Ok; until then, we have to reduce CO2 emissions by 7% each year from now until we reach net zero in 2050. Even if we figured out fusion today (highly unlikely), it won't be deployed at scale by then (a modern fission reactor takes 10 years to build). So there are two choices really: progressively scale down the aviation industry, or ignore the climate crisis and let 100s of millions in poor countries die from climate-related causes.
We could actually do that using fission alone, since we have we enough fuel for 70,000 years or so, and nuclear waste is free of green house gases. It would just be very expensive up front, as nuclear plants tend to be.