UNDERSTAND CARBON OFFSETS
A carbon offset is purchased with payment proportionate to the greenhouse gas (CO2e) produced by your journey. The money goes to a project that will balance out the environmental impact by reducing the generation of green house gases elsewhere. This can be done by: building solar arrays, wind farms, capturing the methane from cows passing gas ... seriously, flatulent cows are a big issue ... and many other ways.
One person's flight from San Diego to New York produces approximately 833 pounds of CO2e.
A third of an acre of forest can absorb that amount of carbon from the atmosphere.
If you fund the preservation of that portion of a forest, you will have offset the impact of that trip.
Purchase of carbon offsets goes to projects audited according formal standards to verify they are:
Additional - The reduction wouldn't have occurred without your funds
Permanent - The Greenhouse gasses won't be released again
Doesn't create greenhouse gases elsewhere (Leakage)
Verified Carbon Standard is one of the most popular standards used to do this.
Audited projects are recorded in registries. You can review all the VCS offset projects here.
Checkout this comprehensive guide that explains what a carbon offset is and what you should consider when purchasing one.