Does it matter? Ultimately, these are estimates. Educated, data backed estimates, but still estimates.

One larger than expected volcanic eruption, coral reefs dying faster than expected, whatever, all it takes is one or two things to not go the way they're expected and everything speeds up.

20 years or 25 years, the point is we're all kinda fucked unless we do something about it.

What we need to do has been and will continue to be debated ad nauseam, but we know we must do something.