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© Eugene S. Takle
February 2000, 2002, 2004, 2005, 2006
Introduction
Understanding present and future climates requires that we first understand some characteristics
of past climates. Past climates can be divided into two periods: (1) the recent past when we have had
instruments to measure and record climate variables over a large portion of the globe, and (2) all of the
history of planet Earth before the era of instrumental measurements.
Instrumental records of sufficient spatial coverage to represent global observations began in
about 1880 (some would say 1850 is a better starting date). Although we have scattered
observations before this time, they were not sufficiently widespread to represent global values.
It could be argued that even current measurements do not include some areas of the globe, say
Africa, to be considered to be truly global in scope. There is a period prior to the instrumental
record when we have anecdotal evidence of climate conditions, but these observations were regional
in scope and lacked a quantitative base. Examples include locations of glacier termini, date of
lake freeze-up, and memorable floods and droughts.
Examples of Proxy Data
Climates that are reconstructed by means other than instrumental measurements are called paleoclimates.
Data used to reconstruct climate variables are called proxy data. Examples of proxy data to be discussed
later include:
Climates of the Past
Warm climates of the past may be useful models of what we can expect in future climates. The
Pliocene Optimum occurred 3.3 million to 4.3 million years ago. At that time the CO2 level of
the atmosphere may have been greater than 600 ppm. The Eemian Optimum occurred 125-130 thousand
years ago and had a CO2 level somewhat less than current levels. The mid Holocene
(5,000 to 6,000 years ago) is another period of temporarily and relatively warm conditions.
Paleobotanic proxy data and oxygen isotopes are used to reconstruct evidence of climate
conditions this far in the past. It should be cautioned that large uncertainties accompany
any statements about climatic conditions from this far in the past. In some cases we have
sufficient information about external conditions (External
vs. Internal) to create computer model simulations
of past climates for comparison of sparsely distributed proxy data.
The first graph in Figure 1 shows relative temperature changes over the last 1 million years compared to the present global mean temperature, given as the horizontal dashed line. Variations on these time scales are due to variations in the earth's orbital motion about the sun and are known as the Milankovitch effects. It is notable that for most of the record of the last million years the earth was considerably colder than today. Each minimum of the record corresponded to an ice age. During some of these ice ages, the hydrological balance tipped toward more H2O being resident in ice masses than today. This caused sea level to drop as much as 120 m below current levels.
The second graph of Figure 1 shows the last 11,000 years, a period that contains the Holocene maximum. This period evidently was not accompanied by a CO2 increase as was the case for warm periods in the more distant past. The Younger Dryas was an abrupt cooling that occurred about 12,700-11,500 years ago that returned the Northern Hemisphere to full glacial conditions during a general warming trend as it was emerging from the last full glacial period. Keigwin and Boyle relate such changes to changes in the thermohaline circulation. Graph (c) shows the period of the last 1100 years, which includes the so-called "Little Ice Age", which in fact may have been a more regional rather than global phenomenon. Some evidence suggests that the there was a minimum in solar activity during the period of the Little Ice Age, so it could be that this cool period resulted from reduced solar irradiation.
During the Eemian interglacial (125,000 to 130,000 years ago) temperatures in the North Polar region were estimated to be as much as 8°C warmer than today (Figure 2), with expected impacts on polar ice. For the Holocene climatic optimum, temperatures in polar regions were 3°C warmer than present (Figure 3). Some regions such as Africa may have had up to 300% of the current precipitation during this period. It is notable that during both the Eemian and Holocene periods the maximum departure from current temperatures seem to be in the polar regions.
The paleoclimate program of the National Oceanic and Atmospheric Administration (NOAA) has a wealth of information on paleoclimate research. Start with the NOAA paleoclimate primer and follow the many links imbedded therein.
Borehole Temperatures
Evidence for warming of the planet in the last five centuries can be found in the
geothermal observations from
boreholes on several continents.
The procedure for recovering information about global warming from these records is
described by NOAA.
Data from boreholes at least 200 m deep, and in some cases as much as 600 m deep, have been
analyzed. Data above 20 m have been omitted because they include annual variability which
obscures the global warming trend, and data below 600 m have not been included because they
contain no information about the past 500 years. Comparison of the borehole temperatures
with surface air temperatures in the last 150 years show that the two datasets are in
agreement, and there is some suggestion that the rate of warming in the geothermal data has increased in
the 20th century in agreement with the accelerated warming of the air temperature record.
Coral Skeletons
Ocean coral are very sensitive to changes in sea-surface temperatures (SST). Given
constant
sea water O18 concentration and biological processes, variability in the oxygen
isotopic composition (O18) of coralline aragonite is a function of the sea surface temperature
in which the coral secreted its skeleton. Therefore measurements of O18 in the layered
structure of the coral is a proxy record of sea-surface temperature. Data on
coral from several locations
can be found at the NOAA site .
Tree Ring Reconstruction of
the Last 1,000 Years
Michael E. Mann and Raymond S. Bradley of the Department of Geosciences, University of
Massachusetts and Malcom K. Hughes of the Laboratory of Tree-Ring Research, University of
Arizona reconstruct Northern
Hemisphere temperatures during the past millennium by building on recent studies
using various proxy data networks. They have examined both mean values and uncertainties
in the proxy data. Large uncertainties prior to AD 1400 preclude definitive statements
for that period. However, their results suggest that the last 100 years of the 20th century is anomalous
in the context of at least the past thousand years. The decade of the 1990s was the
warmest of the record, and 1998 the warmest year, at moderately high levels of confidence.
The 20th century warming represents an abrupt and opposite change from the pre-existing
millenial-scale cooling trend that is consistent with long-term astronomical forcing.
Fossil Pollen and Glacier Termini
Fossil pollen records have permitted
vegetation maps to be constructed of species distributions for various times in the past, and
tree rings have been used to determine the existence and geographical distributions of past major droughts.
Geographical distributions of pollen
from past periods can be compared with
climate conditions simulated for past periods by global climate models as checks
on the validity of such models. Glacial termini
(the farthest point down the mountain valley that a glacier extends) have been observed and
recorded by historical records and by the glacial deposits they leave behind.
Measurement of glacial termini reveal glacial retreat on all continents.
(Figure 4)
From these various proxy data, it should be apparent that a wealth of paleoclimate data are available to extend our record of the earth's surface temperature much farther back in time than the beginning of recorded temperatures.