My carbon footprint: flights, damned flights and statistics

A lesson that archaeology has taught me repeatedly is that you can’t solve a problem if you don’t know how big it is. This goes for finishing a PhD, cooking dinner for an entire field team, or getting through the last crates of lithics before the museum closes down for the summer. You need to know how many words you need to write, how many people you need to feed, and how many stone tools are in those boxes.

One problem that I’ve been working on lately is how to reduce my personal contribution to the climate mess we find ourselves in. To do this effectively, I needed to work out my carbon footprint. I knew there was space for reductions but I didn’t how exactly how big my footprint was or what exactly it was made up of. So I decided to find out.

In particular, and because I’m also keen to do what I can to decarbonise my work practices, I wanted to know just how many flights I’ve taken as a PhD student and researcher in archaeology. I started my PhD in October 2011 and I’ve been working as a postdoc since the beginning of 2015, so I decided to do an analysis of my flights for the period January 2012–December 2018. I wasn’t sure how many flights I’d taken in that time. I guessed maybe four or five return flights per year, on average, so maybe seventy flights altogether for the period in question. But I knew I could check, because all my flight confirmations are saved in my email account. I sat down one Sunday in late November and I started collating the information into a simple spreadsheet: date, points of departure and arrival, purpose. Where I had stopovers, I counted each leg of the flight separately.

It turns out I’m not very good at guessing how many flights I’ve taken, because the total was 152. One hundred and fifty two individual flights over seven years, or an average of nearly 22 flights per year.

The next step was to add up the carbon footprint of all of these flights. The first problem I met is that it is extremely difficult to find out how to calculate a carbon footprint accurately. There are many calculators online and they vary wildly in the results they give. Part of this is down to the existence of major complicating factors when calculating the global warming footprint of a flight, including the fact that CO2 is not the only pollutant emitted by airplanes, and that pollutants that are emitted high up in the atmosphere have a different effect from those emitted at ground level. For the purposes of carbon footprint calculation, this is often accounted for by what is called a radiative forcing index (RFI; here is a good explainer by Carbon Brief). There are numerous ways of applying this, which explains much of the difference between different carbon footprint calculators – some don’t apply an RFI at all, others do it only if you ask them to, and the methods and factors used vary significantly.

In the end I settled on Atmosfair to calculate my carbon footprint. The RFI calculation methods they use seemed well-reasoned; similar methods are also used by Other calculators often gave much lower results (especially ICAO), but underestimation seemed like a bigger risk than overestimation.

To cut to the chase, this is the total footprint I calculated for those 152 flights: 47,844 kg CO2-equivalent. Nearly 7 tonnes of CO2-eq per year over the last seven years, just from flights.

That seemed like a lot (it is a lot) but I wanted some context, so I decided to estimate what the rest of my carbon footprint looks like. Again, this is not straightforward, but here goes. I rent a small flat with my partner near the city centre in Bordeaux. We don’t have children or pets. We don’t have a car. We cycle everywhere. We recently went mostly vegetarian, eating meat a maximum of three or four times a month. We are not avid shoppers. Using our energy bills, I calculated the carbon footprint for gas (4500 kWh/year) and electricity (1200 kWh/year) in our flat and it’s about 500 kg for each of us (that’s pretty low, thanks to France’s nuclear energy). Our transport footprint apart from flying is negligible and I’m ignoring it here (it just includes some occasional bus or train trips and very rare taxi trips). The average dietary footprint in France has been estimated at about 1,200 kg – that may well be an overestimate in our case since we’ve mostly cut out meat, but let’s leave it as it is. Let’s allow 2,600 kg per year to account for clothing, services and buying manufactured products (based on the averages for France as found in SI Table 3 of this paper). That all adds up to a personal estimated carbon footprint excluding flying of ca. 4,300 kg per year. It hasn’t been exactly the same for the last seven years – when I lived in the UK my energy footprint was almost certainly higher, for example – but it’ll do as a point of comparison.

So my carbon footprint is made up of something like 6,800 kg CO2-eq per year on flights and around 4,300 kg on everything else. In other words, flights make up about 60% of my global warming contribution for the last seven years. If I want to reduce my carbon footprint, there’s nothing else for it – I have to cut my flying.


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So what does this flying consist of? I’ve put my flights into three categories: Work, Holidays, and Mobility.

Work includes all the flights I’ve taken for study and research in archaeology: to fieldwork, conferences, meetings, and so on. This added up to 90 flights, and 55% of the calculated carbon footprint for flying. I’ve done a lot of work in Russia, so it includes many flights to Saint Petersburg and Moscow from the UK and France, but that’s the furthest I’ve flown for work: all my conferences and research trips have been within Europe.

Holidays includes flights for holidays (rare) and also to visit family and friends outside of my home country of Ireland (more frequent). This made up 20 flights, although they also included the only very long-haul flights I’ve taken in the last seven years, and contributed 32% of the flights footprint.

