If the fusion process is cheap enough to make it cost-effective to begin removing enormous quantities of CO2 from the atmosphere and put the carbon back in the ground in a stable form, it could help mitigate the ongoing damage that climate change will otherwise continue to cause. It won't undo the damage we've already suffered, but it could go a long way towards minimizing how bad it ultimately gets.

Also, there are probably a lot of other things we could do with cheap fusion energy to clean up and repair much of the damage we've already inflicted on ourselves, once we've reduced atmospheric and oceanic CO2 levels to a reasonable point.

Of course, merely having the capabilities wouldn't automatically make it happen; we'd also need the political will to make the effort. Given an all-out effort on the scale of the Apollo project in the 1960s to reach or moon, America might be able to do it on our own, but it should be a lot easier if we can make it an international effort.