water becomes turbid and green and abounds with life.

Filter feeders such as shellfish, corals, and larval zooplankton eat the algae and multiply with abandon. Fish eat the filter feeders-and each other-eventually becoming abundant. Higher predators such as birds, seals, and killer whales eat the fish and likewise prosper. Commercial fishing operations thrive. The central role that algae may eventually play in the human economy when the coal runs out is thus extremely appealing, in that it echos the role they already play in the sea.

Unfortunately, algae are difficult to farm. The fundamental reason why is that they achieve their great fecundity by avoiding responsibility, not by being gifted. A conventional food plant like corn has stems to erect, roots to put down, leaves to unfurl, seeds to generate, and so forth, not to mention longer-term things such as waiting for spring. But algae do none of these things. They lead a short, brutal life of freedom rather than a long one of endless toil, and they just don’t care about tomorrow. As a result, we have to do their chores for them if we want to raise them, and that costs money.

The extra costs wind up being so great that no one can figure out how to farm algae profitably. That isn’t so surprising, for pure algal biomass is a green substitute for coal, a resource that is extremely cheap at the moment and correspondingly hard to beat. But the problem is worse than that, for algae become less productive than conventional agricultural plants, not more productive, once we discount the extra costs of raising them. Algae farming will probably not become a significant industry until both coal supplies and land resources for agriculture have run out.

Algae’s monumental cost problems make it almost impossible to take present-day algae biofuel companies seriously. We want to believe, but we find ourselves thinking instead about all those nutritious government subsidies. The most unforgettable of these subsidies is the high-priority U.S. congressional mandate that the Air Force secure a supply of green jet fuel at any price. Exactly how much the Air Force is paying isn’t public knowledge, but rumors are that it’s about ten times the cost of jet fuel made from petroleum.

Not surprisingly, saltwater agriculture is a rather low priority with startups jockeying to supply this fuel, as is agriculture generally. For example, one of them is reported to be growing algae in plastic bags (made from petroleum) stacked in warehouses. Another isn’t engaging photosynthesis at all but is instead grazing its algae in the dark on cheap sugar, presumably obtained from conventional crops such as beets or cane. Another eschews growing algae completely and plans instead to harvest the fish that eat them, no doubt with the objective of grinding the fish up and processing the happy brew into gasoline and diesel fuel.

The absurdity of green biofuels that never see the sun is actually not funny, as it’s symptomatic of attempts to contravene the laws of economics-presumably out of fear of what those laws may portend. If the big energy companies won’t make algae biofuels, so the reasoning goes, we must do it for them, thus heading off impending catastrophe when the oil runs out. But there is no need at the moment to create saltwater agriculture that spares land and water for food production. If there were, we would have such agriculture. There also is no need to generate as much biomass as we possibly can as a carbon resource. The world is awash with cheap carbon fuels. The world also has agricultural land to spare. It pays farmers not to plant the fields they already have. It struggles to find buyers for subsidized butter and cheese in oversupply.

The energy industry’s sudden interest in algae might also be part of this absurdity, unfortunately. Green politics powerfully encourages “greenwash,” the practice of associating yourself with green causes to look more environmentally friendly than you actually are. Although the investments that the oil majors are presently making in algae look technically legitimate, they might just be public relations expenditures. We can’t tell, for the amounts of money involved, though considerable, are smaller than the potential costs of taxation, regulation, and political vexations that might be visited upon them for not being sufficiently green. Absent some truly unprecedented discovery or breakthrough, it will be hard not to smile knowingly whenever world-famous geneticists begin explaining their strategic algae oil partnerships that involve no farming until sometime way in the future, if ever.

– Robert B. Laughlin shared the Nobel Prize in Physics in 1998. He is a professor at Stanford University and the author of The Crime of Reason, A Different Universe, and Powering the Future.

Excerpted with permission from Powering the Future by Robert B. Laughlin. Available from Basic Books, a member of The Perseus Books Group. Copyright © 2011. Photography by Daniel Mayer (algae on rocks), Malene Thyssen (manure), and Steve Jurvetson (bubbles).