Detecting Arctic climate change using KÖppen climate classification

Ecological impacts of the recent warming trend in the Arctic are already noted as a decrease in tundra area with replacement by shrubs, and changes in the tree line. The potential impact of vegetation changes to feedbacks on the atmospheric climate system is enormous because of their large land surface area and the multiyear memory of the vegetation cover. While much of the vegetation information is anecdotical, and satellite NDVI (Normalized Difference Vegetation Index) estimates began in 1981, the Köppen climate classification can be used to relate surface types to monthly mean air temperature from the NCEP/NCAR reanalysis and station data, and thus serves as proxy data for earlier period. In Köppen classification, tundra area is defined as the warmest summer monthly mean temperature being less than 10°C. We found a decrease in tundra area from the mid 1970s to the present, similar to the trend of the NDVI data. The decreases are largest in NW Canada, and eastern and coastal Siberia. Köppen proxies show an increase in tundra area from the late 1950s to the 1970s. The calculated tundra area minimum in the 1950s was similar to present conditions, and occurred about 10 years after the previous 20th century temperature maximum.