New research shows that human pollution of the atmosphere with acid is now almost back to the level that it was before the pollution started with industrialization in the 1930s. The results come from studies of the Greenland ice sheet and are published in the scientific journal, Environmental Science and Technology.The Greenland ice sheet is a unique archive of the climate and atmospheric composition far back in time. The ice sheet is made up of snow that falls and never melts, but rather remains year after year and is gradually compressed into ice. By drilling ice cores down through the kilometre-thick ice sheet, the researchers can analyse every single annual layer, which can tell us about past climate change and concentration of greenhouse gases and pollutants in the atmosphere.