Since pollen grains are well preserved in the sediment layers that form in the bottom of a pond, lake or ocean, an analysis of the pollen grains in each layer tell us what kinds of plants were growing at the time the sediment was deposited. Inferences can then be made about the climate based on the types of plants found in each layer. Since tree growth is influenced by climatic conditions, patterns in tree-ring widths, density, and isotopic composition reflect variations in climate. In temperate regions where there is a distinct growing season, trees generally produce one ring a year, and thus record the climatic conditions of each year. Trees can grow to be hundreds to thousands of years old and can contain annually-resolved records of climate for centuries to millennia.
Scientists drill through the deep ice to collect ice cores.
These cores contain dust, air bubbles, or isotopes of oxygen, that can be used to interpret the past climate of that area. Between 6 and 11 billion metric tons of sediment accumulate in the ocean and lake basins each year. Scientist drill cores of sediment from the basin floors.