- Ireland’s summer temperatures were up to 1.4 ¬∞C warmer in 8400 BC under a climate warming scenario
- Mean temperatures in 5000 BC exceeded twentieth century values by 2C, with woods growing on Orkney
- Colder conditions returned around 2200 BC with increased rainfall and bog development
- The Medieval Warm Period showed maximum warmth restricted to two-three 20-30 year intervals
- NASA Earth Observatory has identified three cold intervals beginning in 1650, 1770, and 1850
Climate data spanning from 8400 BC through the twentieth century has revealed the scale of temperature fluctuations that challenge assumptions about modern warming. Ireland’s summer temperatures registered up to 1.4 ¬∞C warmer in 8400 BC under a climate warming scenario, while mean temperatures in 5000 BC exceeded second half twentieth century values by 2 ¬∞C. The climatic limit of elm and lime trees moved northwards around 3000 BC, with woodlands potentially growing on Orkney and exposed areas of northern Scotland under less windy conditions than currently exist.
Colder conditions returned around 2200 BC, probably arising from volcanic activity, with mean temperatures falling and rainfall increasing as bogs developed across the landscape. By 200 BC temperatures may have dropped 2°C below the warmest post-glacial period values. Scientists have derived these estimates from tree-ring dating, ice-cores, palynology, population movements, settlement patterns, and written sources after 3400 BC. According to Adelle Thomas (2014) and Crowley and Lowery (2000), average summer temperatures between 8000 BC and roughly 3000 to 2000 BC stood 2-3 °C warmer than today, but the climate cooled by 2°C or more around 2500 BC.
The Medieval Warm Period (~1000-1300 A.D.) has featured prominently in debates about twentieth century warming, with some individual records showing greater warmth than present conditions. New hemispheric composites incorporating additional time series have demonstrated that maximum Medieval warmth was restricted to two-three 20-30 year intervals, with composite values during these times being only comparable to the mid-20th century warm time interval. The Roman Warm Period from approximately 250 BC to AD 400 produced unusually warm weather across Europe and the North Atlantic. NASA Earth Observatory has identified three particularly cold intervals beginning about 1650, 1770, and 1850, with each separated by periods of slight warming.
