History of the British Climate Part I

Yesterday I covered the last 400 000 years of British climatological history. Today I’m going to do something like the previous æon, and possibly all the way back to the beginning of the world. In fact, yeah I’ll do that.

4 543 million years ago, the future Solar System was a swirling disc of dust and gas orbiting a newborn Sun. Jupiter had already formed and was gradually pulling the particles whose times to orbit were in harmony with its own slightly towards itself, leading to them drifting slightly out of phase with it and clumping into fairly insubstantial rings of matter. I’m not sure how warm the belt which would become us was at the time, but it was probably well below freezing point, because if it hadn’t been, there would have been no grains of water ice. On the other hand, there were also comets, so maybe not, but the fact remains that the Sun was dimmer and weaker back then and there were no greenhouse gases in a position to warm the dust and gas which would become Earth. It took seventy to a hundred million years for it to form, and at the beginning it would’ve been slightly more massive, have no permanent moon and the atmosphere would have been briefly high in hydrogen and helium. Within ten million years of its formation, a Mars-sized body which has been christened Theia hit us and shattered the outside layers of the planet, causing them to go into orbit around us and fall together into the body I call Cynthia and most other English speakers call “the Moon”. Clearly there was no such place as Britain at this point and the entire surface of the planet was molten rock heated by the mechanical energy of compression and collision along with radioactivity. The atmosphere would have been substantially superheated steam. Shortly after being hit by a planet-sized body, the atmosphere would in fact have been vaporised rock. It’s possible to determine the climate of the entire planet at this point, as it was quite uniform, meaning that although it makes no sense to talk of Britain, it does make sense to describe how conditions were everywhere. This eon lasted about 500 million years, and during this period the vaporised rock atmosphere would have condensed and fallen onto the surface as drops of lava. Towards the end of the Hadean, life was present, which seems to imply that there was liquid water in at least some places.

The next period is referred to as the Eoarchean, when the pressure was probably dozens of times higher than it is today, more like the solid surface of Venus than today’s Earth. Temperatures were between 0 and 40°C and there may have been ice ages. To quote ELO, “the weather’s fine but there may be a meteor shower”, because this was the time of the Late Heavy Bombardment, when for 300 million years asteroid collisions and other large meteors would have rained very often from the sky, although this has recently been questioned. The atmosphere was high in methane and carbon dioxide, which being greenhouse gases may have ensured that this planet was warm enough for life to survive on it given that the sun was 30% weaker than it is now.

All of this is rather vague and applies to the whole world. The earliest known British rocks are found in Na h-Eileanan Siar, also known as the Western Isles, and have been dated at 3 000 million years old. It isn’t clear that anywhere can be meaningfully called Britain before that date, and there’s no trace of anything else. It was likely to have been a small piece of the surface of the planet with unclear neighbours. The rock concerned is gneiss, which is a common component of continental shields, which are bits of Earth’s surface that haven’t been affected much by continental drift, such as mountain formation or rifting. It would be a bit excessive to call the rocks in Na h-Eileanan “continental shield” because they’re quite small, the nearest substantial example of one being most of Finland and Sweden, but they are the original and only rocks in that small area of these isles.

Even long after this, the island of Great Britain would have been in several parts, making it difficult to describe the nature of its climate. It means imposing the current situation on the past when it’s actually quite transient on a geological time scale. Also, in some areas, including this one, Charnwood, sedimentary rocks were laid down at the bottom of the sea or ocean and the idea of this being Britain is almost meaningless. It also changes the significance of climate, and as far as being at the bottom of a really deep ocean is concerned, almost irrelevant.

