There are several methods that are used to get a sense of what the climate in an area was like in the past. Some methods help determine climate decades ago, 100s of years ago and even thousands to millions of years ago. One method is to examine fossils. Once it is known what type of climate certain plants and animals lived in, that information can be used as a proxy for climate. For example, tropical plants and fish fossils indicate that a region was once dominated by a tropical environment. Uplift and/or continental drift help contribute to changing a climate. For example, the uplift of the Rocky mountains turned a more tropical region into a higher elevation cooler climate.

Another method is to examine sediments. Sediments will contain not only fossils of plants and animals but also pollens and microorganisms. The type of plants, animals, pollens and microorganisms found can be used to infer climate. For example, regions with oil deposits indicate a region that was once dominated by tropical plants and animals since this is the source of the carbon in the oil deposits. A region that is now desert was once dominated by tropical plants in the past as evidenced by the oil deposits. Pollens can be used to determine the type of plant life present. This can also be used to infer climate.

Tree rings can be used to get a sense of weather and climate. Especially for non-tropical trees, each year a tree will lay down a new ring of growth. The health of the ring can indicate if the tree was stressed or normal. Conditions such as droughts, disease, and fires can restrict ring growth. Tree rings can be used to get a sense of rainfall pattern, temperature pattern and stressors.

The presence of coral reefs fossils can be used to assess past climate since particular corals tend to grow in a certain temperature range. The health of coral reefs can also be used to determine stressors that were present such as temperature that were too cool and inadequate light exposure (cloudy water).

Finally, another powerful method used to determine past climates is ice cores. This involves drilling through an ice sheet and examining what is trapped in the ice. Increasing depth is used to examine climate farther back in time. Higher concentration of CO2 can be used to infer a warmer climate since CO2 is more abundant when global temperatures are warmer.