Paleoenvironmental reconstruction (also known as paleoclimate reconstruction) refers to the results and the investigations undertaken to determine what the climate and vegetation were like at a particular time and place in the past. Climate, including vegetation, temperature, and relative humidity, has varied considerably during the time since the earliest human habitation of planet earth, from both natural and cultural (human-made) causes. Researchers rely on modern analogs; that is to say, they compare the findings from the past to those found in current climates around the world. However, there are periods in the very ancient past when the climate was completely different from what is currently being experienced on our planet. In general, those situations appear to be the result of climate conditions that had more extreme seasonal differences than any we’ve experienced today. It is particularly important to recognize that atmospheric carbon dioxide levels were lower in the past than those present today, so ecosystems with less greenhouse gas in the atmosphere likely behaved differently than they do today. Climatologists primarily use paleoenvironmental data to understand how the environment of our world has changed and how modern societies need to prepare for the changes to come. Archaeologists use paleoenvironmental data to help understand the living conditions for the people who lived at an archaeological site. Climatologists benefit from the archaeological studies because they show how humans in the past learned how to adapt or failed to adapt to environmental change, and how they caused environmental changes or made them worse or better by their actions. The data that are collected and interpreted by paleoclimatologists are known as proxies, stand-ins for what can’t be directly measured. We can’t travel back in time to measure the temperature or humidity of a given day or year or century, and there are no written records of climatic changes that would give us those details older than a couple of hundred years. Instead, paleoclimate researchers rely on biological, chemical, and geological traces of past events that were influenced by the climate. The primary proxies used by climate researchers are plant and animal remains because the type of flora and fauna in a region indicates the climate: think of polar bears and palm trees as indicators of local climates. Identifiable traces of plants and animals range in size from whole trees to microscopic diatoms and chemical signatures. The most useful remains are those that are large enough to be identifiable to species; modern science has been able to identify objects as tiny as pollen grains and spores to plant species. Proxy evidence can be biotic, geomorphic, geochemical, or geophysical; they can record environmental data that range in time from yearly, every ten years, every century, every millennium or even multi-millennia. Events such as tree growth and regional vegetation changes leave traces in soils and peat deposits, glacial ice and moraines, cave formations, and in the bottoms of lakes and oceans.