York U. physicist challenges climate change finding

A York University professor has ignited a controversy by challenging a supposed prime example of man-made climate change — jet condensation trails, known as contrails, act like clouds, cooling the Earth during the day and keeping it warmer at night.



Physicist William van Wijngaarden says he found no evidence to support this climate effect in Canadian temperature records for the contrail-free days immediately after the 9/11 terrorist attacks.


That contrasts with a 2002 study by U.S. researchers that concluded the temperature spread between day and night over the lower 48 states increased by 1.5 C over long-term averages between Sept. 11 and 14 in 2001, when commercial air flights were mostly grounded over North America.

Heralded as evidence from a "natural laboratory," the U.S. findings after 9/11 have been widely quoted as demonstrating short-term human impact on climate, since the birth of jet travel in the 1950s, as opposed to the longer buildup of greenhouse gases from burning fossil fuel.

temperature rise too large

  • The York researcher said he decided to double-check the U.S. findings because the claimed temperature rise was so large, almost equal to the global average temperature increase from greenhouse warming.