Climate change is a confusing topic. The environmental movement with Greta Jeanne D’Arc Thunberg in front can give the impression that climate has been nice and stable since times long gone. Then, we humans started to burn fossil fuels and now we are headed for the ditch.

Climate sceptics argue that climate always has changed. Through most of the Earth’s history, there was more CO₂ in the atmosphere than now, and no ice on the poles. During the ice ages temperatures rose and fell with CO₂ following shortly afterwards.

The problem is: They are both right. Kind of. In this blog post we will explore the Earth’s climate through time, on a scale of millions to thousands of years, through the big picture geologists are used to. We will look at the mechanisms behind natural climate change, slow and fast – and what it means for the debate on man-made global warming.

A graph, compiled by Wikipedia, which shows how CO₂ followed temperature during the variations between glacial and interglacial (the very cold and the temperate, like now) periods during the ice ages.

Through most of the Phanerozoic – the time of higher life – Earth has been warmer than today, and CO₂ in the atmosphere much higher. During the Jurassic and Cretaceous, the heyday of the dinosaurs, all continents were lush and green. Dinosaurs and forests thrived even in Antarctica.

Through most of the Earth’s time, the CO₂ content in the atmosphere has been higher than today. The last time CO₂ was almost as low as today was during the last great ice age 360 to 260 million years ago. The graph is compiled by Wikipedia; the individual sources are plotted individually and highlight the uncertainties ins the estimates; however, the overall trend is clear. Note that the time line goes from old at the left to recent at the right.

The Earth’s temperature through the Phanerozoic, compiled by Wikipedia. The compiled graph is not the final truth on historical temperature, and the longer back, the greater uncertainty, but it highlights the main trends: Earth has been warmer than today most of the time, but the ice age around 300 million years ago is clear, and there has been a steady temperature decline the last 50 million years.

Today’s cold climate, with ice caps on both poles, are an outlier, an aberration in a usually warmer world. But it is not the only ice age in history, so let’s use the ice ages as starting point to understand the climate through time.

Earth with North America and the familiar ice caps on Greenland and the Arctic Ocean – a special situation in our planet’s long history. (NASA image from the Apollo 16 mission, via Wikimedia Commons).

The last time Earth had large ice caps on the poles was from around 360 to 260 million years ago, during the late Carboniferous to mid Permian periods. Before that, there was an ice age 450-420 million years ago, straddling the border of the Ordovician-Silurian periods.

The southern hemisphere of the Earth, 300 million years ago. The continents are assembled in the supercontinent Pangaea: Antarctica sits on the bottom of the globe, then as now, with Africa and Australia attached. Image from Dinosaurpictures.org, based on maps from Christoper Scotese

Before that, there were some much more serious ice ages. Around 720 million years ago, the Earth froze over, from tip to toe. Twice. These Snowball Earths are known from fossil glacier moraine sediments, called tillites, from these times. The tillites are found widely distributed on all continents, also continents which were close to Equator back then.

Tillites from Moelv in south Norway. These tillites are younger than the Snowball Earths, around 620 million years old, but show well the hodgepodge of rocks of all sizes, typical of a moraine.

Reconstruction of the the Earth 750 million years ago, just before the last snowballs. Image from Dinosaurpictures.org, based on maps from Christoper Scotese

What causes an ice age? Why does the Earth cool? And, during the ice age we are in now, what makes the ice wax and wane, from glacials, with ice covering most of the high north of America and Eurasia, to warmer interglacials, like today, with glaciers only high in the mountains, and on the poles?

Cooling the Earth is pretty simple: Keep less of the energy that arrives from the sun.

One way to do that is to reflect more of the energy, a.k.a. light, back into space. As anyone who has walked on a black beach in summer knows, dark surfaces absorb heat, while white surfaces reflect the heat back. A dark Earth will absorb heat, and warm up, a light Earth will reflect the energy back and keep cool.

Glaciers are white and making big glaciers will thus cool the Earth. These glaciers should preferably be at low latitude, because light from the sun hits the Earth at high angle at the Equator, and therefore carries more energy than the light that hits at low angle in the polar regions. This is why the tropics are warm and Norway is cold.

