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Indexed under : Earth Sciences

Wikenigma - an Encyclopedia of Unknowns Wikenigma - an Encyclopedia of the Unknown

Aviation contrails (climate impact)

Jet engines used in commercial and military aircraft typically burn carbon-based fuels (e.g. kerosene). When the fuel burns, a by-product is CO2, which is a potent greenhouse gas and contributes towards global warming.

It's relatively straightforward to calculate how much CO2 is produced by a flight. London to New York (one way) for example, typically generates around 350Kg per passenger [ source BA ]. But current atmospheric theory holds that the major climate-changing effects of aviation come not from the CO2 emissions, but from the the water-vapour condensation trails (contrails) which the aircraft often create when flying at high altitudes.

The water vapour almost instantaneously freezes to produce ice crystals which form a long and narrow 'cirrus' cloud. Depending on weather conditions and flight altitude, these clouds can disperse in just a few seconds, or, can last for several hours. They can be short, just a few meters, or may be tens, and sometimes hundreds of kilometers long.

The trails contribute to 'Radiative Forcing' (RF) in the atmosphere. In other words, like any other cloud, they can absorb or reflect incoming radiation from the Sun, and also absorb/reflect/ longer-wavelength light being radiated upwards form the Earth's surface.

Radiative Forcing in clouds is a poorly understood phenomenon (see Cloud forcingplugin-autotooltip__plain plugin-autotooltip_bigCloud forcing

Cloud Forcing - a.k.a. Cloud Radiative Forcing (CRF) or Cloud Radiative Effect (CRE) - refers to the complex effects that cloud cover has on the Earth's 'radiation budget' - in other words how much clouds affect global warming by the Sun.
) and the effects of contrails even less so. Nevertheless, as mentioned above, many climatologists currently think that the RF effects of contrails have considerably more impact than do aircraft CO2 emissions - though estimates greatly vary.

In addition to the RF effects of the trails themselves, the tiny ice crystals in the trails (along with other particulate matter such as soot etc.) can act as 'seeds' leading to the formation of entire sheets of 'normal' cirrus clouds - which can cover hundreds, or even thousands of square kilometers, and have their own, much larger RF contributions. (Note that 'cloud seeding' itself is currently not well understood. See : Cloud ice formationplugin-autotooltip__plain plugin-autotooltip_bigCloud ice formation

The formation of ice crystals have important implications for stratospheric ozone chemistry, cloud dynamics, rock weathering, and hydrate formation etc., however the exact mechanisms by which microscopic particulate matter 'seeds' ice-crystals are u…

Contrails are notoriously difficult to study, as their persistence and size is greatly affected by the prevailing weather conditions at the time of their formation. A unique opportunity for research arose during the COVID19 epidemic, when tens of thousands of flights were grounded for many months. During that period, several studies attempted to gauge the atmospheric effects of the lack of contrails.

But even that opportunity failed to help clarify the overall impact of the trails. One study, for example, found that the lack of trails did have measurable effects - but that the effects were opposite in Summer and Winter. Averaging the effects over a year, they more or less cancelled-out. (See :The climate impact of COVID-19-induced contrail changes Atmos. Chem. Phys., 21, 9405–9416, 2021)

To sum up, the effects of aircraft contrails on Climate Change are currently very poorly quantified :

There are significant scientific uncertainties remaining in quantifying aviation’s non-CO2 impacts on climate. The non-CO2 impacts arise from emissions of oxides of nitrogen (NOx), soot particles, oxidised sulphur species, and water vapour. These emissions result in changes in the chemical composition of the global atmosphere and cloudiness, perturbing the earth-atmosphere radiation budget. The net impact of aviation non-CO2 emissions is a positive radiative forcing (warming), although there are a number of individual positive (warming) and negative (cooling) forcings arising from respective aviation non-CO2 emissions, for which large uncertainties remain.“

Source : Safe and Sustainable Aviation - final report 30(4) European Commission

For extensive technical details see : Aviation and global climate change in the 21st century Atmospheric Environment, 43 (2009) 3520–3537.

Notes :

• There are many other polluting compounds caused by burning aviation fuel at high altitudes, including NOx, SOx, O3, HCs etc.. In general, their impacts are largely unknown.

• The climate implications for future aircraft which could use hydrogen as a fuel have not been accurately quantified - as they will also produce contrails from the water vapour which burning hydrogen with oxygen will produce (though without most of the other pollutants listed above).

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