PLEASE NOTE:


*

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. To subscribe/unsubscribe,
please contact the moderator Benny J Peiser <b.j.peiser@livjm.ac.uk>.
Information circulated on this network is for scholarly and
educational use only. The attached information may not be copied or
reproduced for any other purposes without prior permission of the
copyright holders. The fully indexed archive of the CCNet, from
February 1997 on, can be found at http://abob.libs.uga.edu/bobk/cccmenu.html



CCCMENU CCC for 1999

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.