If I don't fly from London to my sister's wedding in New Zealand she will be upset, I will cause her pain and so that's morally bad. If I do fly to my sister's wedding in New Zealand I will put about four tonnes of carbon dioxide into the atmosphere, which will contribute to climate change, which, according to the World Health Organisation, already causes about 150,000 deaths every year. Clearly that's also morally bad. Which is the morally correct thing to do?

In dilemmas of this kind, always start by thinking about whether they are really inescapable. One escape in this case it to speak with your sister. If she likes New Zealand, she is unlikely to be indifferent to the environmental degradation that is already so much in evidence elsewhere. Plus you can offer to donate the flight cost to a good cause of her choice, in honor of her wedding. In any case, it is much easier for her to understand and accept the decision if she was herself involved in making it or at least in thinking it through.

BTW, I checked your numbers because 4 tonnes seemed like a lot. But you are basically right. A Boeing 747-8 takes a bit over 200 tonnes of fuel (over half its take-off weight), roughly 137 gallons of fuel per passenger. Each gallon produces 20 lbs of carbon dioxide. So that's about 1.3 tonnes per person. But then one tank does not get you there, plus you'll have to fly back as well. So 4 tonnes is a very good estimate. Way too much, indeed.

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