Proxy records are commonly used to reconstruct the Earth's climate beyond the period of direct scientific observation. Some commonly used proxy records
include ice cores, tree rings, ocean sediments, and coral records. Consistent results in the historical climatic data derived from these proxy records indicate the reliability of these records as accurate means of reconstructing past climates.
These records indicate large-scale changes in the Earth's climate in the past, including periods of complete global glaciation (Snowball Earth) and contrasting ice-free periods. During the past 900,000 years, the Earth has fallen into a pattern of glacial-interglacial cycles operating on 100,000-year timescales. Glaciations are long periods of cold temperatures, resulting in the growth of large land and sea ice masses extending outward from the poles. These glaciations tend to come to an abrupt end with rapid warming and consequent ice recession.
The last glaciation, the Wisconsin glaciation, ended approximately 10,000 to 11,000 years ago. During this glaciation, ice caps in the Northern Hemisphere extended to latitudes of approximately 45 degrees N, and were 1.8-2.5 mi. (3-4 km.) thick. Since the end of the Wisconsin glaciation, the Earth's climate has been in a period of relative climatic stability, a period commonly referred to as the Holocene (the past 10,000 years).
Synthesis of proxy records for the past 1,000 years in the Northern Hemisphere shows a gradual cooling for 900 years, culminating in the Little Ice Age (1400-1850). During the Little Ice Age, sea ice throughout the Arctic increased, and alpine glaciers across Europe expanded down valleys, covering farmland and destroying agriculture. Since the end of the Little Ice Age, the world has been in a state of declining ice extent.
Proxy records from the Southern Hemisphere are not as unanimous in their observations of 20th century warming. Some regions in the Southern Hemisphere, including Australia, New Zealand, and South America indicate the 20th century as a period of unprecedented warmth in the last millennium. However, ice core records from Antarctica do not indicate this same 20th-century warming. The data derived from both proxy records, and direct observation, indicate that climate does not always operate on a global scale, rather, changes occur on a more regional scale.
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