TiiiiIiiii IiiiiIiiii

2000 1500 1000 500 0

Thousands of years before present

Figure 1.12: Benthic foram oxygen isotopes from the Ocean Drilling Program Core ODP 849 (see Mix et al in the Further Readings for this chapter). Values are reported relative to the PDB standard. The core is located in the tropical Pacific, but the benthic data is representative of the global climate state. Note that the vertical axis has been reversed, so that upward excursions represent warmer and less icy times.

as the orbital parameters of Mars also show Milankovic cycles. Compared to Earth, however, the study of how these cycles are expressed in the paleoclimate record of Mars is in its very infancy.

1.10.2 Ice core records

The ice at the base of the Antarctic ice sheet is about a million years old, so one can also retrieve a record of past climate by drilling cores into the ice. Many aspects of climate are recorded in the ice, but the ones that will concern us here are stable isotopes of water (JD and J18O), recorded in the ice itself, and composition of past air preserved in bubbles within the ice. The stable isotopes are essentially recorders of temperature; the ice becomes more isotopically light as the polar temperature becomes colder, since more of the heavy isotopes have been distilled out in that case.

The upper panel of Fig. 1.13 shows the JD time series from the Vostok and Epica ice cores in the Antarctic. The Vostok data is systematically below the Epica data because Vostok is further inland, higher and colder, but otherwise tracks the Epica data. This record only covers the period within which the glacial-interglacial cycles have already settled into a 100,000 year cycle; older ice is too distorted to yield a useful record. This record confirms the 100,000 year cycle seen in the marine cores, and also confirms the asymmetry between slow buildup of glaciers and rapid deglaciation. It also shows that there is a strong Antarctic warming and cooling that occurs in association with the glacial/interglacial cycles.

The CO2 data is shown in the lower panel of Fig. 1.13. In the course of the glacial/interglacial cycles, CO2 fluctuates between 180 parts per million (by count of molecules) and 300 parts per million. Moreover, the fluctuation is very nearly synchronous with the warming and cooling. The correlation between CO2 and temperature does not determine which causes which (or whether they mutually reinforce each other), but since CO2 is a greenhouse gas, it is certain that the rise in CO2 warms the planet and reinforces the termination of ice ages, whereas conversely the fall of CO2 enhances cooling and reinforces the onset of glaciation. The origin of the glacial/interglacial CO2 cycle is another of the Big Questions. Our understanding of glacial/interglacial cycles cannot be complete without resolving this question.

Greenland ice also records past climate, as seen in Fig. 1.14. The base of the Greenland ice cap is not as old as the Antarctic ice, so one can only go back in time about 100,000 years here. By way of compensation, though, the rate of snow accumulation in Greenland is much higher than in Antarctica, so one can see the past with much higher time resolution.

The Greenland record records a sharp deglaciation leading to the modern era, consistently with the Antarctic record. However, we can see in Greenland that there are many high frequency temperature fluctuations embedded within the glacial period -especially the period from about 60,000 years ago to 10,000 years ago. These don't have a strict periodicity, but have a time scale on the order of a thousand years, and hence are collectively referred to as millennial variability. The expressions of millennial variability seen in Greenland isotopes are called Dansgaard-Oeschger events after their discoverers. There are many other climate proxies that reflect millennial variability of the sort seen in Greenland isotopes, and these often represent precipitous switches in the climate state - referred to as abrupt climate change.. One of the most striking of these abrupt change events occurs as the planet is coming out of the most recent ice age - the Last Glacial Maximum (or LGM). The spike marked "B" in the figure is the Bolling warm period, and represents a recovery of the climate to full interglacial warmth. However, in the wake of the Bolling the climate abruptly reverted to full glacial temperatures, in an event known as the Younger Dryas (marked "YD" in the figure). The Younger Dryas is believed to have been triggered by a massive draining

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