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Obliquity

FIGURE 12.21. Variations in eccentricity, precession, and obliquity over 300k y, starting 200k y in the past, through the present day and 100k y in to the future. From Berger and Loutre, (1992).

Variations in summer insolation in middle to high latitudes are thought to play a particularly important role in the growth and retreat of ice sheets: melting occurs only during a short time during the summer and ice surface temperature is largely determined by insolation. Thus cool summers in the northern hemisphere, where today most of the Earth's land mass is located, allow snow and ice to persist through to the next winter. In this way large ice sheets can develop over hundreds to thousands of years. Conversely, warmer summers shrink ice sheets by melting more ice than can accumulate during the winter.

Figure 12.22 shows insolation variations as a function of latitude and seasons during various phases of Earth's orbit. These can be calculated very accurately, as was first systematically carried out by Milankovitch. Note that fluctuations of order 30 Wm-2 occur in middle to high

Eccentricity Precession

Obliquity

FIGURE 12.21. Variations in eccentricity, precession, and obliquity over 300k y, starting 200k y in the past, through the present day and 100k y in to the future. From Berger and Loutre, (1992).

FIGURE 12.22. Insolation at the top of the atmosphere computed using the orbital solution of Berger and Loutre (1992). (a) Daily average intensity in Wm-2 contoured against latitude and month, indicating average conditions over the last two million years. (b) Modern insolation plotted as an anomaly from average conditions. (c) Insolation averaged during each maximum of obliquity over the last two million years and shown as an anomaly from average conditions. (d) Similar to (c) but for when Earth is closest to the Sun during northern hemisphere summer solstice.

FIGURE 12.22. Insolation at the top of the atmosphere computed using the orbital solution of Berger and Loutre (1992). (a) Daily average intensity in Wm-2 contoured against latitude and month, indicating average conditions over the last two million years. (b) Modern insolation plotted as an anomaly from average conditions. (c) Insolation averaged during each maximum of obliquity over the last two million years and shown as an anomaly from average conditions. (d) Similar to (c) but for when Earth is closest to the Sun during northern hemisphere summer solstice.

latitudes, a significant signal comparable, for example, to the radiative forcing due to clouds.

Astronomical forcing is an immensely appealing mechanism, offering a seemingly simple explanation of climate variability on timescales of tens to hundreds of thousands of years. It is widely applied in an attempt to rationalize the paleorecord. One of the most convincing pieces of evidence of astronomical periods showing up in the paleorecord are the fluctuations in á18O of calcite found in North Atlantic deep sea cores shown in Fig. 12.18 (left) over the past 2.5My. As discussed previously, an oscillation with a period of about 40ky, that of obliquity, can be seen by eye for the first 2M y of the record. However, the 100ky cycles at the end of the record (see also Fig. 12.19), which are signatures of massive glacial-interglacial cycles, may have little directly to do with orbital forcing, which has very little power at this period. Perhaps the 100Ky cycle is being set by internal dynamics of the ice sheets, almost independently of orbital forcing. Whatever the extent of orbital forcing, it must be significantly amplified by positive feedbacks involving some or all of the following: water vapor, ice-albedo interactions, clouds, ocean circulation, internal ice-sheet dynamics, among many other processes.

In summary, many theories have been put forward to account for the shape and period of oscillations of the kind seen in, for example, Figs. 12.18 and 12.19, but none can account for the observed record and none is generally accepted.

Abrupt Climate Change

As we have seen, over Earth history the climate of the planet has been in markedly different states, ranging from a "greenhouse" to an "icehouse." Moreover the paleorecord suggests that there have been very rapid oscillations between glacial and interglacial conditions. For example, the 50ky record of 518O shown in Fig. 12.18 (right), taken from an ice core in Greenland, reveal many large rightward spikes on millennial timescales (indicating frequent abrupt transitions to warmer followed by a return to colder conditions). These are called Dansgaard-Oeschger events (or D-O for short, after the geochemists Willi Dans-gaard and Hans Oeschger who first noted them) and correspond to abrupt warmings of Greenland by 5-10°C, followed by gradual cooling and then an abrupt drop to cold conditions again. They were probably confined to the N. Atlantic and are less extreme than the difference between glacial and interglacial states. Scientists have also found evidence of millennial timescale fluctuations in the extent of ice-rafted debris deposited in sediments in the North Atlantic—known as Heinrich events (after the marine geologist Hartmut Heinrich). They are thought to be the signature of intermittent advance and retreat of the sea-ice edge. D-O and Heinrich events are examples of what are called "abrupt" climate changes, because they occur on timescales very much shorter (10, 100, 1000 y) than that of external climate forcings, such as Milankovitch cycles, but long compared to the seasons. It is important to realize then that the LGM was not just much colder than today, but that it repeatedly and intermittently swung between frigid and milder climates in just a few decades. Indeed such erratic behavior is a feature of the last 100k y of climate history.

