http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== CCNet-ESSAY: ON THE CAUSE OF ICE-AGES By Fred Hoyle and Chandra Wickramasinghe "The renewal of ice-age conditions would render a large fraction of the world's major food-growing areas inoperable, and so would inevitably lead to the extinction of most of the present human population. Since bolide impacts cannot be called up to order, we must look to a sustained greenhouse effect to maintain the present advantageous world climate. This implies the ability to inject effective greenhouse gases into the atmosphere, the opposite of what environmentalists are erroneously advocating." 1. The Greenhouse Effect The greenhouse effect raises the Earth's temperature by about 40oC above what it would otherwise have been. Without the greenhouse effect the Earth would be locked into a permanent ice-age. This fact gives the lie to those renegade scientists, who in their anxiety to get their hands into the public purse, are seeking to persuade the public that the greenhouse effect is a bad thing greatly to be feared. The reverse is true. The greenhouse effect is an exceedingly good thing, without which those of us who happen to live in Britain would be buried under several hundreds of metres of ice. Water vapour and carbon dioxide are the main greenhouse gases. Carbon dioxide produces essentially the whole of its effect through absorption at infrared wavelengths from about 13.5mm to 17.5mm. Because the blocking by carbon dioxide over this interval is large, the band having steeply-falling wings, additions of carbon dioxide have only a second-order influence on the greenhouse effect and are inconsequential compared to the major factors which control the Earth's climate. The blocking effect of water vapour rises all the way from 17.5mm to almost 100mm. The wavelength 13.5mm is important in two respects. In the energy distribution of radiation emitted at ground and sea-level it marks the halfway point, one-half of the energy being at wavelengths shorter than 13.5mm and one-half at wavelengths longer. It also marks a division in the effectiveness of the blocking of greenhouse gases. Shortward of 13.5mm the blocking is comparatively weak, longward of 13.5mm it is strong, excepting for a partial window from 17.5mm to about 20mm. Shortward of 13.5mm there is a broad weak absorption from water vapour with its minimum in the region of 10mm, together with narrow bands from 03 and CH4. Of these, some current fuss is being made about CH4. But blocking by methane is somewhat shortward of 8mm, which is so far out on the short wavelength tail of the Earth's reradiated spectrum as also to be of no great consequence. Thus the Planck maximum for a reradiated spectrum of, say, an effective temperature 290K is at 17.6mm with respect to energy, and at 12.7mm with respect to maximum photon emission. Thus methane makes its contribution in a region of the reradiated spectrum where there is only 10 percent of the energy, for which reason fluctuations in atmospheric methane can produce only minor effects, like those produced by fluctuations of CO2. The gas that can produce major effects, and towards which one must therefore look for an understanding of large shifts of the Earth's climate, is water vapour. Without the greenhouse effect the Earth's mean temperature, averaged with respect to latitude, between day and night and between land and sea, is given by the formula T= [1.37 x 106 (1-A)/ac]l/4, where 1.37 x 106erg cm-2 s-1 is the solar energy flux outside the Earth, A is an averaged value for the Earth's albedo, c is the velocity of light, and a is the radiation density constant, equal to 7.565 x 10-15 erg cm-3deg-4. Thus for an albedo of 0.4 one would have 245K, very cold indeed. It is known from model calculations of stellar atmospheres that the situation becomes complex and difficult when opacity sources are highly wavelength dependent, as they are for the terrestrial greenhouse effect. The same must arise here so that it seems desirable to seek an approximation with the virtue of physical rectitude rather than to set up a supposedly accurate computation in which approximations of uncertain physical validity are nevertheless made in the end. Owing to the fortunate circumstance that the wavelength 13.5mm has the special properties described above, such a useful approximation lies immediately to hand. Suppose the half of the reradiated energy longward of 13.5mm to be completely blocked by the heavy opacity of the greenhouse gases and suppose the half shortward of 13.5mm to be completely free to escape. Then it is easy to see that the greenhouse effect must raise the Earth's mean temperature by 21/4 above what it would otherwise be, about 292K instead of 245K, a result agreeing very well with experience. One can see that the weak blocking which actually takes place shortward of 13.5mm is approximately compensated by the partial window from 17.5mm to 20mm. With a first approximation that is evidently close to the truth it is possible to calculate the effects of changing individual greenhouse gases as fluctuations from this first approximation, thereby keeping close contact with physical reality. The above remarks concerning the opacity of water vapour refers to a so-called standard atmosphere which is taken to contain 1 cm cm-2 of precipitable water. Reducing the water content appreciably to only a few millimetres of precipitable water weakens the greenhouse, dropping the Earth's mean temperature (for the same A) to about 280K, which corresponds closely to what is required for ice-age conditions. The conclusion is therefore that reducing the average water content of the atmosphere to about a third of its present-day value, while maintaining the albedo, would