Understanding how the climate has changed in the past is essential if we are to put today’s warm climate into its proper perspective. There is a key question in the post-hockey stick era, and that is how warm was the Medieval Warm Period of a thousand years ago and how warm was the Roman Warm Period of a thousand years before that.
Some scientists have looked at the data for the MWP and whilst conceding that it was warm they have doubted its extent, suggesting that it was confined to Europe. This was despite the fact that there are very few temperature proxies outside Europe for the period.
They mapped the spatial and temporal temperature pattern between the 9th and the 20th century using about 150 temperature proxies including tree-rings, ice-cores, marine sediments, lake sediments and speleothems. Most of their data was from Europe and North America, a small minority from Siberia, northern Africa and the Middle East. Their data was compiled to give a resolution of 100 years.
Their conclusion is that there was widespread Medieval warming in the 10th and 11th centuries, followed by a gradual cooling into the 17th century (the Little Ice Age) and then a warming from the 18th century that accelerated in the 20th. Significantly, they find that the warmth of the MWP was as widespread and as uniform as the 20th century. Although it was as warm as it is now it was not possible to make an exact comparison because of the study’s 100 year time resolution.
This paper should be read in conjunction with last years Nature paper by Oppo et al, who looked at a warm region of the ocean called the Indo-Pacific warm pool. Using an analysis of temperature proxies in nearby sediment cores they concluded that the region was also warm during the MWP in Europe. It was in fact as warm as today’s warm period.
The Editor’s summary of this paper said, “The data show that while recent decades have been anomalously warm, they are statistically indistinguishable from temperatures prevailing during the Medieval Warm Period from around AD 1000 to AD 1250,” which I think is an interesting way of summarising it.
Last year Mann et al writing in Science also used an array of temperature proxies to confirm that the MWP was as warm, possibly warmer, than it is today. The paper also went on to explain that in the authors view the causes of the two similarly hot periods were different. See GWPF’s report.
More recently scientists from the University of Saskatchewan and the University of Durham studied bivalve molluscs and used isotopic ratios in their shell as a sea temperature proxie and concluded that the Roman Warm Period was warmer than today. Also see the GWPF’s report.
The scientific literature is pointing towards a consensus on this issue. Nobody, neither scientist nor politician, should describe our current warm climate as unprecedented.