Yes, most airlines offer this option. There are two ways, one is buying the CO2 rights from someone else (that doesn't have a net positive effect on the world, don't do this if you really want to contribute something) the other is donating money to projects specifically to reduce CO2. Mostly that's tree planting projects, in areas that have had deforestation in the past.

There are a few organizations that audit these projects to guarantee they're actually planting trees and planting the right amount etc.