What does two degree Celsius “average global temperature” really mean?

 The world’s leaders want to limit the rise in average temperatures to two degrees Celsius by 2050. This sounds like a very clear statement of ambition. Everyone understands that limiting averaging temperature rises to two degrees would be a good thing, because that it what the scientists advise, isn’t it? The truth is that the ambition is not clear and really I am not sure what it limiting average temperature rises to two degrees Celsius actually means. As usual the politicians have fudged things and I shall explain why I do not understand what they mean. Continue reading