What does this diagram mean?
The B1 and A2 referenced in the chloropleth maps are referring to the emissions scenarios that were presented alongside the IPCC's Third Assessment Report (TAR) in the Special Report on Emissions Scenarios (SRES). These scenarios referred to the rate, timelines, and total volume of carbon and other greenhouse emissions that might be realized by the end of the 21st century.

These were several scenarios (A2, A1B, B1, etc.) that were used with the aid of climate models to determine what the possible climate change impacts of pursuing "Business as Usual" (A2), to rapidly reducing emissions and stabilizing climate (B1). While some researchers still use these emissions scenarios and the climate models that were presented with them for some predictions, most climate scientists now refer to the more recent Fifth Assessment Report's (AR5) Representative Concentration Pathway Scenarios (RCP).

For more information: http://www.nature.com/nature/journal/v463/n7282/full/nature08823.html