In reading the IPCC report I had the thought that though the various scenarios were making certain assumptions about global GHG emission reduction, they were all averaging the 30-odd climate models to get a range of likely values.
But some models are far better than others at modeling certain aspects of the global climate (ocean currents, glaciers, CH4, wild-land fires, etc.). But wouldn't it make more sense to use the models for what they were created for and use the best ones at that?
When you do that, it becomes clear that there is really a floor to the future global average temp and that the IPCC scenarios showing the likely values and a spread up and down from that are covering a truth. The truth that the spread of likely future temperature in any given scenario is actually quite skewed to the high end.
Then there's the fact that none of the traditional models give us good cloud effect predictions at all and that the best estimate fix is that clouds will net-add to at least the near-term average global temp (say a few hundred to a few thousand years out). That raises the floor even higher. Not a lot higher but definitely not lower.
Then there's human nature in large numbers and it's amazingly invariant over time. Lots of people are dumb, really dumb. Take the nation of Germany. Dropped nuclear and went green like crazy and still ended up with a net increase in CO2 emissions a decade later. Eventually, maybe even this year, they will start down the other side and be reducing their CO2 punch, but it's the fact that while going green they went the wrong way for a decade plus is a good warning sign about what large groups of even well-intentioned people do.
At any rate the rest of the world isn't Germany and even if we were the switch from fossil fuels won't go fast enough. Building that much infrastructure in so short a time is too far from practical.
Solar and wind power alone can’t scale up fast enough to generate the vast amounts of electricity that will be needed by midcentury, especially as we convert car engines and the like from fossil fuels to carbon-free energy sources. Even Germany’s concerted recent effort to add renewables—the most ambitious national effort so far—was nowhere near fast enough. A global increase in renewables at a rate matching Germany’s peak success would add about 0.7 trillion kilowatt-hours of clean electricity every year. That’s just over a fifth of the necessary 3.3 trillion annual target.
To put it another way, even if the world were as enthusiastic and technically capable as Germany at the height of its renewables buildup—and neither of these is even close to true in the great majority of countries—decarbonizing the world at that rate would take nearly 150 years.
Are we totally screwed then? Some really smart people think so.
China, given their mode of government, could build nuclear and shut down coal fast enough to get the world 1/5th of the way to where we need to be by 2050. But then they're not moving that fast and they're also doing the old Massive Belt and Road Initiative that is undermining the green things they're doing at home.