Thanks Contrarian for the interesting link
The following is from an article published 15 years ago. So much might be dated. I thought it useful to post here just the same as it outlines in non-technical language the basic processes at work. For copyright reasons I had to cut out portions. (It's a difficult process in a case like this and I hope it's enough)
FAIR USE ONLY
The Economist, 7-13 April 1990, pp.95 - 100
"The Once And Future Weather
THE weather is a peculiar mix of the singular and the repetitive. Every cloudscape is unique, but it is easy to learn which ones are the harbingers of rain and which of thunderstorms. This is because the weather is not random. Its ceaseless change is patterned, coming in cycles ranging from minutes to millennia. A computer that simulated 1,000 years of the world's weather would never see the same day twice. But neither would it see any vast change in the weather's broad characteristics.
Such regularity is called the climate. The climate defines the limits within which weather patterns will vary. A great deal of variation - storms one winter, bitter cold the next, a run of unusually dry summers - can be accommodated within a stable climate. But the climate itself is not immune to the cycles of change seen in the weather. It changes too - slowly but surely.
This sort of change can clearly be seen in recorded history. The most dramatic example is the Little Ice Age, which lasted from the thirteenth century to the end of the eighteenth. Glaciers grew larger in Norway and the Alps' Atlantic pack ice spread further south than it did before or has since. The Thames froze solid around London Bridge. The average temperature in Europe was probably 1øC less than today. ...
In 1800 carbon dioxide (CO2) made up 0.028% of the earth's atmosphere. Now it makes up 0.035%. This tiny fraction of the atmosphere amounts to an enormous number of molecules. And unlike the molecules that make up most of the atmosphere, CO2 is particularly good at absorbing heat. Atmospheric CO2 sits like a duvet around the world, ensuring that not all the warmth which arrives as light from the sun is radiated away. Mankind has been restuffing the duvet, notably with CO2, but also with other heat-absorbing molecules, like methane and the CFCs. ..
Changing the way that the sun heats the earth sounds like a sure way to change the climate. Predictions about the outcome of the experiment are in great demand because climate change - when and if it comes - can have all sorts of consequences. The Little Ice Age changed the pattern of fisheries in the Atlantic and wiped out the English vineyards of the early middle ages. It seems to have been responsible for the crop failures that led to famine in fourteenth-century Europe. ...
Reliable records of the climate are comparatively recent; few are more than a couple of centuries old. But meteorologists are not the only ones who record climate. Crop yields can do so, so can diarists. To measure the cloudiness of summers past, Dr Hubert Lamb, one of the pioneers of climatic history, has even used paintings by British and Dutch artists (among many other things).
Even with such cleverness, historical studies can go only so far. Not much was written down - or painted - about anything more than 3,000 years ago. Archaeology can stretch the record slightly by discovering when people cultivated the tops of hills and when they stuck to warmer valleys, and by showing which animals they ate. But there comes a limit beyond which only non-human witnesses will do. They are the only ones that can speak of the 4 billion years of changing climate which came before the 10,000 years of semi-civilised man. ...
Some of the evidence is inanimate. Glaciers grow and shrink, and in so doing scratch the face of the earth beneath them. Lakes expand, contract and freeze. Storms
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deposit huge amounts of muck quickly. Constant winds build dunes. All this is recorded in the surrounding sediments and the rocks that eventually form from them. But the most telling records of past weather are those written in the patterns of life. Life quickly adapts to the vagaries of its environment, so it bears the imprint of the climate.
A tree, for example, keeps a fine record. For a start there are its rings. Each year a growing tree lays down new wood around its trunk. The concentric rings in a cross-section record the seasons of the year. They also record the climate: when conditions suited the tree, it grew faster, and laid down broader rings. Cold summers leave thin rings, harsh winters curtail growth quickly.
Where a tree is found is significant in itself, if you know the tree's likes and dislikes. Pliny the Elder noticed that beech trees, which in his day grew high in the hills and on the plains of the north, used to grow in Rome itself took this as evidence that Italy had got warmer. If, unlike Pliny, you do not have annalists to tell you where the trees lived, you can study the pollen they left behind. Pollen can be misleading (partly because of the wind) but new methods, which rely on finding "assemblages" of pollens from various plants, are making the technique more reliable.