In the Archean, which lasted fifteen hundred million years, the planet was shrouded in methane clouds and there was practically no free oxygen in the atmosphere. The sedimentary rocks surviving which had been exposed to the atmosphere show no glacial erosion, but they do show evidence of rivers and rain. Therefore it did rain. In fact, presumably there was an enormous rainstorm lasting thousands of years at some point in the late Hadean when the oceans were formed due to the atmosphere and surface getting cool enough for the steam to condense out and persist on the surface, but because the pressure was much higher this would have happened long before the surface temperature dropped below 100°C. It is actually possible to measure the surface temperature by looking at the proportion of oxygen-18 in the rocks. There are two stable isotopes of oxygen: 16 and 18. Because oxygen-18 is heavier, molecules containing it vaporise at a slightly higher temperature. Chert, which is a sedimentary flint-like rock, is silica, i.e. silicon dioxide, containing oxygen, and is present in some Archean deposits, making it possible to measure the temperature where it was laid down. This puts the ocean temperature at 70°C, but this is probably wrong because weathering once it was exposed to the atmosphere would influence this. The degree of weathering which occurred was unaffected by land plants, since there weren’t any – there weren’t any plants in fact – and suggests a surface temperature between 18 and 24°C, so semitropical. The fact that there was neither excessive heat nor excessive cold suggests various things about the planet such as the ratio of methane and carbon dioxide, a relatively transparent atmosphere and only limited land surface, so it seems that not only do we only have bits of Na h-Eileanan available but that may have been partly because there just wasn’t that much land.

The Archean was followed by the Proterozoic, which began around 2 500 million years ago. This was characterised by the evolution of blue-green algæ, which proceeded to release oxygen into the atmosphere and removed carbon dioxide. This may also have reduced the activity of methane-producing organisms, another greenhouse gas, and also oxidised the methane. Incidentally, this hedging language I’m using here is down to my ignorance more than scientists’. Anyway, the consequences of this were that iron began to rust in the ocean, depositing itself in bands of rust on the sea bed, and the temperature of the planet fell, triggering an ice age. It’s theorised that this planet has two relatively stable states climatically, which it switches between: icehouse and hothouse. Icehouse has generally not dominated but can do at certain times and in fact it is at the moment, anthropogenic climate change notwithstanding. The dominant state is hothouse, which is generally warmer than today for millions of years at a stretch. Even so, there does seem to have been an ice age in the early Proterozoic, and at the end of the Proterozoic there was another much more severe one. In between those times the world-wide climate would’ve been warmer than today.

The Cryogenian Period was a crucial time in our planet’s history. It appears that the land was mainly equatorial at the start of this period, which would probably have included the bits of land which were to become these isles. We were situated just south of the Equator, in Laurentia and Baltica, as part of the supercontinent Rodinia, meaning a hot, wet climate, except that we were below sea level, so a very wet climate! The oddity about this time is that glaciers are found at the Equator, i.e. the parts of the supercontinent which were equatorial at the time, and it’s thought that this means that most or all of the planet was covered in ice and as cold as Antarctica. My comment about tropical conditions applies to how things were before this arose. There are a couple of hypotheses about how this happened. One is that Earth may have had an axial tilt as high as 60°, meaning that constant night in the winter and the midnight Sun in the summer would’ve applied to everywhere further from the Equator than today’s Brazil or Israel. Very surprisingly, a snowball Earth can only happen if there’s a lot of equatorial land. Most of the Sun’s heat is absorbed near the Equator, meaning that if there’s a lot of land there the heat would not be absorbed as much, and this would cool down the whole planet.

By Ryan Somma – Life in the Ediacaran SeaUploaded by FunkMonk, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=24277381

The Ediacaran follows the Cryogenian and is for this part of Britain very significant, because it’s from this time, lasting 94 million years from 635 to 541 million years ago, that some of the most famous fossils found in this area date. These can be seen in a local museum and include the feather-like Charnia as seen above, and Bradgatia linfordensis, a lettuce-like organism obviously (to locals) named after Bradgate Park and Newtown Linford, both in Charnwood. Charnodiscus concentricus is another. These are all thought to be “quilted” animals who left no descendants, although some people class them in their own kingdom because they’re quite unlike any animals or plants we’re familiar with. They appeared 600 million years ago and all died out before the Cambrian. They may have had symbiotic algæ in their compartments, meaning that since many of them were also attached to the sea bed, the water must have been sufficiently shallow to allow light to penetrate. Hence Charnwood was still underwater, but the ice must’ve been gone and the water wasn’t particularly deep.

