When the work is done. Human activity causes warmer weekend nights in some places (red) and cooler weekend nights in others (blue).

Weekend Weather

While politicians squabble over how much humans are to blame for the 0.5°C rise in Earth's surface temperature over the last century, a new study suggests that people cause temperature fluctuations on this scale weekly. Because the 7-day week is a human invention, climatologists say this provides proof of our impact on climate.

A "weekend effect" on the weather over large cities has been reported several times. Ironically, clouds seem more likely on people's days off. But because of weak statistics these studies have been considered "a little on the fringe and not trusted," says Piers Forster, a climatologist at the University of Reading, U.K. To test the idea more rigorously, Forster and Susan Soloman, a climatologist at the Aeronomy Laboratory of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, analyzed 40 years' worth of daily temperature readings from 10,000 weather stations worldwide. To reduce the influence of natural weather fluctuations such as heat waves and cold snaps, they examined the difference between the daytime maximum and the nighttime minimum--the so-called diurnal temperature range (DTR).

Sure enough, the DTR fluctuates, mainly due to shifts in nighttime temperatures, at many locations around the world during the weekend, even far away from urban centers, they report online this week in the Proceedings of the National Academy of Sciences. The changes differ from place to place: For example, the nights are warmer during the weekend around Beijing and Mexico City whereas they are cooler on weekends around Tokyo and Montreal. But the most important aspects of their results, says Forster, are that DTR fluctuates on a 7-day cycle--proving that human activity is the cause--and the weekly temperature changes are as big as global warming over the previous century. Forster suspects that weekly cycles of aerosol pollution from industry and automobiles affect temperature by changing cloud cover and precipitation.

The study shows that some aspect of human activity does interact strongly with the climate on shorter time scales, and it may help explain the marked acceleration in global warming in recent decades, says Dian Seidel, a climatologist at NOAA not involved with the current study. But first there are important questions to answer, says Seidel, such as why some places end up with colder rather than warmer weekend nights.

Related sites
Forster's home page
NOAA climate monitoring