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.
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.
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]