Air travel creates a huge amount of carbon dioxide, so San Francisco has installed an offset system for travelers in kiosks at SFO

Climate Passport contributions fund the Garcia River Forest, a reforestation project in Mendocino County where redwood and Douglas fir trees are being added to a forest that had been heavily logged. They also go to the SFCarbon Fund, which is steering the money to Dogpatch Biofuels, a bio-diesel fueling station in southeastern San Francisco.

I would much prefer to buy them through the ticketing process so the offsets could be distributed, although there are certainly advantages to supporting local offsets.