(^) The absorbing gases are water vapor, carbon dioxide (which absorbs IR in the 12 to
17 μm band centered at 15 μm), methane, nitrous oxide, and more. Methane and
nitrous oxide, while smaller contributors to the air mixture, are also stronger IR
absorbers on a per molecule basis than CO 2 . Methane in the air and retained in surface
ecosystems is mostly biologically produced: fermentation in cattle guts; bacterial
methanogenesis in marshes, rice paddies and sediments, etc. It is also a variable but
dominant (∼95 mole %) component of natural gas, some of which is lost to the
atmosphere in production, transport and use. There are several events that could result
in large methane releases, including widespread tundra meltdown and degassing
(adding a huge positive feedback to global warming) and seafloor disturbances that
could release methane–water clathrates buried just below surface sediments of many
continental shelves. Nitrous oxide is generated by fossil-fuel burning and naturally by,
among other things, phytoplankton metabolism.
(^) For the moment, increasing CO 2 is the main concern. Fossil-fuel burning (changing
carbon-isotopic composition assures us that coal, oil, and natural gas are the main
sources) has been and is increasing the carbon dioxide concentration in the
atmosphere (Fig. 16.4), an increase that has strongly accelerated since the middle of
the 20th century. There are additions from burning of biomass (wood, etc.) and
opening of soils. The overall increase is reduced by a third to half, relative to fuel
burned, by dissolution of CO 2 in the ocean, less so by recent reforestation and effects
minor in the short term, but significant on time scales of thousands of years, like
incorporation in soils and weathering of silicate minerals. Warming from absorption
ff
(ff)
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