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.

Some tentative ideas concerning correlations between solar radiation, gravity, and geomagnetism are given. It is suggested that long-term gravitational effects, associated with planetary configurations, may cause variable solar output.

Seasonal changes in the extent of snow cover and pack-ice were analysed in the satellite-derived maps of the Northern Hemisphere obtained during the last 5 yr. Two intervals were studied in detail: summer 1968 to summer 1969 and summer 1971 to summer 1972. These studies demonstrate that the variation in ground albedo as a result of changing snow and ice cover affects the Earth's energy budget to a far greater degree than any possible extraterrestrial mechanism could do on such a short time scale. However, the rapid expansion of snow cover between late September and November suggests possible feed-back process which could be speeded up or slowed down by minute changes in energy and moisture budget at the snow accretion area. These changes may be of any origin, natural or artificial, and on the time scales ranging from days to millenia.

In this context, the close correspondence between the radiometrically dated gross climate changes within the past 150,000 yr and between the curve showing the rate of change in winter insolation for the middle latitudes of the Northern Hemisphere is conspicuous. The periodicity of about 20,000 yr is involved.

An insolation chronology is introduced here, which is based entirely on astronomic factors (on the so called Milankovitch mechanism of Earth orbital elements). It is independent of any geologic or geochronologic dating systems. Two alternating units comprise the insolation chronology. The positive insolation regime (PIR) is an episode defined by progressively increasing winter irradiation in the Northern Hemisphere, whereas the negative insolation regime (NIR) is an episode of progressively decreasing winter irradiation. The PIR closely correlates with the generally warm intervals such as interglacials and temperate interstadials, the NIR with generally cold intervals. This basic pattern interferes with short-term oscillations which may retard or advance the climate response on the scale of centuries.

It is observed that the positive insolation regime designated as PIR 110, which started at 11,000 YBP, has ended recently. The new negative insolation regime, NIR 0/ + 8, will last for the next 8000 yr. Inasmuch within the last radiometrically dated 150,000 yr no NIR is known to correlate with generally warm interval, the prognosis is for a long-lasting global cooling more severe than any experienced hitherto by civilized mankind.

The equilibrium temperature of the Earth is maintained by a balance between the unreflected part of the incoming solar energy, which is absorbed by the Earth-atmosphere system, and the outgoing long-wave radiation escaping from the Earth to space. It has long been suspected that suspended atmospheric particles (aerosols) might affect this balance, primarily by affecting the albedo or reflectivity of the Earth, thereby altering the amount of solar energy absorbed by the Earth. In light of some recent evidence suggesting the existence of an increase in atmospheric particle concentrations (presumably related to man's activities), the need for development of adequate numerical models to study this problem is apparent. Recent numerical models studying the effect of particles on climate are often based on multiple scattering radiative transfer calculations, and use global averages for particle concentrations and optical properties. By contrasting certain existing models, some major problems in modeling studies that attempt to answer the question of the effects of increased atmospheric particles on climate can be illustrated. It will also be apparent that another uncertainty in the results of such studies arises from a lack of adequate observed input data on the geographic and vertical distributions of particle concentrations and their optical properties. Furthermore, a model that could realistically simulate the impact of increasing atmospheric particle concentration on climate must eventually include the simultaneous coupled effects of all the important atmospheric processes, such as fluid motions and cloud microphysics, in addition to the radiative transfer effects. Current modeling studies already do predict that increases in particle concentrations could have a significant effect on climate. Now, it remains for us to develop the kinds of refined models needed to verify or deny these predictions.

On the premise that climate responds in a stable and well-behaved manner to changes of terrestrial or extraterrestrial conditions of environment, the question of anticipating when the present interglacial will break down is explored from two different viewpoints. The concept, that the timing of the breakdown can be inferred through a comparison of the duration of the present interglacial (to date) with the duration of previous interglacials of similar warmth in the upper Quaternary, is examined from the viewpoint of the chronology of changes of solar radiation received at the top of the atmosphere, generally recognized as a likely causative agent in the development of interglacials. It is found that the radiation changes around the time of the present interglacial are somewhat different in character from those that accompanied the last interglacial, but that the radiation changes around the time of the last interglacial were more nearly similar to those which may have accompanied earlier interglacials. From this viewpoint, comparisons drawn between the present and previous interglacials are judged to be of uncertain value as a predictive tool for the future. The principal effects of man's activities on present-day climate are then reviewed. and the thermal effects of anticipated future increases of atmospheric carbon dioxide and particle loading are compared. It is concluded that the net impact of human activities on the climate of future decades and centuries is quite likely to be one of warming, and therefore favorable to the perpetuation of the present interglacial.