# Contrail conspiracy **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [September 23, 2010, 2:45am UTC](https://boards.straightdope.com/t/contrail-conspiracy/554730 "2010-09-23T02:45:58Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![dadums](https://avatars.discourse-cdn.com/v4/letter/d/838e76/32.png) [@dadums](https://boards.straightdope.com/u/dadums)\ **Post date:** [September 23, 2010, 2:45am UTC](https://boards.straightdope.com/t/contrail-conspiracy/554730/1 "2010-09-23T02:45:58Z")
I do not remember the particulars, but after 9/11 and there was not any air traffic, some lab did a test. If I remember correctly, they found the earth was warmer since there were no contrails. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [September 23, 2010, 3:18am UTC](https://boards.straightdope.com/t/contrail-conspiracy/554730/2 "2010-09-23T03:18:39Z")
Original column: [Are jet contrails the latest threat?](http://www.straightdope.com/columns/read/2556/are-jet-contrails-the-latest-threat) ---
**Author:** ![Frazzled](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frazzled/32/3368_2.png) [@Frazzled](https://boards.straightdope.com/u/Frazzled)\ **Post date:** [September 23, 2010, 3:22am UTC](https://boards.straightdope.com/t/contrail-conspiracy/554730/3 "2010-09-23T03:22:46Z")
What you’re thinking of is the theory of Global Dimming: > **[Global dimming](https://en.wikipedia.org/wiki/Global_dimming)** > > Lists > Categories > The first systematic measurements of global direct irradiance at the Earth's surface began in the 1950s. A decline in irradiance was soon observed, and it was given the name of global dimming. It continued from 1950s until 1980s, with an observed reduction of 4–5% per decade, even though solar activity did not vary more than the usual at the time. Global dimming has instead been attributed to an increase in atmospheric particulate matter, predominantly sulfate aerosols, as th... which was also featured on NOVA: > **[NOVA | Dimming the Sun | PBS](https://www.pbs.org/wgbh/nova/sun/)** > > In this companion Web site to the NOVA program Dimming the Sun, find a history of global dimming, learn about clean air technologies, discover how aircraft contrails might affect regional temperatures, read a personal essay by filmmaker David Sington... The upshot of this theory is that pollution, and especially jet contrails are blocking some of the light of the sun from hitting the Earth. 9/11 gave us some real data. Unfortunately I can’t remember what the name is but there was an impact on US temperatures when we were grounding planes that was not felt in Mexico and Canada where air traffic was normal. Global Dimming does _not_ change the theory of Global Warming. Climate change is occuring and there is a balance between the effects of carbon dioxide and pollution. The fear is what will happen when the scales tip. ---
**Author:** ![Frazzled](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frazzled/32/3368_2.png) [@Frazzled](https://boards.straightdope.com/u/Frazzled)\ **Post date:** [September 23, 2010, 2:48pm UTC](https://boards.straightdope.com/t/contrail-conspiracy/554730/4 "2010-09-23T14:48:18Z")
I was just looking at the PBS site I listed above and saw that there was an article posted about the effects of contrails on the climate: [http://www.pbs.org/wgbh/nova/sun/contrail.html](http://www.pbs.org/wgbh/nova/sun/contrail.html). This quote from the article refers exactly to what you were asking > [@](#): > > Another study that took advantage of the grounding gave striking evidence of what contrails can do. David Travis of the University of Wisconsin-Whitewater and two colleagues measured the difference, over those three contrail-free days, between the highest daytime temperature and the lowest nighttime temperature across the continental U.S. They compared those data with the average range in day-night temperatures for the period 1971-2000, again across the contiguous 48 states. Travis’s team discovered that from roughly midday September 11 to midday September 14, the days had become warmer and the nights cooler, with the overall range greater by about two degrees Fahrenheit. ---
**Author:** ![Wheelz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wheelz/32/5718_2.png) [@Wheelz](https://boards.straightdope.com/u/Wheelz)\ **Post date:** [September 23, 2010, 3:16pm UTC](https://boards.straightdope.com/t/contrail-conspiracy/554730/5 "2010-09-23T15:16:35Z")
Interesting, but what makes it a “conspiracy”? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 23, 2010, 3:32pm UTC](https://boards.straightdope.com/t/contrail-conspiracy/554730/6 "2010-09-23T15:32:49Z")
> [@Wheelz](#): > > Interesting, but what makes it a “conspiracy”? The conspiracy is about [chemtrails](http://en.wikipedia.org/wiki/Chemtrail_conspiracy_theory).