Contrail Effects

A radiative forcing is something that disturbs the balance between incoming and outgoing radiation in the atmosphere. A positive forcing warms the surface, and a negative one cools it. Contrails, and the clouds which result from them (see the BBC link, mentioned in the previous post) are the largest radiative forcing associated with aviation. Despite their similar appearances, especially when longer lasting contrails begin spread out and resemble clouds, the radiative forcing for contrails is 9 times larger then for similar shaped, naturally formed cirrus clouds (Burkhardt and Kärcher 2011).

Contrails can be expected to have the same effects as cirrus clouds i.e to reflect incoming short wave radiation and to retain long wave outgoing radiation from the surface - this is due to the small ice crystals that contrails are made up of. Small differences in the properties between contrails and cirrus clouds for example the size of ice crystals however, creates some ambiguity and differences in the forcing effects of contrails. Contrails have a smaller vertical depth and are formed at cooler temperatures meaning that contrail induced cirrus clouds can form and persist high in the troposphere even when there are no natural cirrus clouds.

Ascertaining the global coverage of contrails is difficult as only young contrails, which still exhibit the classic line formation (see background picture to this blog) can be distinguished from natural cirrus cloud. It is impossible to tell from satellite imagery whether cirrus clouds have been induced by aircraft or are natural. It is also thought that global atmospheric circulation maintains a higher level of cirrus cloud over Europe in comparison to the USA, where there is more air traffic (Burkhardt and Kärcher 2011).

The aftermath of 9/11 presented a unique research opportunity in that there were no commercial flights in the 3 days that followed. Travis et al. (2002) compare the diurnal temperature range for the 11th - 14th September 2001 with that from 1971- 2000. They found there was an anomalous increase in diurnal temperature (1.1°C), which they partially attribute to the lack of contrails in this 3 day period. It has also been suggested that the presence of contrails, which ties up water vapour in the atmosphere, means that this vapour can not be used to expand natural cirrus clouds - and so can prevent their expansion and counter their own climatic impact (Burkhardt and Kärcher 2011). 

These examples show the difficulty presented in pinpointing the effects contrails have on radiative forcing and the difficulty of assessing their impacts due to their close correlation with naturally occurring cirrus clouds. 

Contrails as seen from the International Space Station. Credits ESA/NASA

Contrail Formation

Over the next few posts I aim to explore the effects aircraft are having on the environment. I'm going to start with contrails as these are perhaps the most telling signs that a plane is flying overhead, or if a plane has flown by, or if you're really struggling to find one out the window at this point they can be seen in the background picture to this blog! They represent a direct anthropogenic input into the atmosphere. There is extensive literature on them, with some of it being quite contrailversial. Today I am going to look at their formation. 

Contrails are linear ice clouds that form behind both propellor and jet aircraft flying in sufficiently cold air (-40 °C), typically found at high altitudes, i.e at the top of the troposphere. They have similar properties and structure to cirrus clouds, which also form at these altitudes. The length of time a contrail lasts for depends on the humidity of the atmosphere the aircraft is flying through. For instance they will be short lived and evaporate quickly (perhaps seconds) when formed in dry air, however more persistent with the possibility of developing into a cirrus cloud layer in air with relative humidity above ice saturation (Schumann et al. 1999).

Cirrus Clouds

The conditions needed to form a contrail can be described using thermodynamics though the Schmidt-Appleman criterion, which is a function of atmospheric temperature, pressure, aircraft fuel energy content, water vapour exhausted, and the aircraft’s overall propulsion efficiency. The 1999 IPCC report on aviation, which I spoke about in my last post, confirmed the reliability of the Schmidt-Appleman equation to predict the conditions of contrail formation. 

In accordance with the Schmidt-Appleman criterion, a contrail is formed when an increase in relative humidity occurs in an engine plume where warm, moist exhaust is expelled from the engine and mixes with surrounding sufficiently cold atmospheric air that enables humidity to reach liquid saturation in the plume (Schumann 2005). The water vapour attaches itself onto condensation nuclei, and in the process freezes into ice crystals almost instantaneously. 

Along with the expulsion of mostly water vapour and CO2 from the aircraft engine, nitrogen oxides, carbon monoxide, hydrocarbons, sulphuric oxides, organic material, chemi-ions, soot, and small metal particles from the mechanical erosion of the aircraft are also released, however in much smaller amounts. Some of these provide condensation nuclei which aid in the formation of contrails or increase the affinity of water vapour to them, for example chemo-ions which support the coagulation of small initial ice particles. Even if they are not released however, contrails will still form due to the condensation nuclei already present in the upper troposphere estimated to be around 10² - 10⁴ cm^-3 (Schumann 2005). 