Bogs, too, are surprisingly informative. In a recent study Dr Peter Moore of London's Kings College was surprised to find that old peat bogs all over Europe appear to have put on a spurt of growth in 800 BC-600 BC. Since bogs like it cold and wet, that might indicate an event like the Little Ice Age (possibly one that gave Rome the cold climate which Pliny noticed it recovering from). The bogs also suggest a cooling around 400 AD, and a later one which might correspond to the onset of the Little Ice Age itself.
Animals are less abundant and well-preserved than plants, but they provide clues too. Beetles are particularly helpful. Certain sorts clearly indicate certain types of climate, and the fact that beetles fly means that as soon as somewhere suits them they are likely to colonise it.
Bogs, bugs and trees all have a drawback. At best, they can provide little more insight than a set of thermometers and raingauges stretching back over the years. They record the effects of climate change. On causes they are mute. This is particularly unfortunate for those who wish to predict the future. They need to know about the mechanisms that lead from cause to effect. One putative cause of future change - the build up of CO2 and the other greenhouse gases is already known. It is the effects on which everything hinges. Past effects are known; past causes, by and large, are invisible.
Some causes, though, have recently become known. For the past 2m years ice ages have repeatedly frozen the world. During an ice age, snow in fairly high latitudes does not melt during the summer and permanent ice caps form over northern America, Europe and Asia, and on the Andes. The root cause of the ice ages is now established. ...
Why the iceman comes
The sun's energy lies behind almost everything that happens on earth. It allows plants to grow and so to feed animals. It evaporates water to provide rain. Fossilised sunshine, stored in the carbonaceous husks of long dead life, provides the energy in coal and oil. Today's sun drives the weather.
Sunlight does not illuminate the earth even-handedly. It shines directly on the equator, but only obliquely on the poles, so the tropics get hotter. The atmosphere acts to even out this difference by moving warm air to cold places.
If the world were a billiard ball surrounded by an atmosphere, its weather would be simple. Hot air would rise at the tropics and move to the poles, polar air would slide down the surface towards the equator. But the real earth is more complex. For a start, it spins, which gives air currents a tendency to twist. Spin also brings the warming-cooling cycle of day and night. That is exacerbated by the oceans, which are slower to warm up and cool down than the land, and embroiled in their own attempt to move heat out from the tropics. There is geography, which puts mountains in the way of the winds, and life, which changes the way the land reflects sunlight, how much moisture it retains, and so on.
However fraught the details, the sun is still the main cause of the weather. As such it also dominates climate, setting the limits on the weather. Changes in the sunlight reaching the earth change the climate. The sun's brightness does not appear to change much. It fluctuates a little, in cycles 11 years long, but the change in brightness is only 0.1% of the total. There is considerable debate about whether that change influences the weather, which is unlikely to be settled until a few more 11-year cycles have been and gone. Taking a longer look at things, some have suggested that the Little Ice Age might have been linked to particularly low solar activity. They point to the low numbers of sunspots reported by seventeenth-century astronomers.
Even when the sun's output does not vary, the distribution of the energy received by the earth will - because the way the earth faces the sun changes. The earth's axis of rotation is tilted with respect to the plane of its orbit. So from April to September the sun is overhead in the northern tropics and the north pole is permanently illuminated. From October to March the reverse is true and the southern hemisphere enjoys its summer while the north sits through its winter.
But the tilt which causes the seasons is not fixed. The amount by which the planet is tilted with respect to the orbital plane can change from 21.8ø to 24.4ø over a period of 41,000 years. This means that the tropics - the band around the equator in which the sun can be directly overhead - expand and contract, as do the Arctic and Antarctic circles. The broader he tropics, the larger the differences between summer and winter.
As well as swinging up and down, the earth's axis also wobbles like an off-centre spinning top, the poles inscribing circles every 23,000 years. This motion, called the precession of the equinoxes, means that in 12,000 years northern midwinter will fall in June. This can also have an effect on the intensity of the seasons. The earth orbits the sun in an ellipse, which means that at some times of the year it is marginally closer to the sun than at other times. At present, this change runs in step with the southern seasons. In the southern winter, the earth is the furthest from the sun, in southern summer it is nearest, reinforcing the effects of the seasons. The precession of the equinoxes will eventually reverse this, so that the northern seasons are in step with the orbit.
The orbit itself is not stable either. Over a period of 100,000 years, it goes from its most nearly circular to its most eccentrically elliptical. There is a similar cycle that lasts a little over 400,000 years. Since the average distance from the earth to the sun stays the same while the orbit changes, the amount of sunlight received in a year will be constant. But the variation over the year will change. ...
