Mostly False
After the 9/11 terrorist attacks grounded commercial air traffic, "there was a temperature drop while the airplanes weren't flying, for the week afterwards."

Michael J. Gardiner on Thursday, July 26th, 2012 in a debate

Michael Gardiner says moratorium on jet flights after 9/11 caused temperature drop over U.S.

In their first debate last month, three Republicans seeking Rhode Island’s 2nd Congressional District seat -- Michael J. Gardiner, Michael G. Riley and Kara D. Russo -- were discussing the role fossil fuels should play in a national energy strategy when the issue of climate change emerged.

When Russo questioned whether climate change even existed, considering there is "so much . . . propaganda about global warming," Gardiner, a Providence lawyer, said human activity had "clearly" affected climate.

As an example, he pointed to the three days after the 9/11 terrorist attacks when all commercial planes were grounded over the United States.

"We did see in 9/11, we did see I believe there was a temperature drop while the airplanes weren't flying, for the week afterwards, that there was an average temperature drop," he said.  "So I think clearly there are man-made effects."

We wondered whether Gardiner knew of some new research confirming the much-discussed theory in the 11 years since the attacks.

The theory caught wide attention a year after the attacks when University of Wisconsin-Whitewater Prof. David Travis and two colleagues published a study -- here’s a summary -- that found that the absence of jet vapor trails in the skies during the three-day flying hiatus had a climatological effect.

Travis and his colleagues had collected the daily high and low temperatures for those days from 4,000 weather stations across the country and compared the data with the same three-day period in each year from 1971 through 2000.

They determined that without contrails acting as artificial clouds trapping in heat (and to a lesser extent blocking the sun) the average daily temperature range over the continent had widened by about two degrees. In other words, the average low temperature dropped a bit while the average high temperature rose slightly.

Since then other studies have questioned whether the lack of contrails in the skies during those three days after the attacks actually played any role in the widened temperature variations that Travis reported. They have suggested that the temperature change Travis found might have been a coincidence -- the result of unusually clear weather during the period the jets were grounded.

When contacted by PolitiFact Rhode Island, Gardiner said he was relying on past news stories when he made his claim that the temperature had dropped while air flight was suspended.

After reading those news stories again, Gardiner acknowledged in an e-mail that he had misspoken during the candidates’ debate; Travis’ study showed a widened daily temperature range in both directions following the terrorist attacks -- not just a drop in temperature -- and, said Gardiner,  "indeed there is controversy surrounding the observations . . . "

Said Gardiner, "I do a better job of illuminating a path for discussion and thought than 'nailing' a precise scientifically unassailable statement."

Still, while Gardiner stated incorrectly what climatological event had occurred in the days after 9/11, his point for raising it was well-taken.

Gardiner’s underlying point that humans are affecting the climate is accepted by the vast majority of scientists. But he misstated the conclusions of a study that itself has been called into question. We rule his statement Mostly False.