Plus ça change

Hasn’t Earth’s climate always changed? (11 minute read)

Off the coast of Cairns, in Queensland, Australia, lies an island known today as Fitzroy Island. It’s just under three kilometres from the nearest coastline, and 97% of the island is a national park. I’d like to say that I’ve been there and can describe it, but I haven’t. Still, when I look at the pictures, it seems familiar. I’ve visited a fair few tropical islands, and this one looks very much like them. There’s turquoise water lapping at white sand beaches, rocky outcrops, palm trees and forest-clad slopes. You could see such things on the mainland of Australia too, but the fact that it’s an island makes it all the more magical.

It's hard for most of us to imagine that Fitzroy Island wasn’t always an island. But that’s hardly surprising. If you check out the ‘culture and history’ page for Fitzroy Island from the Australian National Parks Service, history starts at 1770 when the island was named by Captain Cook. The history most of us learn about Australia goes back 250 years, and that’s it.

But some Australians learn a different story. The Gunggandji people tell stories about an area of coastal grassland which was flooded by the sea and the formation of an island they call Gulnyjarubay, meaning ‘the fist’. This fist was once connected to a wrist and arm, but they were submerged by the rising sea.

Geologists confirm that this story, passed down orally through hundreds of generations, is accurate. Fitzroy Island was indeed once connected to the mainland – around 12,000 years ago. Then, the sea level rose, and it became an island.

If you have chills running down the back of your spine right now, you aren’t alone. The first time I heard that indigenous Australians told stories about sea level rise more than 10,000 years ago, I found it almost incomprehensible. Stonehenge was built around 5000 years ago, but we aren’t entirely sure why because the knowledge has been lost. The knowledge about Fitzroy Island is millennia older than that and is passed on accurately enough for geologists to confirm it.

Image credit: Melanie Newfield

Fitzroy Island is not the only example of indigenous Australian stories recording the end of the last ice age. There are islands all around the Australian coast where similar stories were told. Australia is surrounded by islands which were once part of the mainland – but why?

The simple answer to the question is this: 10,000 years ago the sea level was lower mainly, but not entirely, because there was less water in the ocean. Where was the missing water? It was in massive ice sheets which covered large areas of North America, Europe, Asia and South America, as well as in expanded Greenland and Antarctic ice sheets. At that time, the Earth was at the end of the most recent ice age.

Ice ages are important in discussions about human-caused climate change. The fact that the climate has changed in the past, without any contribution from humans, is taken by some people to mean that the current climate change is not caused by humans either. So it is useful to understand what we know about past climate fluctuations and what is different about the climate change happening now.

The climate of the Earth has changed many times in its 4.54 billion year history. When the Earth was first formed, it was at its hottest – so hot that the surface of the Earth was molten. The high temperatures resulted from frequent explosions as the Earth collided with rocky debris littering the solar system. It probably took around 20 million years to cool enough for a solid crust to form on the surface of the solid rock – beneath this crust, the rock is still molten today.

Exactly when Earth’s atmosphere and ocean formed is the subject of scientific debate. In fact, the details of what Earth was like over the first half-billion years or so are still uncertain, and since they are unimportant to the current climate change discussion, I won’t go into further detail right now. However, at some stage during the first few million years there was an atmosphere which included water vapour and carbon dioxide, perhaps with ammonia and methane, but little to no oxygen. In fact, there would be little to no oxygen for another two billion years.

The first definite evidence we have for life on Earth dates back 3.5 billion years, although there are indications that life may have been present earlier than this. There is limited information on the temperature of the Earth over the next billion years, but it’s thought to have had an average temperature between 0^oC and 40^oC, with considerable fluctuations. This sounds like a large variation, and it is, but it was stable enough for life to flourish.

During this time, there were microbes producing food from sunlight, a process known as photosynthesis. However, around 2.5 billion years ago certain bacteria evolved a new type of photosynthesis which produced oxygen as a waste product. Oxygen began to build up in the atmosphere. Around this time, there was a period when Earth was very cold. Although it’s difficult to prove cause and effect for events which happened 2.5 billion years ago, it’s now thought that the increase in oxygen was the cause of the low temperatures. There are two ways this might have occurred. The first is that the oxygen produced during photosynthesis comes from carbon dioxide in the atmosphere, so presumably there was a drop in carbon dioxide. The second is that the increase in oxygen may have led to a drop in methane levels in the atmosphere (since oxygen and methane react together). Carbon dioxide and methane are both important greenhouse gases – so less of them means cooler temperatures.

After a couple of hundred million years of ice age, the Earth warmed, and there was more than a billion years of warmer conditions. It’s thought that these warmer conditions were also related to methane, which increased due to greater emissions from the ocean – more specifically, methane was being produced by microbes in the ocean and the chemical composition of seawater at the time meant that most of it was released into the atmosphere. There is also evidence that oxygen levels were lower during this period.

Up to this point, all living things were still only made up of single cells. However, during this warm period multi-celled organisms appeared. So far, we’ve narrowed the time period to somewhere between 2.1 billion and 600 million years ago. These weren’t the complex animals and plants familiar to us, but perhaps were like simple seaweeds.

The billion-year warm period came to an end sometime around 700 million years ago with another period of increased oxygen levels and extreme cold. Again, it’s thought that the increased oxygen levels led to a drop in carbon dioxide and methane, which caused the cooling. Exactly why oxygen levels increased is not entirely clear, but it’s linked to the activity of living things, not chemical processes.