The idea that the cycles of rotation and orbit affect the climate is credited to a Yugoslav astronomer, Milutin Milankovitch, who elaborated it in the first half of the cen-
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tury. For a long time, it was unprovable, because the dates of events in the distant past were so poorly known. In the past 20 years it has risen triumphantly on the back of the most important record of past climate - the fossilised chemistry of the ocean.
Despite the efforts of oil companies, sewage authorities and waste-dumpers, the seas are still mostly composed of water. Each molecule of water contains a pair of hydrogen atoms, which are light, attached to an oxygen atom, which is considerably heavier. But not all oxygen is the same. Some oxygen atoms weigh 16 times as much as a hydrogen atom, and some (about 1 in 1,250) weigh 18 times as much.
When water evaporates the vapour contains a greater proportion of lighter oxygen 16 than heavier oxygen-18. The molecules bearing oxygen-18 will condense from the vapour more readily too. Normally this has no effect on the oceans' composition, because after brief sojourns in clouds and rivers, evaporated sea-water returns whence it came. During an ice age, though, a lot of the water falls as snow on ice caps, and is thus held hostage for the duration. So the water in ice-age oceans is enriched in the heavier "isotope", oxygen-18 (DO), and that in the ice caps in the lighter oxygen-16 (16 O).
To study the frequency of different isotopes in ancient oceans, you can study the life they contained. The seas and oceans abound in single-celled creatures called foraminifera - forams, for short - wrapped in shells of calcium carbonate which are made in part with oxygen from the water. The amount of DO in sea water is reflected in the amount in the shells, which continually drift to the oceans' oozy floors to form layers of sediment. Drill into the ocean floor and remove a column of such sediment and you can build a record of the ice ages.
In 1976 Dr Nicholas Shackleton, from Cambridge University, and his colleagues Dr James Hays of Columbia University in New York and Dr John Imbrie of Brown University in Rhode Island, produced a set of detailed foraminifera records, making use of greatly improved histories of the earth's magnetic field (indicators of which are also held in the sediments) to date the shells. Their record showed that the world's ice cover swelled and shrank to a complicated rhythm, from which three different beats were untangled. There were cycles of 23,000 years, 41,000 years and 100,000 years. These are the Milankovitch cycles. Further work - and comparisons with cores bored through ice caps, which show that ice-age ice has a complementary enrichment in 16O - strengthened their findings.
Oxygen isotope work showed clearly that the Milankovitch cycles were tied to the climate. But it did not show how. After all the cycles do not change the amount of sunlight reaching the earth, just its seasonal variation: how can effects that are dependent on season plunge both north and south into an ice age?
Carbon and conveyors
One part of the answer lies in the atmosphere, which tends to be chemically much the same the world over. Air from the past can be recovered from bubbles in polar ice. The ice is layered and samples have been brought up from strata laid down over the past 140,000 years. That covers several ice ages and the warm "interglacial" period 100,000 years ago. One of the messages of ice-age studies is that such interglacials occur rarely. There is one going on now, but the most recent one before today's was 100,000 years ago, at the last peak of the long Milankovitch cycle. For some unknown reason that cycle appears to be the most important one.
The record shows that during the most recent ice age, the level of carbon dioxide in the atmosphere was considerably lower than it is today; as low as 0.018%. That looks like good evidence for warming caused by the greenhouse effect, because it shows low CO2when the world was cold. The coldness of the ice ages was not due just to low CO2 - the level was not low enough to account for all the cooling. In fact, the ice ages were probably largely self-perpetuating, because the ice caps reflected so much sunlight straight back out into space. But the low level is just the sort of thing that might nudge the climate into such a vicious circle.
Where did the CO2 disappear to? Whenever it is missing from the atmosphere, look for it in the ocean. There is roughly 60 times as much CO2 dissolved in the ocean as there is floating free in the atmosphere. Plants use it for photosynthesis, shelly creatures like the forams make their carbonate carapaces from it. Both processes reduce CO2 to its constituent carbon. When the plants and animals die and sink to the bottom, the carbon is re-oxidised, creating CO2 afresh in the deep waters of the ocean. Eventually that CO2 circulates back up to the surface.
