The well-established medieval warm period (MWP) reached its peak in about 1200 AD and then gave way to the little ice age (LIA) that lasted from about 1400 to 1850 AD. Those periods are well documented in history and accepted by climatologists. The Viking colonisation of Greenland took place during the MWP when lush green vegetation thrived, giving it its name. The Viking settlements collapsed during the LIA, when even the Thames in London froze over.
If the Medieval Warm Period was warmer than today, with no greenhouse gas contribution, what would be so unusual about modern times being warm also?
This presented a serious problem to the supporters of AGW. The MWP peaked at a higher temperature than today and at a time when there was no signiﬁcant human emission of CO2. This naturally raised the question: What would be so unusual about the current warming trend that necessitated the response to link it to human CO2 emissions? In return, the AGW advocates drew attention to a little known 1999 paper using tree ring data to assess past temperatures, eliciting memories of the now infamous presentation of the ‘Hockey Stick’ curve. Tree rings are not a reliable temperature proxy because they are inﬂuenced by many other factors, apart from temperature: rainfall, sunlight, cloudiness, pests, competition from other trees, soil nutrients, frost and snow duration. Nevertheless, the tree ring curve was accepted by the IPCC and replaced their earlier curve. As we know, it has the shape of a hockey stick. Trees grow only on land and 71% of the Earth’s surface is covered by water. The data were only from the Northern Hemisphere, yet presented as the global temperature curve. Quoting John Daly: ‘It was a coup: total, bloodless, and swift, and the hockey stick was greeted with a chorus of approval from the greenhouse effect supporting industry.