Paleoclimate; Ice Core and Lake Sediment Records

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.

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.

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.

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2-7: Paleoclimate; Ice Core and Lake Sediment Records