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The carbon numbers: what is the full potential of HS2?

All transportation infrastructure projects emit CO2 when being built, but only trains earn that CO2 back, because when people switch from high-CO2 modes like cars and planes, the only question is “how long” until it’s all earned back and we start getting gains. Carbon Brief wrote a brilliant piece showing how it’s helped reduce flights and emissions around the world once implemented. In the U.K. we have mostly ignored high-speed rail as a concept, and focused on building more and more roads, which only create more CO2 emissions through induced demand, and never ever earn back the construction CO2.

GPEW have often said “just because we are against HS2, doesn’t mean we support RIS2”, but the amount of time, energy, campaigning, and writing poured into HS2 massively outweighs the effort spent fighting airport expansions or damaging futile road schemes, can only be seen as tacit support however you look at it. For example, according to The Green Party, “HS2 is an act of ecocide”, but when talking about RIS2 the strongest language The Green Party can muster is being “disappointed”.

In what world can we consider 343 miles of train car/truck/plane-busting railway track to be ecocide, but only muster mild disappointment about 4,000 miles of road that lock us into decades more of ever increasing car and truck usage? We’ll talk about airport emissions in this article, and we’ve already written about how RIS2 requires more land than HS2, causes more deforestation than HS2, and only ever increases carbon emissions unlike HS2, even if everyone’s fossil fuel car is quickly swapped for an electric car.

“RIS2 will make carbon emissions from the SRN go up, by about 20 MtCO2, during a
period when we need to make them go down, by about 167 MtCO2. This increase in
CO2 from RIS2 will negate 80% of potential carbon savings from electric vehicles on
the SRN between now and 2032.”

Transport for Quality of Life, The carbon impact of the national roads programme, July 2020

Let’s take a look at the projected carbon emissions for a variety of large infrastructure projects to provide context for the HS2 carbon emission numbers, to help people focus their efforts on the projects most deserving of attention.


Here are some of the worst offending roads and modifications:

ProjectUser Emissions over 60 yearsConstruction Emissions in tCO2e
M6 Junctions 21A-26 (SMART motorway)4,104,829450,918
A428 Black Cat to Caxton Gibbet3,313,499208,380
Lower Thames Crossing3,200,0002,000,000
M54-M6 Link Road1,956,00081,890
A303 Stonehenge1,955,509466,903
A12 Chelmsford1,826,288
A1 Morpeth to Ellingham1,378,28259,000

Let’s look at this information in another way, compared to the net emissions of HS2 after 60 years.

These are just a few of the 50 road projects planned for RIS2. 5 projects have refused appraisals, but of the 45 we have the carbon numbers for, this is a total of 31.8 MtCO2e in user emissions over the next 60 years, on top of 4.2 MtCO2e for construction alone. HS2’s annual operational emissions will be 4.2 MtCO2e, and the construction emissions will be 8-14 MtCO2e for both Phase 1 and Phase 2 (A and B). The operational emissions are far lower due to being an electric railway, which is wildly more efficient than road transportation even when operating at high-speed, even yes even when taking the growth of electric vehicles into consideration.

Let’s compare RIS2 in full to HS2 in full.

This happens because more roads just means more cars, more trucks, more traffic, more emissions. Increasing capacity for trains means less road use, by moving road freight to rail (something some supermarkets already do, but are unable to do much more of due to the existing railways being at capacity). This already shows that the “120 years until carbon-neutral” soundbite is nonsense, and depending on whether HS2 construction emissions are limited to the conservative “central case”, or the optimistic “stretch case”, it will be quicker than 60 years. How much quicker? That depends on policy, and how much we can reduce air travel with high speed trains along these popular routes. Instead of reducing air travel, currently several airports are trying to expand, and that of course is going to mean a huge increase of CO2 emissions for the transport sector.

Something we cannot afford if we want to hit our emissions targets. How bad will it be? Very bad. As domestic flights open up to get holiday makers to the beach quickly (something that we could do with electric trains if we weren’t constrained by capacity), here are some emissions statistics to consider.

ProjectMtCO2 between 2025-2050
Southampton Airport Expansion2.66
Leeds Bradford Airport2.37
Bristol Airport Expansion2.00

This source only shows a 25 year span which makes it hard to compare with HS2 and RIS2 which have reported their emissions over 60 years, but we can do some basic calculations to estimate the emissions over an equivalent timeframe assuming constant demand for flying, gradual improvement per the CCC “Balanced Net Zero” profile of air CO2 intensity, and no further improvement beyond 2050.

ProjectMtCO2 over 60 years (2025-2084)
Southampton Airport Expansion5.15
Leeds Bradford Airport4.58
Bristol Airport Expansion3.87
Source: “NEF Turbulence Expected” emissions, modified to estimate the impact over 60 years.

These numbers are only CO2, which makes them appear lower than they should be, but CO2-equivalent numbers were not available for all of them. Southampton Airport alone would be responsible for 44 MtCO2e over 120 years. When people incorrectly say it would take HS2 “120 years to become carbon neutral”, even if that were true (we’ve established already that it’s not) that would be far better than being 44 MtCO2e up over the same timespan.

