of IR by CO 2 allows the warmer atmosphere to retain more water vapor, which is a
more-abundant IR absorber, generating a multiplier effect. A commonly accepted
radiative balance model predicts a temperature increase of 0.3°C watt m−2 if the
water-vapor multiplier is neglected, and 0.6°C watt−1 m−2 if it is included (it must be
included). It also predicts that doubling of CO 2 should increase the rate of heating
over all by 4 watts m−2, so at twice the pre-industrial level of 270 ppmv CO 2 (= 540
ppmv) we might expect warming on the order of 2.4°C. That is,
Fig. 16.4 Plot of time-series of Mauna Loa Observatory CO 2 concentration (ppm),
1958–2010. This is the widely disseminated “Keeling Curve” established by C.D.
Keeling and maintained by R.F. Keeling
(^) (Data provided by R.F. Keeling, Scripps Institution of Oceanography.)
(^) A tangle of possible feedback effects (the topic of a vast literature) are subsumed (or
ignored) in that. As you worry (that is the right word) about this, keep in mind that
light absorption by a component of a fluid mixture is negatively exponential with
respect to its concentration. That is, if we double CO 2 and get an increase “X” of IR
absorbance or associated warming, it must be doubled again to raise that effect to 2X.
At sufficiently high levels, all IR in the 15 μm band (and some other bands) will
convert to heat in the atmosphere, with the temperature rising enough to sustain
radiation balance at somewhat shorter wavelengths. In the fairly deep past, during the
Eocene, CO 2 was much higher than twice current levels, and life went on. It was
warmer on average, sea level was much higher, with inland seas in places like Kansas,
USA (essentially no ice was stored on land), and there were tree ferns and crocodiles
on Ellesmere Island. No level of CO 2 likely from human activity will be the end of
the world, just of the climate as people and all extratropical species are accustomed to