1 Dec 2007, 3:24pm
Holocene Botany Holocene Climates
by admin

After the Ice Age

Pielou, E.C., After the Ice Age: The Return of Life to Glaciated North America. 1991, Univ. Chicago Press.

Review by Mike Dubrasich

The smartest woman in the world is a little old lady who lives on Vancouver Island in British Columbia. Eighty-ish and now retired, Evelyn putters about her garden, does her shopping, and lives her life in relative obscurity. Her neighbors must know and love her, and they must find her to be very bright, but they may not realize just how smart she is.

Evelyn has described herself as a “naturalist,” but she is better known to the scientific world as Dr. E. C. Pielou, the inventor of mathematical ecology.

Mathematical ecology involves the quantification and statistical analysis of natural phenomena. Dr. Pielou’s book, Introduction to Mathematical Ecology (1969), is the bible of the field. It is not a light read. To get the most out of it requires a background in ecology and statistics at the post-graduate level. However, if you wish to measure something in the environment and do not follow Evelyn’s advice on the matter, you are doing it wrong.

Dr. Pielou holds Ph.D. and D.Sc. degrees from the University of London. She has been a professor at the Yale School of Forestry, Dalhousie University, Halifax, and the University of Lethbridge, Alberta, as well as holding a variety of guest lectureship positions all over. In 1984 she was awarded the Lawson Medal of the Canadian Botanical Association and in 1986 she won the Eminent Ecologist Award of the Ecological Society of America. The ESA has also established the E.C. Pielou Award, a competitive award made annually to a graduate student or recent Ph.D. graduate based on overall quality of the student’s scientific contribution to statistical ecology.

Dr. Pielou has written some popular science books, as well as dozens of technical scientific papers. Her books include:

Ecological Diversity (1975)
The Interpretation of Ecological Data: A Primer on Classification and Ordination (1984)
The World of Northern Evergreens (1988)
Biogeography (1992)
A Naturalist’s Guide to the Arctic (1995)
Fresh Water (1998)
The Energy of Nature (2005)

Her most famous (fame is subjective) “pop” book, and one that perhaps has more significance today than when she wrote it, is After the Ice Age: The Return of Life to Glaciated North America (1991).

In After the Ice Age Dr. Pielou describes the ecological changes that have occurred in North America over the last 20,000 years.

We live in a glacial age, something rare in the history of the Earth. Our current glacial age (the Pleistocene) began two million years ago (roughly). The most recent previous glacial age was during the Permian Epoch, 250 million years ago.

During the Pleistocene vast ice sheets have covered North America and northern Eurasia most of the time, punctuated by periodic interglacial warm periods when the ice has retreated. The previous Pleistocene interglacials have been 10,000 years long (roughly) and have occurred every 100,000 years like clockwork.

Our current interglacial, the Holocene, is 10,000 years old. Prior to the Holocene was the Wisconsin glaciation which lasted 100,000 years. Prior to that was the previous interglacial, the Sangamon, roughly 115,000 to 105,000 years ago (the Sangamon is termed the Eemian in Europe). Many of the Pleistocene glaciations and interglacial periods have been named [here], although the record gets blurrier the farther back one goes.

Ice Ages are thought to be caused by the tectonic drift of land masses over the North or South Poles. When a continent drifts over one or the other pole, ocean water cannot circulate from pole to equator, and thus ocean currents cannot moderate polar temperatures effectively. Ice builds up on the polar land mass, and an Ice Age ensues. The landmass over the South Pole today is Antarctica. Pangaea was the landmass over the South Pole during the Permian Ice Age.

When the Wisconsin Glaciation ice sheets reached their fullest extent and began to retreat (15,000 years ago, roughly) North America was mostly ice and tundra. Our current vegetation has developed since then, as plant species moved north from refugia onto bare land exposed by retreating ice. The colonization by Plants and animals (ecosystems) of what was mostly a lifeless continent is the tale Dr. Pielou tells in After the Ice Ages:

As the ice sheets vanished, they left a growing expanse of bare ground where, at first, nothing lived. Over the millennia, the area has been colonized by an enormous number of plants and animals whose permanent home is in the low-latitude regions that are ice free during the glaciations. The invaders managed to occupy the newly exposed land and water as it became available, and now they form the biosphere of most of North America.