Mobility might seem like a slightly strange category, but more on it later – it includes all the flights I’ve taken to go back to Ireland for personal and family reasons. This included 42 flights, and made up 13% of the flights footprint.

There was a bit of messing round involved where I wasn’t sure how to allocate a flight from a multi-leg journey with several stops, but to be honest it didn’t make much difference to the numbers in the end – one more short-haul flight in one or another category doesn’t change the overall results.

This is a barplot of the CO2-equivalent in kg per year and per category:

Graph of my flight-related carbon footprint for 2012-2018, divided into the three categories of Work, Holidays and Mobility
My estimated carbon footprint from flights for 2012-2018


You can see that work usually makes up the biggest category although in 2014 and 2015 the holidays category becomes a lot bigger. You can also see that the work footprint is a lot smaller in 2014 and 2018, and that the mobility footprint is bigger in 2016–2018 than in previous years.

I’m going to discuss the Holidays and Mobility categories briefly, and then go on to Work.

Holidays make up 32% of the flight-related footprint. The reason my Holidays footprint is so big in 2014 and 2015 is because I took a trip to Australia in 2014–2015 to go and see my family out there after I’d submitted my PhD. That return flight to Sydney has a shocking footprint: over 10,000 kg CO2-eq, or 22% of my entire total for flights for the last seven years. The holidays footprint for 2012 and 2013 is very low because I was working on my PhD and I was broke. It’s been bigger in the last few years because I’ve gone on a few holidays within Europe and to visit my partner’s family. The potential for reducing this part of my footprint is high: both my partner and I are keen to shrink our footprint as much as possible, and this is an obvious place to make changes.

I named the Mobility category after the “Mobility Allowance” that comes with my current postdoctoral funding: these journeys make up 13% of the overall CO2-eq footprint for my flights, although for 2018 they actually constitute the biggest single category. They include flights back to Ireland for Christmases, weddings, funerals, and just to see my family. The reason that their contribution is bigger in 2016–2018 is because I moved to France from the UK in early 2016 and so the carbon footprint of the individual flights increased. These flights are the result of, basically, academic mobility. This is what happens to your carbon footprint when you’re living in a different country from your family but you still fly home to see them. Many people, of course, find themselves much further away from their family than this. And looking at this part of my footprint makes me think. On a personal level, I can cut this part of my footprint: it’s not that difficult to get to Ireland from France by train and ferry (although it is more expensive and much slower). But to take a bigger perspective: I’ve taken on average three return flights a year to go see my family over the last seven years. And that is mirrored many thousands of times over by other researchers and students living away from their families across the world. There is a massive ongoing push for increased mobility in academia from funders and employers, and I don’t deny the intellectual and career benefits it’s given me. But if we are to keep doing this, how can we make this aspect of academia carbon neutral?


* * *


The final category is work.

Overall, the 90 flights I’ve taken for work make up a majority of my flights-related footprint: 55% in total. It’s pretty consistent over the years, except for 2014 and 2018. A little bit of personal history is necessary here to make sense of this. From October 2011 to November 2014 I was working on my PhD. In 2012 and 2013 I took a lot of research trips to Russia, but I spent most of 2014 writing and so didn’t travel much. I spent 2015 working as a postdoctoral research assistant on a large project based in Oxford which involved quite a lot of travel around Europe. I then moved to Bordeaux and spent 2016 and 2017 as a postdoctoral fellow, still travelling, but with very limited funding so I tended to always pick the cheapest travel mode. In early 2018 I started a new fellowship, again in Bordeaux. I actually didn’t take any research trips in 2018 because I was already fully occupied working on papers and desk-based research for my new project. My new fellowship has a very generous budget for research expenses, which means that I now don’t have to take the cheapest options for travel, and so I could use the train to get to conferences in Barcelona, Liège and London. The only flights I took for work in 2018 were in fact to get home from Liège because of train strikes in France. This is why my footprint is so much lower in 2018.

The flights I’ve taken over the last seven years were for various purposes: fieldwork (mainly in Russia), conferences, and occasional meetings and training sessions. Some of them were incredibly useful and vital to the work I’ve done. Some of them weren’t. For some, I could in principle have taken the train without too much effort.

These flights make up a huge part of my footprint, and they represent my personal contribution to, well, carbonising archaeology. So I wanted to know if I could have done things differently. I’ve gone through my work trips and divided them into three categories. The first category contains journeys that I think were worth taking, where I learned a lot and collected useful data, and that would have been a significant hassle to do without flying – all of these were research trips to Russia. The second category contains journeys that I think were worth taking but where I could happily have taken the train: this includes most of the conferences I’ve gone to. The final category contains journeys that, frankly, I don’t think were worth taking, and that I could have skipped without any adverse effect on my or anyone else’s work. In general I could have guessed this before organising the trip, if I’d thought about it hard enough.

