Climate-Change Redux

In a C&EN article on climate change, the average lifetime of carbon dioxide molecules in the atmosphere was said to be many decades, and in other articles and textbooks, the average lifetime for CO₂ is given as 100 years. The implication is that lowering the CO₂ concentration in the atmosphere is not a good measure of the time needed for changes in CO₂ concentration, and the 100-year estimate is not correct anyway.

On a timescale of years, the atmosphere can be considered well mixed, and for a well-mixed system, the average lifetime of any species is the holdup divided by the rate of removal. For CO₂, the atmosphere now holds about 700 gigatons (Gt) of carbon, and the natural process of photosynthesis and absorption in cold ocean waters removes an estimated 100 Gt per year. Without human activities, this would normally be balanced by emissions of 100 Gt per year due to decomposition of organic matter, desorption of CO₂ from warm waters, and volcanic activity.

Dividing 700 Gt by 100 Gt per year gives an average CO₂ lifetime of seven years. The incorrect figure of 100 years may have come from dividing 700 Gt by the estimated anthropogenic contribution of 7 Gt per year, due mostly to fossil fuel combustion. Although 7 Gt per year is a small part of the total emissions of 107 Gt per year, the CO₂ molecules released by burning fossil fuels have the same probability of removal as the naturally emitted molecules and an average lifetime of seven years.

A simple model that neglects the effects of other greenhouse gases and the complicated feedback mechanisms shows why a very long time is needed for major changes in CO₂ concentration of about 2 ppm per year (current value is 380 ppm). The other half is removed by a combination of absorption, adsorption, and increased photosynthesis, making the total removal 103.5 Gt per year. If our yearly CO₂ emissions could be halved, a difficult but achievable goal, the total emissions and removal might be balanced, and the CO₂ level would not change. If we could achieve the more difficult task of lowering our net CO₂ emissions to 1.5 Gt per year (80% reduction) and the natural processes of CO₂ emission and removal do not change appreciably, total emissions would be 2 Gt less than the current removal rate (103.5–101.5).

In one year, this would decrease the CO₂ concentration by only about 0.3% or 1 ppm per year (2/700 x 380). Since the CO₂ concentration is bound to increase for many years as limits in emissions are slowly adopted, it will take a long time to reach the new Intergovernmental Panel on Climate Change goal of 350 ppm CO₂.

Peter Harriott
Ithaca, N.Y.

Thank you for announcing ACS's stance on climate change (C&EN, July 27, 2009, page 5). Despite efforts by propagandists (climate-change deniers) to selectively pick data from larger studies and draw their own conclusions, organizations like ACS need to remain diligent and draw conclusions based on all available data.

Jake Neubauer
Warrensburg, Mo.