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.