Just as oxygen comes in different isotopes, so does carbon: 12C, which is the most common form, rarer 13C, and, rarest of all, radioactive i4C. Because photosynthesis and shell-building both discriminate between 12C and 13C, comparisons of the ratio between the two in surface- and deep dwelling forams can show how CO2 was distributed in the water. By extension, they can show the level of CO2 in the atmosphere.
Dr Shackleton and Dr Nick Pisias, from Oregon state university, have constructed a carbon-isotope record that goes back
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340,000 years, and have compared it to the oxygen-isotope record. The atmospheric CO2level follows the Milankovitch cycles just as the ice ages do. But it is slightly ahead of them, just as it should be if CO2 were causing the move from one stable state to another.
The CO2 in the oceans does not just reflect atmospheric levels. It controls them. If the level in the upper oceans drops, CO2 will be soaked up from the atmosphere to make up the difference. What could make that CO2 level drop? Hyperactive life could use it up. Or CO2 could build up at the bottom of the oceans, at the expense of the shallows. ...
a way in which the deep ceans might be responsible for low atmospheric CO2. It depends on the "North Atlantic Conveyor". In winter, salty water moving up the North Atlantic is cooled by cold winds coming off America. Since cold, salty water is particularly dense, water sinks down to the bottom of the ocean, off Iceland. The heat given up by the water in theconveyor to the winds above is a huge influence on north Atlantic climate, making Lisbon's winters much nicer than Boston's.
The conveyor is the driving force behind the world's deep ocean current - a flow equal to 20 times that of all the world's rivers combined, often moving in the opposite direction to surface currents. The deep current flows south down the Atlantic, then east across the southern Indian Ocean and into the Pacific. All the time, detritus from the shallow ocean is falling into it. When the water returns to the surface in the Pacific, it brings with it CO2 recreated in the depths from fallen carbon. So if the flow stopped,CO2 would build up at the bottom of the oceans, and the shallow waters would necessarily contain less. CO2 would be drawn from the atmosphere to make good the loss, and things would start to cool down.
There is evidence that this is what happened in the ice ages. ....
Forams are prone to the mistake of incorporating a metal, cadmium, into their shells under the misapprehension that it is calcium. The amount of cadmium in a shell reflects the amount dissolved in the water and the distribution of cadmium in the oceans is similar to that of phosphates and nitrates - the nutrients essential to life in the sea. So forams from nutrient-rich water have cadmium-rich shells.
Deep Atlantic water, freshly sunk by the conveyor, is low in nutrients because it was recently surface water, and full of little beasties that eat a lot. Deep water in the Indian and Pacific oceans is better stocked with nutrients, because the water has been at depth long enough for them to trickle down into it. In the last ice age, however, according to bottom-dwelling foram shells, nutrients were equally distributed around the deep waters of the globe. The North Atlantic Conveyor, if it was working at all, was going slow.
A similar conclusion comes from studies of 14C. This radioactive isotope, which is constantly being produced in the atmosphere by solar radiation, is often used to date biological samples - from the shroud of Turin to tree rings. It can also be used to date the deep waters of the oceans. At present, deep waters in the Pacific are about 1,500 years "older" than surface waters. It is 1,500 years since they were biologically active shallow waters. In the most recent ice age, the difference in ages was 2,000 years. Water was staying deep for longer.
To Dr Broecker and some of his colleagues, this makes a strong case for regarding the North Atlantic Conveyor as a sort of climate switch. When it is on, deep water keeps moving, CO2 is constantly brought up from the depths, and the world stays warm. When it is off, deep water sits tight, there is a net transfer of CO2 from the atmosphere to the abyss, and the world gets cold.
The idea that ocean circulation (and related chemical and biological changes) provides the key to understanding the ice ages is widely believed. ...
The lesson of the ice ages is that the way in which the oceans and the atmosphere interact is the single biggest factor determining climate changes. It is likely to fix the extent and course of greenhouse warming. Just as historians of climate look to the sea, so must modern climatologists. The largest unanswered questions about the greenhouse effect lie not in the atmosphere, but in the puzzling interplay of chemistry, physics and biology in the oceans - a puzzle the historians seem to be helping to solve. ...
What made the ice ages quite so cold? The CO2 level cannot on its own account for the whole 10-12 øC drop in global temperatures. Ice cores reveal lower levels of methane and higher levels of dust in the ice-age atmosphere than there are today. Dust acts in a contrary way to CO2 - it stops sunlight getting in in the first place, rather than trapping heat.
