Frozen Earth: The Once and Future Story of Ice Ages (38 page)

Thus it is likely that uplift and weathering of the Himalayas has affected atmospheric CO
₂
and played a part in the gradual cooling of our planet over the past 35 million years.
In the most comprehensive computer simulations of climate the influence of greenhouse gases is quite clear: the coldest temperatures and greatest ice cover always appear in trials with the lowest amounts of CO
₂
.
If mountain building
is
implicated in the initiation of the Pleistocene Ice Age through its effect on chemical weathering, then it may have played a role in ice ages of the more distant past, too.

And if greenhouse gases are so important for ice age climate, what does that say about the future?
Today, of course, the concern is about increasing CO
₂
and global warming, not cooling.
Over the past few thousand years, and accelerating over the past few hundred years, clearing land for agriculture, burning forests, and especially burning fossil fuels, has added CO
₂
to the atmosphere more quickly than it can be taken up by weathering or photosynthesis or dissolution in the ocean.
Its concentration has risen by about one third, and is still increasing rapidly, now almost entirely due to burning of fossil fuel.
During the last interglacial period, some 120,000 years ago, CO
₂
levels (and temperatures) were similar to those of today.
But the greenhouse gas content of the atmosphere did not rise any higher—in fact, it decreased as the Earth began cooling into the glacial interval that culminated only 20,000 years ago.
At present, already at a peak concentration, CO
₂
is being added to the atmosphere a hundred times faster
than it was during any of the natural increases that can be observed in ice cores.

Even critics of global warming don’t dispute the greenhouse property of CO
₂
—it is a well-known fact of physics.
It is difficult to comprehend how further additions to the atmosphere at current rates could fail to raise global temperatures and possibly influence the course of the Pleistocene Ice Age.
And there is an additional possible source of climate surprise that may arise because of the increasing temperatures: methane.
We saw in chapter 8 that release of large amounts of methane has been suggested as a cause for the end of Snowball Earth ice ages and the very warm periods that immediately followed.
Today, there are great stores of methane locked up as “hydrates”—icy crystals that contain large amounts of methane in a relatively small volume.
The methane in this unusual form of ice comes primarily from bacterial action, and it forms solid layers in the permafrost of cold regions and also in ocean sediments along the margins of continents.
The hydrate crystals can only exist over a restricted range of low temperatures and moderate pressures, and decompose easily when conditions change.

As global temperatures increase, heat penetrates slowly into the arctic permafrost and the oceans.
Both gradually warm up, and at some point, the hydrates will become unstable and begin to release their trapped methane, which could trigger abruptly increased warming.
Even though methane has a short lifetime in the atmosphere, its greenhouse effect would produce a sharp upward temperature spike and could be prolonged if the very large amounts of existing hydrate were to decompose sporadically over a period of time.

There are indications in the geological record that sudden bursts of methane have been released into the atmosphere in the past.
The physical evidence includes “pockmarked” sediments in the Arctic and sub-Arctic—areas where detailed mapping of the seafloor shows multiple craters up to a hundred meters across, interpreted to be the result of rapid release of large bubbles of methane gas, probably due to the decomposition of hydrate layers.
Destruction of the hydrates most
likely resulted from the gradual warming of seawater during the present interglacial period.
The chemical composition of some ocean sediments also points to large-scale methane release.
Carbon in the methane produced by bacteria has a very distinctive isotopic makeup, and when it is released into seawater that signature is transferred to organisms living in the water, and eventually gets preserved in the sediments.
Along the central California coast and elsewhere, recent sediments show series of isotopic “spikes” that appear to be attributable to abrupt injection of methane into seawater—presumably from the decomposition of hydrates.
And much farther back in the geologic record, about 55 million years ago, one of the largest recorded abrupt increases in ocean water temperature—7 to 8 degrees Celsius—is accompanied by similar isotopic evidence for methane release.
Most scientists have concluded that huge volumes of methane hydrates must have suddenly decomposed, for reasons that are still unclear, and that the methane release was responsible for the sudden temperature increase that followed.

