Milankovic cycles on Earth

Earth's precessional cycle is shown in Figure 7.12. The precession angle increases at a nearly constant rate, completing a cycle every 22,000 years. Though the variation in rate is not evident over any one cycle, the rate is not exactly constant, and therefore the phase drifts over the course of hundreds of thousands of years.

The precessional cycle is very rapid, and the precession angle has changed markedly even over historical times. Eight thousand years ago, when the first Sumerians poured into the valleys of the Tigris and Euphrates, the star we now call Polaris (the "Pole Star", in the tail of the Little Bear) was about 40o of arc away from the star that the the North Polar axis then pointed to,and about which the constellations rotated at the time. The consequences of precession for change in seasonality are potentially highly consequential. In Figure 7.12, the July insolation at 65N is shown as a general indication of the magnitude of the seasonality effect; high northern July insolation in the precessional cycle goes with low January northern insolation, weak southern January (summer) insolation, and relatively strong southern July (winter) insolation. Ten thousand years ago, the Northern Hemisphere summer insolation was fully 40W/m2 greater than at present, and so the northern summers should have been considerably warmer than today, while the northern winters should have been considerably colder. The effect should show up especially over land, which is dark enough to absorb most of the solar radiation and has low enough thermal inertia to respond nearly instantaneously to seasonal changes. The climate system in its full glory is nonlinear and complex, so the response of climate to this change in seasonality could show up in any number of unexpected ways, and not simply as an enhancement of the Northern Hemisphere seasonal cycle over land.

The event which is most likely to be a recent manifestation of the precessional cycle is the "Climatic Optimum," covering the period of about 5000 to 7000 years ago (see Chapter 1). The term is most often used to refer to a period of generally warmer Eurasian temperatures. The "optimum" is sometimes said to be about 1-2K warmer than present, but it is difficult to get reliable estimates of global mean temperatures, or even annual means. What is certain is that some regions during some seasons were warmer than they were at recent pre-industrial times. At about the same time, the Sahara, which is now a torrid desert, experienced a period of greening, with currently dry riverbeds ("wadis") filled with water, and a teeming variety of animal life and flora not known at present. The greening of the Sahara is thought to be associated with atmospheric circulation systems known as "monsoons," forced to a greater extent by the enhanced heating of Northern Hemisphere subtropical land. A central question, though, is why the greening of the Sahara, and the Climatic Optimum occurred several thousand years after the precessional peak in Northern Hemisphere insolation. There are some indications that the warming may have begun as much as 10,000 years ago, but the question of the physics accounting for the time delay in response remains unsettled. Candidates for the necessary inertia in climate response include vegetation adaptation, land ice, and deep ocean heat storage.

Looking further back in time, the obliquity and eccentricity variations become significant, though of course, the precession cycle also continues to have a large effect. The Earth's obliquity and eccentricity cycle is shown in Figure 7.13. The amplitude of the obliquity cycle varies considerably over time, but it's dominant period is on the order of 40,000 years. The Earth's obliquity varies narrowly in a range from about 22o to 24.5o. At present, the Earth is in the

—Precession angle —65N July Insolation

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Figure 7.12: Evolution of precession angle relative to the Northern Hemisphere Summer Solstice, and the associated July insolation at 65N. Data taken from Berger and Loutre (1991).

middle of its obliquity range. Eccentricity varies on a longer time scale of approximately 100,000 years. However, in Figure 7.13 there are also hints of 400,000 year cycle of eccentricity, whose fingerprint consists of two high eccentricity cycles followed by two low eccentricity cycles. This visual impression is borne out by spectral analysis. Currently, the Earth is near the low end of its eccentricity range, though it has gotten quite close to zero during the past two million years. At the other extreme, Earth's eccentricity has gotten as high as .055, or more than half that of Mars.