The general shift from colder, dustier conditions to warmer, less dusty conditions over the last 10ky or so seen in Fig. 12.23, is generally interpreted as the result of orbital-scale changes in obliquity and precession. Obliquity reached a maximum

10ky ago (see Fig. 12.21), enhancing the seasonal cycle and producing a maximum of summer insolation at all latitudes in the northern hemisphere (Fig. 12.22c), so making it less likely that ice survives the summer. Atmospheric CO2 concentrations may also have played a role (although it is not known to what extent they are a cause or an effect), increasing from 190 ppm to 280 ppm (Fig. 12.19). The combination of increased summer insolation and increased CO2 concentrations probably triggered melting of the massive northern ice sheets, with ice-albedo feedbacks helping to amplify the shifts. It is thought that huge inland lakes were formed, many times the volume of the present Great Lakes, which may have intermittently and suddenly discharged into the Arctic/Atlantic Ocean. As can be seen in Fig. 12.23, the warming trend after the last ice age was not monotonic but involved large, short-timescale excursions. Evidence from the deposits of pollen of the plant Dryas octopetala, which thrives today in cold tundra in Scandinavia, tells us that 12ky ago or so, warming after the LGM was punctuated by a spell of bitter cold, a period now known as the Younger Dryas. Further evidence for this cold period, together with numerous other fluctuations, come from Greenland ice cores such as that shown in Fig. 12.18 (right). Along with the longer-term trends, one observes (Fig. 12.23) spectacular, shorter-term shifts, of which the Younger Dryas is but one.

After the last ice age came to an end, the climate warmed up dramatically to reach present day conditions around 10ky ago. Since then climate has settled in to a relatively quiescent mode up until the present day. This period—the last 10ky—is known as the Holocene. There was a warm climatic optimum between 9k and 5k y ago, during which, for example, El Nino appears to have been largely absent. The relatively benign climate of the Holocene is perhaps the central reason for the explosion in the development of human social and

FIGURE 12.23. The transition from the Last Glacial Maximum to the relatively ice-free conditions of the Holocene took roughly ten thousand years. In certain regions this transition was punctuated by rapid climate variations having timescales of decades to millennia. Shown is the GISP2 ice-core (Grootes and Stuiver, 1997) with shading indicating the return to glacial-like conditions, a period known as the Younger Dryas. The Younger Dryas is a prominent feature of many North Atlantic and European climate records and its presence can be detected in climate records across much of the Northern Hemisphere.

FIGURE 12.23. The transition from the Last Glacial Maximum to the relatively ice-free conditions of the Holocene took roughly ten thousand years. In certain regions this transition was punctuated by rapid climate variations having timescales of decades to millennia. Shown is the GISP2 ice-core (Grootes and Stuiver, 1997) with shading indicating the return to glacial-like conditions, a period known as the Younger Dryas. The Younger Dryas is a prominent feature of many North Atlantic and European climate records and its presence can be detected in climate records across much of the Northern Hemisphere.

economic structures, farming, and agriculture. Before the Holocene, agriculture was perhaps impossible in much of Northern Europe, because the variance in climate was so great.

A commonly held view is that an important mechanism behind rapid climate shifts is fluctuation of the ocean's thermohaline circulation discussed in Chapter 11.

The thermohaline circulation may have been sensitive to freshwater discharge from inland lakes formed from melting ice. The discharge of fresh water may have occurred intermittently and perhaps involved large volumes of fresh water sufficient to alter the surface salinity and hence buoyancy of the surface ocean. Let us return to Fig. 11.28 (bottom), which shows the ocean's meridional overturning circulation (MOC), with a deep-sinking branch in the northern North Atlantic. Warm, salty water is converted into colder, fresher water by heat loss to the atmosphere and fresh water supply from precipitation and ice flow from the Arctic. As discussed in Chapter 11, in the present climate the MOC carries heat poleward, helping to keep the North Atlantic ice-free. But what might have happened if, for some reason, fresh water supply to polar convection sites was increased, as was likely in the melt after the LGM, reducing salinity and so making it more difficult for the ocean to overturn? One might expect the MOC to decrease in strength,12 with a concomitant reduction in the supply of heat to northern latitudes by ocean circulation, perhaps inducing cooling and accounting for the abrupt temperature fluctuations observed in the record.