produce an ice-age. 2. Ice-age Conditions Ice-age conditions were dry, dusty and cold. The great deposits of loess, wind-blown soil, in E. Europe and China, imply a climate that was dusty in the lower atmosphere, a situation implying a low precipitation rate. Low precipitation is not a handicap to the accumulation of large glaciers, which will grow even at annual precipitation rates as little as a few centimetres per year, provided the temperature is low enough to prevent summer melting. During the ice-ages the whole Earth was cooled, including the tropics. This is proved by glaciers extending down to about 10,000 feet on tropical mountains, mountains which at present do not hold glaciers, such as the mountains on the island of Hawaii. The need for the whole Earth to be appreciably cooled disposes of astronomical theories of the cause of ice-ages, in particular of the Milankovitch theory of small oscillations of the tilt of the Earth's rotation axis to the plane of the ecliptic, and of small oscillations in the eccentricity of the Earth's orbit. Neither of these effects produces any change in the amount of solar energy incident on the Earth and so could not lead to widespread cooling. Oscillations of tilt merely produce slight latitude variations in the incidence of solar energy, which are in any case much smaller than the transport in latitude of heat by atmospheric storms and ocean currents. Indeed the transport of oceanic heat towards the poles gives a far larger effect than would easily buffer slight latitude variations of insolation. Oscillations in eccentricity of the Earth's orbit produce small shifts of solar energy between one geographical hemisphere and the other, and so should tend to cool one hemisphere and warm the other. But ice-ages occur contemporaneously in both hemispheres, not alternatively, a disproof that was already well-understood more than half a century ago. Claims in favour of the astronomical theory, made from numerical computer studies, say more about the work of computer studies than they do about ice-ages. If we were to imagine such an atmospheric state being brought about today, evaporation from the relatively warm surface layers of the ocean would quickly resupply water vapour to a typical amount of 1 cm of precipitable water per cm2 and the cooling due to a reduced greenhouse effect would quickly be gone. Thus it is the heat of the ocean which saves us from the possibility of an immediate onset of ice-age conditions. Reckoning the heat of the ocean as being the energy content above freezing point, which can be thought of as available heat, almost all is contained in a surface layer with depth no greater than a few hundred metres, the amount being equivalent to a supply of sunlight over a time interval of a few years, say 3 to 5 years. It is because the ocean has this back storage of heat that we do not drop almost immediately into an ice-age. In distant geological periods the heat storage in the oceans was considerably greater than it is at present. Today the ocean bottom waters are close to freezing, whereas only 50 million years ago the bottom temperature was about 15'C and the available oceanic heat was then equivalent to a 50 year supply of sunlight. The difference has been caused by drifting continents, especially by the positioning of Antarctica and Greenland at or close to the poles. Melt water from arctic glaciers has gradually filled the lower ocean with water close to freezing, greatly reducing the margin of safety against ice-age conditions developing. This is why the past million years has been essentially a continuing ice-age, broken occasionally by short-lived interglacials. It is also why those who have engaged in lurid talk over an enhanced greenhouse effect raising the Earth's temperature by a degree or two should be seen as both demented and dangerous. The problem for the present swollen human species is of a drift back into an ice-age, not away from an ice-age. Manifestly, we need all the greenhouse we can get, even to the extent of the British Isles becoming good for the growing of vines. The present-day situation is best seen as one of neutral equilibrium unlike an ice-age which is a position of stable equilibrium. The present-day situation is one in which over relatively short intervals the world climate stays the way it is, but over longer intervals can be subject to drift. Looking through climatic records for the recent millennium the drift over a century or two is by 1-2oC. Drift from the present-day down by 10o C into an ice-age requires an excess of about ten downward steps over upward steps, say each step of 1o C. With a century between steps, random shifts would bring on the next ice-age in an interval of about 10,000 years, the typical length of an interglacial. Without some artificial means of giving positive feedback to the climate, such an eventual drift into ice-age conditions appears inevitable. All this is on the assumption of a fixed albedo, a point which now requires consideration. 