How effectively methane has contributed to the warming of the Earth during the present interglacial period is still a topic of debate.
What its role will be in the future is also uncertain.
But two things are clear: first, there are very large stocks of this gas stored both on land at high latitudes and along the continental shelves almost everywhere; and second, there is an undeniable correlation in the ice-core data between increased temperature and increased methane in the atmosphere.
Even if methane is not the immediate cause, it follows temperature increases very closely and must amplify them.

In spite of the well-documented rise in atmospheric CO
₂
and the possibility that large amounts of methane gas will also be released, the consensus view until recently has been that the current warm interglacial period will end soon (in geological terms) and that the Earth is headed toward another glacial episode.
This conclusion was based mainly on examination of the past climate record—the peak of the last glaciation was twenty thousand years ago, and over the past million years or so the warm interglacial periods that separate major ice
advances have typically lasted only ten or twenty thousand years.
Man’s additions of CO
₂
to the atmosphere may prolong the warm climate of the current interglacial period a bit, but at current rates of usage, our supply of fossil fuels will run out in a few centuries anyway.
Elevated levels of carbon dioxide will linger in the atmosphere long after that, but will gradually decrease, reducing the greenhouse effect.
Inexorably, the fluctuations in the Earth’s orbit will draw us into the next glacial episode, and ice sheets will once again build up from centers in Scandinavia, Canada, and Russia.

Or will they?
It is possible that the consensus view is wrong.
We have seen how the glacial-interglacial cycles of the past million years have closely followed the 100,000-year timescale of the eccentricity of the Earth’s orbit, its tendency to be more or less elliptical.
Although exactly why climate tracks eccentricity is not known with certainty, the correlation is clear.
And a close look at how the Earth’s orbit will change in the future shows that its eccentricity will decrease steadily to almost zero about 30,000 years from now.
This is apparent even in James Croll’s original graph, reproduced in chapter 5 (figure 11).
It is something that has not happened for hundreds of thousands of years.
The practical effect is that the variability in the amount of solar radiation received by the Earth will be much less over the next 50,000 years or so than it has been through the past few glacial-interglacial cycles.
Coupled with persisting high levels of CO
₂
, this could push the next glacial advance far into the future.
Some computer simulations suggest that under these conditions, significant glaciation will not occur before sixty or seventy thousand years from now, and even then the ice will not be as extensive as it was during the previous few glacial advances.
And there is yet another possibility.
If CO
₂
emissions are not curbed, global warming could completely melt the Greenland glaciers and a substantial part of the Antarctic ice sheet.
This would not happen instantaneously; the melting would continue over many human generations.
Nevertheless, the consequences for mankind would be serious: the sea level would rise by nearly sixty meters, flooding vast areas of the
continents, including most parts of present-day cities like New York and London; weather patterns worldwide would be altered drastically, disrupting agriculture in unpredictable ways; the frequency and intensity of hurricanes would increase because they draw their energy from warm ocean water, which would be far more widespread than currently.
Warming would be reinforced by the loss of highly reflective ice and snow, and possibly by the decomposition of unstable methane hydrates.
The elevated temperatures coupled with complete loss of continental ice sheets might constitute a threshold-crossing event that would thrust the Earth into a regime from which the glaciers could not quickly recover, even with the return of greater eccentricity and lower CO
₂
levels.
Only a few hundred years after Louis Agassiz announced his theory of a global ice age, mankind may inadvertently bring the Pleistocene Ice Age to a premature close, ushering in another long period of ice-free existence for our planet.

