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The Ancient Climate

The climate has varied widely down the ages, from tropical over most of the planet, to the ice-age conditions that we have today.

Post-Cambrian times Ice ages What causes ice ages?

Key to Climate column

The Climate column in the main tables clearly gives the crudest possible representation of the climate at different times. This table gives the key. More detail is given in www.scotese.com.

Average Global temp (°c)		Wet	Medium (or unknown)	Dry
No data
>22	v. hot
22	hot
17	warm
12	cool
ICE

Surprisingly there really do seem to have been long periods when the climate was actually very much the same worldwide. We appear to be in a fairly rare situation at the moment, with land stretching almost from pole to pole, and the climate varying between extremes.

Now Scotese is the acknowledged expert on palaeo mapping, or continental drift. But I’m less persuaded of his authority on climates. On the other hand he has produced a far more comprehensive and intelligible account than I’ve found elsewhere. If a scientist has the courtesy to report back to us on what he’s been doing with our money, then we have to applaud it – and make the most of it.

Climate change down the aeons

Technically, we are still in an ice age because we have ice at the poles. This means that we have a much wider range of climate than was often the case in the past.

It turns out that we always get ice ages during periods of continental collision and supercontinent formation (more). We’re in the middle of one of these at the moment, even though the next supercontinent is not due to form for another 250 million years. The reason seems to be that large land masses block the ocean currents, and stop them feeding their huge amounts of heat up to the polar regions.

In between ice ages, the land even at the poles can be very pleasant. Dinosaur and palm-tree fossils have been found in Australia, dating to the time when Australia was down near the South Pole. So both were living quite happily with 6 months of constant daylight and 6 months of total darkness.

How do they know what the climate was like?

Well ‘know’ is possibly over-stating it. But it’s a detective job.

There is actually a great deal of information buried in the rocks. For example, coal needs warm wet conditions to form (more). So where there’s a lot of coal the climate, at that place and at that time, must have been warm and wet. By contrast, salt pans need it hot and dry. There’s masses of salt under parts of Cheshire in England, so ‘England’s green and pleasant land’ certainly wasn’t always like that. Deserts also need it dry, even if not necessarily hot.

Ice leaves all sorts of evidence around, from the scrape marks as the ice grinds slowly down the valley, to ‘moraines’ of assorted detritus that they leave along the sides and down at the snout where they melt. Denmark is built entirely on the ‘terminal’ moraine at the end of the glacier that carved out the Baltic.

Now and again you get a huge boulder of a type of rock that doesn’t occur for hundreds of miles. When I was young, these were total mysteries. It’s remarkable how recently the penny dropped that ice could have been responsible.

When plants emerged from the sea to colonise the land, they began to provide additional evidence. Plant fossils are better than animal fossils because they can’t escape climate changes. They have to endure or die. Some ancient plants are still remarkably similar to their modern counterparts. So palaeo-climatologists infer that the conditions that they could tolerate are probably not too different either.

The Hadean Aeon (0-4 kMy)

For the young planet, we’re not discussing effete topics like “would we have been able to go out without a coat?”. We’re interested in more brutal matters such as “would there have been any oxygen to breathe?” (Answer: no.)

For the first hundred million years or so (wild guess) our planet was far too hot for either carbon or water to survive on the surface. It was all swilling around in the atmosphere as carbon dioxide and steam. This picture depicts the gradual calming down of the planet, as the Hadean Aeon merges into the Archaean.

I’ve read that, at a certain point during the Hadean, the atmosphere became cool enough for the moisture in the atmosphere to start tipping down as rain. As soon as the rain hit the hot surface, it boiled off again of course. But in doing so it gradually cooled the surface layer. The rain will have continued to pour down for millions of years (another wild guess) until the surface became cool enough to stop boiling it off. This may be why the geologists found evidence of liquid water very much earlier than they had dreamed possible (more).

