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Recent History
Once Daniel Fahrenheit invented the modern thermometer in 1714 and the Fahrenheit scale of measuring temperature became standardized, records of temperatures for various locations started being kept. From these records scientists have been able to track global temperature variations for the past 300 years. In geologic time, this period is very short, but even these records show minor temperature cycles have occurred.
Tree Measures
Certain long-lived trees have been used to determine temperature variations that occurred during the time the tree was alive. In general, trees grow faster in warm years and slower in cool years. Some coastal redwoods are over 1,000 years old. By gauging the growth rings on long-lived trees, scientists can date which periods of the trees' lives were warmer or cooler. Comparing the findings from ancient trees in different parts of the world can indicate if the temperature cycles were localized or global.
Ice Measures
In certain areas, such as Greenland, Alaska and other subarctic regions, scientists bore into glaciers to determine temperature trends. Glaciers form when more snow falls in winter than can melt in the summer. The snow piles up and grows into glaciers. Just as tree rings show a tree's age, the winter and summer cycles are etched as layers in glacial ice. Glaciers grow more rapidly in cool cycles and very slowly in warm cycles.
Biological Clues
Most plants grow better in a specific temperature zone than in areas that are too hot or cool. That's why palm trees don't grow in Minnesota or white pines in Florida. So when scientists find fossil clues or other evidence to show warm-climate species of plants once existed farther into the cooler regions of the planet than they currently grow, it's an indication of past global warming. By dating these plants, it's possible to determine the historic time period when temperatures were warmer or cooler around the world.