Earth’s climate system has changed dramatically in the past. The Figure shows a reconstruction from the Last Glacial Maximum (20,000 years ago) illustrating the large ice sheets (more than 3 km thick) that covered North America and Northern Europe at that time. Temperature reconstructions (color bar), based e.g. on fossils of marine microorganisms found in sea floor sediments and pollen from land plants found in lake sediments, are important to understand the long term behavior of the climate system and to test climate models that are used for projections of future climate.

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