Rodinia was breaking up at this time, so there would’ve been a network of shallow seas, which sounds like the situation as it was here. Rodinia was an unusual supercontinent because it seems to have formed by the landmasses moving all the way round the world and colliding with each other on the opposite side to where they originated, which meant they had a long time to erode and the land surface was quite flat. The network of seas would have increased rainfall on the land, since much more of it would’ve been closer to the sea. This may in fact have been part of what triggered the earlier ice age. The temperature of the Ediacaran was still around 2°C cooler than the average for the Holocene, so it looks like the weather here would’ve been cold, wet and rainy. Plus ça change!

The Cambrian was warmer, around 8°C warmer than the Holocene average, and in fact this set a precedent for the generally warmer temperatures of the Phanerozoic, our current eon. During the next period, the Ordivician, sea levels rose by a hardly believable six hundred metres. This ended as a new supercontinent, Gondwana, reached the South Pole and a new ice age started, lasting twenty million years. A gamma ray burst may then have cause the mass extinction at the end of the period, meaning that it may have rained concentrated nitric acid.

Around 400 million years ago, three mini-continents collided to form the British Isles as we know them today, and it begins to become more meaningful to talk about British climate. These were Laurentia, which is effectively all of Scotland, Avalonia, which is England and Wales, and Armorica, which is Brittany, Devon and Cornwall plus a lot of other land such as Iberia. Glen Mòr, the fault along which Loch Ness is situated, continues into Ireland and therefore I imagine Ireland was also in two halves before this. Avalonia began as a volcanic island chain north of Gondwana. Britain was about 30° south of the Equator then. It drifted gradually north, crossing the Equator about 300 million years ago, and over this time other land collided with the forming Pangæa, meaning that it was increasingly far from the sea. This is about the time the Carboniferous started and the future Britain became covered in the rainforests which would become the coal measures, so Britain was hot and swampy, and the oxygen content of the air was so high that lightning strikes would have ignited wet vegetation, so there would be many forest fires even though conditions were damp. Around 305 million years ago, climate got cooler and drier and sea level fell, leading to retreat of coal forests from higher ground and the emergence of fragmented rain forests, which were no longer able to maintain their genetic diversity and there was a lot of inbreeding, shrinking of the size of, for example, horsetails, to cope with the conditions and a new ice age started in the Southern Hemisphere, although not severe enough to make Britain cold.

By this time, Pangæa was forming, as were the Pennines. Hot dry desert conditions took over from rainforest, with presumably an intermediate phase which today would be like the Serengeti, although with very different flora and fauna the details are not obvious. The late Permian was a peculiar time climatically, as the interior of Pangæa seemed to have extreme temperature variations so that it was both very hot and very cold at different times of year, and it’s been suggested that this was a cause of the Great Dying, where almost all life on Earth became extinct. Britain was now in the northern tropics, and as such was in the same zone as the Sahara is now. The Scottish Highlands at the time would’ve been as high as the Himalayas and formed part of a range which extended southwest into the Little Atlas and Appalachians. There might also have been a rain shadow desert to the east, making it even drier than it would’ve been without them, but the monsoon conditions which prevailed to the southeast might make it heavily forested.

In the Triassic there were salt flats in Cheshire, hence the salt mines which existed there in historical times, and red sandstone forming in what is now the Southwest, hence the very red soils in that area. Towards the end of the Triassic, the sea level began to rise again, converting much of the isles into a subtropical shallow sea and many of the hills and mountains as they existed then into islands, such as the Mendips.

