Everybody on Kos who wants to solve the ecological crisis would do very well to understand the above diagram of US energy production and consumption.
The X axis represents total energy produced or consumed. The units are quads; which are 10^18 joules. As we can see, the US consumed more than 100 quads or 10^20 joules of energy in 2005. US Photosynthesis - the process by which plants and microorganisms build biomass - consumed 80 quads. This means that if we burned every fresh plant grown in our country with 100% efficiency we could not fill our current energy demand. The line called "forest potential" - represents the maximum amount of energy that could be sustainably harvested from our nation's forests. This represents 5 quads. These two lines show the utter futility of biomass schemes as a single "ecological" source for our nation's energy. We would need to reduce US energy consumption by something like 80-95% to have any hope of meeting our energy needs through the burning of biological material.
The Y axis is Energy Return on Investment (EROI). This is the ratio of energy produced by a product or process over the energy consumed in building or extracting it. It is hard to read, but "tar sands" is way in the bottom left with a EROI of less than 1 - implying it takes more energy to extract than it produces. Also in this category are various highly subsidized products like ethanol biodiesel - whose EROI hovers around 1. Coal still remains king, with EROI values oil hasn't seen in decades. However, high EROI should not be confused with sustainability or ecological friendliness. Just because coal can be extracted with little energy cost, does not mean there aren't huge impacts from pollution, strip mining, global warming, etc. Natural gas and oil remain large parts of the US energy budget, with EROI values of around 20:1
The much vaunted renewable and clean sources of all plot in the lower left portion of the figure. Murphy and Hall choose the EROI value of 3:1 as the "Minimum Required for Civilization". The idea here is that a society with too inefficient an energy source would need to put so much of its efforts into making energy that it could not function. Both nuclear and solar plot perilously close to this line. This is because the large amount of energy requires to enrich uranium and make solar cells respectively. Thus if we wanted to dump fossil fuels in favor of solar and/or nuclear, serious difficulties would likely emerge with the efficiency of these technologies.
Wind, hydro, and firewood plot at respectable EROI values, but are very small portions of the current US energy budget. All of these have serious limitations for upscaling. Firewood is discussed above. Apart from the huge environmental impacts of dams, most rivers in the US are currently dammed. Thus there is not very much room for additional hydro power, and certainly nowhere near the 100 Quads needed for US energy consumption. Wind may have some promise. But as is shown, it currently accounts for less than 1% of US energy production. To get it anywhere near 100 Quads, vast tracts of would need to be converted to wind farms, with huge environmental impacts. Modern windmills require certain rare metals as well as very large quantities of concrete. Thus its not clear that so many windmills could even be produced at all, let alone sustainably.
The lesson that I take from this sort of data is that our society is nowhere close to a green "magic bullet" that could meet US energy demand through sustainable means. Its important to remember that prior to the industrial revolution, wood, wind, and flowing water were the principle sources of energy harvested by human beings. The idea that we can return to those sources of energy while maintaining an industrialized society may be a dangerous fantasy. As no renewable looks remotely promising when faced with 100 quads of energy demand, tackling energy demand might be the only way to stave off the global ecological crisis. Politically, however, this move is almost certain to be deeply unpopular. There is no way to substantially lower US energy demand without great sacrifice, to say nothing of the developing world that seeks to emulate a US lifestyle. Thus platitudes about the need to invest in Green Jobs may sound reassuring, but as serious solutions to the ecological crisis, they are not remotely realistic. If a serious solution is desired, a more revolutionary course is needed.