Whenever anyone drives a car, the car burns plant matter that was created by solar power and converted into petroleum a long time ago. Nobody knows how much petroleum is in the ground but there's less every day. As we've become more and more conscious of the possibility of catching all the fish in the sea or cutting all the trees in the forest, we've begun talking about sustainable yield.
Sustainable yield is the amount of something, be it fish, or trees, or whatever, that you can take from a given area without ever running out. You have to leave enough fish or trees to breed more for you to catch next year. If you take too many, you'll eventually take them all and there won't be any more. If you don't take enough, the extra fish or trees will die and be wasted.
Although there are some who believe that oil is constantly formed deep in the earth, most oil scientists believe that oil isn't being formed any more. On that basis, the "sustainable yield" of oil is zero.
Suppose that fish lived forever but that no fish were born. If you took one fish per year, eventually they'd all be gone; the sustainable yield of any resource that doesn't reproduce is zero. If no oil is being formed, the sustainable yield is zero. Since we refuse to cut our petroleum use to zero, we're pumping oil above its sustainable yield and will eventually run out.
We're being urged to replace petroleum with "renewable resources." A resource is renewable if it replenishes itself at the rate we consume it. That is, a forest is claimed to be renewable if we harvest the trees at about the rate they grow.
Weeds always seem to grow back when we cut them; using biofuels to replace petroleum seems to have no cost. The idea is to harvest plants and extract automobile fuel from them. This shortcuts the natural processes by which long-ago plants were turned into petroleum.
Unfortunately, there's one teensy little problem: biofuels are no more sustainable than petroleum.
Jan Baptist van Helmont (1577-1644) got credit for performing the first experiment to determine where plant mass came from. He carefully weighed the soil in a pot and planted a willow tree in it. When the tree weighed about 170 pounds, he found that relatively little soil weight had been lost from the pot.
He concluded that the water he gave the tree had turned into plant matter. Helmont ignored his own data that burning 62 pounds of oak charcoal produced 61 pounds of carbon dioxide and one pound of ash, some of the mass of the tree had come from gases in the air. He also ignored the ancient knowledge that crop yield declines when crops are grown repeatedly in the same field unless manure is added to the soil to put back what the plant takes out.
Helmont should have known better; we know that when a plant grows, the plant takes something out of the soil. If the plant dies and rots in situ, whatever the plant took gets recycled, but if someone cuts the plant and takes it away, the soil loses something. Eventually the soil runs out of whatever the plant takes and those plants won't grow there any more.
We know this from observing the ethanol boondoggle. Corn makes heavy demands on soil. The only way we can keep harvesting corn from the same farm year after year is to pour on fertilizer.
Corn fertilizer is made from guess what? Petroleum! As George Will put it:
In 2005, America used 15 percent of its corn crop to supplant less than 2 percent of its gasoline use.
Something is clearly wrong here. As scientists who don't live in corn-growing states have looked into the facts behind making ethanol from corn, they've found that it takes so much petroleum to grow corn in the first place that we get very little more energy out of the ethanol than we put into the corn.
If we burn corn instead of eating it, food prices go up. Ethanol use is based on politics, not on facts.
Using corn to make ethanol has pushed food prices up; people are talking about other plants such as sugar cane or switch grass. George Will wrote:
In Indonesia alone, 44 million acres [of forest] have been razed to make way for production of palm oil.
Biofuels not only tempt investors to destroy forests, they can't work in the long run. No matter what plant you use, every plant takes something from the soil. Unless you put back whatever the plant takes from the soil, that plant won't grow there any more when it's gone.
We have a choice. Continue powering our cars by taking petroleum out of the ground, or power our cars by taking whatever plants need to grow out of the ground.
The bottom line is, biofuels are not sustainable because they deplete the soil. Crop rotation helps, but every plant that's hauled away takes something.
Biofuels are renewable only to the extent that we figure out what each plant takes from the soil and put it back. Depending on the energy cost of making whatever it is we have to put back, we may or may not gain any energy in the process.
Ignore the hype. Biofuels are neither sustainable nor renewable without a lot more work than anyone's talking about.