They based their work on a range of possible targets for stabilizing carbon-dioxide concentrations in the atmosphere, but focused on a particular range: 450 molecules for every million in the atmosphere parts per million by volume (ppm) to 600 ppm. Concentrations as of 2007 stand at about 383 ppm, according to the Global Carbon Project, compared with 280 ppm at the start of the Industrial Revolution.
The 450-600 range is of special interest, since 450 by the year 2100 represents a target widely cited in negotiations for a new global climate treaty, which its architects hope will be ready to take over when the 1997 Kyoto Protocol's first five-year "commitment period" expires in 2012. The 600 is a business-as-usual emissions path.
In short, if CO2 concentrations peak at 450-600 ppm, declines in rainfall during the dry season in regions such as the US Southwest or the Mediterranean are comparable to the Dust Bowl drought in the US during the 1930s and those seasonal declines persist for millenniums. Sea level from heat expansion alone rises by an average of up to three feet through the year 3000.
For simplicity's sake, they didn't take a large number of secondary effects into account. And projections of melting of Greenland's and Antarctica's ice caps are still too uncertain to include. The levels Solomon's team projected are a minimum, based on better-understood physical processes. The results appear in the issue of the Proceedings of the National Academy of Sciences for the week of Jan. 26, 2009.