3. The Albedo The remarkable feature of the Earth's albedo is that atmospheric water does not lift A close to unity. If even a very small fraction of even a very dry atmosphere were to condense into tiny ice crystals, this would happen. The mass exclusion coefficient, through the scattering back into space of sunlight, produced by dielectric crystals with radii of a few tenths of a micrometre, is about 3000 cm2 g-1 . Thus a condensation of only 0.1 percent of the water in a very dry atmosphere with only 1 mm of precipitable would yet contribute about 0.3 to A. Essentially no water must be condensed into ice crystals if A is to be appreciably less than unity. Otherwise the Earth would appear from the outside as an intensely bright white planet with an albedo even higher than Venus, while below the haze of ice crystals it would be exceedingly cold at ground-level. The saving grace is that ice crystals do not form in supersaturated water vapour except at very low temperatures, below say -50oC. For the Earth's emission into space of radiation at wavelengths longer than 20mm we can think of a photosphere at which the optical depth out into space is of order unity. If only radiation were involved in determining the water vapour temperature at this photosphere the temperature would be of order 290 t -1/4 where t was the optical depth from ground level up to the photosphere, suitably averaged at wavelengths longer than 20mm. In a typical atmosphere t would be about 19, leading to a photospheric temperature for water vapour (and hence for surrounding air) of as little as 163K, i.e. -110oC, far below that needed for ice crystal formation. The circumstance that ice crystals do not form profusely except under special conditions in Antarctica shows that calculating for radiation only cannot be correct. A convective transport of energy from ground-level to the water-vapour photosphere is required. This cannot be carried by air movements but must come from the upward transport of the latent heat of condensation of the water vapour itself. To keep the photospheric water vapour temperature above -50oC, and so to prevent ice crystal formation, the transport of water vapour must be such as would lead to an annual precipitation rate of about 50 cm. For comparison, the present-day world-wide average of the precipitation rate is about 80 cm of rain, sufficient to prevent ice crystal formation, but not by a wide margin. Let the world climate drift downward, however, sufficiently for the surface layers of the ocean to cool to the point where an annual average rainfall of 50 cm cannot be maintained and the consequent formation of an atmospheric haze of ice crystals would plunge the Earth immediately back into an ice-age. 4. Emergence from an Ice-age The cooling of the ocean over the past 50 million years eventually made an ice-age the norm of the Earth's climate, as it has been throughout most of the Pleistocene, with brief changes only during interglacials lasting for times of about 10,000 years or less. Left to itself, it is hard to see how anything internal to the Earth could ever break the stable grip of an ice-age. Thus to understand the cause of interglacials we must look to catastrophic events. The impact of a comet-sized object into a major ocean appears essential to the ending of an ice-age. An object of mass 1016g would have sufficient energy to throw up some 1020g of water into the stratosphere, immediately creating a powerful greenhouse effect as the water spread around the world to give some 10 g of precipitable water per cm2. Such a greenhouse effect lasting for some months, and at a lesser level for several years, would produce a sufficient warming of the surface waters of the ocean to jerk the Earth almost discontinuously out of a long drawn-out ice-age into the beginning of an interglacial. The 18O/16O analysis of Greenland ice cores shows that an immense melting of glacier ice began about 13,000 years ago and was essentially completed within a millenium. But this information is slow-moving in time, although it possesses the great merit of being of world-wide significance. On a more restricted geographical scale, fossil insect records show that the summer temperature in Britain rose by 10oC or more in as little as 50 years, an essentially decisive indication of a catastrophic event as its cause. The fossil insect record also shows that a second catastrophic event of a similar nature occurred 10,000 years ago, again with a major temperature rise in only a few decades. It is therefore cometary impacts that we must thank for the equable spell of climate in which human history and civilisation has prospered so spectacularly. The renewal of ice-age conditions would render a large fraction of the world's major food-growing areas inoperable, and so would inevitably lead to the extinction of most of the present human population. Since bolide impacts cannot be called up to order, we must look to a sustained greenhouse effect to maintain the present advantageous world climate. This implies the ability to inject effective greenhouse gases into the atmosphere, the opposite of what environmentalists are erroneously advocating. 5. Conclusions Ice-age conditions are dry and cold, the local temperature being reduced over the entire Earth. The high atmosphere probably had a haze of small ice crystals while the lower atmosphere was dusty. Such conditions were stable, capable of persisting until a large bolide hit one of the major oceans. The water then thrown high into the stratosphere provided a large temporary greenhouse effect, but sufficient to produce a warming of the world ocean down to a depth of a few hundred metres. It is this warming that maintains the resulting interglacial period. The interglacial climate possesses only neutral equilibrium however. It experiences random walk both up and down, until a situation arises in which the number of steps downward become sufficient for the Earth to fall back into the ice-age trap. Thereafter only a further large bolide impact can produce a departure from the grey, drab iceage conditions. This will be so in the future unless Man finds an effective way to maintain a suitably large greenhouse effect. Copyright 1999, Fred Hoyle and Chandra Wickramasinghe ---------------------------------------- THE CAMBRIDGE-CONFERENCE NETWORK (CCNet) ---------------------------------------- The CCNet is a scholarly electronic network. 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