How does a biotechnology company like the ones described here make petroleum from plants? And what effect will all this new petroleum have on global warming?

All fossil fuels contain energy stored in the form of hydrocarbons. Different hydrocarbons have different uses. Methane, the simplest hydrocarbon, is the main component of natural gas, while various complex hydrocarbons make up petroleum. Gas and petroleum refiners process these hydrocarbons into an array of fuels like gasoline, diesel fuel, and jet fuel.

The hydrocarbon in fossil fuels began as a different type of molecule, carbohydrate. This carbohydrate was produced by plants, then converted by a geological process to hydrocarbon. So far, fossil fuels have been the only game in town for obtaining hydrocarbon. Renewable fuels currently in use, such as ethanol, belong to yet another chemical class and do not have the properties that make hydrocarbon so useful as a fuel source.

Producing renewable hydrocarbon begins with naturally occurring microbes called methanogens. (Technically, methanogens are not bacteria, although they are commonly described as such.) These microbes are found in many environments, including the guts of many animals, such as cattle and humans. Here they are notorious for the gas they produce as they feed on indigestible organic compounds.

These microbes convert one type of organic compound (carbohydrate) to another (hydrocarbon). The chemical pathways to do this are already in place, and through genetic engineering, biotechnologists aim to make organisms that produce different kinds of hydrocarbons which can be refined into fuel.

Plants are mostly made of a type of carbohydrate called cellulose, which is the favorite food of these microbes. This means that almost all plant products, including waste like straw, stover, and sawdust, can theoretically be converted into fuel.

Burning fossil fuels increases the amount of carbon dioxide in the atmosphere because the carbon was stored in the fuel, then released when the fuel is burned. Renewable hydrocarbons, on the other hand, have no net effect on atmospheric carbon dioxide. This is because the plants at the beginning of the cycle made their cellulose out of carbon they removed from the atmosphere. The carbon dioxide released when renewable hydrocarbons burn is equal to the amount that was previously removed by the plant sources. Therefore, these fuels not only have a practically limitless supply, they do not contribute directly to levels of atmospheric carbon dioxide thought to be involved in global warming. Their "carbon footprint" consists only of the carbon released by refining and transporting them. If these processes can be made to take their energy from renewables, the carbon footprint will theoretically be zero.