Despite much research on the formation of contrails, there is comparatively less information on the way that aircraft affect already existing cirrus clouds, and the way in which contrails turn into cirrus clouds when the conditions allow for it. This is due to the microphysical properties of the ice crystals of cirrus clouds being difficult to investigate at  high altitudes. Where it was once thought that soot from ageing aircraft were more likely to result in contrails increasing in size to clouds (Schröder et al. 2000),  a more recent study concluded that reducing or even eliminating soot in the upper troposphere would still lead to contrail formation (Kärcher and Yu 2009). 

The following link is taken from the BBC and shows a series of images of aircraft contrails progressively turning into cloud over the UK. 

Finally, the following video shows a contrail coming out of an a380 (the worlds largest passenger airliner!) really well - although difficult to picture, it shows that not all of the contrail is coming out of the aircraft but that the water vapour created in the exhaust plume is also interacting with condensation nuclei already in the atmosphere. The video further emphasises the atmospheric conditions needed for contrail formation, it appears to have been following the aircraft for some time - and that tells us something about relative humidity of the atmosphere. 

A Long Flight Ahead

Through the course of this blog I aim to travel through the history of commercial aviation, the environmental impacts of air travel, how environmental concerns associated with it are perceived today, and how the aviation industry is responding to such concerns - this is a very loose structure and I'm sure other things will pop up (there may be some unexpected turbulence for instance).

I thought a fitting way into it all would be to share this video published by British Airways on their YouTube channel last week, where Willie Walsh, CEO of the International Airlines Group (AIG), the parent company of British Airways, discuses the  current challenges the aviation industry faces with respect to climate change.

He consolidates the point I made in my previous post on how despite only accounting for around 2% of CO₂ emissions  - aviation has become a "whipping boy" to current climate issues as it is recognised its contribution will only increase. He calls on governments to work together in developing a global scheme covering aviation which can be adhered to by all airlines, and that assistance (read funding?!) should be provided much like it is in developing sustainable fuels for cars, to the airline industry where for the foreseeable future, there will be a reliance on a liquid based, carbon fuel.  

Concerns on the impact air travel was having on the environment were only extensively addressed in the 1999 Intergovernmental Panel on Climate Change (IPCC) report on 'Aviation and the Global Atmosphere'. This was despite calls being made in the late 60s and 70s on the potential impact of contrails relating to aviation, however this was only on the effect supersonic aircraft had on stratospheric ozone (Lee et al. 2009) (which doesn't really help us!). 

In a short space of time our knowledge of the environmental impacts of aviation have increased greatly to become one of the more hotly debated environmental issues of the 21st century. 10 years on from the IPCC aviation report, airlines were to pledge to cut CO₂ emissions to half the 2005 level by 2050 - this was championed by none other than Willie Walsh with the hopes that if it was approved, it would be added to the agenda at the 2009 Copenhagen Summit, marking concrete efforts to address aviation and climate change. 

Introduction: Prepare doors for takeoff

As part of a university module I am taking called Global Environmental Change (which coincidentally has the same code as this flight of discovery I am about to embark on, GEOG3057), I aim to explore the issues surrounding air travel and the impacts it has on the environment. To me, commercial aviation is one of our greatest achievements as a humankind. It involves people from all over the world coming, working, talking, arguing, collaborating, engineering and building things together. It also involves flying through the sky, which is quite neat. 

Currently the aviation sector is responsible for 2% of global annual CO₂ emissions. This may seem insignificant in relation to the  2.2 billion people that gain from flying and the 32 million jobs the aviation industry provides, but it's actually quite a lot of  CO₂. In 2010 for example, aviation produced 760 million tonnes of it (and I feel guilty about having to work in the library, with the lights on when there's plenty of sunlight coming through the windows!).

Global air travel is showing no signs of slowing down despite increasing concern for the impact it is having on the environment, in particular on the atmosphere. Our reliance on air travel in the world today as a means of transporting goods and people for business or leisure in a timely fashion means that wherever possible, infrastructure will always be built to accommodate its growth. No better is this demonstrated than in China, where 82 new airports are being planned to be built by 2015 as outlined in the country's most recent 5 year plan.  

It is estimated that passenger air travel will grow at an annual rate of 5% from 1990 to 2015 with associated annual CO2 emission increases of 2-3%, and this is despite strict regulation and more environmentally friendly aircraft. And that is just looking at CO₂! Most of the growth in aviation will come from outside the US and EU considering that together they are currently responsible for 48% of CO₂ emissions from flights originating within them. 

Its a shame we can't all take the swanky AGV train to Australia

Over the next few months, I hope to address the environmental concerns associated with  aviation. I’m interested in what I will discover in light of the predicted growth of the aviation sector and what this may equate to, perhaps in my lifetime concerning the future of air travel.