Some people reading this will say “They all sound bad, let’s scrap the lot!” Whilst that is perfectly understandable, it does not solve the problem: people and things currently get around in an incredibly dirty way, and electric cars aren’t going to suddenly fix it. Even if petrol car was replaced with an electric car, the 7 hour drive or 5 hour train between Glasgow and London has millions of people choosing the 3 hour flight, but with HS2 completed in full we’d see the train times match those of a flight (probably quicker considering you end up in the city instead of an airport on the outskirts.) And yes, HS2 trains will go to Scotland, not just Birmingham.

High-speed rail has displaced air travel along routes that were previously more regularly flown, so it would be a little strange to expect things to be uniquely different in the U.K, especially when the Eurostar has put a huge dent in the flights between London and Paris.

If HS2 is priced competitively with flights, using airline style ticketing systems, people will naturally use the most effective method of transportation, eventually resulting in a 50% drop. If we pair that with a French-style domestic air ban along routes serviced by HS2, we’d make that impact bigger, and have the carbon emission savings sooner, which is the sort of urgent change we need to limit the devastating effects of the climate crisis.

When faced with this sort of information, a common response is “we can just stop moving around as much so we don’t need more trains, more airport expansion, or more roads” A global pandemic didn’t stop people flying, moving around the country, commuting to work, or needing to move physical goods around in trucks; road use is already up to 113% of pre-pandemic levels and long-distance leisure rail travel was back above 90% by September 2021. It’s unlikely that simply wagging a finger and saying “stop it!” is going to work particularly well, especially in a decade or two once COVID-19 is far behind us. We do need to reduce travel overall – maybe we can achieve a 20% reduction through amazing policy, bike lanes, LTNs, buses, trams, etc. For what is left we need to build the low-carbon efficient mode of transport we want people to use, to help transition trips that would have been cars, trucks and planes, over to this new intercity train service- or to the existing local train network that has now been unclogged – allowing for more passenger and freight trips on the existing lines.

HS2 is brilliantly in line with existing GPEW policies:

Policy About HS2 Impact
TR010 Sustainable modes, minimum environmental and community impact, safety, health Sustainable, less impact than alternatives, clean, safe, small land footprint
TR011 Reduce journeys and distance, use sustainable modes, improve sustainability, integrate modes HS2 is lower-CO2 than alternatives; this will improve as grid decarbonises.
TR030 Hierarchy of modes HS2 is rail: better than car or plane; makes space for rail freight: better than truck
TR031 Planning at most local level possible HS2 separates out local from long-distance trains, meaning the local ones can be devolved to Birmingham / Manchester / Leeds; local focus may mean national-scale investment is under-emphasised
TR033 Infrastructure investment should be based on whole life environmental impact HS2 is designed for a 12-+-year life. It pays back the investment
TR041 Shift from fossil fuel use HS2 is powered by rapidly-decarbonising grid electricity, whilst planes and trucks will be fossil-fuel powered for a long time.
TR047 Favour electric transport HS2 is powered by grid electricity, so gets cleaner as the grid does.
TR200 Majority of car journeys must move to public transport HS2 adds capacity for shift from car to rail.
TR233 Priority to expand rail use HS2 contributes to points c and e. It creates capacity to allow for b, c, d
TR240 Rail electrification HS2 is fully grid electric from day 1
TR242 Invest to expand rail network and services HS2 is a sensible investment in rail capacity – on its own lines and on the classic network
TR244 New N-S high speed line HS2 is a N-S high-speed line. It creates, not limits, local rail capacity.
TR330 Freight needs to move back to rail and water from road and air HS2 provides freight capacity by freeing up the classic rail network
TR350 Invest to transfer freight to rail HS2 is an investment in rail network capacity that can be used for more freight trains
TR351 Core rail freight network without loss of passenger capacity HS2 frees up key parts of the freight network.
TR510 Reduction in air travel HS2 offers alternative to major domestic air routes – particularly London – Scottish Central Belt.
TR511 Manage air demand HS2 offers alternative to major domestic air routes.
TR512 Substitute air with low-impact modes HS2 is 6-10gCO2/km vs air ( low-CO2 alternative to domestic air
TR532 Public transport to airports HS2 is public transport, reducing car travel to airports

Put simply, airports and roads will only increase carbon emissions, trains will decrease carbon emissions, so suggesting all these infrastructure projects are “just as bad as each other” is a case of false equivalence.

If you continue to campaign against HS2 that’s up to you, but demonizing this project to such extreme levels whilst barely mentioning airport expansion, and only being “slightly disappointed” by RIS2, has two horrendous outcomes. Firstly, it results in the sort of fear and confusion amongst the public that results in HS2 workers being assaulted. Secondly, it helps the road and air industry continue to burn the planet to a crisp with weak promises of jet-zero or other greenwashing about shiny future-tech that just keeps the status quo going, instead of providing the modal shift we need to decarbonise transportation.

[Article amended on 16th Sept 2021 to correct a typo and clarify that it’s long-distance rail travel that is back to 90% of pre-pandemic levels.]