This excellent review by Bill McKibben recounts the brief history of IPCC reports. McKibben accurately, in my opinion, describes AR4 as essentially echoing the conclusion from AR2: that humans are causing the globe to warm. What's happened in AR3 and AR4 is that the scientific uncertainty has been reduced.
The SPM seems like it was rushed a bit. (Instructions to editors were left in the final copy.) McKibben notes how this, and the media focus on the shifting certainty, have obscured other points:
More important findings were ignored in accounts of the report and in some cases were obscured by the document's very poor prose, which is much more opaque than its predecessors. Those findings include:
- The amount of carbon in the atmosphere is now increasing at a faster rate even than before.
- Temperature increases would be considerably higher than they have been so far were it not for the blanket of soot and other pollution that is temporarily helping to cool the planet.
- Alternative explanations for some of the warming (for example, sunspot activity and the "urban heat island effect," the raising of temperatures in cities caused by high building densities and the use of heat-retaining materials such as concrete and asphalt) are now known to be relatively negligible.
- Almost everything frozen on earth is melting. Heavy rainfalls are becoming more common since the air is warmer and therefore holds more water than cold air, and "cold days, cold nights and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent."
McKibben goes on the describe how the conservative wording of the SPM, a natural outcome of the process that makes it, obscures these findings and the warnings of what will come.
The point, as mentioned below, is this: the Earth is committed to some warming based on what we've already put in the atmosphere but we have about 10 years to prevent the real bad circumstances from happening. There is a very large "procrastination penalty". The SPM mentions all this but its hidden behind the dry prose.