Actually, nuclear is pretty green. While in use, it only outputs steam and purified water. Despite the fact it has two big downsides (initial costs, the fact that when it goes bad, it goes REALLY bad), it's currently our best form of energy until we get fusion off the ground (and we have no idea how long that'll take).
That isn't at all what it's about. Think of it like this. We're aiming for no more than 2C, which doesn't sound like that big of a deal. Just two years ago, we had a heat wave that spiked nearly 10C over the average, then the next year, we had a nice cold snap that made for a very mild summer. This doesn't prove anything, either pro or con, about global warming, since it's entirely localized conditions that fluctuate more wildly than global averages.
The concern of the global average is that it's a relative constant, and a 2 degree difference there makes for wide, sweeping changes across the globe. Case in point, the difference between our current climate, and the muggy, CO2 rich atmosphere of the Jurrassic was about 12C. 5C, which is our worst case scenario that must be avoided at all costs, with highly acidic, hot ocean conditions, would be enough to raise some havoc on our current global economic structure.
