The relevant text is: Without prompt aggressive limits on CO2 emissions, the Earth will likely warm by an average of 4°-5° by the century's end.
The source is given in the popup that appears when you hover the mouse over the cartoon, and is given as the latest IPCC Fifth Assessment Report (AR5). That's where XKCD goes wrong.
What is the IPCC projection based on? Computer models. So how are the models tracking climate? Very poorly:
I’ve updated our comparison of 90 climate models versus observations for global average surface temperatures through 2013, and we still see that >95% of the models have over-forecast the warming trend since 1979, whether we use their own surface temperature dataset (HadCRUT4), or our satellite dataset of lower tropospheric temperatures (UAH):The models are simply terrible - two out of 90 are close to the actual recorded temperatures over the last three decades, and they're getting worse over time, not better. In fact, AR5 weakens the case for global warming:
Whether humans are the cause of 100% of the observed warming or not, the conclusion is that global warming isn’t as bad as was predicted. That should have major policy implications…assuming policy is still informed by facts more than emotions and political aspirations.
Figure 1 shows that observations particularly since 2005 are on the low end of the envelope that contains 90% of the climate model simulations. Observations in 2011-2012 are below the 5-95% envelope of the CMIP5 simulations. The trend in the model simulations is substantially larger than the observed trend over the past 15 years.
Emphasis mine. The observed temperatures are no longer within the error bands of the model projections. The idea that this unexplained hiatus will suddenly end and temperatures for the rest of the century will increase even faster than the IPCC projected is certainly novel but is hard to justify.
- Figure 1. After expecting an increase of 0.2oC per decade in the early decades of the 21st century from the AR4 statements, the rate of warming over the past 15 years is only ~0.05C.
- The IPCC AR5 bases its projection for the period 2016-2036 of 0.10 to 0.23oC per decade on expert judgment, rather than on the climate model results
- The IPCC does not have a convincing or confident explanation for the hiatus in warming.
And so XKCD disappoints. Wherever we will be in 86 years, it's certainly not going to be where XKCD and the IPCC say we will.
UPDATE 10 June 2014 18:27: Goober opens up a righteous can of whoop-ass on the topic. Recommended.