Of the 90 flights I took for work over the last seven years, in retrospect I think only 44 of them were essential. I could have cancelled 26 of them without losing out on much, and I could have replaced 20 of them with train journeys.

I’m leaving out 2018 from the following calculation because I already used low-carbon transport for most of my trips that year. But if I recalculate my carbon footprint for the imaginary world where I cancelled some of my work trips and took the train for others, then my work footprint for 2011–2017 drops significantly – in fact, from 26,212 kg CO2-eq to 16,018 kg CO2-eq, or by 39%. That’s a big saving. And that’s without doing anything particularly radical, such as taking the train to Russia rather than flying, and it’s without cancelling truly useful research trips or conferences. If anything I would probably have benefited from travelling the way I do in this imaginary world: I would have wasted less time on travel that wasn’t useful, and I would probably have planned the trips that I did take more carefully.


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For me, this has been a really useful exercise. It took me a long time to write this blogpost, not just because it involved a lot of research but also because it took me a while to get my head round the results. When I first calculated my flight-related footprint I was taken aback at how much bigger it was than I thought, and frankly pretty ashamed at my profligacy. Later I started to see it differently. I can’t change the past. But I can change my behaviour from now on.

I hope that what I’ve done here can help start some conversations and provoke other people into thinking constructively about their own carbon footprints. Recently I’ve had quite a few conversations and Twitter interactions with friends and colleagues about work-related travel where people tell me that they feel really guilty about it. Please don’t. Everyone has different circumstances, and we all have to weigh up our travel decisions for ourselves (if they’re even our decisions to make, which they aren’t always). Feeling guilty just isn’t useful. Lots of us are in situations where there just aren’t straightforward ways to fulfil our work requirements with low-carbon travel, and lots of us have commitments to field projects in farflung places that are impossible to get to overland.

On the other hand, I think that many of us can cut our travel-related carbon footprints and I think it is worth us all thinking about what we can do. If you want to do this I strongly recommend taking some time to calculate your past travel-related footprint (try doing it just for the past year or two), or starting a simple spreadsheet of journeys that you take from now on. For calculating the carbon footprint of flights I do recommend Atmosfair, although I’d be interested to hear if anyone has a properly informed opinion about the differences between various calculators. It’s also very much worth getting an idea of the differences between modes of transport for taking particular trips: for Europe the Ecopassenger website is excellent for this. (Note that to you need to change the settings after calculation to include the “climate factor” if you want a full estimate of the flight footprint including RFI). This really brought home to me the amazing efficiency of trains (especially in Western Europe with its clean electric network) compared with kerosene-guzzling air travel. For example, a trip from Bordeaux to London has a footprint of 14  kg CO2-eq if done by train, vs 168 kg CO2-eq by air – and these short trips, as I’ve found, soon add up.


Ecopassenger comparison of different modes of transport for a journey from Bordeaux to London
Ecopassenger comparison of transport options for a journey from Bordeaux to London including calculation of “climate factor” (RFI) for flights


We all have different constraints in terms of our funding, workloads and personal responsibilities: we have to figure out for ourselves what we can and want to do. But if, like me, you’re interested in reducing the carbon impacts of our discipline then it’s important to get an understanding of what those carbon impacts are, based on numbers rather than gut feeling or guesses. Our own work-related carbon footprints, which are the ones we understand the best, are a very good place to start. For me this exercise has been confronting but in the end it’s also been empowering: now that I know how big my carbon footprint is, I can do something to reduce it and I know how much I’m reducing it by. This is amazingly satisfying. It means I can make more informed decisions about what I’m doing and concentrate on making reductions where it counts.

My experience of calculating my own footprint also appears to confirm that, if we’re to decarbonise our discipline, the most important thing we need to focus on is travel. There are lots of other things to talk about too: everything from lab practices to conference catering. But it seems very likely to me that the biggest single element of our disciplinary footprint derives from air travel. There are very many archaeologists who travel a lot less by plane than I have over the last few years; at the same time, there are quite a few who travel much more. If we want to make meaningful reductions in archaeology’s contribution to global warming, this is where we should start.

I’m still a long way from understanding the total carbon footprint of archaeology and how we can reduce it as a discipline. But I’ve been brilliantly encouraged by these calculations, because they suggest that in my case at least there is a lot of space for reductions in my work-related footprint. If that’s true for me, it’s true for other people too. If we’re to fully decarbonise our discipline we will need to make some very difficult choices, but there is also plenty of low-hanging fruit there for the taking if we choose to change our practices. For most of us, however, there are stumbling blocks to making these changes, including funding, academic culture, project management practices, personal responsibilities and employer policies. These aren’t necessarily insurmountable but they need consideration. I’ll be writing more in the future on what I think we can do about these issues – in the meantime, if anyone has any thoughts on any of this I’d love to hear them.