Oxygen isotopic analysis and absolute dating of deep-sea cores show that temperatures as high as those of today occurred for only about 10% of the time during the past half million years. The shortness of the high temperature intervals (“hypsithermals”) suggests a precarious environmental balance, a condition which makes man's interference with the environment during the present hypsithermal extremely critical. This precarious balance must be stabilized if a new glaciation or total deglaciation is to be avoided.

Pollen studies of the four European interglacial intervals indicate a strong similarity in vegetational sequence: a pretemperate phase (I) marks the late glacial, early temperate (II) and late temperate (III) phases mark the interglacial proper, and a posttemperate phase (IV) represents the beginning of the next cold period. A grossly similar record is now known for the last interglacial (Sangamon) of central and southeastern United States. The Holocene sequence in both Europe and America are completely typical of the interglacial sequence, although much more is known of the geographic variations.

Estimates for the duration of the interglacials range from 10,000 to more than 30,000 yr, according to counts of the annually laminated sediments (organic varves).

The Holocene has already run a course of at least 10,000 yr. If it is like earlier interglacials, it will end soon, giving way to gradually developing cold conditions, which may not lead to glacial maxima for tens of thousands of years.

A “glacial cycle” is defined as a major global climatic oscillation of the order of 105 yr, developed within an “ice age” sensu lato which may last 106–107 yr and which recurs at widely spaced intervals in geologic time (ca. 2 × 108 yr). The ice age situation thus only persists during 5–10% of all geologic time and is preconditioned by geotectonics and not by extraterrestrial controls.

The repetitive nature of the glacial cycle is demonstrated by independent records from (a) glacigenic, ice-melt deposits, i.e., tills and fluvioglacial terrace accumulations; (b) eolian by-products, i.e., loess; (c) marine terraces and beach deposits, eustasy inversely reflecting the ice volumes; and (d) deep-sea deposits, biologically and lithologically reflecting the climatic events. Only the marine section offers a continuous, uninterrupted record. Different chronometric techniques confirm a major cycle of the order of 105 yr (eight repetitions in the last 700,000 yr), but there are multiple types of modulation, in part related to self-accelerating feedback, either negative (retardation) or positive, leading to acceleration.

Within any given cycle there is an Interglacial, Anaglacial, Pleniglacial and Kataglacial phase, characteristics of which are repeated on a small scale in minor cycles. Their timing is variable in a latitudinal sense, apparently steered by radiation changes that first affect the tropics.

Interglacials are defined in their classical stratotype areas of NW Europe, by sedimentary sequences characterized by the pollen of deciduous forests, pointing to climates at least as warm as those of the present time. The present cycle began ca. 10,000 YBP, with the start of the Holocene epoch, and the contemporary warm phase is seen as a typical interglacial stage. Such warm peaks characteristically last about 10,000 yr. Symptoms of the expected ensuing glaciation range from a global fall in temperature since mid Holocene, to tropical desiccation (growth of deserts) and high latitude retreat of tree lines.

Much of Baffin Island is close to the modern glaciation limit and climatic changes within the last decade are already being reflected in snow cover extent. Statistical analysis of glacierized and ice-free corries indicates that changes in direct solar radiation due to astronomical factors are inadequate to account for glacierization of those at present ice-free. These and other sources of evidence demonstrate the need for augmented winter snowfall in order to increase the extent of glacierization. The pattern of glacial history in this area is for maximum ice extent during the early glacial phase (>68,000, <137,000 BP), followed by a reduction in ice volume during the cold pleniglacial (>24,000, < 68,000 BP) and then a limited late glacial advance (the Cockburn Stade, ca. 8,000 BP) due to increased precipitation. The Barnes Ice Cap did not disappear in the Holocene as it did in the last interglacial. The area is highly suitable for long-term monitoring of climatic change and glacial response.

Consideration of the history of Holocene climate in the Rocky Mountains indicates that the over-all trend during the past 2500 yr has been toward increasing warmth, interrupted by cooler times of minor advances of cirque glaciers. Comparison of Holocene climatic history with the record of past interglacials in the region suggests that the present interglacial is not complete and that the climate may become first warmer and subsequently wetter before it is completed. Correlation of the timing of the regional glacial-interglacial record for the past 140,000 yr with the record of major sea level changes and with the calculated changes in the earth's insolation suggest that the present interglacial may be completed within a few millenia and that it may be followed by a significant cooling of the climate.

The Pleistocene/Holocene boundary is easily recognized in deep-sea cores from the South Atlantic as an upward change from predominantly shelf and meroplanktonic diatoms to holoplanktonic forms. Those cores with high sedimentation rates show several climatic fluctuations during the Holocene. One core (E 7-1) from the Scotia Sea has a sufficiently expanded Holocene to reveal three distinct coolings at approximately 4600 yr BP, 2800 yr BP and 1000 yr BP. This compares favorably with the results of other workers.

