Paleoclimate is the circumstance of much earlier mean meterological condtions, for times prior to instrumental weather measurements. Most discussions of paleoclimate relate to prehistoric times, where written records of meteorology are not extant. Paleoclimatologists use information from natural climate proxies, such as tree rings, ice cores, corals, and ocean and lake sediments, that record variations in past climate.
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Records of past climate from these proxy records are important for several reasons. Instrumental records of climate are limited in many parts of the world to the past 100 years or less, and are too short to assess whether climate variability, events, and trends of the 20th and 21st centuries are representative of the long-term natural variability of past centuries and millennia. For example, was the 1930s Dust Bowl drought, a widespread and severe event in the United States, a rare occurrence or have similar events occurred in past centuries?
Knowledge of the long-term natural variability of the Earth Climate system, and its causes, will also allow an understanding of the roles of natural climate variability and human-induced climate change in the current and future climate. In particular, reconstructed temperatures from proxy data for the past 1000 years have allowed an assessment of the warming over recent decades and indicate that at least part of this warming is due to the impact of human activities on climate, according to the Intergovernmental Panel on Climate Change.
Geological evidence demonstrates that the Earth's climate is dynamic, and has varied widely from our everyday experience. Over the past two million years, numerous glacial periods have covered much of the northern hemisphere in glacial ice, dropped sea level as much as 125 meters, and significantly cooled even tropical regions. In the more distant past, the Cretaceous Period was significantly warmer than today, with less polar ice, raising sea levels and allowing warm weather organisms to thrive at higher latitudes than previous. In the past 150 years, instrumental weather records indicate that the Earth has warmed by approximately 0.6°C.
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