Ronald Bailey has a nice little summary of the IPCC's latest Summary with some background info. The bottom line: their best guess is that 2100 will be 3 degrees (Celsius) warmer with sea levels 11 inches higher.

The Summary says the temperature is "likely to be in the range 2 to 4.5 degrees Celsius (3.5 to 8 degrees Fahrenheit) with a best estimate of about 3 degrees Celsius (5.4 degrees Fahrenheit) and is very unlikely to be less than 1.5 degrees Celsius (2.7 degrees Fahrenheit). The Summary adds, "Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values." Basically, IPCC global temperature projections are back to where they were in 1990 in the FAR.

However, the IPCC's new Summary continues the trend of lowering sea level increases that is found in its previous scientific assessments. By 2100 sea level is expected to rise between 28 to 43 centimeters (11 to 16 inches). The report notes that sea level rose about 7 inches during the 20th century.

The new estimate for sea level rise has proved to be one of the more controversial aspects of the IPCC Summary. For example, Stefan Rahmstorf from the University of Potsdam in Germany, is a co-author of a brief report in Science this week that suggests the previous IPCC projections "may in some respects even have underestimated the change, in particular for sea level." He thinks sea level could rise as much as 55 inches over the next century.

There is much argument to be had about proposed policy responses that try to reduce CO₂ emissions and the like, but I think we can much more easily agree that there is a fair chance we will not achieve the global cooperation necessary to noticably slow global warming. With that in mind, we should, at the very least, be making contigency plans and hedging our financial bets so as to be ready for a warmer world. For example, if we think parts of Bangledesh will be flooded (more than usual), then let's think about how we might help the Bangledeshis build things like dikes. By themselves, national policies like a carbon tax, if they prove to be too little, too late, will leave our descendents saying, "Well, they meant well..."