Theories that invoke changes in the ocean's MOC as an explanation of abrupt climate change signals, although appealing, are not fully worked out. Sea ice, with its very strong albedo and insulating feedbacks that dramatically affect atmospheric temperature, is a potential amplifier of climate change. Moreover sea ice can also grow and (or) melt rapidly because of these positive feedbacks and so is likely to be an important factor in abrupt climate change. A wind field change could also account for the observed correlation between reconstructed Greenland temperatures and deep sea cores, with changes in ocean circulation being driven directly by the wind. Moreover, the wind field is likely to be very sensitive to the presence or absence of ice, because of its elevation, roughness, and albedo properties.

12.3.6. Global warming

Since the 1950s, scientists have been concerned about the increasing atmospheric concentrations of CO2 brought about by human activities (cf. Fig. 1.3). The problem, of course, is that the carbon locked up in the oil and coal fields, the result of burial of tropical forests over tens of millions of years, are very likely to be returned to the atmosphere in a few centuries. As already mentioned, by the end of this century atmospheric CO2 concentrations are likely to reach 600 ppm, not present on the Earth for perhaps 10My (Fig. 12.14). There is concern that global warming will result and indeed warming induced by human activity appears to be already underway. Figure 12.24 shows temperature reconstructions of northern hemisphere surface air temperature during the last 1100 y together with the instrumental record over the past 150 y or so. The spread between the reconstructions indicates a lower bound on the uncertainty in these estimates. Even after taking due note of uncertainty and that the temperature scale is in tenths °C, the rapid rise in the late twentieth century is alarming and, should it continue, cause for concern.

Global warming could occur gradually, over the course of a few centuries. However, some scientists speculate that the climate might be pushed into a more erratic state that could trigger abrupt change. If the atmosphere were to warm, so the argument goes, it would contain more water vapor, resulting in an enhancement of meridional water vapor transport, enhanced precipitation over the pole, a suppression of ocean convection, a reduction in the intensity of the MOC, thence a reduction in the meridional ocean heat transport, and so an abrupt cooling of the high-latitude climate.

Even though the possibility of imminent abrupt climate change is small and, as far as we know, even less likely to occur in warm periods such as our own, it must be taken very seriously because the impacts on the environment and humanity would be so large, the more so if the transition were to be very abrupt. As we have seen,

12There is a commonly held misconception that a weakening of the Atlantic MOC is synonymous with a weakening of the Gulf Stream. As discussed in detail in Chapter 10, the Gulf Stream is a wind-driven phenomenon whose strength depends on the wind and is not directly related to buoyancy supply.

FIGURE 12.24. Estimates of Northern Hemisphere surface air temperature during the last 1100 years. Temperatures obtained from instruments (Jones and Moberg, 2003) are shown in black. Colored curves indicate different proxy reconstructions of temperature. Proxies, such as tree rings, ice cores, and corals, are necessary for estimating temperature before widespread instrumental coverage, before about 1850. The spread between the reconstructions indicates a lower-bound on the uncertainty in these estimates. All records have been smoothed using a 20-year running average and adjusted to have zero-mean between 1900 and 1960.

FIGURE 12.24. Estimates of Northern Hemisphere surface air temperature during the last 1100 years. Temperatures obtained from instruments (Jones and Moberg, 2003) are shown in black. Colored curves indicate different proxy reconstructions of temperature. Proxies, such as tree rings, ice cores, and corals, are necessary for estimating temperature before widespread instrumental coverage, before about 1850. The spread between the reconstructions indicates a lower-bound on the uncertainty in these estimates. All records have been smoothed using a 20-year running average and adjusted to have zero-mean between 1900 and 1960.

the paleorecord suggests that such events have happened very rapidly in the past (on timescales as short as a decade). Moreover, climate models support the idea that the ocean's MOC, with its coupling to ice and the hydrological cycle, is a sensitive component of the climate system. We simply do not know the likelihood of an abrupt climate shift occuring in the future or, should it do so, the extent to which human activities may have played a role.

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