(XO+XKTbtal X

Figure 4.39 Ratio of active members of the chemical family to the total abundance of the chemical family. Values are from a two-dimensional model; solid lines represent the partitioning at 50°N, and the dotted line represents the partitioning at 8°N. (After Solomon el al. [140J, Figure 1.)

XO and X—making the chemical family more effective at destroying O,. One can see that virtually all of the inorganic iodine in the. stratosphere is in the form of the I and 10 radicals. At the other extreme is fluorine, which, owing to the stability of the HF molecule, has an ([FO| + [F])/ftotal F| ratio of approximately zero throughout the stratosphere—and is therefore unimportant for O, loss.

Ground-based measurements of IO absorption imply total stratospheric iodine abundances of 0.2 (+ 0.3/-0.2) pptv. This result, combined with recent laboratory measurements of the rate of the reactions of 10 with other halogen species, suggests that iodine chemistry is not responsible for the observed reductions in lower stratospheric O, during the last several decades [141j.

An aside: environmental engineering

As evidence mounts that human activity is changing the atmosphere, it is becoming increasingly clear that mankind has the power to intentionally engineer the climate. Is this a good idea?

Leaving aside the legal and ethical questions, imagine that we live on an Earthlike (class M) planet that is a bit too cool, and we want to warm it. We might try releasing CFCs. These molecules are nontoxic, have large infrared cross-sections, and absorb in regions of the spectrum (the "window" region, 8--12 uni) that the atmosphere is largely transparent to. As a result, even in parts per trillion by volume abundances, these species are potent greenhouse gases [142], and their release would be expected to warm the planet.

Up to the mid-1980s, it was generally agreed that this was the end of the story. Since then, however, we have learned much about the atmospheric effects of CFCs. As we have shown, the release of CFCs leads to an increase in stratospheric chlorine and a reduction of O, in the lower stratosphere. Os however is itself a powerful greenhouse gas. It turns out that the reduction in lower-stratospheric O, can offset much if not all of the anticipated wanning due to CFCs 1143,144J. Thus, it is not clear how much, if any, surface warming would actually result from the release of CFCs.

The lesson here is that any attempt at environmental engineering must be undertaken with care. Our knowledge of the atmosphere is often incomplete and unknown factors might not only cause the desired outcome to not occur: it might cause the opposite outcome to occur!

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