Scientists can reconstruct the cyclical expansion and contraction of polar caps and other ice masses from ice core samples taken from Greenland and Antarctica. When snow falls on high, cold glaciers the air trapped between snow grains is eventually transformed into air bubbles as the snow is compressed into ice from the weight of subsequent accumulations. The ratio of two oxygen molecules with different molecular weights (O16 and O18 isotopes) is a proxy record for the temperature conditions that existed when the snow was deposited. By studying these air bubbles and ice, scientists have been able to determine that the ice buildup from 90,000 years ago to 20,000 years ago was quite variable and was followed by a (geologically speaking) fairly rapid 10,000-year transition to the (current) climatically very stable Holocene period. The Holocene is the 10,000-year interglacial period in which human civilization developed and modern plant and animal distributions shifted into their current states (Eddy and Oeschger 1993). These ice cores also provide information on the presence of CO2, an important greenhouse-effect gas. Carbon dioxide was in much lower concentrations during cold periods than in interglacials (which is similar for the greenhouse gas methane, CH4). This implies an amplifying effect, or a positive feedback, because less of these gases during glacials means less infrared radiative heat trapped in the atmosphere—thus amplifying the cooling—and vice versa during interglacials. The ice cores also show that concentrations of CO2 and CH4 and temperature were remarkably constant for about the past 10,000 years (before A.D. 1700), particularly when compared with the longer record. That relative constancy in chemical composition of the greenhouse gases held until the industrial age during the last few centuries.
The transition from extensive glaciation of the Ice Age to the more hospitable landscapes of the Holocene took from 5000 to 10,000 years, during which time the average global temperature increased 5-7°C and the sea level rose some 100 meters. Thus we estimate that natural rates of warming on a sustained global basis are about 0.5°C to 1°C per thousand years. In addition to the sustained rates, there is growing evidence of rapid so-called abrupt nonlinear changes as well (Severinghaus and Brook 1999). Both the slower and more rapid changes were large enough to have radically influenced where species live and to have potentially contributed to the well-known extinctions of large animals (e.g., woolly mammoths, sabertooth cats, and enormous salamanders).
A large interdisciplinary team of scientists, including ecologists, palynologists (scientists who study pollen), paleontologists (scientists who study prehistoric life, especially fossils), climatologists, and geologists, formed a research consortium (Cooperative Holocene Mapping Project 1988, Wright et al. 1993) to study the dramatic ecological changes accompanying the transition from Ice Age to the recent interglacial period. One group of these researchers used a variety of proxy indicators to reconstruct vegetation patterns over the past 18,000 years for a significant fraction of the Earth's land areas. In particular, cores of fossil pollen from dozens of sites around North America clearly showed how boreal, coniferous tree pollen, now the dominant pollen type in the boreal zone in central Canada, was a prime pollen type during the last Ice Age (15,000-20,000 years ago) in what are now the mixed hardwood and Corn Belt regions of the United States. During the last Ice Age, most of Canada was under ice; pollen cores indicate that as the ice receded, boreal trees moved northward, "chasing" the ice cap. One interpretation of this information was that biological communities moved intact with a changing climate. In fact, Darwin (1859) asserted as much:
As the arctic forms moved first southward and afterward backward to the north, in unison with the changing climate, they will not have been exposed during their long migrations to any great diversity of temperature; and as they all migrated in a body together, their mutual relations will not have been much disturbed. Hence, in accordance with the principles inculcated in this volume, these forms will not have been liable to much modification.
If this were true, the principal ecological concern over the prospect of future climate change would be that human land-use patterns might block what had previously been the free-ranging movement of natural communities in response to climate change. The Cooperative Holocene Mapping Project, however, investigated multiple pollen types, including not only boreal species but also herbs and more arid (xeric) species, as well as oaks and other mesic species.
They discovered that during the transition from the last Ice Age to the present interglacial, nearly all species moved north, as expected. During a significant portion of the transition period, however, the distribution and combinations of pollen types provided no analogous associations to today's vegetation communities (Overpeck et al. 1992). That is, whereas all species moved, they moved at different rates and directions, not as groups. Consequently, the groupings of species during the transition period were often dissimilar to those present today. The relevance of this is that, in the future, ecosystems will not necessarily move as a unit as climate changes (even if there were time and space enough for such a migration).
Past vegetation responses to climate change at a sustained average rate of 1°C per millennium indicates that credible predictions of future vegetation changes cannot neglect transient (i.e., time-evolving) dynamics of the ecological system. Furthermore, because the forecasted global average rate of temperature increase over the next century or two exceeds those typical of the sustained average rates experienced during the last 120,000 years, it is unlikely that paleo-climatic conditions reconstructed from millennial time scale conditions would be near analogs for a rapidly changing anthropogeni-cally warmed world (e.g., see Schneider and Root 1998, from which some of this chapter is adapted). Future climates may not only be quite different from more recent past climates, they may also be quite different from those inferred from paleoclimatic data and from those to which some existing species are evolutionarily adapted. Therefore, past changes do provide a backdrop or context to gauge future changes, but not primarily as a spatial analog; rather as means to verify the behavior of models of climate or ecosystem dynamics that are then used to project the future conditions given the rapid time-evolving patterns of anthropogenic forcing (Crowley 1993, Schneider 1993a).
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