Earth’s climate has undoubtedly changed throughout its long history. The amount of energy from the sun has fluctuated, as have the amount of sunlight reflected into space and the amount of heat trapped by the atmosphere. Fossils reveal some clues to our climatological history, but they cannot tell the whole story. Fortunately, scientists have additional ways of knowing about the climates of the past.
For example, coral animals produce annual growth rings in their skeletons, providing records about water temperature and salinity that may span a few hundred years. Likewise, tree rings can reveal the temperature and rainfall in a particular region over the past few thousand years.
But what about records spanning a longer period? Here, climate scientists turn to a nonliving source of information: ice cores. Scientists have drilled into the thick ice sheets covering Greenland and Antarctica, collecting core samples spanning hundreds of thousands of years. Thanks to seasonal changes in the amount of snowfall, the cores have annual layers that researchers can count to measure time. The thickness of each layer indicates the amount and type of precipitation for each year. Moreover, bubbles in the cores contain air samples trapped when the snow was compressed into ice. Analyzing these bubbles reveals a continuous record of changes in the gas composition of the atmosphere. Other data, ranging from oxygen isotopes to pollen, yield valuable clues about the prevailing temperature at the time the ice formed.