The suffocating carbon dioxide blanket was much more persistent however. When I first started following this story, the scientists couldn’t understand why our planet didn’t freeze over – and stay that way – after the end of the hot phase. It seems that the young Sun was a bit like a newly-lit open fire. It was not drawing properly yet, so it was not providing a great deal of heat. Then they realised that they had forgotten the greenhouse effect, and everything was explained.

The Precambrian Period (4kMy- 500My)

We start the great Precambrian Period with the greenhouse blanket still fully intact.

There was still not a whiff of oxygen around. We tend to forget that oxygen is a terribly reactive gas. Any free oxygen will certainly find something to combine with very quickly. If there is any free oxygen, it’s because something is pushing it out in enormous quantities. And that something is life. To be more precise, it’s bacteria – even today. You might think that it’s plants that produce oxygen. But in fact the ‘chloroplasts’ that work the trick are actually captured bacteria.

To be honest, a ‘eukaryotic’ cell (see below) captured a photosynthetic bacterium, thousands of millions of years ago, and the ‘bacteria’ are now thoroughly incorporated into the larger cell. But the evidence of their origin persists.

So the gradual thinning of the CO₂ blanket couldn’t start until bacteria had mastered the very difficult trick of photosynthesis (more). We tend to regard bacteria as pretty primitive beasties, and compared with the eukaryotic cells that we are made of, so they are (more). But the more the scientists find out about them, the more advanced they appear. And the ‘cyano-bacteria’ that provide most of the world’s oxygen are more sophisticated than most.

This picture is not very clear (I can’t remember where I got it from). But the smaller organism is a bacterium, complete with the propulsive system that many bacteria have. The larger one is a eukaryotic cell, with its various ‘organelles’. These are actually little factories and each has a specific job to do. The ‘ex-cyanobacterium’ that does the photosynthesis is the ‘chloroplast’. Animal cells don’t have these.

According to Scotese, the amount of carbon dioxide in the atmosphere had dropped to around 20% by 3½ thousand years ago. Today of course it’s almost zero.

The first evidence of free oxygen in the environment is provided by the ‘Banded Iron formations’ or BIFs (more). These appear from around 3 to 3½ thousand million years ago (I think). But at first, all the oxygen went into oxygenating the oceans (in which of course the cyanos were living). Only when the oceans were pretty well replete did significant amounts start leaking into the atmosphere.

Oxygenating the atmosphere seems to have started some 2000 million years ago. And even then we’re talking around 1% if we’re lucky. Initially the process was very slow, because the Sun’s ultra-violet rays broke it down into atomic oxygen (or something) and took it out of circulation. Only when enough had built up to create the ozone layer that protects us today, did the build-up of atmospheric oxygen begin to speed up.

It’s easy to forget that what actually releases oxygen into the atmosphere is not the amount that the cyanos produce. Any carbon lying around in an oxygenated environment will quickly get eaten by other simpler bacteria. And in doing so they will absorb the same amount of oxygen that the cyanos released when they extracted the carbon in the first place.

What ensures that there’s any left to oxygenate the wider environment is carbon burial. So in fact we owe our plenitude of oxygen to the huge amounts of carbon that lay buried in the ground – in particular the coal (more) and oil that have been laid down at various times. We would not be wise to dig up too much of it.

That’s about all we can say about the Precambrian climate. There’s virtually no information on how warm/wet/windy it might have been at various times. And little use that we could make of the information anyway.

Post-Cambrian times

Now we come to the Cambrian Period (545-505 My) immediately after the Great Cambrian Explosion, when large creatures appear on the scene. This is a trilobite (more).There is still very little information on what the climate was like. But ‘equable’ seems to sum up the little that is known.

And these conditions continued until about half way through the Ordovician (505-440 My). Then there was a ice age that lasted for the rest of the period. However the equatorial regions remained warm and sunny. The sea level was low (more), which will have left most of the coastal margins high and dry. However some must still have been flooded because this is the time when plants invaded the land.

