Which of the following is an accurate description of a method used to study short-term climate change?(A) The oxygen-isotope ratios of glacial ice record changes in global temperature for the past 800,000 years.(B) All of the possible answers are correct.(C) The color of a tree ring indicates the rate of growth in a given year.(D) Pollen is useful for studying modern-day climate but cannot be used to study ancient climate because pollen is not preserved in sediment.

Tree rings form because during each growth season new water and food conducting cells (tracheids) are added around the perimeter of the tree trunk. Cells in the spring growth tend to be larger with thinner walls than the previous set of cells produced at the end of the previous summer. Over the course of the growing season, successive rings of cells become smaller with increasingly thick walls. In winter, growth ceases and no new cells are laid down. Then when the new growing season begins, thin-walled large cells form again producing a clear line between the old wood and the new wood because of the difference in texture.

Tree rings provide a record of past climate because their width is determined by tree growth rate, which in turn is determined by environmental conditions. Since one ring is produced every year (usually) the ages of the climatic events can be worked out very precisely by counting back. Records from young trees, old trees, house and ship timbers and fossil trees can be tied together by identifying sections with the same sequence of climatic event, the records overlap in time where the climatic patterns they record match up.