There was some interesting engagement between commenters on the Kevin Anderson thread and the good professor himself. Hat tips to all concerned.
My own contribution to the comments was limited - having been snowbound since the weekend, there was a certain amount of merrymaking in the village last night by way of cheering ourselves up. The one comment I did make was to note that a temperature rise of four degrees by 2060 is extremely high in the light of the temperatures observed since the millennium. Prof Anderson's response was to refer commenters to the Phil Trans A special edition that started the thread off.
If we look at the introductory article, by New et al., there is indeed some explanation of why four degrees is considered a number that should be discussed.
The 2009 Copenhagen Accord recognized the scientific view ‘that the increase in global temperature should be below 2 degrees Celsius’ despite growing views that this might be too high. At the same time, the continued rise in greenhouse gas emissions in the past decade and the delays in a comprehensive global emissions reduction agreement have made achieving this target extremely difficult, arguably impossible, raising the likelihood of global temperature rises of 3◦C or 4◦C within this century. Yet, there are few studies that assess the potential impacts and consequences of a warming of 4◦C or greater in a systematic manner. Papers in this themed issue provide an initial picture of the challenges facing a world that warms by 4◦C or more...
In other words, we think that CO2 emissions are going to be higher than expected therefore we need to look at higher temperature rises.
But hold on, my point was that 4 degrees by 2060 (perhaps 5 or 6 degrees per century) is high in the light of recent temperature trends. As readers of Lucia's blog know, even a trend of 2 degrees per century is on the cusp of falsification, so 5 or 6 is surely falsified at a very level of confidence.
If the trend is already falsified what is the point of looking at it, other than as part of a PR campaign?