By the time we get to the Silurian(440-410 My), conditions had warmed again – although there was till some ice near the South Pole. But there was an arid belt south of the Equator which provided clear and sunny skies, just the conditions to allow coral reefs to thrive. This is also the time when creepy-crawlies invaded the land (arthropods to be precise). The period bowed out with a short sharp cold snap, or even an ice age.

At the start of the Devonian (410-360 My), the planet was warming up again, but conditions tended to be dry. The sea level was rising, and conditions were wetter by the middle of the period. At this time, Arctic Canada was on the Equator. Trees appeared around the middle of the period. This picture is of one of the earliest. The great coal forests seem to have started to appear at around this time. It must also have been a period when the amount of oxygen in the atmosphere started its dramatic increase, though I’ve seen nothing written about this. By this time the sea level must have been really high, because Scotese says that warm shallow seas “covered” much of North America, Siberia and Australia.

By the end of the Devonian, Pangaea was beginning to assemble (more), and sure enough, glaciers appeared around the South Pole. However nearer the Equator, conditions were warm and moist. And this when the coal forests started to get serious. The oxygen level rose dramatically. It’s probably no co-incidence that, shortly after this, insects discovered flight (more).

But the real heyday of the coal forests was the Carboniferous (360-290 My) (more). As Pangaea moved northwards, different areas acquired an ideal climate for them, viz, warm and wet. There was plenty of swamp for the dead trees to be buried in, well away from that dreadful oxygen. As the link shows, the trees of the time were quite different from modern trees. However by the end of the period the first conifers appeared.

Oxygen levels reached crazy heights (more) during the Carboniferous. As the link shows, this allowed insects to grow huge. Dragonflies, for example, grew to wingspans of around 2 feet.

By the end of the Carboniferous, Pangaea was building and a new ice age was on the way.

By the start of the Permian (290-245 My), the ice age was gripping the southern hemisphere. Central Pangaea dried out. The coal forests disappeared, to be replaced by deserts. However there were still rainforests in China, because it was passing through the Equator at the time.

The flying reptiles (more) appear in the fossil record at around this time. But they appear suddenly and fully developed. So they must have evolved some time before, presumably during the Carboniferous, while the oxygen level was still high.

At the end of the Permian came the greatest mass extinction of all time, the P-T event, when 99% of all life perished. Impact aficionados are looking for impact signs, but as far as I know nothing convincing has yet been found. But there were huge flood-basalt flows at the time, the Siberian Traps (more). These caused massive global warming as inconceivable amounts of carbon dioxide and other greenhouse gases poured out from the Earth’s interior. Many scientists regard this as more than enough explanation for the extinction.

Conditions were still very hot during the early Triassic (245-200 My). Indeed it may have been one of the hottest periods in Earth history (or possibly not quite, see below). The interior of Pangaea was still dry, and the polar regions were warm even in the winter. However later in the period, things cooled to something more comfortable. Conditions were warm even at the poles. This is the time when dinosaurs and primitive mammals first appeared.

The Jurassic (200-145 My) was of course the heyday of the dinosaurs. The interior of Pangaea was very dry and hot. There were widespread deserts. However China was surrounded by moisture-bearing winds and was lush and verdant. It’s probably no co-incidence that China is proving a veritable cornucopia of exciting fossil finds. Towards the end of the period the conditions changed, as Pangaea began to break up. It became less dry, and the polar regions suffered winter snows.

Also towards the end of the period, oxygen levels rose again. Dinosaurs, flying reptiles and insects all got huge. Flying dinosaurs, or birds, appeared (Archaeopteryx). The denser atmosphere must have helped these pioneering flyers no end.

The early Cretaceous (145-65 My) produced a mild ice age. There was snow and ice during the winter. However the weather soon warmed up, and became warmer than today. The equatorial regions must have been a little hot for the dinosaurs because they “migrated between the warm temperate and the cool temperate zones as the seasons changed”.

