The Concept of Commitment - The Changing Arctic Environment: The Arctic Messenger - page 383

Appendix V
The Concept of Commitment
Climate scientists are confident that warming and sea level change will continue for centur-
ies because of their knowledge of the carbon cycle in the case of carbon dioxide and of the
lifetime of other Green House Gases (GHGs) in the atmosphere. For example, the lifetimes
for methane and hydrochlorofluorocarbon-22 (as a representative of these types of chemic-
als) is about 12 years and for nitrous oxide about 110 years. The concentration of any GHG
in the atmosphere depends on how much of the gas is being added to the atmosphere in com-
parison to how much is being removed. Therefore, for these substances (excluding carbon
dioxide), it is not difficult to calculate the atmospheric concentration that would result from
a reduction in the emissions of the gas. Stabilization and even a return to preindustrial levels
(if emissions are eliminated) are possible within decades or a few centuries for substances
with a short atmospheric lifetime.
However, this is not the case with carbon dioxide, for which an atmospheric lifetime
cannot be calculated. Unfortunately, carbon dioxide is by far the most important GHG. Until
the age ofindustrialisation, the concentration ofcarbon dioxide in the atmosphere was main-
tained over the long term at a roughly constant level (approximately 280 ppm) through the
carbon cycle, which involves gas exchange between the atmosphere, the ocean and the litho-
sphere and which is made up of slow and fast components. At present, the oceans are ab-
sorbing much of the “excess” carbon dioxide we are adding to the atmosphere - but not all.
About 20% will remain in the atmosphere for many millennia.
What about that 375 billion tonnes of carbon that humankind has injected into the at-
mosphere through our CO 2 emissions since the beginning of the Industrial Revolution? The
oceans are a very significant component of the carbon cycle. In fact, the oceans are thought