The idea that ice ages are due to changes in Earth's orbital parameters is nearly as old as the discovery of ice ages themselves. The idea has gained currency, but it is nearly as hard to justify today on basic physical principles as it was when first proposed. The main reason for its acceptance is circumstantial, in that increasingly detailed data on the observed rhythm of the ice ages shows the unmistakable imprint of the calculated rhythm of the orbital forcing. James Croll first proposed in the 1870's that changes in the Earth's eccentricity led to ice ages, and his idea was refined a half century later by Milutin Milankovic, whose name is now generally attached to the theory. The centerpiece of Milankovic's idea is that ice ages require the accumulation of snow on land, and that this in turn is favored by mild summers (limiting melting of old snow and ice) and warmer, but still sub-freezing, winters (favoring snow accumulation, since warmer air contains more water). The gaping hole in Milankovic's theory is that it predicts that ice ages should follow the precessional cycle. In particular, the Northern Hemisphere and Southern Hemisphere should have ice ages in alternation every 10,000 years, with the severity of the ice ages modulated by the eccentricity cycle. This is not at all what is observed. Figure 7.14 shows the Antarctic temperature record for the past 400,000 years, together with eccentricity and the July insolation at 65N. Numerous other temperature proxies worldwide show that the Northern Hemisphere temperature, and global glacier ice volume, is nearly in phase with the Antarctic temperature record, so that the Antarctic temperature can be taken as an index of when the world is in an ice age. The dominant signal in the climate response is an approximately 100,000 year spacing in the major interglacial warm periods, and a similar spacing in the coldest glacial periods. Crudely speaking, each interglacial corresponds to a peak in eccentricity, and a time within which (during parts of the precessional cycle) the Northern Hemisphere seasonality is unusually strong. This is somewhat reminiscent of the Milankovic mechanism, but what filters out the high frequency precessional cycle? Why does the entire Earth fall into an ice age at the same time, rather than alternating between hemispheres? A closer examination of the 65N July insolation strongly suggests that major global deglaciations occur when the Northern Hemisphere seasonality is weak, suggesting that the Earth listens to the Northern hemisphere forcing more than the Southern, in deciding when to have an ice age. This probably has something to do with the fact that the Northern hemisphere has more land, and hence more seasonality, than the Summer, but the precise way this asymmetry influences global glaciation remains largely obscure.

The problem is not that the amplitude of radiative forcing associated with Milankovic cycles is small: it amounts to an enormous 100W/m2, with the amplitude determined by the eccentricity cycle. The problem is that the forcing occurs on the fast precessional time scale, whereas the climate response is predominately on a much slower 100,000 year time scale. One does not so much need an amplifier of Milankovic forcing, as a "rectifier," which is sensitive to the amplitude of the precessional variation, rather than to its mean. Recall that atmospheric CO2 is observed to vary on the glacial-interglacial time scale. Certainly, this is a major piece of the puzzle, since the drop in CO2 during glacial times is sufficient to account for a major portion of the cooling of the climate, particularly in the Southern Hemisphere (see Chapter 4). CO2 is a globalizing effect, and (insofar as it is linked to the glacial-interglacial physical climate changes) an amplifying feedback. The circumstantial role of CO2 in ice ages is also a reprise of an old idea. The 19th century physicist Tyndall, whose work on infrared spectroscopy is at the foundations of our current understanding of the greenhouse effect, was primarily interested in explaining the ice ages, and the

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Figure 7.13: Evolution of the Earth's obliquity and eccentricity. Data taken from Berger and Loutre (1991).

association reappeared later in the work of Chamberlain. The mechanism of the CO2 cycle not known, but almost certainly involves CO2 storage in the deep ocean. The lack of a theory for the glacial-interglacial CO2 cycle is the central impediment to a theory of the ice ages. The presence of ice does seem to be a prerequisite for a strong climate response to orbital forcing. Before the onset of permanent polar ice at the beginning of the Pleistocene, response to orbital forcing was weak (see Chapter 1). Besides CO2, ocean circulations can potentially play a major role in globalizing and rectifying the Northern Hemisphere signal, through direct heat transport as well as indirect effects on CO2. The answer to the mystery of the ice ages lies somewhere in the space: ice,ocean, CO2, but how the system works its miracles to yield a 100,000 year cycle is still unknown.

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