The Cretaceous bowed out with the famous impact, plus of course the devastating Deccan Traps (more). Between them they killed off the dinosaurs and the flying reptiles in the K-T extinction.

The Extinction left the field clear for the mammals and the birds, but they weren’t without their problems. The Paleocene (65-56 My) started out with possibly the greatest surge of global warming so far, leading to the hottest conditions of the post-Cambrian era. Palm trees grew in Greenland, which by that time was pretty well where it is today. There were mangrove swamps in southern Australia, which at the time was around 65° south. Today the area is around 30°, and presumably far too cool for mangrove swamps.

The mammals that came through the extinction were about the size of rats (more). This makes perfect sense. Small creatures cope with heat much better than large ones.

Towards the end of the period there was a short very hot period, caused by ‘methane outpourings’. Around this time, serious primates appeared.

It was still hot during the early Eocene (56-35 My). Alligators swam in swamps near the North Pole, and palm trees grew in southern Alaska. Much of ‘Eurasia’ was warm and humid. Later in the period India was covered by tropical rainforest and warm temperate forests covered much of Australia.

However the writing was on the wall, because by the end of the period the oxygen level was falling, and ice was beginning to form at the South Pole. The descent into the current ice age had begun.

During the Oligocene (35-23 My) the Antarctic ice sheet began to grow, although there was still no ice in the North. Warm temperate forests covered much of the northern hemisphere. Genetic evidence suggests that the first apes appeared around this time. However towards the end of the period, it warmed up again, and the Antarctic ice melted.

During the early Miocene (23-5 My), the climate was similar to today’s only warmer. There were palm trees in England and northern Europe. We had the same climatic bands as today – cold at the poles and hot at the Equator.

However, about half way through the period, around 15 My, the descent into the ice age got serious. The temperature began to drop sharply and the Antarctic refroze.

Scotese runs out at this point, and I have yet to find a good source for carrying the climate story on. However Wikipedia carries an article on the current ice age, which shows every sign of good scholarship. So we will have to content ourselves with that.

This diagram comes from the Wikipedia article. It shows how the temperature has changed since the end of the dinosaur era. The actual temperature of past ages is difficult to arrive at, and it involves assumptions that may or may not be valid. So scientists don’t normally attempt it. Instead they rely on ‘proxies’ such as ‘Benthic δ¹⁸O (per mil)’.

(Feel free to skip this paragraph unless you want to know a bit more.) This particular proxy is actually a measure of the amount of water that is locked up in ice. Water molecules that are made with the slightly heavier oxygen isotope oxygen-18 don’t freeze quite so readily as molecules made with the normal oxygen-16. So during an ice age, the ocean waters are ‘enriched’ with oxygen-18 water. The fossilised shells of sea creatures living at the time are also enriched with oxygen-18. So scientists can analyse shells, taken from sea-floor deposits of different ages, and provide a chart of how the amount of ice worldwide varied over time. For consistency, they rely on a single creature that is found more or less everywhere, viz. foraminifera (forams for short). A single mud core can give a splendid record in its own right. But the scientists have taken many, at many different sites. And in general they all agree remarkably well. How closely this relates to actual temperature is something of a problem. But it’s the best we can do.

The Ice age

Exactly when the recent ice age should be deemed to have started depends on who you consult. The Pliocene (5-1.6 My) sees a continuing gradual descent into it. However the fluctuations between warm and cold are beginning to get serious.

In the early Pleistocene (1.6 My-10 ky), some sources report a clear ‘interglacial’ from 1.5 to 1.3 My, followed by a full glacial period. another interglacial between 800 and 600 ky. But Wikipedia charts a slow and steady cooling, starting over 3 million years ago. The cooling is accompanied by increasingly regular and vicious fluctuations between much ice and less ice.

