Studying Climate Change
As far back as can be seen in the geologic record, global and regional climates have continually undergone change. For example, some areas in the state of Nebraska used to be desert, but now the sand dunes are covered with a thin layer of vegetation. In other parts of the world, such as the Middle East, a temperate climate became much drier over time. Thousands of years ago, Iraq was a lush, fertile land, and today it is mostly desert.
In order to describe trends in climate change, scientists need climate information from the past. 
Scientists study past climates by analyzing data from ice cores, fossils, the pollen record, and tree rings.
Ice Cores
Figure 35 shows one of the best tools for studying climate change: ice cores. In very cold regions such as Antarctica, snow falls each year, forming a new layer of the glacier. By drilling down into the glacier and extracting a core of ice, scientists can obtain a record that spans up to hundreds of thousands of years. The composition of each layer in the ice core reveals how cold it was, how dry and dusty it was, and how much carbon dioxide was in the atmosphere when the layer formed.
Fossils
The most abundant isotope of oxygen is oxygen-16. Oxygen-18, which has two more neutrons, is much rarer. During periods of cold climates, ocean water becomes richer in oxygen-18. Plants and animals living in the water incorporate that ratio of isotopes in their body parts. By analyzing the oxygen content in marine fossils, scientists can study climate change over long time periods, ranging from tens of millions of years to hundreds of millions of years.
Pollen and Tree Rings
Scientists use pollen records and tree rings to assess climate changes over shorter time periods, ranging from decades to centuries. Different kinds of plants thrive in hot or cold temperatures or in wet or dry climates. By identifying the kinds of pollen found in ice cores or lake sediments, scientists can learn about the climate history. Tree rings also reveal information about past climates. Depending on how favorable the climate is, trees grow by different amounts. Thick tree rings indicate warm and moist climates. Thin rings indicate cold and dry climates.
Figure 35 Ice cores from Antarctica reveal a climate record that goes back more than 800,000 years.
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