xkcd2

4.5 Degrees

Without prompt, aggressive limits on CO2 emissions, the Earth will likely warm by an average of 4°-5°C by the century's end. How big a change is that? [[The panel shows a temperature scale.]] In the coldest part of the last ice age, Earth's average temperature was 4.5°C below the 20th century norm. Let's call a 4.5°C difference one "Ice Age unit." ((These are along the time line, spaced out for clarity.)) -2 IAU <-- snowball Earth (-4 IAU) -1 IAU 20,000 years ago ((There's an inset panel, showing a picture of a glacier.)) My neighborhood: Half a mile of ice 0 Average during modern times ((Another inset panel.)) [[A character is in the foreground of a green field with a skyline in the distance.]] My neighborhood: Character: Hi! 0.20 Where we are today +1 IAU Where we'll be in 86 years ((Another inset panel.)) My neighborhood: ? +2 IAU Cretaceous hothouse +200m sea level rise No glaciers Palm trees at the poles

The good news is that according to the latest IPCC report, if we enact aggressive emissions limits now, we could hold the warming to 2°C. That's only HALF an ice age unit, which is probably no big deal.