Vegan

I would imagine the flights people take to travel to these conferences also have a significant climate impact, probably more than from the food served at the conference

that said, I'm not sure a conference has to be a perfect representation of the values the conference might hold, especially if the burden in the current society creates restriction ... the conference after all is trying to help find ways to alter society so we don't destroy our world, and I would assume better and greener public transportation is one of those policy positions being advocated - you can't take a train that isn't there

though vegan food is more materially achievable than the transportation alternatives, it's not an insignificant cultural problem as people are not generally on board with the changes to diet, and substitutes (like transportation infrastructure) are not to the point of eliminating that discomfort such that people don't feel it's a burden to eat vegan (though in a vegan community this sounds absurd, we all are people who very much decided it's worth making that change and felt the burden was not too high, but we are not representative of the average person, let alone the average climate scientist or conference-goer, though they probably are more likely to be vegan than the average person off the street).