The annual meeting of the European Astronomical Society took place in Lyon, France, in 2019, but in 2020 it was held online only due the COVID-19 pandemic. The carbon footprint of the virtual meeting was roughly 3,000 times smaller than the face-to-face one, providing encouragement for more ecologically minded conferencing.
The scientific evidence that we live in a climate emergency calls for urgent action . As a 8 society, we are collectively failing to live within our environmental boundaries with possibly 9 catastrophic consequences for human civilisation 1 . The time to address these issues is now 1, . The United Nations Emissions Gap Report from 2019 states that each year a global 10 reduction of emissions of 7.6% is required to limit the average global temperature rise to 1.5°C 3 -the target that was outlined in the Paris agreement in 2016 . At the current rate of 11 emissions, we will exceed the 'carbon budget' to meet this goal within the next eight years . 12 While ultimately system change is required to solve the climate crisis, there is also the responsibility of individuals to reduce our emissions. This applies in particular to astronomers who rely heavily on fossil fuel energy for, e.g. computation, telescope operation and travel , , ,. To future-proof astronomy, we must recognise impending environmental change, 14 15 financial uncertainties and the need for moral introspection, which threaten to hinder the continuation of the discipline. At the same time, the advancement and sharing of knowledge in general is becoming even more vital as we face a global threat.
Conferences are a vital element of astrophysical research and collaboration, but the air travel often connected with face-to-face conferences is a major source of concern. Following last year's annual European Astronomical Society meeting in Lyon (EWASS2019), we conducted a short survey among participants who had agreed to receive such communication via email (719 out of 1240), to estimate the current, collective carbon emissions generated through travel by attendees. In establishing this initial estimate it was hoped that guidance could be developed to reduce future travel-related emissions. The anonymous questionnaire was very simple and only asked for the participants' origin and final destination and their main mode of transport. After two weeks we had collected 267 (22% of all participants) valid responses.
Just over two thirds of the respondents (66.9%) indicated that they arrived in Lyon by airplane, 27.8% arrived by train and the remaining 5.3% used other means of transport such as car, bus, metro, bike or by foot. 86.5% returned directly to their origin after the conference using the same means of transport. Of those who did not, the modal split was similar to the inbound journey.
We computed the CO 2 -equivalent emissions (CO2eq) associated with every plane or train trip using the online Travel Footprint Calculator with its default settings and we refer to the 16 accompanying paper for a discussion on the pitfalls of the methods used in these 17 calculations (e.g. assumptions about the Radiative Forcing Index). For car trips, we used Google Maps to compute the shortest road distance and assumed emissions of 110 g/km . 18 The result of this computation is shown in Fig. 1 The majority of trips (~80%) produced CO2eq emissions of less than 1000 kg per trip. Conversely, the intercontinental flights (~ 10% of all trips) produced 50% of the total emissions of respondents.
In addition we also looked up the equivalent train travel time for all European travellers who arrived by plane, except for those where a train connection does (currently) not exist. This resulted in 159 theoretically possible train connections. 135 (85%) of these would take less than 24 hours; 114 (72%) less than 15 hours. We show a histogram of the latter together with the (looked up) train travel times for those 74 respondents who actually journeyed by train in Fig. 2 week of conferencing (Monday, 9 am till Friday 5 pm) is to be combined with train travel, however, this requires flexibility for the participants to spend weekend days for travel, which may not always be possible. Another obstacle for choosing the train instead of the plane are the often higher costs of train tickets compared to equivalent airfares. This does not reflect the real costs of both modes of transport, however, since air fares do not include VAT or taxes on energy, and also do not usually include compensation for the significant environmental damage that airplanes create. On the other hand, some universities, including the Dutch universities of Leiden and Groningen and the Belgian university of Ghent, already have environmental-friendly travel policies in place that require researchers to take the train for connections that take less than six hours and recommend to take the train for connections that take up to eight hours.
Lastly, we estimated the total travel emissions associated with EWASS 2019 by correcting, country-per-country, for incompleteness in our survey responses. For example, to compute the total travel emissions for all Dutch participants, we computed the average emissions of Dutch respondents for both their outbound and return trip (673 kg) and multiplied this by the total number of participants from this country (78). This assumes a similar modal split (plane vs. train) for those participants who did not fill in the travel survey. The total travel emissions for all 1240 participants is then an estimated 1855 t CO2eq. This is of the same order of magnitude as the yearly emissions of a medium-sized astronomical research institute (e.g. problem and the responsibility that we have that the additional emissions burden involved in one average astronomer participating in a single conference in Europe is roughly equal to the average total annual per capita emissions in India.
We note that our brief analysis does not include other emissions / waste associated with single-use items (such as plastic bottles, conference booklets, other give-aways), extra food production, energy for the building etc. We estimate that these factors are only a small fraction of the travel-related emissions, however (e.g. based on K. Jahnke's analysis of the carbon footprint of the Max-Planck-Institute for Astronomy ).
We can also compare the EWASS 2019 travel emissions to EAS 2020 --the first online-only version of the European Astronomical Society's annual meeting: With 1777 participants, EAS 2020 was the largest online astronomy meeting to date -a change that impacted not only carbon emissions, but also diversity and inclusivity, as a forthcoming article will illustrate. 681/1777 participants (38%) answered the EAS 2020 exit survey and of those, 57% said they would have taken the plane if they had travelled to Leiden. This number is lower than last year's (67%) which may be connected to the fact that Leiden (via Rotterdam or Utrecht) is well connected to the high-speed rail network (e.g. direct connections to Paris, Brussels, London, Cologne, Frankfurt). Nevertheless, this result may also give reason to hope that astronomers are becoming more aware of the environmental damage that flying causes.
Of course EAS 2020 was not entirely carbon-neutral either, since virtual conferencing consumes a considerable amount of electrical energy both at the end user's site and for running the network infrastructure (and servers). A simple estimation can be made based on the fraction of people joining each of the five days of the conference (~80%, based on the exit survey), the number of hours people were online per day (5.5 hours, based on the exit survey), the data rate of a Zoom webinar (1.2 Mbps downstream ) and the electricity use 22 per Gigabyte transferred over the internet (0.06 kWh/GB for 2015 ). In addition, electrical 23 energy is required for running the participants' laptops (~ 30 W) and the Zoom server itself.
For the latter we estimate that a single 24-core Xeon machine would suffice which consumes approximately 300 W of electrical power. The total electrical energy consumption for EAS 2020 is then 1173 kWh (laptops), 1263 kWh (network) and 15 kWh (Zoom servers). With the CO2eq emission intensity for electricity generation (240 g CO2eq/kWh ) we arrive at a total 24 carbon footprint for EAS 2020 of 588 kg -roughly the emissions of a single return trip by airplane from Liverpool to Lyon. 25 We conclude that the internet-related emissions of EAS 2020 were insignificant compared to the travel-related emissions alone of EWASS 2019. This is comparable to other recent estimates for large international conference, e.g. a virtual annual meeting of the American Geophysical Union was calculated to emit less than 0.1% of the travel emissions of the real AGU 2019 meeting .