One Million More Reasons Not to Fly?

Cumulative Effects
By J.B. Ruhl

Gazing out the window of an exit row seat on a plane trip back from a conference last week, I was reminded of a news item I saw in a recent issue of Scientific American ("Hot Trails," Sept. 2006, page 28). It turns out that in the days following 9/11, when jet traffic was basically zero, average daytime temperatures for the nation rose slightly and nighttime temperatures dropped more substantially. This finding supported a long held hypothesis that jet contrails reduce the temperature range by cooling the temperature during the day and heating it at night. Contrails are condensation trails that act essentially as thin cloud barriers that both reflect sunlight and block the earth's heat from rising. During the day the former effect is dominant, and during the night the latter effect is exclusive (because, obviously, there is no sunlight to reflect).

Armed with this proof, the ingenious legal mind might suggest ways of shifting flights from night to day as a way of countering global climate warming. We could auction off limited night slots, allow trading of night flight rights, etc. The net effect should be to take advantage of the daytime cooling effect of contrails and reduce their nightime heating effect.

Not so fast. The contrail effect is a classic "cumulative effects" phenomenon--the aggregate effects of many small individual similar events. It is an unfortunate term because it suggests that aggregation effects are linear. But is one contrail's effect of "x" on temperature simply aggregated, so that 100 contrails in a region equal an effect of 1oox? Most likely not. We already know that time matters. Space probably matters too. And the aggregation of effect in any time-space context may exhibit nonlinear properties, such that 100 contrails in a region over a defined time have an effect of 50x or 150x. Perhaps, for example, the number of daytime contrails is just below the threshold at which any more will tip the dominance over to the heat barrier effect. And maybe the number of nighttime contrails is well above the nonlinear threshold at which fewer contrails make a difference. Then our ingenious market-based solution would lead to exactly the wrong result. After all, all we really know is the effects of contrail numbers lumped around two points--zero contrails and status quo. It is amazing we know even that--it was the result of a one-time (we hope) disaster; it is not likely the FAA ever would have allowed anyone to test the hypothesis by banning all flights for a week. We clearly do not know what happens, therefore, between zero and status quo or beyond.

Cumulative effects problems are devilishly hard for law and policy to crack. They have way of growing on you, and once we are aware of them, they are very hard to study. It seems unlikely they behave in neat, linear fashion as the individual events aggregate over time and space. And it seems unlikely that the identified cumulative effects system has effects limited to what is being observed. For example, even if shifting contrails from night to day would help in the temperature range sense, maybe the increased day ground traffic would trigger some other cumulative effect threshold, and maybe the reduced job employment at night would trigger another. Who knows? How do we model it?

With this post, following a vacation of sorts from blogging, I start a new series on cumulative effects. How do they behave in contexts relevant to law and policy? What pressing problems of the day are, at bottom, cumulative effects problems? What devices can law employ to manage them? Comments are welcome along the way.