The climate impact of transport is surprisingly complicated

Emissions in action. Image: Getty.

The 2020s will have to involve some very big decisions about transport – the UK’s most polluting sector. The UK government’s response so far has been erratic, choosing to intervene to prevent the collapse of Flybe (Europe’s biggest regional airline) and give the green light for the high-speed rail project, HS2.

Decarbonising transport would eliminate 26 per cent of UK CO₂ emissions that come from how people get around. But Prime Minister Boris Johnson recently said that doing this poses “difficult and complicated” questions. On this, Johnson is almost certainly right.

The gilets jaunes protests against fuel duty rises in France show the delicate balancing act between decisive climate action and continued economic growth and convenience. But shouldn’t the government allow a regional flight operator to fail and invest in high-speed rail instead? The answer is not so simple.

Carbon footprints can be misleading

Aviation is one of the fastest growing fossil fuel consumers, with airlines contributing about 3.5 per cent of all man-made greenhouse gas emissions. This might seem small, but a single transatlantic flight from London to New York can grow your personal carbon footprint by as much as the entire heating budget of the average European.

At high altitudes, contrails – the white lines we see in the sky – are formed in the wake of aircraft. These high altitude clouds are too thin to reflect much sunlight, but the ice crystals inside them can trap heat. Unlike low-level cloud, which has a net-cooling effect, contrails contribute significantly to global warming, effectively boosting the aviation industry’s share of greenhouse gas emissions to around 4.9 per cent.

For the most part, the environmental benefit of high-speed rail is taken for granted. Most, but not all, research suggests that high-speed rail can offset emissions from aviation if it can attract enough passengers from alternative air routes. But the relative climate impacts of aviation to other modes of transport depend on more than just engines and altitude.

We can compare the emissions of different forms of transport by calculating the emissions produced by each one when moving one passenger one kilometre. This effectively compares how much CO₂ leaves each vehicle’s exhaust, but it ignores greenhouse gas emissions from the building and maintenance of the vehicles, the infrastructure – such as tracks, runways and airports – and the production of fuel.

The warming effects of different greenhouse gases happen over different time periods, from a few days of short intensive warming to centuries of gentle influence. In order to provide a common unit to measure the impact of different gases, warming effects are standardised over a given time period. The time period normally used is 100 years.

But if it were five years, the effect of contrails would account for more global warming than all the cars in the world. They raise the temperature of the atmosphere in short, intense bursts. On longer timescales, like 20 years, the short term effects are less important and make aviation look considerably better – with flying looking potentially less damaging than some cars over the same distance.

This is still not the whole story though. The energy inputs for different modes of travel vary. The direct burning of fossil fuels in engines, for example jet kerosene in aircraft, emits greenhouse gases. In electrically powered high-speed rail, operating the train produces no emissions, except from the fossil fuels used to generate that electricity elsewhere.

Developing HS2 will mean deploying stations, tracks and centres of communication, and they’ll need ongoing maintenance. These all need energy and material investments, which will create further greenhouse gas emissions through manufacture, transport, and use. That could increase the carbon footprint of rail by between 1.8 and 2.5 times, over just accounting for the operation of the trains. For aviation, the same infrastructure requirements are relatively small, and are responsible for a 1.2–1.3 increase, with road transport showing a 1.4–1.6 increase.

Comparing life cycles

A life cycle approach gives a better understanding of where emissions are occurring and compares transport modes on a much more level playing field. This helps us understand that most greenhouse gas emissions in air and road travel come from flying and driving, whereas in rail travel, the climate effects are dominated by those emissions produced building the infrastructure itself. Emissions from operating trains are generally lower because of the heavy reliance on electricity. But there are still emissions from the manufacture and maintenance of renewable energy technologies to consider.

All modes of high-speed travel come with a cost to the environment. Being able to accurately compare the energy requirements and emissions of different transport options is the first step towards addressing their climate impact.

