The CO2 content of the atmosphere is controlled by exchanges of carbon between many reservoirs of various residence times, ranging from seconds to millions of years. Human respiration of carbon that comes from consumption of animals and herbaceous plants constitutes a rapidly cycling system (t<5 yr) that has very little net effect on atmospheric CO2 concentration. The primary mode in which human activities are influencing the carbon dioxide content of the atmosphere is by pumping fossil carbon from the geologically isolated fossil carbon reservoir to the atmospheric and surface carbon reservoirs, where it remains and cycles for decades to centuries before being sequestered once again into reservoirs with longer residence times such as woody biomass, soil carbon, and oceanic carbon. Human fossil fuel burning adds nearly 30 billion metric tons (IPCC AR4) of CO2 equivalent annually to the surface/atmosphere system that would otherwise be stuck in the million year timescale deep fossil carbon reservoir, a rate of around 60,000 times that at which carbon is added to the fossil reservoir. Human respiration (~2.5 billion metric tons according to Mr. Luder) merely recycles carbon that was already in that rapidly cycling surface/atmosphere system. We can breathe easy; it is primarily fossil fuel emissions of carbon (and to a lesser degree deforestation of old-growth forests) that science, business, and society need to work toward reducing, in order to mitigate the human footprint on Earth's carbon cycle and radiative energy balance. I commend ETH-Zürich, the Schweizer Bundesrat, and the Swiss people for taking such bold steps to address the problem of anthropogenic climate change, and hope that the world will take note of Switzerland's leadership on this issue.

Mark A. Miller - 12.11.09

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