Comparing past and present climate – a tool to distinguish between natural and human-induced climate change

Abstract

Variations in climate over the last approx. 2000 years and a contrast in the winter climate regime of NW Europe and West Greenland may be ascribed to variations in the teleconnection pattern expressed by the North Atlantic Oscillation (NAO). This well-known climate phenomenon is, among others, reflected in an increased inflow of warm Atlantic Water into the Labrador Sea under conditions of a colder European winter regime (negative NAO indices). Cooling of the Labrador Sea appears to have been a feature typical for a milder European winter climate (positive NAO indices). This pattern is seen both on shorter, (sub)decadal timescales of the last approx. 60 years and on a longer, centennial timescale through the last millennia. During a significant part of the 20th century warming, episodes of milder winters in Europe may be related to Atlantic patterns of natural climate variability similarly as recorded in the past millennia. However, records for recent decades (since the mid 1980s) show a different pattern indicating a significant change in North Atlantic ocean- and atmospheric circulations. This seemingly changed behaviour of the climate system corresponds in timing to the episode of the most extensive global temperature increase, suggesting an increasing anthropogenic effect on the climate development of the past ca. 20 years.