Following the terrorist attack on the World Trade Centre in September 2001, commercial flights were grounded across the United States for three days and three nights. This absence of aircraft contrails provided an unprecedented opportunity to study their contribution to global warming.
It has been argued that contrails reflect heat back from the Earth’s surface and atmosphere, but allow solar radiation to penetrate, thus resulting in net global warming. The difference between peak daytime and minimum night-time temperatures widened by 1% during those three days.
However, recent studies have found that a variation of 1°C in September is not unusual and could simply be attributed to low cloud cover. Indeed, climate models have suggested that 200 times the current number of flights over the USA would be needed to have a significant impact on temperature variations.
This does does not mean that contrails are no cause for concern, though. It is expected that air traffic will continue to grow by 2–5% per year and could triple by 2050, so the effect of contrails on climate change cannot be ignored and may prove significant in the future.