In recent times, the levels of certain ice core impurities have risen dramatically because of human activities. For example, the amounts of sulphuric acid have been elevated since the initiation of the industrialization mainly due to extended fossil fuel burning, and the concentrations of lead in the ice has risen significantly because of its use as a gasoline additive. Human pollution has, however, a long record. For example, the mining activities during the Romans Empire caused elevated levels of lead in the atmosphere and in the ice sheets.
Sulphate and nitrate levels during the last century
Sulphate and nitrate has been measured in samples from the Greenland ice cores.
The volcanic imprint is seen in the early part of the record: Katmai (1912), Hekla (1947) and Bezymianny (1956) all caused elevated levels of sulphate, but during the 1940s through the 1970s, the increased use of sulphur-rich fossils fuels caused the background level of sulphate to rise to a level similar to or exceeding the volcanic peaks. In this period, sulphate pollution caused acid rain and severe damage to trees in some areas. Initiatives to cut emissions and clean the sulphate from exhaust gasses were initiated, and by 2000, the sulphate level in Greenland precipitation had dropped to levels similar to those a century earlier.
Nitrate is formed from nitrous oxides produced in vehicle engines during combustion of oil-based fuels. The sharp rise in the amount of cars in the 1950s and 1960s is strongly reflected in the nitrate curve, but although the number of cars has continued to increase, the nitrate level in Greenland ice cores has been almost constant since then, probably due to the increased use of catalytic converters.
The natural level of dust in ice sheets is extremely low (for central Greenland as low as 1 part in 1012, or pg/g), but nevertheless, the lead concentration can be measured. To measure the concentration of lead, scientists need considerable amounts of ice (typically in the kilogram range) due to the very low concentrations. Also, extreme care must be taken not to contaminate the samples.
The ice cores show that already in Roman times, human activities caused a 3-5-fold rise in the lead content of polar snow. The isotopic composition of lead differs between regions, and analysis of the isotopic composition of lead in samples from the Roman times point the Rio Tinto area of southwestern Spain as the most likely source area.
Concentrations rose again from 800 A.D., but from the mid-1900s, the use of lead additives in gasoline caused a dramatic increase in the lead content of Greenland snow, reaching more than 400 pg/g in the 1960s. Since then, the use of lead additives has been restricted, and the levels are now approaching pre-industrial levels.
Ice Core Drilling Projects