Heinburg, Energy Bulletin 2007
"In 1996 the European Environment Council said that the global average surface temperature increase should be held to a maximum of 2 degrees C above pre-industrial levels, and that to accomplish this the atmospheric concentration of carbon dioxide (CO2) will have to be stabilized at 550 parts per million (the current concentration is 380 ppm, though the addition of other greenhouse gases raises the figure to the equivalent of 440 to 450 ppm of CO2). But recent studies have tended to suggest that, in order to achieve the 2 degree cap, much lower CO2 levels will be needed. One study by researchers at the Potsdam Institute for Climate Impact in Germany concluded that—again, to keep the temperature from increasing more than 2 degrees C—the atmospheric concentration target should be 440 ppm of CO2 equivalents, implying that the atmospheric concentration of greenhouse gases will need to be stabilized at current levels. But, to make the challenge even more difficult, it turns out that the biosphere’s ability to absorb carbon is being reduced by human activity, and this must be factored into the equation; by 2030, this carbon-absorbing ability will have been reduced from the current 4 billion tons per year to 2.7 billion. Thus if an equilibrium level of atmospheric carbon is to be maintained through 2030, emissions will have to be reduced from the current annual level of 7 billion tons to 2.7 billion tons, a reduction of 60 percent. It is hard to imagine how, if that translated to a 60 percent reduction in energy consumption, it could mean anything but economic ruin for the world."