A Brief Note on Airplane, Clouds, and Global Warming

We recently linked to a new paper by D.S. Lee of Manchester Metropolitan University (U.K.), along with 20 coauthors, on global aviation and climate change. This seems very timely given that flying has been greatly affected by the coronavirus pandemic.

According to the United Nations’ International Civil Aviation Organization (ICAO), passenger revenue loss to U.S. airlines due to COVID-19 totaled $71 billion through September. Worldwide passenger numbers are down around 60 percent. The big legacy carriers are nearing fatal financial trouble, which is likely to provoke a major debate about the future role of air travel.

Part of that debate will be over the contribution of aviation to global warming. If Democrats win in November and take the Senate, there will be tremendous pressure from the left to commit the U.S. to “net-zero” carbon dioxide emissions, and every time the government will have a chance to intervene on transportation, climate change will come up.

Besides the carbon dioxide emissions associated with the combustion of jet fuel, aircraft artificially increase cloudiness when their contrails grow into large cirrus (ice crystal) clouds.

As D.S. Lee and coauthors show in their study, the net effect of these clouds is warming, particularly at night. This most evident under calm conditions on winter nights, when cloud decks greatly inhibit the rapid cooling that would normally occur.

Cirrus and other clouds reflect away a substantial amount of incoming solar radiation, but their net daytime effect is still a slight warming. At night, there is no sun, so more cirrus clouds only serve to enhance warming.

Lee and his colleagues show that the non-CO₂ warming effects from aviation comprise about two thirds of their total warming, which itself is about 3.5 percent of the total human contribution.

That means aviation itself—via cloud generation—is a contributor to warming beyond just the greenhouse gas (CO₂) effect, which will make rescuing the industry a large political football.

Noting that the contrail effect is mainly in winter and at night, is this necessarily a bad thing? A slight warming of the winter night will have effects on the emissions from carbon dioxide associated with heating your house. Lee et al merely conclude that “aviation emissions and cloud effects remain a continued focus of anthropogenic climate change research and policy discussions”. Indeed.