evidence: On the polar ice sheets of Greenland and Antarctica, scientists can extract ice cores up to 3km deep, taking us back 123,000 years in Greenland and over 800,000 years in Antarctica. [...] Antarctic ice cores show us that the concentration of carbon dioxide was stable over the last millennium until the early 19th century. [...] Ice cores can also help us identify more recent changes in the Earth's atmosphere, such as the radioactive material released during nuclear testing in the 1950s, or the results of introducing unleaded petrol in the 1980s.