History of Sorts

The grounding of all commercial aircraft within U.S. airspace for the 3 days following the 11 September 2001 terrorist attacks provides a unique opportunity to study the potential role of jet aircraft contrails in climate.

Dr. David Travis, a renowned climatologist, and professor of geography, played a key role in identifying and studying the phenomenon of global dimming, which refers to the gradual reduction in the amount of solar radiation reaching the Earth’s surface. His research contributed significantly to our understanding of how human activities, particularly the emission of aerosols and pollutants, impact the Earth’s climate system.

In a documentary by the BBC Dr. Travis noted:
“Around the twelfth, later on in the day, when I was driving to work, and I noticed how bright blue and clear the sky was. And at first I didn’t think about it, then I realised the sky was unusually clear.”

Key Findings of Dr. David Travis on Global Dimming:

9/11 and the Impact of Contrails:
One of Dr. Travis’s most important studies on global dimming came from an unexpected event—the grounding of flights in the U.S. after the September 11, 2001 attacks. For three days, no commercial jets were flying over the U.S., and this provided a unique opportunity to study the impact of airplane contrails (the cloud-like trails left by aircraft) on solar radiation.

• Discovery: Travis found that during the three-day period without contrails, there was a noticeable increase in the diurnal temperature range (the difference between daytime and nighttime temperatures). The range widened by about 1.1°C (2°F)—a significant finding in climate studies.
• Conclusion: Airplane contrails were trapping heat at night and reflecting solar radiation during the day, contributing to a dimming effect. Without the contrails, more sunlight reached the Earth during the day, and more heat escaped at night.

Aerosols and Global Dimming:
Dr. Travis’s work ties into broader studies on how aerosols, such as those produced by fossil fuel combustion, lead to global dimming. Aerosols and pollutants in the atmosphere act as a barrier to sunlight, scattering and reflecting solar radiation away from the Earth’s surface.

Global Dimming Trends: Global dimming has been observed worldwide, particularly during the mid-20th century, peaking in the 1970s and then showing a slight reversal in recent decades due to cleaner air policies in many industrialized countries.

Climate Change Link:
Travis’s research highlighted a complex interaction between global dimming and global warming. Aerosols responsible for dimming also mask the full impact of greenhouse gases by cooling the atmosphere slightly. As pollution levels decrease, the full warming effect of CO₂ becomes more apparent, leading to a more rapid rise in temperatures.

Implications:

• Temperature Rise: The findings suggested that without the cooling effect of pollution, the warming due to greenhouse gases could be worse than previously predicted.
• Weather Patterns: The cooling and dimming effect from aerosols can alter weather patterns, potentially influencing monsoons and droughts in various regions.
• Policy Impact: Dr. Travis’s work, along with other researchers’ contributions, has had implications for environmental and climate policy, emphasizing the need to address both greenhouse gases and aerosol pollution comprehensively.

Overall, Dr. David Travis’s findings have been crucial in understanding the complex interplay between human activities, pollution, and climate change through the lens of global dimming.

Sources

https://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml

https://journals.ametsoc.org/view/journals/clim/17/5/1520-0442_2004_017_1123_rviudt_2.0.co_2.xml

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