The Past Million Years

In the past 2.5 million years, the so-called Quaternary period, the Earth's climate has been marked by temperature swings between extended glacial periods, which were characterized by thick ice sheets covering large parts of North America, Northern Europe and Siberia, and interglacial times characterised by an ice covering only in Antarctica and sometimes Greenland, as is the case today. A schematic illustrating these glacial and interglacial swings and putting the temperature trends of this period in an historical perspective is displayed in Fig. 5.

Drilling is a valuable way of gaining information about the Pleistocene period. Sediment cores reaching a few million years back in time and ice core records dating back to about 800,000 years have been extensively analysed. The ice at the base of the Antarctic is about one million years old. The composition of the past air is

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Fig. 5 Temperature trends for the last 10 Million years. Note the different scalings on the time axis: the twentieth century is shown in linear scale. Earlier periods are shown in terms of increasing powers of ten but are linear within each period (figure adapted from Bureau of Meteorology, Australia)

preserved in bubbles within the ice and stable isotopes, derived from the ice itself, are essentially recorders of the temperature at that time. Changes in the atmospheric CO2 concentrations over the past 800 kyr, are illustrated as a composite of EPICA Dome C, Vostok and Taylor Dome ice cores together with the EPICA Dome C temperature anomaly record in Fig. 6; this figure is taken from Luthi et al. (2008). Similar data for methane can be found in Loulergue et al. (2008). Based on the analysis of ice records like the ones shown in Fig. 6 and an alignment with many marine sediment cores, it is clear that on time scales of tens of thousands of years or longer, temperature and some other climate parameters vary coherently with one another and that the variations are global in extent. Atmospheric CO2 and methane concentrations have risen and fallen synchronously with temperature. The correlation between CO2 and temperature does not, however, fully determine the underlying causes and which as detailed time assessments reveal (Pierrehumbert 2009), for many parts of the record CO2 is leading temperature by a few hundred years, thereby suggesting that there are different mechanisms acting as compared to those dominant in the present-day anthropogenic greenhouse warming. Over the last

Ch4 Ice Core

Fig. 6 Composite CO2 (upper curve) and CH4 (lower curve) concentrations from EPICA Dome C, Vostok and Taylor Dome ice cores cover the past 800 kyr. (from Luthi et al. 2008 and Loulergue et al. 2008). The temperature anomaly (middle curve) is the deviation from the mean temperature of the last millennium (Jouzel et al. 2007)

Fig. 6 Composite CO2 (upper curve) and CH4 (lower curve) concentrations from EPICA Dome C, Vostok and Taylor Dome ice cores cover the past 800 kyr. (from Luthi et al. 2008 and Loulergue et al. 2008). The temperature anomaly (middle curve) is the deviation from the mean temperature of the last millennium (Jouzel et al. 2007)

800,000 years, the Milankowitch cycles were the dominant causes and pace-makers for climate variability. Global temperatures cooled at irregular rates during the extended glacial epochs and rose much faster at the beginning of the interglacials. These interglacials recurred during the recorded period at intervals of roughly 100,000 years and had durations typically of about 15,000-20,000 years. The last glacial maximum occurred around 20,000 years ago. The pronounced climatic swings during this Quarternary period are believed to be driven by subtle variations in the orbit parameters, which affect the summer insolation at high Northern latitudes. When the summer insolation was relatively weak for longer periods, the winter snow could partially survive, leading to thick ice sheets over a time span of thousands of years, with the ice-albedo effect amplifying the orbital forcing. Although its correlation with temperature is not totally resolved, because CO2 is a greenhouse gas, it is fairly certain that a rise in CO2 concentration warms the planet and reinforces the termination of ice ages, whereas a decrease in CO2 concentration enhances cooling and reinforces the onset of glaciation.

The data shown in Fig. 6 are the longest and most carefully studied records of polar-ice climate histories published. The record tells us that the highest temperature occurred during this period around 130 kyr ago. The CO2 concentration during the past 800 kyr never exceeded the present-day value of 385 ppm. For the many interesting details, such as short-term fluctuations as disclosed by the analysis of the sediment and ice core records, the articles by Jouzel et al. (2007), Luthi et al. (2008), Loulergue et al. (2008) and the book by Pierrehumbert (2009) should be consulted. Interesting findings concerning the important Asian monsoon over the past quarter million years are reported by Wang et al. (2008).

The oceans, which cover more than two-thirds of our blue planet, definitely contribute as strongly as the large ice sheets to the timing of climate events. These waters move in a global circulation system, which is driven by subtle density differences and thereby transport huge amounts of heat energy. Ocean circulation is thus an active and highly nonlinear player in the global climate game. Increasingly clear evidence implicates ocean circulation in abrupt and dramatic climate shifts, such as sudden temperature changes in Greenland on the order of 5-10°C and massive surges of icebergs into the North Atlantic Ocean - events that have occurred repeatedly during the last glacial cycle (Rahmstorf 2002).

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