The primeval forests of Central Europe are still inhabited by relatively ecologically demanding, thermo- and hydrophilous snail fauna of interglacial character, which differs from the assemblages of Holocene climatic optimum and of the last interglacial optimum by the absence of some southern (exotic) elements.

In the deforested areas inhabited by man, the less demanding mesic to steppe fauna predominates. It differs from analogous early glacial communities only in comprising modern, climatically demanding species, which are unknown from the Pleistocene. They found a suitable habitat in Central Europe thanks to human interference. The dwindling formation of tufa spring deposits and their partial destruction both in occupied and unoccupied areas indicate that the Holocene warm interval is in the late phase.

The recent slope deposits are reminiscent of early glacial sections both by their lithology and fauna reflecting existence of large open spaces. Whereas in early glacials the deforestation resulted from the change of climate, the recent one is almost entirely a product of man.

In assessing the present-day position within the interglacial cycle, we should take into consideration all these conclusions and always keep in mind the share of man's influence in the development observed.

The present assemblages of Ostracods in Central Europe resemble the assemblages known from the earliest phases of Holocene. This observation supports the view that the termination of the present warm interval is to be expected in the near future.

The nine-banded armadillos (Dasypus novemcinctus) have been moving northward in the Great Plains region from the late 1800s to the 1950s but now seem to be retreating from their lately acquired northern range. The armadillos have a nontypical homoiothermic blood system which makes them fairly vulnerable to cold climates. Many other adjustments of animal ranges have taken place in the Holocene, even during the past few centuries and evidence indicates that in many cases climate changes played an important role.

We are now living under interglacial climatic conditions, the Present Interglacial or Flandrian Interglacial Age. It will certainly be followed by the Future Ice Age. The major cold/warm changes seem to have a cyclicity of 10,500 yr. We have been in the second cycle (characterized by cooler climate) after the Last Ice Age for 2200 yr and will continue to be so for another 8300 yr. By analogy with the conditions during the Last Interglacial it is concluded that this cycle will remain moderately warm. With the end of the third cycle at about 18,800 years AP, the Present Interglacial will end and the First Future Glacial Age begin. Further information about the climatic conditions during the “cold” cycle 117,700–107,200 y. a. is necessary, however, before a really well-founded prediction can be made.

Samples representing the post-Eemian cool interval (approx 110,000 yr BP) were taken from 15 cores on the eastern flank of the mid-Atlantic Ridge from 42°N to 61°N lat. The floral-faunal assemblages in these samples characterize the surface paleooceanography which we consider a possible analog to the cooling expected to follow the present warm interval. The derived paleooceanographic map indicates at least a 16° lat southward displacement of cooled water masses from today. The present position of the Transitional water mass which gives N. Europe its equitable climate was then occupied by Subpolar water while the Polar Front extended south of Iceland. The result was a drop in annual temperature of the NE Atlantic surface water by at least 5°C.

Distinct correlative calcium carbonate oscillations are evident in two deep-sea cores taken in the Saharan windblown dust zone of the eastern equatorial Atlantic. Using the presence and absence of Globorotalia menardii for stratigraphic control and correlating these cores with radiometrically dated cores from the Caribbean the carbonate oscillations have two distinct periods. Two major oscillations about 117,000 yr and 100,000 yr long, and shorter oscillations averaging about 14,000 yr, occur. The carbonate curves are interpreted to indicate a pronounced increase in wind stress and probable climatic deterioration after the beginning of the last interglacial (post-Eemian). This is also reflected in the carbonate curves for the previous major climatic cycle. If the Holocene warm period is analogous to the Eemian (Barbados Terrace III) then we may expect a pronounced climatic deterioration in the next few thousand years.

In North Atlantic deep-sea core V23-82, changing surface water conditions are revealed by changing composition of fauna and flora. Sedimentation rate seems to vary little so that approximate dating of climatic events within the X zone of Ericson is possible. These include an interglacial temperature maximum at 124,000 YBP; start of the post-Eemian cooling at about 116,000 YBP; and peak cool conditions at about 110,000 YBP, followed by warming.

Drop in summer and winter temperatures about two-thirds of the way towards full glacial values and a significant drop of salinity is suggested by quantitative paleoenvironmental analysis of the 110,000 YBP cold episode.

If the Eemian is taken as the analog of the present interglacial, a point in time 116,000 YBP becomes the historical model for today's ocean, and the North Atlantic is now approaching a time of severe cooling.

Renewed development of the Laurentide and Scandinavian ice sheets would have large effects on global climate. By one model, growth of a new ice sheet would be slow and would begin with systematic and easily measurable changes in remote regions of Northeastern Canada. By another model, growth of a new ice sheet would begin rapidly with the development of thin ice cover over a large area. Clearly, immediate global impact would be much greater with the second model than with the first.

Because sea level fluctuations are the reciprocal of change in ice volume, interaction between Pleistocene coral reefs and sea level events provides an estimate of the dynamics of the ocean-cryosphere system. The data suggest rapid growth of continental sheets.

