How big a change would 4.5 degrees celsius be?
Randall Munroe of the XKCD webcomic has tried to better visualize how what might seem to be relatively small temperature differences can lead to big changes on our planet. He uses 4.5 degrees celsius (8°F) as a unit of measurement, showing on the negative side how just 1 of those units was enough to create an ice age and burrow his neighborhood (he lives in Boston) under half a mile of ice. Minus 4 of those units and you reach snowball Earth, with ice from pole to pole.
On the positive side, we don't know what 1 unit will be like, but 2 units and we're back to the hottest conditions of the cretaceous period, with sea level being 200 meters above what it is today, no glaciers, and palm trees at the poles. Not good...
So the idea is to take something abstract to most people (4.5 degrees celsius) and turn it into something concrete (halfway to having palm trees at the poles).
For more on where our climate might be headed, check out our coverage of the latest IPCC report. (Business as usual would lead to "temperature increases of between 3.7 celsius (C) and 4.8C, which is far above the 2C threshold that scientists agree would lead to much more catastrophic warming.")