Palaeoclimate reconstruction from isotope data

The isotope 18O is present in natural oxygen at a concentration of about 1 part in 500 compared with the more abundant isotope 16O. When water evaporates, water containing the lighter isotope is more easily vaporised, so that water vapour in the atmosphere contains less 18O compared with sea water. Similar separation occurs in the process of condensation when ice crystals form in clouds. The amount of separation between the two oxygen isotopes in these processes depends on the temperatures at which evaporation and condensation occur. Measurements on snowfall in different places can be used to calibrate the method; it is found that the concentration of 18O varies by about 0.7 of a part per 1000 for each degree of change in average temperature at the surface. Information is therefore available in the ice cores taken from polar ice caps concerning the variation in atmospheric temperature in polar regions during the whole period when the ice core was laid down.

Since the ice caps are formed from accumulated snowfall which contains less 18O compared with sea water, the concentration of 18O in water from the oceans provides a measure of the total volume of the ice in the ice caps; it changes by about 1 part in 1000 between the maximum ice extent of the ice ages and the warm periods in between. Information about the 18O content of ocean water at different times is locked up in corals and in cores of sediment taken from the ocean bottom, which contain carbonates from fossils of plankton and small sea creatures from past centuries and millennia. Measurements of radioactive isotopes, such as the carbon isotope 14C, and correlations with other significant past events enable the corals and sediment cores to be dated. Since the separation between the oxygen isotopes which occurs as these creatures are formed also depends on the temperature of the sea water (although the dependence is weaker than the other dependencies considered above) information is also available about the distribution of ocean surface temperature at different times in the past.

was formed - gases such as carbon dioxide or methane. Dust particles that may have come from volcanoes or from the sea surface are also contained within the ice. Further information is provided by analysis of the ice itself. Small quantities of different oxygen isotopes and of the heavy isotope of hydrogen (deuterium) are contained in the ice. The ratios of these isotopes that are present depend sensitively on the temperatures at which evaporation and condensation took place for the water in the clouds from which the ice originated (see box). These in turn are dependent on the average temperature near the surface of the Earth. A temperature record for the polar regions can therefore be constructed from analyses of the ice cores. The associated changes in global average temperature are estimated to be about half the changes in the polar regions.

Such a reconstruction from a Vostok core for the temperature and the carbon dioxide content is shown in Figure 4.6a for the past 160 000 years, which includes the last major ice age that began about 120 000 years ago and began to come to an end about 20 000 years ago. Figure 4.6b extends the record to

650 000 years ago. The close connections that exist between temperature, carbon dioxide and methane concentrations are evident in Figure 4.6. Note also from Figure 4.6 the likely growth of atmospheric carbon dioxide during the twenty-first century, taking it to levels that are unlikely to have been exceeded during the past 20 million years.

Further information over the past million years is available from investigations of the composition of ocean sediments. Fossils of plankton and other small sea creatures deposited in these sediments also contain different isotopes of oxygen. In particular the amount of the heavier isotope of oxygen (18O) compared with the more abundant isotope (16O) is sensitive both to the temperature at which the fossils were formed and to the total volume of ice in the world's ice caps at the time of the fossils' formation that is linked to the global sea level. For instance, from such data it can be deduced that the sea level at the last glacial maximum, 20 000 years ago, was about 120 m lower than today and that during the last interglacial period, about 125 000 years ago, it was likely between 4 and 6 m higher than today due to some melting of the polar ice caps in both Greenland and Antarctica.

From the variety of palaeoclimate data available, variations in the volume of ice in the ice caps can be reconstructed over the greater part of the last million years (Figures 4.6b, lower curve, and 4.7c). In this record six or seven major ice ages can be identified with warmer periods in between, the period between these major ice ages being approximately 100 000 years. Other cycles are also evident in the record.

The most obvious place to look for the cause of regular cycles in climate is outside the Earth, in the Sun's radiation. Has this varied in the past in a cyclic way? So far as is known the output of the Sun itself has not changed to any large extent over the last million years or so. But because of variations in the Earth's orbit, the distribution of solar radiation has varied in a more or less regular way during this period.

Three regular variations occur in the orbit of the Earth around the Sun (Figure 4.7a). The Earth's orbit, although nearly circular, is actually an ellipse. The eccentricity of the ellipse (which is related to the ratio between the greatest and the least diameters) varies with a period of about 100 000 years; that is the slowest of the three variations. The Earth also spins on its own axis, the axis of spin being tilted with respect to the axis of the Earth's orbit, the angle of tilt varying between 21.6° and 24.5° (currently it is 23.5°) with a period of about 41 000 years. The third variation is of the time of year when the Earth is closest to the Sun (the Earth's perihelion). The time of perihelion moves through the months of the year with a period of about 23 000 years (see also Figure 5.19); in the present configuration, the Earth is closest to the Sun in January.

21.6 degrees

24.5 degrees

Earth

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&200-tfl

400-

600-

21.6 degrees

24.5 degrees

Earth

Ice volume

Figure 4.7 Variations in the Earth's orbit (a), in its eccentricity, the orientation of its spin axis (between 21.6° and 24.5°) and the longitude of perihelion (i.e. the time of year when the Earth is closest to the Sun; see also Figure 5.19), cause changes in the average amount of summer sunshine (in millions of joules per square metre per day) near the poles (b). These changes appear as cycles in the climate record in terms of the volume of ice in the ice caps (c).

As the Earth's orbit changes its relationship to the Sun, although the total quantity of solar radiation reaching the Earth varies very little, the distribution of that radiation with latitude and season over the Earth's surface changes considerably. The changes are especially large in polar regions where the variations in summer sunshine, for instance, reach about 10% (Figure 4.7b). James Croll, a British scientist, first pointed out in 1867 that the major ice ages of the past might be linked with these regular variations in the seasonal distribution of solar radiation reaching the Earth . His ideas were developed in 1920 by Milutin Milankovitch, a climatologist from Yugoslavia, whose name is usually linked with the theory. Inspection by eye of the relationship between the variations of polar summer sunshine and global ice volume shown in Figure 4.7 suggests a significant connection. Careful study of the correlation between the two curves confirms this and demonstrates that 60% of the variance in the climatic record of global ice volume falls close to the three frequencies of regular variations in the Earth's orbit, thus providing support for the Milankovitch theory.4

Summer sunshine

Ice volume

More careful study of the relationship between the ice ages and the Earth's orbital variations shows that the size of the climate changes is larger than might be expected from forcing by the radiation changes alone. Other processes that enhance the effect of the radiation changes (in other words, positive feedback processes) have to be introduced to explain the climate variations. One such feedback arises from the changes in carbon dioxide influencing atmospheric temperature through the greenhouse effect, illustrated by the strong correlation observed in the climatic record between average atmospheric temperature and carbon dioxide concentration (Figure 4.6). Such a correlation does not, of course, prove the existence of the greenhouse feedback; in fact part of the correlation arises because the atmospheric carbon dioxide concentration is itself influenced, through biological feedbacks (see Chapter 3), by factors related to the average global temperature.5 Further, since Antarctic temperature started to rise several centuries before atmospheric carbon dioxide during past glacial terminations, it is clear that carbon dioxide variations have not provided the trigger for the end of glacial periods. However, as we shall see in Chapter 5, climates of the past cannot be modelled successfully without taking the greenhouse feedback into account.6

An obvious question to ask is when, on the Milankovitch theory, is the next ice age due? It so happens that we are currently in a period of relatively small solar radiation variations and the best projections for the long term are of a longer than normal interglacial period leading to the beginning of a new ice age perhaps in 50 000 years' time.7

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