Our knowledge of precisely what went on for the first four billion years of Earth’s history is fairly imprecise. But for the last 500 million years or so, we have better information. Within the last few years, scientists at the Smithsonian Institution have drawn together diverse sources of information to construct a better picture. What they show is a number of fluctuations, with temperatures up to 4^oC lower than today rising to more than 15^oC higher. When the first land plants developed, around 450 million years ago, they triggered a period of rapid cooling, at least by biological standards. Scientists believe that this is because they changed the way rocks weathered, triggering chemical reactions which absorbed carbon dioxide from the atmosphere. The Earth soon warmed again, and when dinosaurs walked the Earth, it was much warmer than today.

The more recent the time period, the more we know about the temperature fluctuations. If we look at the last 500 million years, we see a few ups and downs, with the last 50 million years generally getting cooler. However, if we look closer, we see that the temperature isn’t going down steadily, but fluctuating almost constantly. The closer we look, the more fluctuations we see, but the size of each fluctuation is less. The last million years, for example, show a cycle of the temperature rising and falling around 5^oC, repeating every 100,000 years or so.

Most of the big fluctuations I’ve written about are thought to have a strong connection to the actions of living things. However, there are other forces involved, such as the action of volcanoes (emitting more or less carbon dioxide) and the movement of continents (the different thermal properties of land and water mean that if the land is in different places it can affect Earth’s temperature). The 100,000 year cycle is different. It relates to subtle changes in the Earth’s orbit, known as Milankovitch cycles after the Serbian physicist who first recognised them.

We usually think of the Earth’s movement as being consistent, but it actually changes in a number of ways. One variation is the shape of the Earth’s orbit around the sun, which is almost, but not quite, circular. The degree to which it’s not circular varies over a period of around 100,000 years. Another is the tilt of the Earth. The axis on which the Earth rotates is tilted in relation to its orbit, which is what causes the seasons (I’ve linked to a video explaining this here). The tilt varies slightly with a cycle of around 41,000 years. Yet another variation is a wobble in the rotation, which means that seasons can be more extreme in one hemisphere than the other (the video explaining this made me feel seasick, but if you’ve ever seen a spinning top wobble, you’ve got the picture). The wobble varies over 25,000 years. These aren’t the only variations, but they give you a picture of the various subtle changes which influence our climate over periods of tens to hundreds of thousands of years.

The sea level rise observed by the Gunggandji people in Australia, when Fitzroy Island was formed, was connected to these Milankovitch cycles. But how do we know that the current warming we are seeing isn’t also a part of that picture?

First, there’s the time scale. Milankovitch cycles operate over tens of thousands of years. They don’t warm the climate by more than a degree over 150 years, which is what we have experienced since we started burning fossil fuels. Secondly, Milankovitch cycles actually predict that the Earth should now be cooling slightly. Then there’s carbon dioxide, which is far higher than at any time in the past 800,000 years, as a result of fossil fuel emissions.

When we look at the last million or so years and the Milankovitch cycles, it’s hard to imagine humans influencing the climate. On this time scale, climate change has nothing to do with living things. However, the more we learn about the past climates of the Earth, the clearer it becomes that living things have long played a role in climate change. Ever since the development of photosynthesis, life been changing Earth’s climate. The most remarkable thing is that the climate has never changed enough to wipe out life completely.

The reason that the temperature of the Earth has stayed within a certain range is not, as many assume, our proximity to the sun. The surface of the Moon can range from unimaginably cold at minus 173^oC to well above boiling at 127^oC – and that is just within the space of a lunar day. Theoretically, the Earth could also fluctuate to unsurvivable temperatures. But it doesn’t.

The reason for the Earth’s relative temperature stability over billions of years relates to a process involving carbon dioxide. I’ve written before about the carbon cycle – where carbon dioxide from the atmosphere is taken up by plants and incorporated into their tissues. When the plant dies and decays this carbon is released back into the atmosphere – or the plant is eaten and the carbon gets incorporated into whatever ate the plants. But there is another carbon cycle, one which occurs on geological time scales. When volcanoes erupt, they release carbon dioxide into the atmosphere. Carbon dioxide dissolves in rainwater to form carbonic acid, which can dissolve certain types of rock. The resulting chemicals, which contain the carbon which was once in the atmosphere, are washed into the ocean and sink to the bottom, where they form new rocks. These rocks lock up the carbon until it is released again by volcanoes.

The chemical reactions in this slow carbon cycle are temperature-sensitive. As the temperature increases, the rate of the reaction increases, removing more carbon dioxide from the atmosphere. Removing carbon dioxide reduces the temperature, which slows the rate of carbon dioxide removal, causing temperatures to increase.

This carbon cycle effectively acts as a thermostat for the Earth, keeping temperatures within a certain range. It’s not a particularly precise thermostat, over geological time. Various factors related to water, erosion and the movement of continents, as well as the actions of living things, mean that temperatures do still fluctuate, and by far more than we are used to.

So, yes, over billions of years, the Earth’s climate has changed. It has changed many times and it has changed a lot. But for most of that time, life on Earth was mostly microbes. Our species is a newcomer, only around 300,000 years old. We’ve survived through a few ice ages. But since we developed agriculture, around 15,000-10,000 years ago, we’ve had a reasonably stable climate. We’ve never faced climate change of the scale and speed that we are facing today.