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PL Perspectives

Perspectives on computing and technology from and for those with an interest in programming languages.

A broad scientific consensus warns that human emissions of greenhouse gases are warming the earth. This is not a problem that can be left to future generations: The UN’s Intergovernmental Panel on Climate Change (IPCC) says a 40% decrease in emissions is needed by 2030 to avoid irreversible damage. Achieving reductions on this scale will require urgent and sustained commitment at all levels of society—not only national, state, and city governments, but also universities, companies, and scientific societies.

Indeed, scientific societies have an especially important role to play, since, for many of their members, travel to conferences may be a substantial or even dominant part of their individual contribution to climate change. A single round-trip flight from Philadelphia to Paris emits the equivalent of about 1.7 tons of carbon dioxide (CO2e) per passenger, which is a significant fraction of the total yearly emissions for an average resident of the US (16.5 tons) or Europe (7 tons). Moreover, these emissions have no near-term technological fix, since jet fuel is difficult to replace with renewable energy sources.

How should ACM respond to these facts? Two years ago, SIGPLAN convened an ad hoc Climate Committee to research this question. After investigating many options and intensive and wide-ranging discussions, we are putting forward two concrete proposals. First, all ACM conferences should publicly account for the CO2e emitted as a result of putting them on. Second, ACM should put a price on  carbon in conference budgets, to create pressure on organizers to reduce their footprints.

Mandate public accountability

Our first proposal is modest: Every ACM-sponsored conference should publicly report its carbon footprint, including from travel. Public accounting will nudge conference organizers and attendees to change their behavior. By analogy, chain restaurants in the US are required by law to post calorie counts of food items on menu boards, and studies show that enlightened customers order, on average, up to 50 fewer calories a day as a result.

As a case in point, SIGPLAN has been considering how to reduce emissions over the past couple of years, informed by an accounting of its own CO2e expenditures. This discussion has led the organizers of POPL and ICFP to switch from in-person to on-line program committee meetings (joining a trend among other SIGs). This discussion has also prompted increased investments in livestreaming and video recording to support remote participation.

Carbon offsets

SIGPLAN has also explored how to mitigate conference emissions by purchasing carbon offsets. A carbon offset is sold by a vendor who uses the funds to finance an activity that permanently removes or avoids emitting some amount of greenhouse gases. The veracity and permanence of this activity is certified by a watchdog organization. (Typically, planting trees is not a certifiable activity, since it’s hard to guarantee the trees won’t be cut down, but installing methane capture devices on landfill sites or buying fuel-efficient stoves to replace open-fire cooking in poor communities can be.)

Many organizations, including companies such as Google, Dell, Microsoft, General Motors, Delta Airlines, Lyft, and Expedia, universities, and academic societies, now buy carbon offsets in order to reduce their net footprint. ACM conferences can do the same, and the purchases can be mentioned in the public accounting we are proposing. As of now, ACM conferences may offer attendees the option to pay for an offset during registration. Ultimately, an offset charge could be rolled into the registration fee.

We acknowledge that carbon offsets are a controversial tool. Offsets are a useful short-term expedient, not a replacement for more difficult cutting.

Data gathering

Beyond encouraging conferences to trim their emissions, public accounting will generate useful data for future organizers. The data should be made available in a central place, in a uniform format, suitably anonymized. Most conferences’ footprints will be dominated by participants’ air travel, but the data gathered should go beyond this to include air travel to in-person PC meetings and estimated hotel and food CO2e footprints.

ACM should develop or support tools to gather and publicize this data. SIGPLAN recently built an air-travel-focused CO2e calculator for conferences. Users can upload conference registration data, and the calculator will estimate the CO2e cost of air travel for those sufficiently far away. This calculator could be expanded to account for other forms of conference emissions.

Put a Price on Carbon

Public accounting of CO2e expenditures will incentivize some reductions, but, by itself, it will not induce major changes. Science is a fundamentally social process, and the conference system accelerates scientific research through high-bandwidth interaction, direct dissemination of results, network building, and serendipitous cross-fertilization. Organizers and attendees will be reticent to make changes that might threaten these benefits.

Hard choices

Visualization of CO2e footprint, per conference

Carbon footprint per participant for travel to recent SIGPLAN conferences. The smallest dot (ICFP 14, in Gothenburg, Sweden) represents .9 tons of CO2e per participant; the largest dot (ICFP 16, in Nara Japan) represents 1.94 tons per participant.

To illustrate the challenges, consider the problem of choosing a conference location that minimizes emissions from participants’ travel. Using recent registration data from four SIGPLAN conferences, the following diagram shows an estimated per-participant CO2e footprint for each instance of each conference over the past ten years (excluding a few for which we had trouble getting data), with larger dots representing more emissions. Eyeballing this diagram, it seems that carbon-conscious organizers should hold all of these conferences in either the northeast US or western Europe every year.

