Time to kill the Contrail Multiplier Zombie

It is time for a zombie hunt. A dead science hypothesis from 1999 — that contrails multiply carbon emission — lives on. This living dead story is haunting newspaper articles and social media sites with a strange idea that a contrail caused by a jet flying above 25,000 feet has three times the effect on global warming than the jet’s actual carbon emissions. This zombie science is leading well-meaning consumers attempting to reduce their carbon footprint to drive when they should be flying. It is discouraging others from trusting the science of global warming at all. Science has proven beyond any reasonable doubt that contrails do not have any significant long-term effect on global warming.


Is everything you have learned about contrails and aviation’s contribution to global warming wrong? According to the science contrails have little effect and aviation is often the most environmentally friendly way to travel. 


In its 2016 environmental report ICAO (International Civil Aviation Organization, a United Nations agency), once again reviewed the best available science to dispel the idea that a “contrail multiplier” adds significantly to aviation’s contribution to global warming. But instead of rejoicing over the scientific clarity and technology advances that are transforming aviation into one of the most emissions friendly way to travel long distances, the opposite happened.

Despite all of the available studies, statistics and facts, social media is full of dire warnings about the need to stop flying immediately. The Flygskam[1] movement pushed by activist Greta Thunberg is perhaps the best-known example of this. In the alternate reality of science deniers, restricting or banning flying will somehow magically solve our climate crisis. Welcome to the post truth world, where opinion is pushed as fact, science is ignored, and emotion is used as a tactic to cloud reality for political or economic gain.

The Facts:

Today aviation produces only 2.5% of man-made CO2 emissions. As a percentage of overall emissions, aviation has grown by only 0.5% in 30 years. This growth is in step with the free market global economy that has made aviation the predominant form of long distance transportation. This growing prosperity has also, unfortunately, doubled man-made greenhouse gas emissions.

Planet earth is on fire and reducing man-made emissions is a universally accepted goal. Aviation is responding to the challenge with breathtaking technological advances and international cooperation. Already, new jet aircraft being delivered to the airlines are breaking the 2 litre/100 km per seat benchmark. Unprecedented international cooperation has forged a path to neutral aviation emission growth from 2020 on, and is working toward an absolute 50% reduction and net zero emissions by 2050. On the horizon is hybrid electric technology that promises a 95% drop in traditional aviation emissions. Other technologies, including alternate fuels, all electric flight and hydrogen fuel, combined with offset programs will move aviation to neutral emissions growth, followed by net zero emissions in the years to come.

The ICAO Carbon Emissions Calculator says it all. To help travelers, ICAO has assembled and updates a gold standard database on actual fuel burn data reported by airlines from member states. This enables users to calculate carbon emissions produced per seat on every airline route on the planet. If you want to know what the carbon cost is to fly between two cities today, the ICAO emissions calculator is the most accurate source. According to its results, if you have to travel long distances and want to do so with the smallest carbon footprint, in most parts of the world, please fly.

A full description of how ICAO calculates emissions can be found here on the ICAO website[2].

Enter the Contrail Multiplier Zombie

Despite ICAO’s amazing database showing the real world cost and the carbon efficiency of air travel, what is used most often by the press is completely different. For example, a recent article from The Guardian on aviation carbon emission ignores the ICAO real world numbers and instead used an “estimator” from a lobby group called Atmosfair. This is a not for Profit “climate justice” group that take donations, is involved with selling carbon offsets and has a distinctly political message. Unlike ICAO, Atmosfair, a German non-profit group, actively lobbies for donations, sells carbon offsets and publicizes its own version of aviation emissions calculations.

For a trip from New York to London, ICAO shows a carbon emission per seat of 335 KG, yet Atmosfair, and the many media sources that reference it instead of ICAO, shows an extraordinary threefold difference. A bit of digging on the Atmosfair website confirms that it does indeed multiply all aviation CO2 emissions by a factor of 3 due to the long-disproven “Contrail Effect”. The contrail zombie lives on!  Atmosfair justifies its continued use of the non-existent contrail effect multiplier by pointing to an old German government document from 2008 which is conveniently no longer available. The German Government, in fact, endorses ICAO and the latest science from the EGU shows only a 1.15 multiple.

The readers of The Guardian, or a number of other mainstream media websites and newspapers over the last few years are likely unaware that they have just been fed a political message rather than scientific fact. Journalist and readers alike are being hoodwinked by lobby groups like Atmosfair who are denying the science to promote anti-aviation messages as part of their fund raising efforts.

The success of science deniers in keeping the “contrail multiplier alive” is hard to understand. Is it lack of knowledge? Is it isolation from real information on aviation and climate change? Could it be the manifestation of a well-crafted anti-globalization or political narrative?  The answer could be simpler than any of that; wishful thinking and the inability to accept cold, hard science over more dramatic beliefs.

ICAOs database driven calculator side by side with the Atmosfair “Carbon Estimator”.  The zombie science of contrail multipliers is used to pump the emissions number and stands in stark contrast to ICAOs carefully managed database of actual CO2 created by aviation fuel consumption.  


It’s All About the Science

What is the scientific method that shows aviation to be the most carbon efficient way to travel? In science you ask a question, then establish a hypothesis, a theory. Next, through research and observation, you reach a conclusion that supports or refutes the hypothesis/question. Science deniers, in response to the question, seem to cherry pick the conclusion that they want to be true and ignore the rest of the science. Such appears to be the case with the contrail multiplier zombie. Other aviation emissions myths follow a similar “belief over science” theme. The fallacy that aviation will make up 25% of emissions by the year 2050 is a another good example of this.

