A bunch of biological activity happens, organism reproduce, grow, die. Some of this biomass turns into oil, natural gas, or coal. I’ve left out few details.
During certain periods in the Earth’s history, this happened at a much larger scale that usual, and in certain geographic and geological settings, leading to the eventual formation of huge underground oil reserves, coal fields, gas reservoirs, or bitumen deposits. By the way, some of these 10 million year or so long moments in geological history were probably regional extinction events.
That is how we get fossil Carbon based fuels, for the most part (again, I oversimplify).
An alternative, it seems, is to intervene early in the process. Take the organisms out of the system early, when they have just grown, and turn them into biofuels. Trees or other material can be burned, plant tissues can be converted to liquid fuel or gas, etc. This method is inherently limited compared to using fossil fuels because the fossil fuels were generated over tens or hundreds of millions of years, while this form of biofuel is being generated real time. In order to continue to use energy at the rate we currently use it, with all the energy coming from biofuels, we’d have to be scraping a huge percentage of the output of photosynthesis every day.
To put this in perspective, consider that the total amount of energy that natural systems using photosynthesis on the Earth produce is about six times of what we humans use in energy, from fossil fuels, nuclear, hydro, and various clean energy sources. In other words, if we used only biofuels for our energy, we would have to use one sixth of the energy the entire natural world currently produces, assuming efficiency matched to what nature does. It is likely that some of that use would enhance natural production, or could be used harvested more efficiently, but the differences can’t be large. Maybe we’d only need a seventh, instead of a sixth, of the Earth’s natural photosynthesized production. Or, maybe we would be using it less efficiently and thus need more.
Having said that, there is a certain amount of potential biofuel that goes from some use or another into the trash (or sewer effluence). When we capture that energy, we might be reducing a carbon sink, but we are at the same time using a non-fossil Carbon based fuel source. This includes using discarded cooking oil, or burning sawdust or trash in waste to energy plants.
Justin Gillis at the New York Times has a writeup on a recent report that seems to confirm that there are severe limitations to the use of biofuels. You can read Gillis’ writeup here. The report is here. Following are a few excerpts from the NYT piece.
Western governments have made a wrong turn in energy policy by supporting the large-scale conversion of plants into fuel and should reconsider that strategy, according to a new report from a prominent environmental think tank.
Turning plant matter into liquid fuel or electricity is so inefficient that the approach is unlikely ever to supply a substantial fraction of global energy demand, the report found. It added that continuing to pursue this strategy — which has already led to billions of dollars of investment — is likely to use up vast tracts of fertile land that could be devoted to helping feed the world’s growing population….
The report follows several years of rising concern among scientists about biofuel policies in the United States and Europe, and is the strongest call yet by the World Resources Institute, known for nonpartisan analysis of environmental issues, to urge governments to reconsider those policies.