As climate change due to anthropogenic forcing is predicted to be greatest at high latitudes, the North European - North Atlantic region is a key area for climate and research on past climates. It is important to understand past patterns of climate variability to assess model results for future climate scenarios in this critical region.
Reconstructing temperatures from pollen
Anne Bjune and John Birks from the Bjerknes Centre for Climate Research and Heikki Seppä from the University of Helsinki have reconstructed past mean July temperature based on pollen found in lake sediments from 11 small lakes located in the middle boreal, northern boreal, low-alpine, or low-arctic zones of northern Norway, northern Sweden, northern Finland, and north-west Russia and makes a transect through different modern climate.
Regional temperature trends
The temperature reconstructions from the 11 sites indicate similar patterns as those documented by most other proxy data from the region during the last two millennia. The most obvious trend in the inferred temperatures from this study is a general decrease in all Tjul data during the last 2000 years.
Medieval Warm Period
Pollen-inferred Tjul values on average 0.18 ± 0.56°C higher than present are inferred between 0 and 1100 AD, and temperatures -0.2 ± 0.47°C below present are inferred between 1100 and 1900 AD. No consistent temperature peak is observed during the ‘Medieval Warm Period’, ca. 900-1200 AD.
Little Ice Age
From the data presented in this paper the cooler period between 1100 and 1900 AD corresponds in general with the ‘Little Ice Age’ (LIA). Consistently with independent stable isotopic data, the composite pollen-based record suggests that the coldest periods of the LIA date to 1500 – 1600 AD and 1800 – 1850 AD.
Abrupt warming during the 20th century
An abrupt warming occurred at about 1900 AD and the 20th century is the warmest century since about 1000 AD (950 cal yr BP).