There is an endless twitter argument going on right now over the perennial question: "which future scenario is the correct one?" As the lead author of the US Natl Climate Asst chapter on scenarios, I have an informed opinion: NONE. science2017.globalchange.gov/chapter/4/
We can compare our emissions over the last 15-20 years with scenarios over that time, and when we do, we see that we are currently tracking with the higher scenarios. Will we maintain that trajectory? I sincerely hope not. 
Here's the thing, though: by studying the outcome of various temperature thresholds such as +1.5, 2, 3oC** we are dynamically altering their probability - like Schrodinger's poor cat. The act of observing the impacts will hopefully inspire us to avoid at least some of them.
** not RCP scenarios, thresholds: b/c that's how global targets are framed. Yes, CMIP global climate model simulations are for RCPs/SSPs. Section 4.2.2 explains. Not perfect for impacts that depend more on rate of change, but still--better than nothing! science2017.globalchange.gov/chapter/4/
So rather than arguing over which scenario is realistic or probable, let's remember: all that really matters is cumulative carbon (no matter where or how it is produced) and its impact on global temperature. Source: nap.edu/catalog/12877/…
And although there is no "magic threshold" below which we'll all be fine and above which the world will end, we also know this: each additional GtC carries with it an increasing cost. So the more we reduce, the better off nearly 8 billion of us** will be.
** except for those whose power and wealth depends primarily on the extraction, processing, combustion, or sale of items that burn, fossil fuels and who are unwilling or unable to alter their business paradigm for the clean energy future. Source: en.wikipedia.org/wiki/Fortune_G…
When it comes to scenarios, people, the bottom line is this: WE are the biggest uncertainty. What choices will we make? Which political parties will we elect? What new tech will be invented? And most of all, this: ARE WE WILLING AND ABLE TO CHANGE? Our future is up to US.
So rather than arguing over which scenario is wrong or right, how about we try our absolute best to do everything we can to make the lower scenarios as likely as possible and the higher scenarios as unlikely as possible. Our future is in our hands.