For the last 450 thousand years, Wikipedia also provides this chart. It’s a bit messy, but the overall message is clear. The peaks represent the short warm periods, and the troughs represent the much longer glacials. However, as we keep saying, don’t expect things to be simple because they are not.

This paragraph gives a bit more detail. Feel free to skip it. The top two plots show how the actual estimated temperature varied at two places in Antarctica. ‘Zero’ is clearly today’s temperature. Scientists are happy that the temperatures near the poles reflect those worldwide, except that the polar fluctuations are much larger. So look at the trends, but don’t take too much notice of the actual figures. The bottom plot gives the amount of ice around at the time, calculated using the foraminifera mentioned earlier.

We can see that the jumps between warm and cold are pretty sudden, being complete within ten thousand years or so. Then the ice gradually increases its grip over the next 80 or 90 thousand years, reaching its glacial maximum just before the end. However there are also smaller fluctuations that are much more rapid still. And look at the spike that occurred 250 thousand years ago! I’ve also read that some of the minor fluctuations can take the planet from quite good conditions to full-on ice, or out again, in less than 100 years.

A word of warning, concerning the timeline charts. Different sources have different ideas about what constitutes an interglacial. I’ve done my best, but you may well find a source that disagrees with what I’ve put.

What causes ice ages?

If you ask most experts what causes ice ages, they will bang on about ‘Milankovitch cycles’ and other minor effects. We won’t go into them because there are plenty of web sites that explain them far better than I can. In any case, none of these can be the underlying cause of ice ages (a) because their effects are too small and (b) because they operate on far too short a time scale – a hundred thousand years or so at the outside.

Our timeline shows clearly that ice ages actually come every hundred million years or so. In between, there are long periods when there is no vestige of serious ice anywhere on the planet. As we’ve mentioned before, a hundred million years ago there were dinosaurs and tropical plants thriving at both poles. They must have been there for a long time, tens of millions of years perhaps. To be sure they had to adapt to 6 months of darkness, which can’t have been much fun. But they did not have to adapt to cold.

There is only one thing that operates on a long timescale like this, and that is continental drift. So the fundamental cause of ice ages must have to do with the disposition of the land masses. I’m not sure that even the scientists understand exactly how the land masses work the trick. But it must involve the amount of heat that the ocean currents are able to transport polewards. Today the Gulf Stream carries huge amounts of heat up the coast of northern Europe, for which those of us who live there are eternally grateful. But it is as nothing compared with what would be needed to warm the arctic again.

The present ice age started some 15-20 million years ago, depending on what criterion you adopt. It was certainly at full strength by 4 million years ago. And we are still in its grip today. We may think that we are out of the ice age now, because there is no ice where most of us live. But there are still huge amounts of ice around, therefore we are still in it.

Once the major forces have driven the planet into ice age conditions, then the Milankovitch cycles and other effects come into play. They may not be able to generate an ice age, but they can trigger great changes in its severity. Scientists probably don’t have much detail on earlier ice ages, but this one is characterised by rapid fluctuations. At one moment (in geological time), much of Europe and North America is balmy. Hippos and other exotic animals bathe in the Thames. The next moment the ice has returned and the same area is tundra or even ice. We may still be in one long ice age, but the difference between the ‘glacials’ and the ‘interglacials’ is huge.

The mud cores that we mentioned earlier may not be able to tell us what the temperature was at any particular time. But they can separate the glacials from the interglacials with great precision. The first chart (above) shows the mud-core record over the past 5 million years. Up until about a million years ago, it indicates a 41 thousand year cycle, which matches one of Milankovitch’s cycles (the ‘tilt’ cycle) perfectly. But then the real-life cycles switch abruptly to 100 thousand years. This more or less matches another of Milankovitch’s cycles (eccentricity). However I’ve read that there are problems, because Milankovitch’s 100 ky cycle doesn’t match the ice-age cycle accurately enough to carry conviction.

So on all aspects of the caus(es) of ice ages, we really have to say that the jury is still out.