In the loess series of Central Europe the last interglacial is recorded by the parabrownearth soil accompanied by land snails and pollen of thermophilous deciduous forests. The termination of forest environment is marked by restricted eolian sedimentation and development of chernozemic steppe soil, followed in turn by rapid deposition of calcareous air-borne silt in the so called “Marker” horizon. Hillwash loams and loess interrupted by the weak rendzina-type soil B1f were then formed. The sequence is overlain by the interstadial soil complex which is correlated with the Barbados II Terrace because of its stratigraphic position and warmth-loving snail fauna. Through most of the section bracketed by the interglacial and interstadial soils, the sediments display the reversed declination but positive inclination. The top of the reversed interval is here informally called the Brno magnetostratigraphic horizon. It correlates reasonably well with the upper boundary of the Blake event estimated to be 108,000 yr old.

The vertebrate and the snail faunas of chernozemic soil and of the loess together with the pedogenetic character of strata point to the harsh continental climate with the large temperature variation, dry seasons and partly, with torrential summer rains.

If the remarkably periodic deposition of loess series has to continue, following the pattern observed through the last 350,000 yr or so, then the shift to expressed continental climate in this part of Europe is to be expected soon.

Planktonic foraminiferal studies have been carried out on 28 piston cores of late Pleistocene age from the western Gulf of Mexico, an area of high sedimentation rates. For the interval between 73 × 103 and 95 × 103 yr BP, two of these cores have sedimentation rates of 12 and 15 cm/1000 yr. Calculation of the speed of faunal changes within this interval reveals an extremely rapid paleoclimatic-paleooceanographic change at approximately 90 × 103 years BP. Several species including distinctly warm-sensitive forms, then disappeared from the Gulf of Mexico in less than 350 yr, leaving a depleted planktonic foraminiferal fauna greatly dominated by only three species with little apparent temperature preference. This fauna existed for 2.5 × 103 yr after which distinctly cooler water elements increased in abundance rapidly and formed a high frequency peak approx 83.5–85 × 103 years BP. This increase in cooler water elements reflects either a return to more stable environmental conditions or a lag in their migration to the Gulf of Mexico after the severe climatic cooling, rather than further cooling.

The faunal event in the Gulf of Mexico correlates with an even more spectacular event recorded in the Greenland ice sheet by a drop in 18O values within a time interval of only about 100 yr (Dansgaard et al., 1971, 1972). A possibly correlative climatic event of similarly rapid nature has also been reported for speleothems from southern France (Duplessy et al., 1970).

The paleoclimatic event is closely associated stratigraphically with a widespread volcanic ash layer, although it is possibly significant that the increased volcanism occurred 1000 yr after the paleoclimatic event. A rapid lowering of the lysocline occurs simultaneously with the paleoclimatic event although faunal diversity is low in the succeeding fauna despite decreased calcium carbonate solution. Both the association with volcanism and changes in the position of the lysocline may be significant in consideration of mechanisms of such rapid climatic changes. In turn, such rapid paleoclimatic-paleooceanographic changes as observed in tropical Gulf of Mexico cores, in the Greenland ice sheet and in caves of southern France must be considered in the evaluation of causal mechanisms of glacial and interglacial oscillations.

A spectacular 10‰ drop in the 18O16O ratio (δ) in precipitation indicative of rapid cooling occurred at Camp Century, Greenland, 89,500 y.a. The timing is based upon an assumption of δ periodicity being constant in time. Onset of the Wisconsin pleniglacial is reflected by 6‰ δ drop at 73,000 BP. Similar event might have occurred 109,000 y.a., but the core at corresponding depth is missing. Could it also happen today?

A reduction in the surface salinity of the North Atlantic, by causing an extension of sea ice, could initiate the next Glacial period. Such a salinity reduction could be the result of a slight persistent change in surface pressure in the Caribbean area, that reduces the transfer of water vapor from the Atlantic to the Pacific across the Isthmus of Panama.

Wilson's theory of ice ages implies that the present interglacial will end with, or at least be interrupted by, an Antarctic ice sheet “surge”. Such surges in the past would have caused distinctive rises of sea level: by 10–30 m, in 100 yr or much less, and precisely at the break of climate at the end of each interglacial. Lithostratigraphic, pollen-analytic and radiometric evidence hinting at such a rise (to 17 m?) late in the last interglacial (at about 95,000 BP?) is found in the Spencer's Point formation in Bermuda, the Ladson and Canepatch formations in S. Carolina, the Norfolk formation in Virginia, and above the Walker interglacial swamp in Washington, DC. The strongest evidence that could be found against this rise would be pollen diagrams up toward 17 m which showed continuously freshwater conditions late in the interglacial. Features that might be explained by a surge occur in the Camp Century ice core, in Atlantic, Caribbean and Gulf of Mexico marine cores, and in the Orgnac stalagmite.