Fig. 9a: Decay of a small pulse of CO2 added to today's atmosphere, based on analytic approximation to the Bern carbon cycle model (Joos F et al., An efficient and accurate representation of complex oceanc and biospheric models of anthropogenic carbon uptake, Tellus, 48B, 397-417, 1996; Shine et al., Alternatives to the global warming potential for comparing climate impacts of emissions of greenhouse gases, Clim. Change, 68, 281-302, 2005, see equation given in figure). In this approximation of the carbon cycle, about 1/3 of the anthropogenic CO2 emissions remain in the atmosphere after 100 years, and 1/5 after 1000 years.

Processes not accounted for by the model, including burial of carbon in the ocean sediments from perturbation of the CaCO₃ cycle and silicate weathering, remove CO₂ on longer time scales (Archer, 2005), but for practical purposes 500 - 1000 years is "forever", as it is long enough for ice sheets to respond and it is many human generations. Indeed, [this equation] should be viewed as an approximate lower bound for the portion of fossil fule CO₂ emissions that remain airborne.

Integration over the period 1750 - 2005 of the product of this equation and fossil fuel emissions yields a present airborne fossil fuel CO₂ amount of approx. 80 ppm (Kharecha PA, Hansen JE, 2007).

The observed atmospheric CO₂ increase is approx. 100 ppm, the difference presumably due to the net consequence of deforestation and biospheric uptake not incorporated in the carbon cycle model, and, in part, imprecision of the carbon cycle model. This calculation provides a check on the reasonableness of the carbon cycle model approximation of [the given equation] ...

Archer D, Fate of fossil-fuel CO₂ in geologic time, J. Geophys. Res., 110, C09S05, doi:10.1029/2004JC002625, 2005
Kharecha PA, Hansen JE, Implications of "peak oil" for atmospheric CO₂ and climate, 2007 arXiv:0704.2782v3 [physics.ao-ph] (in cache).

Source: Fig. 9a and page 2302 in Hansen J. et al., Dangerous human-made interference with climate: a GISS model-study, Atmospheric Chemistry and Physics, Vol. 7 (2007), pp. 2287-2312 (in cache).

"... estimates of the characteristic 'residence time' of a molecule of carbon dioxide in the atmosphere involve a complicated mélange of factors, leading to the conclusion that

• although almost half of newly added carbon dioxide molecules remain for only a decade or two,
• roughly a third stay for a century or more, and
• fully one fifth for a millennium

This is why the residence time of such molecules is generally characterized as a century."

Source: Robert M. May (Former President of the Royal Society (2000-2005), Chief Scientific Adviser to the British Government (1995-2000), Oxford, England), How Long Will They Stay (in cache) -in repsonse to Freeman Dyson, "The Question of Global Warming", New York Review of Books (NYBooks), Volume 55, Number 15, October 9, 2008 (in cache).

• Letters to the Editor of the New York Review of Books
  • D. A. Pratt, reply by Freeman Dyson, The Brief Life of a Molecule July 17, 2008 (in cache)

    Freeman Dyson:
    You can get a rough estimate of the lifetime of a carbon dioxide molecule in the atmosphere by dividing the total mass of carbon in the atmosphere by the mass that is absorbed in photosynthesis by land vegetation each year. I do not know the exact numbers. Roughly,

    • the total atmospheric carbon is eight hundred gigatons and
    • photosynthesis absorbs seventy gigatons of carbon per year,

    giving a lifetime of about twelve years. This is the average time that a carbon dioxide molecule spends in the atmosphere before it is absorbed by a land plant. I used this lifetime to estimate how long it would take for a major change in the land vegetation to produce a major change in the atmosphere.

    This calculation completely ignores the ocean. In reality the flow of carbon dioxide into the ocean is about twice as large as the flow into land vegetation. So the lifetime of a carbon dioxide molecule in the atmosphere is really only about five years. Between two jumps into the land vegetation, an average molecule jumps twice into and out of the ocean.
    I ignored the ocean in my estimate because I was considering only land management and not ocean management as a way of controlling carbon dioxide in the atmosphere. It is possible that ocean management may turn out to be technically more effective, but land management is politically easier because each country owns its own land.

  • Robert M. May, reply by Freeman Dyson, How Long Will They Stay? October 9, 2008 (in cache)
  • William D. Nordhaus, Dimitri Zenghelis , and Leigh Sullivan, reply by Freeman Dyson, ‘The Question of Global Warming’: An Exchange September 25, 2008 (in cache)
• Michael D. Lemonick, Freeman Dyson Takes on the Climate Establishment, Interview, Yale Environment 360, 2015 (in cache).

  Princeton physicist Freeman Dyson has been roundly criticized for insisting global warming is not an urgent problem, with many climate scientists dismissing him as woefully ill-informed. In an interview with Yale Environment 360, Dyson explains his iconoclastic views and why he believes they have stirred such controversy.

• The Anthropogenic Greenhouse Era Began Thousands of Years Ago, Climatic Change 61: 261-293, 2003.
• Debate over the Early Anthropogenic Hypothesis, 2005.
• How Did Humans First Alter Global Climate?, Scientific American, vol.292, No.3:46-70, 2005.

  A bold hypothesis suggests that our ancestors' farming practices started warming the earth thousands of years before industrial society did.