The Earth's past climate-including temperature and elements in the atmosphere-has recently been studied by analyzing ice samples from Greenland and Antarctica. The air bubbles in the ice have shown that, over the past 160,000 years, there has been a close correlation between temperature changes and level of natural greenhouse gases carbon dioxide and methane. One recent analysis from Greenland showed that at the end of the last glacial period (when the great ice sheets began to retreat to their present position), temperatures in southern Greenland rose from 5 to 7 degrees in about 100 years. Air bubbles are not the only method of determining characteristics of the Earth.s ancient climate history. Analysis of dust layers from ancient volcanic activity is another such method; as is the study of ice cores, which interpret past solar activity that may have affected our climate. This passage states that: