In an exclusive interview with the Guardian, Dr Rajendra Pachauri, chair of the Intergovernmental Panel on Climate Change, said it would be hypocritical to apologise for the false claim that ­Himalayan glaciers could melt away by 2035, because he was not personally responsible for that part of the report. "You can't expect me to be personally responsible for every word in a 3,000 page report," he said.

It was an isolated mistake, down to human error and "totally out of character" for the panel.

There is much debate over whether the Medieval Warm Period was global in extent or not. The MWP is most clearly expressed in parts of North America, the North Atlantic and Europe and parts of Asia. For it to be global in extent the MWP would need to be seen clearly in more records from the tropical regions and the Southern Hemisphere. There are very few palaeoclimatic records for these latter two regions.

Of course, if the MWP was shown to be global in extent and as warm or warmer than today (based on an equivalent coverage over the NH and SH) then obviously the late-20th century warmth would not be unprecedented. On the other hand, if the MWP was global, but was less warm that today, then current warmth would be unprecedented.

We know from the instrumental temperature record that the two hemispheres do not always follow one another. We cannot, therefore, make the assumption that temperatures in the global average will be similar to those in the northern hemisphere.