Between the dust, the methane, the CO2 and the fact that the ice caps themselves contribute to cooling by reflecting sunlight away, there seems to be enough to account for the global cooling of the ice ages. But what controls the dust and methane levels? Are they consequences of changes in CO2 levels, acting to amplify the change that gives them birth? Or are they controlled by something else - the way ice sheets grow, or the ecology of the tundra, or some other uncharted pathway through the oceans?
Warming and warning ...
One clear message of the ice ages (beyond the fact that CO2, in the atmosphere and the oceans, exercises a great deal of control over the climate) is that change can be sudden and can buck trends. For 6,000 years during the most recent deglaciation, the world slowly warmed up as summers in the northern hemisphere became warmer. Then, 12,000 years ago, everything changed. For 1,000 years the glaciers started to grow again in Europe and in North America. Then the warming resumed.
Dr Broecker has an explanation. Much of the water from the melting American glaciers ran down to the gulf of Mexico, but some formed a vast lake in southern Canada (larger than Lake Superior), cut off from the Mississippi basin by high land and from the eastern seaboard by glaciers. Eventually, the retreating ice opened up a route by which the lake could drain into the Atlantic and the vast influx of fresh water upset the ocean's balance again. The salt levels dropped, the conveyor stopped, and the cold weather returned.
The resumed warming was well-nigh
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cataclysmic. Measurements of oxygen isotopes in the Greenland ice cap show the temperature rising by 5øC in 20 years. It is as though all the warming that should have gone on during the cold millennium took place at once. If Dr Broecker is right, that change was brought about by a trigger little more dramatic than damming a large river (the right large river at the right time, of course). If he is wrong, it was brought about by something else. But it still happened; a warming faster even than the direst predictions of greenhouse doom-mongers. ...
Fixed air models
Dr John Mitchell, of Britain's Meteorological Office, has shown that the mid-Holocene was not like the greenhouse. His technique reveals the most practical application of climate history. He took a computer program like those which forecast today's weather and changed its assumptions, so that it modelled a world with the seasonal variations in sunshine typical 9,000 years ago. The computer then ran through a number of years under those assumptions. Averaging the results provided a picture of climate under those conditions.
He compared the model with records of the real Holocene climate and found that it fitted pretty well. He then ran the model with today's orbital conditions and an increased level of CO2. The climate came out differently. This time the only way to check that the results are right is to wait and see.
The problem is that the present greenhouse effect will tend to move the world's climate into regions uncharted by the historians of ice-age climate. As far as they are concerned, the present interglacial, from the Holocene to now, is as warm as the world getsÄand it only manages that every 100,000 years. Some believe that the interglacial is already on the wane and that, were it not for man's CO2, the glaciers would be preparing to regain their old dominions. By stopping this man appears to be creating a "super-interglacial". Though the links between ocean, atmosphere, life and climate will still be there, the effects will be unlike any seen before.
This is not good news for computer modellers. ...
... the changes caused by CO2 are more far-reaching. CO2 heats the atmosphere from inside, not outside like the sun. It affects the layering of the atmosphere, it migrates to and from the ocean, and it influences the way plants grow.
The recent past provides no greenhouse world against which to check greenhouse models. But long before the ice ages, between 200m and 65m years ago in the Jurassic and Cretaceous periods, the CO2 level was high and the world warm. A model of today's climate which, mutatis mutandis, was able accurately to describe the Cretaceous, would clearly be robust.
Recreating the Cretaceous climate in sufficient detail calls for new tools. Life in the Cretaceous was different, which creates a problem. If there are no spruces, one cannot see the fossil of a spruce and use knowledge of spruces to deduce what the climate was. This means more use has to be made of inanimate evidence. Sediments regularly stripped of their minerals by monsoon rains form identifiable rocks. The dunes and salt flats of deserts are preserved. The rocks provide broad outlines of the climate.
Living things still have a role to play, since the basic mechanisms of life are unlikely to have changed much. Researchers can use the fact that, by and large, plants in warm climates use more 13C in photosynthesis than those in colder climates. The ratio of the two isotopes in the plant (or in the bones of an animal that ate it) can thus reveal something of the average temperature. ...[p.100]"
Edited to add image:
[ 13 March 2005: Message edited by: VanLuke ]
Email this thread to someone!
posted 16 February 2005 03:56 PM
but your heart is in the right place. Sorry! 
If you think anyone who is religious is a religious nut, though, you have a lot to learn. 
