TU-BERLIN Erläuterungen zur Vorlesung TWK an der TU-Berlin
Inst. f. Ökologie
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PD Dr. H. Kehl
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Sargasso Sea Surface Temperature (Bluemle et al., 1999)
 
 

Abb. A2-33/01: Sargasso Sea Surface Temperature.

"Figure 4.


This graph shows the Sargasso Sea surface temperature, which was derived from oxygen isotope ratios. This is an indicator of evaporation and, therefore, a proxy for sea-surface temperature. The Sargasso Sea is a two-million-mi2 body of water in the North Atlantic Ocean that lies roughly between the West Indies and the Azores from approximately 20-35oN. It is relatively static through its vertical column so that potential interference from mixing with other water masses and sediment sources is minimal. The isotopic ratios are derived from biotic debris that has precipitated onto the sea floor. Wide and abrupt variations in temperature are indicated. The relative temperature variations of the Little Ice Age (LIA) and the Medieval Warm Period (MWP) are prominently recorded in the data. Note that the temperature has been increasing since about 300 years before present (1700 A.D.) The horizontal line is the average temperature for this 3000-year period.
After Keigwin, L. D., 1996, The Little Ice Age and Medieval Warm Period in the Sargasso Sea: Science, v. 274, p. 1504-1508.
- [date of access: 05.03.08] 

The entirety of Holocene climatic history can be characterized as a sequence of 10 or more global-scale "little ice ages," fairly irregularly spaced, each lasting a few centuries, and separated by global warming events.

Direct instrumental measurements

Direct instrumental measurements indicate that the average temperature at the Earth's surface increased about 0.8oC from 1866 until 1998 (Figure 2). During this same time, the concentration of CO2 in the atmosphere increased from 280 to 353 parts per million volume. Because this period of time very nearly coincides with the industrial revolution, the supposition arose that the warming was caused by human activities. Most of the warming, however, took place before most of the CO2 increase occurred. Statistical analyses of the climate record since 1860 show that significant interannual and interdecadal variability occurred. This suggests that the warming had causes other than an increase in greenhouse gases alone. The increase in temperatures recorded by direct measurements may be part of a longer-term warming trend that began after the Little Ice Age and before the Industrial Epoch. Many poorly understood factors influence atmospheric CO2 concentrations. For example, because the current increase follows a 300-year warming trend, the observed increases in CO2 are of a magnitude that can be explained by oceans giving off gases naturally as temperatures rise.

Conclusions

A review of research on past temperatures and variations led us to the following conclusions:

  • Climate is in continual flux: the average annual temperature is usually either rising or falling and the temperature is never static for a long period of time.
  • Observed climatic changes occurred over widespread areas, probably on the global scale.
  • Climate changes must be judged against the natural climatic variability that occurs on a comparable time scale. The Little Ice Age, Medieval Warm Period, and similar events are part of this natural variability. These events correspond to global changes of 1-2oC.
  • Global temperatures appear to be rising, irrespective of any human influence, as Earth continues to emerge from the Little Ice Age. If the temperature increase during the past 130 years reflects recovery from the Little Ice Age, it is not unreasonable to expect the temperature to rise another 2 to 2.5 degrees Celsius to a level comparable with that of the Medieval Warm Period about 800 years ago. The Holocene Epoch, as a whole, has been a remarkably stable period with few extremes of either rising or falling temperatures, as were common during Pleistocene glacial and interglacial periods. Nevertheless, the Holocene has been, and still is, a time of fluctuating climate.
  • Climatic changes measured during the last 100 years are not unique or even unusual when compared with the frequency, rate, and magnitude of changes that have taken place since the beginning of the Holocene Epoch. Recent fluctuations in temperature, both upward and downward, are well within the limits observed in nature prior to human influence.

Editors note:
This article was summarized from "Rate and Magnitude of Past Global Climate Changes," which was published in Environmental Geosciences, volume 6, number 2, 1999, pages 63-75. The authors are John P. Bluemle (State Geologist of North Dakota, Bismarck, ND), Joseph M. Sabel (geologist with the U.S. Coast Guard in Oakland, CA), and Wibjörn Karlén (Professor of Physical Geography at the University of Stockholm, Sweden). In the Environmental Geosciences article the authors include citations to more than 70 peer-reviewed reports."

Adapted from: GLOBAL WARMING: A GEOLOGICAL PERSPECTIVE - [date of access: 17.05.07] 

 

See also:
Discussion of the mentioned basic publication Keigwin, L. D., 1996, The Little Ice Age and Medieval Warm Period in the Sargasso Sea.- Science, v. 274, p. 1504-1508 - inClimate Audit by Steve McIntyre and others "Sargasso Sea dO18"
- [date of access: 17.12.07] 

 
 
 
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