Documentary data provide evidence of Stockholm average winter to spring temperatures in the eighteenth and nineteenth centuries

Abstract

Swedish archives provide several types of documentary sources relating to port activities in Stockholm for the eighteenth and nineteenth centuries. These documentary sources reflect sea ice conditions in the harbour inlet and correlate well with late-winter to early-spring temperatures. Instrumental measurements of temperature in Stockholm began in 1756, which allow for careful empirical assessment of the proxies from that date. After combining proxy series from several sources to derive a mean time series, calibration and verification trials are made and a preliminary January-April temperature reconstruction is developed from 1692 to 1892. This series, which explains 67% of the temperature variance, is further verified against independent temperature data from Uppsala, which go back to 1722. This additional verification of the reconstruction also assesses the quality of the early instrumental data from Uppsala, which has potential homogeneity problems before 1739 as a result of the thermometer being located indoors. Our analysis suggests that before this date, the instrumental data may be 'too warm' and need correction. Together, the documentary and instrumental data identify the post-1990 period as the warmest in three centuries. Continuing assessment of the historical archives should result in some of the documentary records being extended back into the early sixteenth century, allowing the future development of a southern Swedish winter temperature reconstruction for the last -500 years.