The following photo is taken from this website and will be removed on request:

This is the “Barrow Kipper”, or rather a monument to where it was found in 1851. Barrow-upon-Soar is about an hour’s walk from where I’m sitting and between 200 and 150 million years ago was underwater, over the entire Jurassic Period. This particular plesiosaur was formerly classed as a Rhomaleousaurus but now as an Atychodracon, from the Early Jurassic, looking something like this but with a bigger head:

It used to be thought that plesiosaurs had to climb ashore to lay their eggs, so this suggests that there was land nearby, but fossils have since been found of pregnant ones, and their limbs were arranged in such a way that they would’ve had to have dragged themselves along the shore quite roughly. However, although it isn’t from precisely the same time, a few miles away in Rutland, the largest and most complete dinosaur fossil ever found in Britain was unearthed, a Cetiosaurus, like a mini-“Brontosaurus”, suggesting that this area was an archipelago of smaller islands or just near a beach. There is a famous traditional song called ‘Ashby De La Zouch By The Sea’, which has often made me wonder whether that particular nearby Leicestershire village ever was.

I am of course a Southerner, and as such Leicestershire will always be slightly foreign to me. My mother is from Maidstone, a place sufficiently famous for its Iguanodon finding that the animal is actually on their coat of arms:

These dinosaurs, dating from 157 million years ago, are also found, along with very many others, on the Isle Of Wight. It’s tempting to telescope all these findings into an imaginary scenario where they’re all simultaneous just because they’re all Jurassic, but in reality the Jurassic Period lasted fifty-six million years, almost as long as the time since the non-avian dinosaurs became extinct, and the Isle Of Wight dinosaurs are mainly early Cretaceous. There were, however, coral reefs in Yorkshire. In the Cretaceous, the situation was once again one of rising sea level with lagoons and streams. To the extent that these isles existed at that point, they were substantially united. That is, Ireland and Great Britain formed a single island, which was intermittently joined to the mainland and still steadily drifting north.

The Late Cretaceous climate was warmer than today’s at the same latitude, which was about the same as Madrid and Rome, although it had been cooling for millions of years. When the Chicxulub Impactor hit, the widespread fires would have raised carbon dioxide levels tenfold and caused a greenhouse effect heating the planet by 7.5°C. In the Palæocene the climate was slightly cooler and drier due to dust in the atmosphere reflecting heat into space, but tropical forests then developed all over the world, even in the Arctic, where the water was lukewarm. The Eocene would’ve involved warm swamps in many parts of Britain.

At this point I’ll repeat something I said a few days ago about Europe. Europe over the Cenozoic, that is, since the extinction of the non-avian dinosaurs, has been gradually transitioning from an archipelago to a large peninsula, and the scattered islands of the region have shown a trend of joining together to build a subcontinent, for want of a better word. Looking at Great Britain and Ireland in this way, they are late developers, or outliers which show how the rest of the region used to be. There’s a common, and correct, idea that before the end of the last Ice Age and for several thousand years after that, Ireland and Great Britain formed a peninsula, and this is true, but there has been a kind of seesawing appearance and disappearance of sea around us and the level of the land at the moment has been pushed down by the recent weight of ice and is gradually springing back up. Hence it does make sense to speak of the British Isles, or perhaps an island comprising Ireland and Great Britain plus low-lying land in between, in the earlier Cenozoic, and moreover to see them as the westernmost members of a collection of islands a bit like the Caribbean or Indonesia in arrangement, although that may be a bit of an exaggeration. The North European Plain, though, was underwater for quite some time, Iberia ceased to be an island around the start of the Cenozoic and the Italian-Illyrian region was also separate for a long interval.

In the Neogene, Britain arrived in its present position and is no longer drifting north. Hence the climate began to approach how it is today although it would’ve been somewhat warmer still. Finally, the Pliocene saw a general drying out and the Pleistocene brings me to the start of yesterday’s post.

I can’t completely guarantee that all of this is accurate as I know a little, but some of it is disputed and I’m probably in the Dunning-Kruger trough at this point where I haven’t reached the point of realising how little I really know and how wrong I’m actually being. Nonetheless, it’s nice to imagine how our climate could’ve been more Mediterranean or Caribbean in particular in the geological past, and also, wouldn’t it be nice to holiday at home but do it using a time machine so we could get to the really sunny and warm climates which this part of the world, so to speak, used to experience?