Global temperatures peaked during the Holocene 8,000 to 10,000 years ago (roughly) during an interval known as the hypsithermal or Climatic Optimum. Since then global temperatures have been falling as we drift unsteadily into another 100,000 year glaciation. The general cooling of the last nine millennia is known as the Neoglaciation:

The Milankovitch cycle proceeds inexorably; we are now embarked on the next glaciation, the neoglaciation as it is called. It had no well-marked beginning. The changes in the earth’s orbit around the sun which make up the Milankovitch cycle caused the start of a trend toward cooler summers at high northern latitudes as early as 9k or 10k B.P. [before present]. But this cooling trend was masked, to begin with, by the warming that resulted from the slow disappearance of the continental ice sheets. The great mass of ice disappeared so slowly that the last of it melted away long after the astronomically determined cooling. The neoglaciation can be said to have begun when the cooling caught up with the warming.

The great ice sheets, in North America the Laurentide and Cordilleran, created their own weather. Chilled air rushed across the vast frozen surface in what are called katabatic winds:

When the ice sheets were big, they must have been the source of strong katabatic winds. A katabatic wind is the flow of cold and therefore dense air down a sloping surface of land or ice; strong katabatic winds flow off the world’s two existing ice sheets, in Greenland and Antarctica; at one Antarctic station a mean hourly wind speed of 156 kilometers per hour has been reported. It seems certain that similar winds blew off North American ice sheets. The summits of the ice domes were probably at an altitude of 3,500 meters above sea level. Contact with the ice cooled the air above them, and smooth slopes fell away in all directions. The conditions would therefore have been ideal for fierce katabatic winds.

The frozen gales created frozen deserts of drifting sand dunes. Today these loess soils deposited by katabatic winds cover much of the Great Plains and Columbia Basin, and northern Eurasia, and grow most of the wheat, soybeans, corn, and other major food crops on the planet.

But not for long. Neoglaciation is upon us. The downward trend is not linear; there are cycles within cycles within cycles. The most recent expression of neoglaciation was the Little Ice Age of 1250 to 1850 (roughly):

It was noted previously (and see chapter 1 and fig. 1.6) that there have been some respites in the neoglaciation because the little ice age cycle went some way toward masking the general cooling trend. The most recent respite was the Little Climatic Optimum, which peaked about 1.8k B.P. It was the last warm spell before the current one, which began somewhat more than 100 years ago; between them came the Little Ice Age.

During the Little Ice Age the Earth’s temperatures plunged to levels not seen for 11,500 years, when climbing out of the grips of the Wisconsin Glacciation. The planet has been slightly warmed since the Little Ice Age (~ 0.5 degrees C over the last 100 years). Despite this respite, the current trend in temperature over any lengthy period is downward. We are about to enter the Little Bigger Ice Age, which may last 600 years or longer. Summer, as we know it today, may not return for another 100,000 years.

This fact is not well-understood today. There is much popular discussion about the retreat of glaciers in Glacier Park for instance, even though those glaciers were entirely melted during the hypsithermal and have come back into existence since then. There is wide concern over warming polar bear habitat, even though the permafrost region is growing, not shrinking:

Muskeg has also grown up over much of the [Hudson Bay lowlands] where permafrost prevails and has presumably spread southward, following the permafrost, as neoglaciation has progressed. …

The replacement of “ordinary” forest, in which dead plants decay and surplus water runs off, by muskeg, in which neither of these things happen, is known as paludification. It amounts to a deterioration or degradation of the vegetation in the sense that plant growth, and indeed the whole rate of materials cycling, is slowed. Paludification thus seems to be one of the ways in which life slows down when a new glaciation starts.

Some lessons that may be drawn:

1. Current vegetation in North America is relatively new. Our forests have NOT been here for millions of years.

2. Current plant communities are invasive, with largely accidental plant “associations,” and are decidely transitional.

All that can be said with certainty is that the geographical ranges of all species of plants are changing all the time.

3. There are no “intact” ecosystems. Intactness is not a biological/ecological phenomenon. It does not exist in the real world.

4. Biodiversity is also an accident and transitional. There is no “balanced” or “best” collection of plants and animals in any location.

5. The earth is cooling, not warming, and that process is inexorable, as far as we know.

It could be that Dr. E. C. Pielou is wrong about all the above, but that’s doubtful when you consider that she is the smartest woman on the planet. After the Ice Age is an important book, in part because it puts climate change into a historical perspective, and history is about to repeat itself.

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