CO2e footprint by participant location

Breakdown of continent-of-origin for participants in each conference. Colored bars represent percentages of participants whose home city is in each continent.

The second diagram tells a different story. The horizontal colored bar at the bottom represents the continent on which each conference was held, and each vertical bar gives a breakdown of the participants in that conference, colored according to the continent of their work address. From a superficial glance at the colors, it is clear that, though a minority come from far away, the majority of participants are local to the region where each conference is being held. This suggests that locating conferences every year in the same one or two places would significantly damage the research community by discouraging participation from other parts of the world. Indeed, one might conclude that, from the point of view of strengthening the research community, conferences should move around as much as possible. These disparate perspectives suggest that reducing conference emissions will require genuinely painful compromises. The impulse to avoid thinking about it is entirely understandable!

However, the present trajectory of world emissions is unsustainable: hard choices will have to be made, and soon. How do we motivate organizers to make them?

A cost for carbon

This dilemma is a microcosm of one faced by all of society. To address it, many policy experts advocate some form of carbon pricing (either taxes or cap-and-trade schemes), imposing a concrete, immediate cost on emissions. Doing so makes manifest the hidden environmental cost of emissions, incentivizing CO2e-reducing changes without specifying exactly which, and thus allowing for creative and efficient responses. Continuing the junk-food analogy, some municipalities such as New York city and Philadelphia have imposed a per-calorie tax on soft drinks, with some studies finding that doing so indeed reduced consumption.

Thus, our second proposal is that ACM should impose a surcharge on conference budgets based on their carbon footprint. The charge should be low to start and increase steadily and predictably. Conference organizers can then choose how best to balance their budgets—whether by decreasing per-participant emissions, decreasing (physical) participation, increasing registration fees, soliciting additional corporate sponsorship, or other means.

Ideally, governments will eventually impose carbon pricing uniformly, and all CO2e-intensive activities will have to pay it. But ACM can send a strong message about the importance of this issue by acting now.

Precipitating change

What should ACM do with the funds collected from this surcharge?

One can imagine many good uses, including carbon offsetting, defraying the costs of virtualizing conferences (livestreaming, etc.), and supporting “green” computing research. As an example of the last, ACM could help fund a cross-cutting research initiative specifically aimed at understanding how to best replace or approximate the socializing and networking aspects of conferences in a virtual setting. But the very first concern should be to find ways of reducing the financial burden of such a surcharge on those who may be disproportionately affected by it—students without grant support, participants from developing areas, etc. That is, wealthier participants should subsidize the carbon footprints of less wealthy ones.

Carbon pricing will also stimulate creative rethinking of the conference model itself—for example, seriously considering alternatives such as rapid-turnaround, journal-only publishing models, once-a-year megaconferences, and entirely virtual conferences.


The climate crisis is too important to ignore and too urgent to leave for world leaders to address at their own pace: Organizations at every scale must confront their own contributions, raise awareness among their members, and establish new ways of doing business in the lower-carbon future that is coming. We in ACM can play our part.

Benjamin C. Pierce is a Professor in the Department of Computer and Information Science at the University of Pennsylvania.  He is the current Vice Chair of SIGPLAN, and chairs SIGPLAN’s ad hoc committee on climate change.  He is the author of several popular graduate texts, including Types and Programming Languages, and is the creator and editor of the Software Foundations ebook series on software verification in Coq.

Michael Hicks is a Professor in the Computer Science department at the University of Maryland, and is the Past Chair of ACM SIGPLAN. His research focuses on using programming languages techniques to improve the security, reliability, and availability of software.

Jens Palsberg is a Professor and a former Department Chair of Computer Science at University of California, Los Angeles (UCLA). He is the Chair of ACM SIGPLAN, former Editor-in-chief of TOPLAS, and a former PC Chair and a former General Chair of POPL. In 2012 he received the SIGPLAN Distinguished Service Award.

Crista Lopes is a Professor in the School of Computer Science at University of California, Irvine (UCI), with research interests in Programming Languages, Software Engineering, and Distributed Virtual Environments. She is an ACM Distinguished Scientist, an IEEE Fellow, a twice-elected member of the SIGPLAN Executive Committee, and Editor in Chief of The Art, Science, and Engineering of Programming.

Disclaimer: These posts are written by individual contributors to share their thoughts on the SIGPLAN blog for the benefit of the community. Any views or opinions represented in this blog are personal, belong solely to the blog author and do not represent those of ACM SIGPLAN or its parent organization, ACM.