Long-term (1850-2010) trends of black carbon deposition in the European Arctic

The climate warming effect of black carbon (BC) is amplified in the Arctic due to its deposition on light surfaces, decreasing reflectivity and hastening snow and ice melt. Monitoring indicates that Arctic atmospheric BC concentrations have declined by 40 % between 1990 and 2009. However, ca. 90 % of BC is wet-deposited in the Arctic and therefore mostly not recorded by atmospheric measurements. Consequently, atmospheric BC concentration and BC deposition trends may diverge. To get a comprehensive view on the climate impact of BC in the Arctic measurements of BC deposition are essential. Ice cores and lake sediments accumulate direct evidence of contamination deposition in chronological order. Despite their importance in deciphering the role of BC in Arctic climate change, relatively few deposition records exist at present. We analyzed a 300-year ice core from Svalbard and four 150-year lake sediment records from northern Finland. Unexpectedly, the ice core and lake sediment records show a pronounced increase in BC deposition from ca. 1970 to the present. The observed increase may be caused by within-Arctic emissions, such as Russian flaring. Additionally, chemical transport model results suggest that the increase may be caused by increased scavenging efficiency of BC due to increased temperatures and precipitation. These results contradict the prevailing understanding of declining BC values in the Arctic based on atmospheric monitoring, Greenland ice cores and modelling. The fact that a similar increasing BC deposition trend was recorded in two separate environmental archives receiving partly different atmospheric transport, highlights the plausibility of such a trend, and implies that it may also be observable at other Arctic locations. Thus, BC may have exerted a more significant Arctic warming impact than currently considered. More BC deposition records are urgently needed to comprehensively decipher the significance of BC in past, present and future Arctic climate change.