Y'know, big tinfoil hat time since conspiracies are really hard to keep, but if I was a climate scientist who wanted people to take climate change more seriously, (as we certainly should!) it wouldn't be a horrible strategy to try and get everyone on board to underpredict. Exceeding the worst case prediction definitely adds a sense of direness that might not be there if things were just going exactly as they'd predicted.

Of course, I don't actually think that's the case, because it'd be nearly impossible to make such a large conspiracy without it leaking, but if there was ever a global conspiracy to try to save humankind, that'd make for a good one.