Check out these two graphs of past, present and future climate change:
They both provide a compelling visual picture of potential future warming in the context of past temperatures.
I wanted to use one of these graphs for a climate presentation recently, and there are pro’s and cons to each. Most importantly (to me):
- The bottom figure includes the “constant composition” scenario (C3 in yellow), which I find a useless distraction. It has no realistic value, as it implies an abrupt and arbitrary CO2 emission reduction, but continued aerosol emissions. (Aerosol particles have a short atmospheric residence time, so emissions have to be sustained in order to keep the concentration constant. OTOH, CO2 has a very long lifetime and with roughly 70% lower emissions its concentration would remain constant). It shows the amount of unrealized warming, but it most definitely is not an (even remotely) plausible scenario, and thus should not be presented as such.
- As for the proxy temperatures going back to the Middle Ages (or even further), I prefer the (mini) spaghetti graph of the lower figure over the top figure, which features only the Mann et al (2008) reconstruction.
Steve Easterbrook reminded me of my dilemma with his post comparing the top figure with one on which the bottom figure is based. He notes a few other differences:
- Different scenario’s are used in each (e.g. the top fig includes the “fossil intensive” A1FI (sort of business as usual) scenario, whereas the bottom fig doesn’t. A2 is worst case shown in the bottom graph, whereas it’s the middle of the road in the top graph.)
- Different temperature baseline (pre-industrial versus 1995-2004 period)
I chose to show the top graph, mainly because of the bothersome inclusion of the C3 “scenario” in the other one.