There are a lot of things we still don't know. It's pretty obvious that humans have an impact on global warming, but the true long term effects are not all known. For example, if you look at natural cycles, we are due for another "little ice age". If that's the case, some global warming might hit the spot in the future.
Or not.
We really don't know. It might even speed up the natural cycle, which probably wouldn't be a good thing.
It's a lot like the US economy. Economists act like they know what's going to happen, but we are in uncharted territory. When financial advisors say "past performance is no guarantee of future performance", they are serious and you should listen.
Past performance doesn't mean squat when our economy is in an unknown state.
![Er... what? o_O o_O](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)