Emissions from the burning of a jet engine’s fuel include carbon dioxide (CO2), nitrogen oxides (NOx), water vapor (H2O), hydrocarbons, sulfur oxides (SOx), and black carbon (soot).
These gases and particles interact at cruising altitude in the upper troposphere and lower stratosphere in different ways to that at ground level, altering the concentration of atmospheric greenhouse gases. Ozone (O3) forms from NOx, and condensation trails, or contrails, form when water vapor condenses rapidly into tiny water droplets that freeze as millions of ice crystals. Both ozone and contrails prevent heat from escaping the atmosphere, and add to the global warming effect.
All these emissions affect global warming either directly (CO2) or indirectly (NOx, SOx, black carbon); by known (CO2) or inexact (NOx, H2O) amounts; and either briefly (NOx, H2O) or for decades (CO2).
Radiative forcing is a measure of global warming. It’s the difference between the sunlight energy absorbed by the Earth and the energy radiated back to space. Greenhouse gases cause radiative forcing, altering the Earth’s radiative equilibrium by increasing the temperature of the climate system.
The Radiative Forcing Index (RFI) is the ratio of total radiative forcing to that from CO2 emissions alone. It is a measure of aircraft-induced climate change other than that from the release of fossil carbon alone.
The International Panel on Climate Change Special Report on Aviation in 1999 estimated an RFI range for aviation emissions of 2 to 4, with a best estimate of 2.7. And the IPCC Fourth Assessment Report in 2007 reported it as ranging between 1.9 and 4.7.
But the warming caused by contrails and induced cirrus cloudiness — the biggest unknown when calculating the RFI — is excluded from the calculation of these RFI estimates.
The effect of aviation emissions on induced cirrus clouds is usually ignored in reporting the overall warming of aviation emissions because of high uncertainty. While an accurate measure hasn’t been pinned down because the mechanisms remain poorly understood, the IPCC Fifth Assessment Report said confidence is growing that it is real. And Carbon Brief reports that “research has indicated their impact on global warming could dwarf that of CO2 from aviation”.
So there is a very real risk that aviation’s total contribution to warming is much more than currently reported.
We look forward to experiencing other cultures and experiencing what the travel industry loves to promote as “exotic” locations. Tourist numbers are growing. In 2017 the UN counted 1.3 billion international arrivals — an increase of 8% in European arrivals. In 2016 in Asia arrivals increased 9%.
But the package is no longer what it’s cracked up to be. Even from the get-go. As the planet warms, air travel will become bumpier, and severe storms that ground aircraft will become more frequent, as will extreme heatwaves making it too hot for safe aircraft use. There will be no thrill in joining the growing crush of tourists on St Marks bridge in Venice, or in staying in an Airbnb in Barcelona and many other cities, where the “sharing” accommodation economy and rising rents have driven the local people and urban culture out of the inner-city. What pleasure will there be in holidaying in a hurricane-devastated Florida, in taking that trade trip to China only to land in a flooded Guangdong, or visiting the Philipines, sitting there in Asia’s cyclone disaster alley? Air travel emissions will help drown the Maldives, Tuvalu and hundreds of tourist destinations this century.
Even Australian tourism destinations are at risk from rising sea levels, including Fremantle, Port Douglas, Noosa, Byron Bay, and the Gold Coast, as well as Sydney and Hobart airports. Scientists at the not-for-profit organisation Climate Central estimate that 275 million people worldwide live in areas that will eventually be flooded at 3°C of global warming.
By 2100, chronic flooding will be occurring in the United States from Maine to Texas and along parts of the west coast including Oakland, California. It will affect as many as 670 coastal communities, including Cambridge Massachusetts, Miami and St Petersburg Florida, and four of New York City’s five boroughs.
A recent study reported that “between 2009 and 2013, tourism’s global carbon footprint [excluding non-CO2 emissions such as contrails] has increased from 3.9 to 4.5gT CO2-e, four times more than previously estimated, accounting for about 8% of global greenhouse gas emissions … Driven by the desire for exotic travel experiences and an increasing reliance on aviation and luxury amenities, affluence has turned tourism into a carbon-intensive consumption category. Global demand for tourism is outstripping the decarbonisation of tourism operations, and, as a result, is accelerating global carbon emissions.”
The purple dotted curve •• IPCC 2 plots an emissions reduction pathway to zero carbon by 2050 (with a 66% probability of 2°C warming) from the IPCC’s Fifth Assessment Report
The red dotted curve •• IPCC 3 plots an emissions reduction pathway to zero carbon by 2060 (with a 50% probability of 2°C warming) from the IPCC’s Fifth Assessment Report
The yellow dotted curve •• INDCs plots an emissions reduction pathway to zero carbon by 2100 (with a 100% probability of 3 to 4°C warming) based on the INDCs announced at the UN Framework Convention on Climate Change, COP 21
Source: Global Commons Institute
It is not possible to consider as viable any emissions reduction path that allows unconstrained international aviation emissions.
Sir David King, the United Kingdom’s permanent Special Representative for Climate Change, presented the dotted line pathways to zero emissions, on the graph above, to the International Energy Agency at their Paris headquarters on 29 January 2016 as part of a presentation Towards decarbonising the global economy: the direction of travel after COP-21.
He used it to illustrate the likely warming outcomes of four emissions reduction pathways, including that of the INDCs announced as part of the Paris Agreement after the UN Framework Convention on Climate Change, Conference of Parties 21 (COP-21), in Paris in 2015. When aviation emissions, growing at the industry-projected rate of 5% annually, are added onto each of these pathways, the Global Commons Institute plotted the dashed lines leading to warming of over 5°C by 2100.