Back in 2015, I got pretty serious about reducing or offsetting my carbon footprint. I don’t have kids, I don’t own a car and I don’t eat meat, so I already had three of the biggies covered. To make up for my electricity use, I started buying credits from a nonprofit that funds wind turbines and other renewable energy projects in New England. Then it was time to examine my habit of boarding kerosene-fueled jet aircraft.