Post-Sept. 11 skies offer clues to contrails, climate change
Released: May 10, 2002
Few people likely think twice about jet contrails, a common sight streaking across blue summer skies. But a new study may have us thinking of these billowy white trails literally as “pseudo-clouds” capable of altering the climate below.
David Travis, a climatologist at the University of Wisconsin-Whitewater, conducted a study that offers some of the first evidence of this long-held theory. The results appear to indicate that jet contrails (short for condensation trails) are leveling off “diurnal temperature ranges” in certain regions of North America, essentially making average days cooler and nights warmer than normal.
The contrails and climate connection was almost impossible to quantify before the tragedy of September 11 and the resulting three-day shutdown of all commercial airline traffic. The satellite data from September 11-13 provided scientists with a view of virtually contrail-free skies for the first time in a half century.
Travis’ research team, which includes Penn State University geographer Andrew Carleton and UW-Whitewater undergraduate Ryan Lauritsen, used satellite images to compare cloud cover from those three days to 30 years of data for mid-September. Then they reviewed daytime and nighttime surface air temperatures across North America collected from 4,000 weather stations.
The group then calculated the 30-year climate “norm” for those early-fall days, using temperature data from the same sources between 1971 and 2000. A final step was to calculate the “diurnal temperature range,” which is the difference between the warmest spike during daytime and the coldest point of night.
When compared against the 30-year record, the group found that diurnal temperature ranges on September 11-13, 2001 expanded as much as 3-5 degrees Fahrenheit. That was more than double any random year-to-year variation over that time.
More importantly, Travis says, they found sharp regional differences in temperature change — ones that correspond closely with where contrails actually form. Contrails, which are formed by a combination of below-freezing temperatures and high atmospheric moisture, are most common through the nation’s midsection, the northeast and the northwest. The biggest temperature changes, as much as 5 degrees, were in those regions.
“Scientists have been noticing unusual changes in diurnal temperatures for quite some time, but can’t explain why,” says Travis.
“We’re providing one possible explanation here. Maybe jet contrail coverage is one of the reasons for this shrinking temperature range.”
What’s the significance of all this? Travis says the findings may complicate the global warming debate, since in some regions contrails actually offset the temperature increases predicted in global warming models. The study also underscores the point that not all climate drivers are “global,” and factors like contrails can make a real difference on a regional scale.
The U.S. military first began contrail studies in the 1960s because of the fact that they give away fighter jet locations. Travis says there is no evidence contrails pollute, and some have argued that reduced diurnal temperature ranges could save heating and cooling costs in major cities.
But, Travis adds, it would be harder to find more tangible, visible proof that human activity affects climate. “Unlike greenhouse gases, we can all look up in the sky and see contrails and imagine how they might increase cloud coverage,” he says.
Past satellite studies have shown that contrails don’t form just skinny lines in the sky, Travis says. They can spread out and persist for up to 12 hours, sometimes covering more than half the sky when occurring in large groups. They are formed because airplane exhaust holds the basic ingredients of cloud formation, namely moisture and particulate matter. Some studies have suggested they can “seed” real clouds and trigger rainfall.
Travis first began studying contrails in the 1980s as an Indiana University graduate student under Carleton. “Like most people, I thought, ‘how can these skinny little vapor trails affect climate?’ But if you consider we have thousands of commercial flights a day in America, you have the potential for significant artificial cloud cover.”
- Brian Mattmiller,npa@uww.edu
