Dear Science,

When all those people avoided driving on I-5 two months ago, did that have any noticeable effect on the climate?


Yes, actually.

Which isn't surprising. When all airliners were grounded after the September 11 attacks, nighttime temperatures dropped—particularly near major airports. Without planes in the sky, a major source of emissions disappeared. Emissions from airplanes cause our nights to be warmer.

To answer your question, I could compare the number of cars on the road any given day with how hot it gets and look for a correlation. As everyone knows, correlation is not causation; a hotter day with more traffic doesn't tell us what causes what. But rarely are we given an opportunity like the major construction project that occurred from August 10 to 25. It's not every day you can simply shut down a large chunk of a local freeway and see how the climate responds.

What happened to the local climate when drivers were instructed to cut back on unnecessary trips, carpool, take the bus or train, telecommute, walk, or ride a bicycle? Science sat down for an edifying evening with Excel, the National Weather Service, and R (a free statistical package) to find out. On the weekdays between August 10 and 25, not much changed. But on both weekends, something interesting happened. The average and maximum daily temperatures were lower, below historical averages, and statistically significantly below those of all the other nonconstruction weekends (by a two-tailed Student's t-test with unequal variance) from August of this and last year. The changes were modest, the data sets tiny, and the data subject to the variation of the climate. (In other words, do I believe the change? Maybe. Maybe not.) Still, it appears reducing car capacity, given adequate warning, might actually cause the weekends to be cooler.

As much as the commutes to and from work, it's the little trips—to the grocery store, to the bank, to the movie-rental place, to the library, to the amateur porn festivals on Lower Queen Anne—that bloat local traffic. It's unreasonable to expect everyone to live within walking distance of work. But we should at least plan communities where these little tasks, these "unnecessary trips" that presumably vaporized on those construction weekends in August, do not require a car.

As any local skier or boarder can tell you, a few degrees makes the difference between epic snow or a slushy rain in the mountains. Might the Saturday drive to the slopes create the slush? Maintaining or expanding car capacity might come with an unexpected cost; we may lose our snowpack.

Warmly Yours, Science

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