How do you make a glacier near Equator? The same way you get the famous snow on Kilimanjaro: Build a tall mountain – or a very long chain of big mountains, which starts with the Himalayas in the east, and continues through the Zagros mountains in Iran, the Caucasus and the Alps in the west.

After breaking lose from Antarctica and drifting northwards, the India continental plate began its collision into Asia around 55 million years ago, with the main mountain building from around 45 million years. Today, the Himalayas is one of the biggest and tallest mountain chains to ever have crisscrossed the Earth.

The Earth 50 and 35 million years ago. 50 million years ago (upper image) India has started to close the ocean in front of Asia, and the incipient collision has started to pile up the emerging Himalayas on both sides. 35 million years ago, the Himalayas are well established and the Tibet Plateau is white. Images from Dinosaurpictures.org, based on maps from Christoper Scotese

But, glaciers in the Himalayas are far from enough to cool the Earth. Two even more important things happened on the big scene of plate tectonics:

Around 30 million years ago, Antarctica became isolated as a fully independent continent around the South Pole. Antarctica had been hanging around the bottom of the Earth for over 300 million years, back when the earth’s continents were assembled in the supercontinent Pangaea. When Pangaea split up, one continent after another broke loose and drifted northwards; India, Australia, Africa, and finally South America cut the chains around 30 million years ago and there became a deep ocean around Antarctica. The ocean currents could circle around Antarctica without being forced into warmer climate zones. Antarctica became isolated, circled by the cold current. The continent cooled and became covered with ice.

The Earth seen from below, 35 million years ago. Australia has broken loose from Antarctica, and the Drake Passage has split off South America, allowing the continent to be isolated by the cold ocean currents. Image from Dinosaurpictures.org, based on maps from Christoper Scotese

Later, the South American continent moved up towards North America and gradually closed the isthmus in Panama, finally shutting the Caribbean off from the Pacific three million years ago. With the seaway between the continents closed, the ocean currents changed, and in the Atlantic, the warm current went northwards from the Caribbean towards Northern Europe. We know it as the Gulf Stream, and without it, Norway would have been as icy as Greenland. This change in ocean current patterns also led to North America, North Europe and Siberia to become buried in ice caps, and the Arctic Ocean got covered by sea ice. Ironically, the warm water carried by the Gulf Stream played a big role, by increasing the precipitation – and thus the snow fall – here north.

A second way to make an ice age is to lift the insulating blanket on the Earth, a.k.a. to reduce the amount of greenhouse gases in the atmosphere. To understand this, we must do a detour into the mechanisms of the greenhouse effect, and how nature itself regulates it.

The Earth receives heat by infrared radiation from the sun, but because the Earth’s surface isn’t a perfect black, it radiates much of that energy back into space. CO₂ works by trapping some of that energy, keeping it as heat in the atmosphere. We call CO₂ a greenhouse gas, because it has the same effect as the glass in a greenhouse.

CO₂ is only around 410 ppm of the atmosphere – 410 parts per million, up from 280 ppm before the industrial revolution. 410 molecules of CO₂ per million gas molecules does not sound like much, but without this trace of CO₂, the average temperature on Earth would have been a shivering minus 18^o C, instead of the comfortable plus 15^o C we actually have. The effect of CO₂ is very strong from the start, and then the effect decreases rapidly, which is why the near 50% rise in CO₂since the Industrial Revolution would has not boiled the Earth.

CO₂ is not the only or even the strongest greenhouse gas, but it is the one, which can start to warm the Earth’s atmosphere from within, starting to roll the ball. The warming caused by CO₂ will in turn evaporate some water from the sea. Water vapor is a nearly twice a potent greenhouse gas as CO₂. Think of CO₂ as the volume switch, with water vapor as the amplifier.

How then, does the temperature not run away when CO₂ makes it start rising?

In the short term, the reason is that water molecules stay in the atmosphere for only nine days on average, while CO₂ stays much longer.

