[dmurray]

There are a few "carbon footprint calculators" out there which can give you an idea based on your lifestyle.

Here's one that looks reasonable: https://www.carbonfootprint.com/calculator.aspx. The largest share of my emissions was from flights, making up 7 tons of my 14 ton total.

So you might think the best single change I could make would be to stop flying. But the same website quotes $8/ton to offset my carbon footprint. So an even better single change would be to donate $100/year to some initiative to upgrade cooking stoves in Africa, or whatever the current best-value project is.