I like xkcd. It's a fun, nerdy webcomic that almost everyone has seen. It is usually interesting and is often insightful. Today, however, it is just nuts. I refer you to a comic which looks more like a Greenpeace scare poster than the normal xkcd fare:
There is so much wrong with this comic, I couldn't hope to give an exhaustive list. It's so wrong people on both sides of the global warming debate should all be able to agree it's wrong. That's an accomplishment.
I've been reading xkcd for years. I never thought to sign up for its forum. This comic changed that. I was so shocked by this comic I immediately registered an account for the xkcd forum and left a comment. I think it speaks for itself:
I'm not looking for a debate on global warming, but there is so much wrong with this comic. I'll just focus on the big point. Projections of future warming depend on two things: 1) The level of future greenhouse gases; 2) The planet's sensitivity to greenhouse gases.
The former is described by Representative Concentration Pathways (RCPs), estimates of how greenhouse gas levels might evolve in the future. These are given in the form of RCPx, where x is the change in radiative forcing from pre-industrial times to 2100. The IPCC uses four RCPs: RCP2.6, RCP4.5, RCP6 and RCP8.5. Of these, only RCP2.6 requires prompt, aggressive action.
Once you've chosen an RCP, there is still the question of the planet's sensitivity. I won't delve into that save to point out the IPCC gives the likely range for the sensitivity as 1.5-4.5°C. You can't simply ignore that level of uncertainty.
Regardless, I'll just accept the IPCC's values. That's convenient because the IPCC gives estimated temperature changes for each RCP as of 2100. The IPCC has this to say about the worst case scenario, RCP8.5:
Warming above 4°C by 2081–2100 is unlikely in all RCPs (high confidence) except RCP8.5. Under the latter, the 4°C global temperature level is exceeded in more than half of ensemble members, and is assessed to be about as likely as not (medium confidence).
And that's in reference to temperatures of 1850-1900. Even the IPCC agrees there's basically no chance we'll see 4°C of warming relative to current temperatures (by 2100), even in the worst case scenario. We certainly won't see 5°C of warming. Moreover, plenty of climate scientists think RCP8.5 is unrealistic, saying RCP6 is more like what we'd see with no climate related policies. If they're right, there's no chance we'll see even 4°C of warming by 2100. None at all.
Even if you feel RCP8.5 is an accurate estimation of what would happen with no climate related policies, we don't need prompt, aggressive action to avoid it. Worst case scenario, without prompt, aggressive action, we can still follow RCP4.5 or RCP6. According to the IPCC, neither of those will result in the sort of warming this comic indicates.
In other words, this comic cites the IPCC, yet it exaggerates even the worst case scenario offered by the IPCC while portraying itself as discussing the best case scenario.
Note: Here's a link to the IPCC report chapter I quoted. I couldn't include it in my comment their because it made my comment get flagged as spam.