Contrails are clouds caused by airplane exhausts, which geologists contend decrease daily temperature ranges on Earth. Following the 2001 World Trade Center attack, cancelled domestic flights triggered the first absence of contrails in decades. Resultant exceptional data capacitated causal inference analysis by propensity score matching. Estimated contrail effect was 6.8981°F.
Barron, Dean S. and Brown, Joe H.
"Contrails: Causal Inference Using Propensity Scores,"
Journal of Modern Applied Statistical Methods:
2, Article 18.
Available at: http://digitalcommons.wayne.edu/jmasm/vol14/iss2/18