What Has Been Learned About Future Climate Changes From Research On Past Climate Changes Over The History Of The Earth?


BACKGROUND INFORMATION: Long-term records from around the globe indicate that during glacial periods (extending back from more than 10,000 years ago) the Earth's climate was significantly different than it is today, with rapid fluctuations in character. The period of warmth over the past 10,000 years appears to be unique over the past 100,000 years (or more). A wide variety of records of past climate history are providing an information base that can help improve understanding of the variability of the Earth's natural climate. Instrumental climate records exist for only a few centuries. This makes the use of reconstruction techniques essential for providing the context for recognizing human-induced changes in climate, and disentangling these from natural variability.

A Warmer Climate Three Million Years Ago May Have Resulted In A Sea Level 30 Meters Higher Than Today's

Reconstruction of marine and terrestrial records throughout the Northern Hemisphere during the middle Pliocene (about 3 to 4 million years ago) suggest that sea level was at least 25 meters higher than it is today. This is the last time period in the Earth's history when global temperatures were as warm as those predicted by climate models for the time when there has been a doubling of the CO2 concentration. Analytical and modeling studies of this period have been used to provide insights into the potential consequences of future climate change. One study indicated that 3 million years ago the northern forests extended to the margin of the Arctic Ocean, sea level was approximately 30 meters higher than today because of a reduction in ice volume in the polar regions, and the globally averaged temperature was 3.5°C warmer than it is today.
Reference: Modeling of Middle Pliocene Climate with the NCAR GENESIS General Circulation Model, Sloan, L. C., T. J. Crowley, and D. Pollard, Global and Planetary Change, Vol. 9, pp. 169-195, 1994.

Sediment Record From Central Asia Reveals That The Climatic Response As The Oceans And Ice Sheets For Over 250,000 Years

The center of the Asian continent, far from oceans and ice sheets, plays a critical role in determining the global climate. The region generates important weather patterns in both winter and summer seasons, including the monsoons of Asia. Recent studies of the sediment record from Lake Baikal, including magnetic, radiochemical, and biogenic components, have clarified the response of this highly seasonal environment to solar forcing. Rather than a direct linear response to changes in solar radiation, the paleoclimate indicators from Lake Baikal show the same intricate pattern of climatic variation as the oceans and ice sheets. In particular, the indicators correlate extremely well with global ice volumes over the last 250,000 years, as indicated by the marine oxygen-isotope records.
Reference: A Rock Magnetic Record from Lake Baikal, Siberia: Evidence for Late Quaternary Climate Change, Peck, J. A., J. W. King, S. M. Colman, and V. A. Kravchinsky, Earth and Planetary Science Letters, Vol. 122, pp. 221-238, 1994.

Massive Iceberg Discharges Linked To Past Global Climate Change

Evidence has been found for episodic outflows of icebergs from continental ice sheets during the last ice age. These outflows flooded the North Atlantic, creating a significant freshening of the salt water as they melted. The six or so great iceberg discharges in the North Atlantic region were apparently not isolated events, but associated with other simultaneous environmental changes. These included surges in mountain glaciers in the Chilean Andes and in the Alps of New Zealand and changes in paleovegetation from Florida to British Columbia, in plankton records from the Sulu Sea, in ice core records from Greenland and Antarctica, and in lake levels in Africa. There is some evidence that these events may be linked to the precession of the seasons caused by changes in the Earth's orbit around the Sun. These massive iceberg discharges and associated releases of freshwater into polar oceans can rapidly disrupt the normal pattern of the thermohaline circulation that warms the North Atlantic, thereby causing rapid cooling of this region that can last for many centuries, disrupting the global climate.
Reference: Massive Iceberg Discharges as Triggers for Global Climate Change, Broecker, W. S., Nature, Vol. 372, pp. 421-424, 1994.

