Greenland ice records human efforts to cut pollution. But after about 1990, different chemical pollutants no longer followed the same path. A new study, jointly conducted by University of Science and Technology of China, South Dakota State University and University of Washington and published in Advances in Atmospheric Sciences , follows sulfate, nitrate and chloride through a century of rising industrial emissions and the decades of pollution controls that followed.
The study used a 79-meter ice core drilled at Summit in central Greenland, in 2007. Its annual chemical record covers the 1772-2006 period. In 2014 , Dr. Lei Geng, the corresponding author of the current study, and his colleagues used the same core to show that the Industrial Revolution has led to rise in atmospheric acidity, with consequences on reactive nitrogen cycles. That study also noted the decline of sulfate in Greenland snow since approximately 1980 in response to efforts, such as the U.S. Clean Air Act, to reduce air pollution. The new work further compares the ice chemical records, year by year, with historical emissions from North America, Europe and the former Soviet Union.
The comparisons indicate that before 1970, the ice core records tracked relatively well with emissions from industrial source regions. Sulfate and nitrate began rising around 1900, while chloride increase began around the middle of the twentieth century. Sulfate and nitrate reached the highest levels around the 1970s. Several lines of evidence, including statistical comparisons with regional inventories, identify North America as the main source region for the human-produced pollution deposited at Summit, with a secondary contribution from Europe.
After 1980, pollution controls reduced emissions in North America and Europe. Sulfate and nitrate in the ice initially fell as well, but that their responses to precursor emissions diverged, especially after about 1990. Sulfate in snow fell faster than North American sulfur dioxide emissions. By early twenty-first century, average sulfate concentration has come down close to the preindustrial level despite the fact that North American emissions remained substantial. While for nitrate in snow, its concentration stayed relatively high although human-produced nitrogen oxide emissions also moderately declined. Regarding chloride, a recently noted halogen specie important for atmospheric oxidation once activated in the air, its trends reflected the combined effect of variations in sea salt, human emissions and atmospheric reactions.
The paper discusses possible reasons for different responses of ice core impurities to emissions. Emissions cuts changed atmospheric acidity and oxidation conditions. With reduced acidity, more sulfur dioxide may have oxidized into sulfate inside clouds near the source region, producing larger particles that rain and snow could remove before they reached Greenland. Changes in North Atlantic circulation may also have weakened northward transport. Additional inputs from lightning, wildfires, thawing permafrost and shipping may have maintained the nitrate level, despite reduced nitrogen oxide emissions. Changing acidity can also affect long-range atmospheric transport efficiency of nitrate travels by altering the nitrate gas-particle distribution. Chloride is further influenced by reactions that release it from sea-salt particles. The available evidence cannot yet show how much each process contributed.
"The reasons for these changing trends of the aerosol components also matter for climate." says Dr. Geng, "Sulfate aerosols cool the atmosphere. If climate models cannot correctly reproduce the rapid post-1990 decline seen in the ice core records, they may underestimate how the contribution to recent Arctic warming from reduced aerosol cooling."
The researchers now would like ice-core concentration and isotope records that extend into the 2020s. Those records could test the proposed explanations and improve chemical-climate models. The broader goal is to use ice cores to track both human action and the atmosphere's response. Today's efforts to improve air quality and mitigate climate change may likewise leave a recognizable signal in the ice layers of the future.
On the Post-1970 Changes in Greenland Ice Core Sulfate and Other Chemical Impurities
28-Aug-2026