40,000 bananas, 8 tons of paper cups, and a whole lot of emissions. Inside the Boston Marathon’s climate footprint.

There are shirts and medals for runners, jackets for volunteers, goodie bags upon completion. There is water to be consumed en route and synthetic gels for runners seeking another burst of energy. Except when it comes to gathering 40,000 people to run together, in the same place, there’s nothing simple about it. Part of the beauty of running is its simplicity. You need a road, a will to move, and that’s about it. Then there are the runners themselves — so many runners, from so many places — traveling from...