Pressured by society’s concerns
about future global warming, climatology has recently become a more predictive
science. Unfortunately, climatologists face daunting challenges in attempting
to forecast particulars of climatic change by way of models based on imperfectly
understood relationships among air, sea, land and life. And the present moment
in geologic time provides only one state of the global system for study. In
the past few years, students of the geologic record have uncovered evidence
of past states and sudden transitions for Earth’s ecosystem that are unparalleled
in the shallow history of the modern world.
Clearly, we must decipher the messages of Earth’s deeper history, unsettling though they may be, to help us anticipate future patterns of climatic change and their biotic consequences.
The geologic record of the recent past is most precise in its offerings. Stabilized sand dunes point to desert conditions for the Nebraska-Colorado border less than a 1,000 years ago. More general are indications that large areas of the American West considered dry today have usually been even drier during the past 10,000 years. The record of glaciers shows that regional climatic changes can be shockingly abrupt. A thickening of annual layers in an ice core extracted from Greenland’s ice cap indicates that about 14,680 years ago the average temperature of the North Atlantic region rose by about 7 degrees Celsius within just three to five years!
The fossil record has also yielded surprises. Pollen from lake sediments reveals that every time continental glaciers have expanded or contracted during the ice age in which we still live, communities of land plants have been reassembled kaleidoscopically through independent migrations of species. Thus, ecologists have had to acknowledge that their classic biomes — characteristic groupings of plants in the modern world — are not ancient communities of species that have evolved in concert but are transitory assemblages, each consisting of a group of species that happen to be adapted to a particular climatic zone that did not exist 15,000 years ago. The glacial expansions and contractions themselves have been linked to Milankovich cycles — periodic changes in Earth’s rotational movements. Orbital forcing has a weak effect on solar heating of Earth, however, and it remains to be discovered how its signals are amplified so as to cause massive glaciers to wax and wane. Only experiments that nature has conducted on a vast geographic scale over thousands of years could be expected to reveal these subtle relationships among solar radiation, climate and vegetation.
Those of us who see in the geologic record convincing evidence that the origin of the Isthmus of Panama triggered the modern ice age are attributing profound climatic changes at high latitudes to the emplacement of a skinny neck of land near the equator. Shifting patterns of heat transport are part of the puzzle, as are positive feedbacks such as the cooling associated with replacement of evergreen forest by tundra and with changes in Earth’s albedo arising from sea ice formation.
Fossilized organic compounds and plant leaves indicate that changes in greenhouse warming have had a surprisingly weak effect on climates during the Cenozoic Era. The degree to which photosynthetic plankton fractionate carbon isotopes while assimilating carbon dioxide varies with the ambient concentration of this greenhouse gas. That certain of these organisms produce alkenones — organic compounds that undergo little alteration after burial and thus retain their original isotopic composition — reflects the concentration of carbon dioxide in the atmosphere. Careful study of fossilized alkenones indicates that this concentration has remained close to its present-day level throughout the past 15 million years.
Studies of fossilized leaves of land plants yield similar results for a much larger portion of Cenozoic time. The spatial density of stomates, which are pores through which gases pass to and from leaves, decreases with an increase in ambient carbon dioxide. This relationship has been quantified for the living fossil genera Ginkgo and Metasequoia through greenhouse experiments and study of museum specimens collected at various times since the start of the Industrial Revolution. Fossil leaves interpreted in this light indicate near-modern levels for globally warm Miocene and Eocene times. Thus, it turns out that about 50 million years ago, palm trees grew in Wyoming and alligators lived inside the Arctic Circle without benefit of exceptional greenhouse warming. To date, all efforts to model the balmy polar temperatures of the Eocene have met with frustration.
More tractable has been a remarkable pulse of global warming documented at the Paleocene-Eocene boundary. Here a worldwide spike of isotopically light carbon seems best explained by a sudden release of methane from the melting of icy bodies in the seafloor known as gas hydrates. Rapid oxidation of the methane apparently produced a brief jump in the concentration of atmospheric carbon dioxide that is recorded in stomates of fossil leaves. What, we may ask, will happen to submarine gas hydrates during the global warming anticipated for our immediate future?
If these deep records of climatic change fail to impress, then contemplate the Snowball Earth scenario. Even if the entire world did not freeze over more than half a billion years ago, as some geologists assert, there is no question that continental glaciers abruptly spread over tropical terrains throughout the world. And let us not forget that the apocalyptic vision of a nuclear winter — a worldwide freeze-up that might occur if the dust of a massive war screens out the sun — sprang from the idea that a so-called impact winter wiped out the dinosaurs about 65 million years ago when debris from Earth’s collision with an extraterrestrial body blackened the skies.