Past Climate Changes At The North Pole Tied To Climate Changes At The South Pole Over The Same Period

Ice cores have been obtained from the Greenland and Antarctic Ice Sheets that have layers of snow spanning the period of the last ice age from 20,000 to 105,000 years ago. Past changes in the climate appear to have been more rapid and more numerous in Greenland than in Antarctica. By counting changes in a number of variables recorded in the layers of snow, 22 warming events of short duration have been identified in the Greenland ice cores and 9 warming events of short duration in the Antarctic ice cores. In the Greenland ice core records, the short warming events are characterized by very rapid warming and rapid cooling transitions at the beginning and end of the events. In Antarctica, related short warming events are characterized by slow warming at the onset of the warming period, and slow cooling at the end of the brief warming period. The preliminary results of this work suggest that warming events occurred in Antarctica whenever warming events in Greenland lasted longer than 2,000 years. The evidence suggests that partial melting of continental ice sheets (e.g., on North America) and changes in ocean circulation were, at least in part, responsible for the climatic teleconnection between the north and south polar regions. There is evidence that North Atlantic Deep Water (NADW) production slowed or ceased during much of the Earth's last glacial period, stopping the transport of heat northward by the Gulf Stream to warm northern Europe. Resumption of NADW production during this cold period has been suggested as the immediate cause of rapid warming during the 22 warming events recorded in the Greenland ice cores.
Reference: Climate Correlations between Greenland and Antarctica during the Past 100,000 Years, Bender, M., T. Sowers, M. Dickson, J. Orchardo, P. Grootes, P. A. Mayewski, and D. A. Meese, Nature, Vol. 372, p. 663-666, 1994.

Greenland Ice Cores Suggest That Large Abrupt Shifts In Oxygen Isotope Ratios Occurred During Melting Of The Ice Sheets

The termination phase of the last ice age was characterized by a series of abrupt returns to glacial climate, the best-known of which was the Younger Dryas event that lasted from about 11,000 to 10,000 years ago. Oxygen isotope data from the Greenland ice cores suggest that temperature shifts of 7 to 10°C occurred over only a few decades and that dust concentrations and the rate of snow accumulation in these cores show an even more rapid transition. A general circulation model that considered a complex variety of processes which could influence isotope ratios was used to attempt to reproduce the measured variability the ratio. The model results suggest that the variability cannot be explained by changes in the North Atlantic thermohaline circulation alone, and that a number of moisture sources contribute to snowfall over Greenland. In addition, evidence of climate change in the Southern Hemisphere during the same period suggests that any explanation must take into account a climatic change much more widespread than the North Atlantic region alone.
Reference: Glacial-Interglacial Changes in Moisture Sources for Greenland: Influences on the Ice Core Record of Climate, Charles, C. D., D. Rind, J. Jouzel, R. D. Kaster, and R. G. Fairbanks, Science, Vol. 263, pp. 508-518, 1994.

Increased Volcanism Linked To Climatic Cooling During The Period From 5000 To 7000 B.C.

Sulfate concentrations measured in Greenland ice core samples suggest that there were up to three times as many volcanic events during the period of 5000 to 7000 B.C. as over the past two millennia. In addition, these eruptions appear to have produced up to five times higher concentrations of sulfate than the largest known historical eruptions. The results suggest that increased volcanism occurred at circum-arctic locations during the millennia following deglaciation. These findings support the suggestion that magma chambers respond to the release of crustal stresses following deglaciation and that this may lead to more explosive volcanism. The overall magnitude and duration of these effects is still uncertain.
Reference: Record of Volcanism Since 7000 B.C. from the GISP2 Greenland Ice Core and Implications for the Volcano-Climate System, Zielinski, G. A., P. A. Mayewski, L. D. Meeker, S. Whitlow, M. S. Twickler, M. Morrison, D. A. Meese, A. J. Gow, and R. B. Alley, Science, Vol. 264, pp. 948-952, 1994.

Evidence From The Great Basin Indicates A Much Moister Climate Thousands Of Years Ago

Studies of changes in paleo-vegetation and the rise and fall of pluvial lakes (i.e., lakes filled by precipitation and run off) in the Great Basin indicate that the climate was much moister thousands of years ago and fluctuated significantly over past millennia. These results suggest that the lack of modern day climate fluctuations of the magnitude found in the historical record is unusual. The history of regional paleo floods in Arizona and Utah reveals that the largest floods clustered into distinct time intervals that coincided with periods of cool, moist climate and frequent El Niño episodes.
Reference: A 30,000 Year Record of Vegetation Dynamics at a Semi-arid Locale in the Great Basin, Nowak, C. L., R S. Nowak, R. J. Tausch, and P. E. Wigand, Journal of Vegetation Science, Vol. 5, pp. 579-590, 1994.

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Last updated 04/10/96