SUGGESTIONS FOR FURTHER READING

ICE AGES AND GLACIATION, GENERAL

John C.
Crowell,
Pre-Mesozoic Ice Ages: Their Bearing on Understanding the Climate System
(Boulder, CO: Geological Society of America, 1999).
This is Memoir 192 of the Geological Society of America.
Crowell has spent a distinguished career studying ice ages and here uses his immense expertise to sift through and summarize the disparate evidence for each of the Earth’s ancient ice ages and to search for their causes and connections to the climate system.

M.J.
Hambrey,
Glacial Environments
(Vancouver: University of British Columbia Press, 1994).
A well-illustrated treatment of the effects of glaciers on landscape.

J.
Imbrie and K.P.
Imbrie,
Ice Ages: Solving the Mystery
(Short Hills, NJ: Enslow, 1979).
A well-written account of how ideas about ice ages developed, with insights (by one of the participants) into the work on sediment cores that confirmed the astronomical controls on Pleistocene glaciation.

R.A.
Muller and Gordon J.
Macdonald,
Ice Ages and Astronomical Causes
(New York: Springer, 2000).
A technical and mathematical analysis of the evidence for astronomical control of ice ages.

LOUIS AGASSIZ

Louis Agassiz,
Studies on Glaciers, Preceded by the Discourse of Neuchâtel,
ed.
and trans.
by Albert V.
Carozzi (New York: Hafner, 1967).
This is an English translation of Agassiz’s famous
Études sur les glaciers,
originally published in 1840.
It also includes the text of Agassiz’s address to the Natural History Society of Switzerland in Neuchâtel in 1837.
The translation includes the magnificent drawings that accompanied the original book.

Edward Lurie,
Louis Agassiz: A Life in Science
(Chicago: University of Chicago Press, 1960).
A comprehensive scholarly account of Louis Agassiz’s life.
However, it focuses mainly on his contributions to biology, with very little discussion devoted to the theory of ice ages.

Jules Marcou,
Life, Letters, and Works of Louis Agassiz
(New York: Macmillan, 1896).
A very detailed account of Agassiz’s life written by a colleague and personal friend who is perhaps a little biased in his treatment.
Although the book is written in English, Marcou reproduced many of Agassiz’s letters in their original French.

JAMES CROLL

James Croll,
Climate and Time in Their Geological Relations: A Theory of Secular Changes of the Earth’s Climate
(London: Daldy, Isbister, 1875).
Croll’s masterpiece, in which he brings together his ideas about the Earth’s climate.

J.C.
Irons,
Autobiographical Sketch of James Croll, with Memoir of his Life and Work
(London: Edward Stanford, 1896).
A sympathetic account written by a friend who wished to make the remarkable details of Croll’s life known to a wider audience.
It includes a listing of all of Croll’s publications.
It is still the only biography available.

MILUTIN MILANKOVITCH

Milutin Milankovitch,
Cannon of Insolation and the Ice-Age Problem
(Jerusalem: Israel Program for Scientific Translations, 1969).
Originally published in 1941 in Belgrade as
Kanon der Erbestrahlund and seine Anwendung auf das Eiseitenproblem,
this was Milankovitch’s culminating effort to bring together all of his calculations and ideas about the Earth’s climate.
Much
more mathematically based than James Croll’s
Climate and Time,
it is, like that earlier book, a masterpiece.

Milutin Milankovitch,
Milutin Milankovitch 1879–1958
(European Geophysical Society, 1995).
This slim volume documenting Milankovitch’s life was put together by his son, Vasko, after his father’s death.
It draws heavily on Milankovitch’s autobiography and is well-illustrated with photographs.

THE CHANNELED SCABLANDS

V.
R.
Baker, ed.
Catastrophic Flooding: The Origin of the Channeled Scabland.
Benchmark papers in Geology, 55 (Stroudsburg, PA: Dowden, Hutchinson & Ross, 1981).
This compilation includes the important scientific papers (mostly authored by J.
Harlan Bretz) that led to the acceptance of a catastrophic flood origin for the Channeled Scablands, each preceded by a commentary written by the editor.