Governments often try to encourage people to change their behaviour and reduce the number of flights they take. But in the case of HS2, the continued availability of regional flights means that only 4 per cent of drivers and only 1 per cent of aeroplane passengers are likely to change their behaviour.

It’s easy to point the finger at aviation and view rail as a low carbon alternative. But governments need to consider and carefully balance the true climate impacts of a transport project, in every phase of its development.

The Conversation

Laurie Wright, Senior Lecturer, Warsash School of Maritime Science and Engineering, Solent University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.


CityMetric is now City Monitor! Come see us at our new home

City Monitor is now live in beta at

CityMetric is now City Monitor, a name that reflects both a ramping up of our ambitions as well as our membership in a network of like-minded publications from New Statesman Media Group. Our new site is now live in beta, so please visit us there going forward. Here’s what CityMetric readers should know about this exciting transition.  

Regular CityMetric readers may have already noticed a few changes around here since the spring. CityMetric’s beloved founding editor, Jonn Elledge, has moved on to some new adventures, and a new team has formed to take the site into the future. It’s led by yours truly – I’m Sommer Mathis, the editor-in-chief of City Monitor. Hello!

My background includes having served as the founding editor of CityLab, editor-in-chief of Atlas Obscura, and editor-in-chief of DCist, a local news publication in the District of Columbia. I’ve been reporting on and writing about cities in one way or another for the past 15 years. To me, there is no more important story in the world right now than how cities are changing and adapting to an increasingly challenging global landscape. The majority of the world’s population lives in cities, and if we’re ever going to be able to tackle the most pressing issues currently facing our planet – the climate emergency, rising inequality, the Covid-19 pandemic ­­­– cities are going to have to lead the way.

That’s why City Monitor is now a global publication dedicated to the future of cities everywhere – not just in the UK (nor for that matter just in the US, where I live). Our mission is to help our readers, many of whom are in leadership positions around the globe, navigate how cities are changing and discover what’s next in the world of urban policy. We’ll do that through original reporting, expert opinion and most crucially, a data-driven approach that emphasises evidence and rigorous analysis. We want to arm local decision-makers and those they work in concert with – whether that’s elected officials, bureaucratic leaders, policy advocates, neighbourhood activists, academics and researchers, entrepreneurs, or plain-old engaged citizens – with real insights and potential answers to tough problems. Subjects we cover include transportation, infrastructure, housing, urban design, public safety, the environment, the economy, and much more.

The City Monitor team is made up of some of the most experienced urban policy journalists in the world. Our managing editor is Adam Sneed, also a CityLab alum where he served as a senior associate editor. Before that he was a technology reporter at Politico. Allison Arieff is City Monitor’s senior editor. She was previously editorial director of the urban planning and policy think tank SPUR, as well as a contributing columnist for The New York Times. Staff writer Jake Blumgart most recently covered development, housing and politics for WHYY, the local public radio station in Philadelphia. And our data reporter is Alexandra Kanik, whose previous roles include data reporting for Louisville Public Media in Kentucky and PublicSource in Pittsburgh, Pennsylvania.

Our team will continue to grow in the coming weeks, and we’ll also be collaborating closely with our editorial colleagues across New Statesman Media Group. In fact, we’re launching a whole network of new publications, covering topics such as the clean energy transition, foreign direct investment, technology, banks and more. Many of these sectors will frequently overlap with our cities coverage, and a key part of our plan is make the most of the expertise that all of these newsrooms combined will bring to bear on our journalism.

Please visit going forward, where you can also sign up for our free email newsletter.

As for CityMetric, some of its archives have already been moved over to the new website, and the rest will follow not long after. If you’re looking for a favourite piece from CityMetric’s past, for a time you’ll still be able to find it here, but before long the whole archive will move over to City Monitor.

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I want to thank and congratulate Jonn Elledge on a brilliant run. Everything we do from here on out will be building on the legacy of his work, and the community that he built here at CityMetric. Cheers, Jonn!

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Sommer Mathis is editor-in-chief of City Monitor.