In the long term, the Earth has its own thermostat, which keeps the temperature fairly stable, and it is driven by plate tectonics. The thermostat works by coupling several chemical reactions, “feedbacks” in science-ish. I apologize in advance for exposing you to some chemical equations:

Volcanoes spew CO₂ out in the atmosphere. This CO₂ reacts with silicate minerals in rocks, following the basic chemical reaction:

2CO₂ + 3H₂O + CaSiO₃ -> Ca²⁺+ 2HCO₃^– + H₄ SiO₄

These ions, calcium, carbonic acid and silica acid, flow into the sea by rivers and there, the calcium ions and the carbonic acid combines to precipitate limestone, water – and also CO₂ .

Ca²⁺ + 2HCO₃^– -> CaCO₃ + CO₂ + H₂O

But, at the end of the reactions, half of the CO₂ that went in is stored safely as limestone on the sea floor. Most of this limestone is really made as the shells of microorganisms, but it can also be deposited by pure chemistry.

Less CO₂ in the atmosphere leads to less greenhouse effect, and the Earth cools. Lower temperature slows down the chemical reaction between silicate rocks and CO₂, because this, like many chemical reactions, depend on temperature. The CO₂ extraction from the atmosphere halts. Volcanoes, steadily puffing along, refill the atmosphere with CO₂, and the temperature goes up again…and so goes the carousel, around and around.

This CO₂ that volcanoes spew out comes from the limestone. It rides along when the ocean plates plunge into the Earth, by subduction. Heated in the deep, the limestone returns to CO₂, which goes back into the atmosphere by volcanoes.

This cycle has kept Earth’s surface temperature more or less stable through time. Emphasize more or less. It prevents the temperature from becoming burning hot or plunge into the freezer and makes it possible for liquid water to exist on the surface – a need to have for life as we know it. Without plate tectonics, life could probably not exist.

But, because plate tectonics is so slow, the thermostat rarely keeps the temperature in perfect balance. If lots of fresh rock is exposed at the same time, the thermostat will extract CO₂ faster than volcanoes can replace it and cool the Earth. And that is what happened when the large mountain chains, from the Himalayas to the Alps rose: They exposed lots of fresh rock for erosion, which reacted with CO₂ and stored it as limestone faster than subduction could recycle it.

These mechanisms combined are how the Earth cooled and plunged into the current ice age. Earth will stay in this ice age, until plate tectonics recycle the limestone in volcanoes or opens the Panama isthmus, to again change the ocean currents – or until we humans heat the planet by by burning coal and oil and refill the atmosphere with CO₂.

Plate tectonics is also important to explain how the previous ice age came to be:

The Carboniferous to Permian was the time when most of the continents on Earth assembled into one large continent, Pangaea – the “all Earth”. Pangaea was centered in the southern hemisphere, with Antarctica around the south pole, then as now, and with the South America, Australia, India and Africa continents-to-be in a crescent around it. Only north America and Asia were in the northern hemisphere.

Just as today, the large land mass around the south pole became the cold bed for glaciers, but assembling Pangaea also meant clashing continents, which created mountain chains along the seams. These mountains probably had the same effect on the “tectonic thermostat” as the Himalayas later.

Then, there was another important cooling agent: life itself.

The late Carboniferous marks a major step in the evolution of plants. It was when plants evolved lignine, the compound that make plant stems stiff, and which enabled them to grow to tall trees.

The Carboniferous Earth, a vast land of swamps and forests, of primitive plants, which later became the coal that fired the industrial revolution (Leo Wehrli [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons).

Like all plants, trees make their building blocks through photosynthesis, the well-known chemical reaction:

CO₂ + H₂O -> C₆H₁₂O₆ + O₂.

Forests covered the Earth and through some tens of millions of years drew CO₂ from the atmosphere into the trees, which eventually became buried in the ground as coal. Estimates of how much are uncertain – because they are estimates – but the big forestation probably lowered the CO₂ content from at least 1000 ppm to temporarily around 300 ppm; from far above today’s level, to the same level as just before the industrial revolution. Removing all this CO₂ from the atmosphere reduced the greenhouse effect. Life itself helped plunge Earth into an ice age, which also shows how the Earth’s climate is closely tied to evolution.

Most of the coal on earth comes from the Carboniferous period. The coal gave the period its name. When we burn fossil fuels today, we actually return CO₂ to the atmosphere it once came from. In the next post, we explore how that CO₂ became fossil fuels – and how CO₂ saved the Earth from becoming an eternal snowball.

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