The contrail issue is an example of the challenges and opportunities for misunderstanding of the scientific method in relation to climate change. In 1999 the IPCC was asked by ICAO (both of them are UN agencies) to put a box around the uncertainties that high altitude emissions and contrails have on aviation’s contribution to global warming. Given the uncertainties of the science at the time, the 1999 IPCC report proposed a 1.9 times multiplier be used on aviation carbon emissions until the science was better understood.

However, by 2005, James Hansen of NASA’s Goddard Institute for Space Studies in New York used observational data analysis to conclude that there is no multiplier.

In a PBS Nova Newsletter on the subject, Hansen was quoted as saying “I think our main concern about aircraft will be their CO2 emissions, not contrails, which are a pretty small climate forcing.”

Later work published by NASA agreed. Their consensus now is that: “contrails may not increase atmospheric warming much on a global basis.”

By 2016 ICAO had dropped any mention of the use of a multiplier for forced radiation heating by contrails, as it was not supported by scientific observations (ICAOs environmental report, page 101). This was a consensus built over decades of extensive scientific work. Along the way a number of ideas, including restricting flight levels to 25,000 feet or less were considered but refuted, as they would actually increase fuel burn and carbon emissions that would outweigh any long term effects from contrails.

Although contrails may only have a small long term effect, if any, to this day the science is not settled and new studies and scientific enquires continue. A recent study ( ACP – Contrail cirrus radiative forcing for future air traffic (copernicus.org) ) shows that contrails could increase three fold by 2050. This is due to more flights in regions where persistent contrails form and an increase in aviation traffic. Combined with decreasing emissions from improved jet engine efficiency, contrails could bump aviation’s contribution to global warming up slightly by the year 2050. This is a far cry from a three times multiplier and may be less that the warming effects creating by new road and rail infrastructure.  Ultimately any environmental assessment of passenger transportation that includes infrastructure ( roads/rail lines) and supply chains will show aviation to be a clear winner for most long distance trips.


If science is denied in support of a political agenda, these protestors could make the climate crisis worse.  It is time to put the science before the politics.


Yet science deniers still repeat the initial contrail theory from 1999, and additional, even more extreme hypotheses, while ignoring the updated science and newer, more accurate data. Today, decades of observations and scientific work, including from the FAA and NASA, are used by ICAO ( endorsed by the IPPC) to support its current position of not using a contrail multiplier.

But the Contrail zombie still stalks the internet and hoodwinks journalists and consumers to this day. Those who wish to deny aviation its leading role in safe long distance transportation continue to push a narrative that forced radiation heating caused by contrails multiples CO2 emissions many times over. At the same time they ignore force radiation multipliers from other forms of transportation, including the proven science that roads and rail lines can create heat island effects with long term effects.

Truth Matters

Science deniers and the their dependence on the contrail Zombie damage the fight against global warming. Misinformation and myths about aviation emissions are harmful. They break the trust between the public and the people and institutions that may unknowingly be repeating falsehoods. It distracts from real facts and clouds the science of climate change with emotionally driven misdirection. These falsehoods can interfere with public buy-in, international cooperation, and corrode support for the new technology and infrastructure such as new airports needed to fight global warming. It misdirects the behavioral changes needed to fight global warming.

Those that attack aviation by conjuring long dead zombie theories and myths need to be called out. Lobby groups that deny science and promote misinformation for profit need to be confronted. Journalists need to investigate, and look deeper into the data they are weaving into their stories.

The fight against global warming can only be won by believing the science, with international cooperation and by killing zombie science. The science says aviation is the most carbon efficient way to travel to most destinations. If you must travel, please fly and buy real carbon offsets. But most all, kill Zombies!



Related Posts:

Time to Get Real About Aviation and Global Warming – Friends of Pickering Airport

Pickering Airport and CORSIA – Opening the door to a clean aviation economy. – Friends of Pickering Airport

Aviation 25% of carbon emissions by 2050 is a Myth – Friends of Pickering Airport

Canadian Airports Will Be Carbon Neutral – Friends of Pickering Airport

Canada’s Carbon Neutral Aviation Challenge – Friends of Pickering Airport

Surprising Aviation facts in the age of Climate Anxiety – Friends of Pickering Airport

Can an EcoHack Cut Global Warming from Aviation in Half? – Friends of Pickering Airport

Greta’s Canadian Oops: Why Emissions Will Go Up If Flying Is Banned – Friends of Pickering Airport

The infrastructure paradox: How building Pickering airport can help fight global warming. – Friends of Pickering Airport




ICAO Carbon Emissions Calculator

The latest science on contrails from the EGU

An example of how Zombie Science is used: How your flight emits as much CO2 as many people do in a year | Environment | The Guardian

Methodology ICAO Carbon Calculator_v11-2018.pdf

James Hansen – Wikipedia

Environmental Protection (icao.int)  (see page 101 on contrails)

Concept for a hybrid-electric plane may reduce aviation’s air pollution problem | MIT News | Massachusetts Institute of Technology



Asphalt Roads Exacerbate Climate Change

Climate change and flying: what share of global CO2 emissions come from aviation? – Our World in Data

IATA – Climate Change

The Contrail Effect | NOVA | PBS

[1] Flight shame or flygskam is an anti-flying social movement, with the aim of reducing the environmental impact of aviation. It started in 2018 in Sweden and gained traction the following year throughout northern Europe. Flygskam is a Swedish word that literally means “flight shame”. The movement discourages people from flying to lower carbon emissions to thwart climate change.


[2] https://www.icao.int/environmental-protection/CarbonOffset/Documents/Methodology%20ICAO%20Carbon%20Calculator_v11-2018.pdf