One thing you can count on in science is that virtually every issue is more complicated than it first seems. Take biodiesel as an example. It can be made from virtually any plant oil by reacting it with methanol. It produces roughly the same amount of carbon dioxide as does petroleum-based diesel when it burns, but since whatever crop is used to produce the oil takes up carbon dioxide from the air through photosynthesis when it grows, there is a smaller contribution to the greenhouse effect. Plus of course, plants are a renewable source of energy while petroleum will eventually run out. But while crops do indeed take up carbon dioxide from the air, growing them causes another greenhouse gas, nitrous oxide, to be produced. Nitrous oxide actually is two to three hundred times as potent a greenhouse gas as carbon dioxide. It forms when bacteria in the soil react with nitrogen containing compounds, both those applied as fertilizer and those produced by crops such as legumes that are capable of converting nitrogen from the air into compounds they can use as nutrients. Incidentally, whether the fertilizer is synthetic or good old-fashioned manure has little impact on nitrous oxide production. Since trees do not produce much nitrous oxide, one study has already concluded that greenhouse emissions could be cut by a third if cars were run on conventional fuel and trees were planted on the land that would otherwise be used to grow crops destined for biodiesel production.
But biodiesel production is hot on the environmental agenda and Brazilian rain forests are being cleared at a frightening rate to make room for fields of soybeans to supply the oil needed for the growing biofuel industry. To be sure, not all the clearing is for soybeans, most of it is to create pastureland to raise animals, a financially lucrative proposition. Both of these have an environmental impact, but calculations show that fields of soybeans may have a greater effect on climate than pastures. How can this be? Clearing of trees causes less rainfall since in a rainforest evaporation of water from trees is a major cause of cloud formation. But replacing trees with soybeans results in less rain than replacing them with pasture. That鈥檚 because soy plants reflect sunlight more efficiently than fields of grass meaning that there is less heating of the surface, less moisture rising into the air, fewer clouds and less rain. Right now we can only guess at what a fourfold reduction in rain caused by soybean fields when compared with pastureland will mean, or indeed what the eventual global consequences of cutting down rainforests will be. It is unlikely to be a pretty picture. Switching to biodiesel from petroleum diesel may not be as 鈥済reen鈥� a process as commonly portrayed. As I said, issues always get more complex on further scrutiny.
