Climate Extremes

This panoply of proxy climate indicators records a startling array of paleoclimates in Earth's history.

Climatic conditions on Earth have ranged from extreme icehouse conditions with, potentially, the entire planet covered in glaciers (a paleoclimate known as Snowball Earth) to extreme hothouse conditions, with atmospheric carbon dioxide concentrations as much as 20 times higher than those at present, and tropical forests extending nearly pole to pole. Earth's climate has also apparently resided everywhere in between these extremes and at times moved rapidly from one to another.

Whether it is because the current climate falls relatively in the middle of the climate spectrum, or because extremes are more likely to be preserved in the geological record, or because understanding of extremes may provide the greatest insight into the climate system as a whole, the extreme paleoclimate events are the most studied. In the realm of extreme warmth, there were the hothouse climates of the Cenomanian/Turonian boundary (90 million years ago) and the Early Eocene Climatic Optimum (52 million years ago), or nearer-term warm climates like the Miocene Climatic Optimum (14 million years ago), the mid-Pliocene warm period (3.5 million years ago) or the Altithermal of the middle Holocene (5,000 years ago).

At the other end of the spectrum lie the extreme cooling of Snowball Earth (630 million years ago), the rapid inception of large Antarctic ice sheets (35 million years ago), or the peak glaciation of the last glacial maximum (18,000 years ago). Equally fascinating, though even more difficult to quantify, are transient or abrupt climate changes such as Pleistocene Heinrich and Dansgaard-Oeschger events where circum North Atlantic temperature changed by as much as 9 degrees F (5 degrees C) in 30 or 40 years or the Initial Eocene Thermal Maximum, when temperatures in the Arctic Ocean reached 73 degrees F (23 degrees C) for 50,000 to 100,000 years. Climate extremes such as these in Earth's history, and incomplete explanations for them, help show that the current climate samples a very finite portion of Earth's climatic possibilities, and if scientists wish to have a solid understanding of what the climate of the future may hold, paleocli-mates must first be understood.

sEE ALsO: Cenozoic Era; CLIMAP Project; Climatic Data, Proxy Records; Earth's Climate History; Ice Ages; Meso-zoic Era; Paleozoic Era; Snowball Earth.

BIBLIOGRAPHY. R.S. Bradley, Paleoclimatology: Reconstructing Climates of the Quaternary (Harcourt Academic Press, 1999); T.J. Crowley and G.R. North, Paleoclimatology (Oxford University Press, 1995).

Jacob Sewall Virginia Tech

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