Freshman Seminar: Introduction to the Science of Climate Change Prof. Jordi Miralda-Escudé, F 9:30

Lecture 6: Earlier history of the Earth climate.

In Lecture 5 we saw that over the last several hundreds of thousands of years, the Earth climate has experienced many ice ages with much colder conditions than today, and interglacial periods in between with similar climate as today. At much earlier times, ice ages did not occur and Earth was generally much warmer.

Evidence for a warm Earth many millions of years ago:

• Measurements of oxygen isotopes in sea-floor sediments indicate that there was practically no permanent ice on the Earth more than 35 million years ago. The continent of Antarctica was completely ice free before that time. The mass of ice in Antarctica has been gradually increasing since 35 million years ago, indicating a gradual cooling tendency over this period of time (with shorter timescale fluctuations occurring like the ice ages in the more recent period).
• We also find fossil evidence of broad-leafed evergreen trees adapted to warm weather at very high latitudes above the arctic and antartic circles, as well as reptiles adapted to warmth. These are species that at present can only survive near the tropics, but 50 to 100 million years ago they seemed to live happily in places like Siberia and Antarctica.
• Sea-level was also hundreds of meters higher than at present. This was in part due to all the water that is in ice today being in the ocean, part due to the fact that water expands as it warms up, and part also due to a different shape of continents and ocean basins compared to the present.

Why was carbon dioxide higher many millions of years ago?

We have seen in the last lecture that the reduction of carbon dioxide over the short timescale of an ice age is probably explained by an exchange of carbon dioxide between the atmosphere and ocean.
However, the much larger change of carbon dioxide over the last 100 million years needs to be explained as a change in the total reservoir of carbon in the ocean, which is large compared to the atmosphere and soil reservoirs.

Over long timescales, the carbon reservoir in the atmosphere-ocean-soil is changed by input of carbon from volcanoes and other outgassing from the mantle, and output of carbon by burial of sea-floor sediments containing carbonate rocks, and eventual subduction of these ocean sediments into the mantle.

This means there are two reasons why carbon dioxide could have been higher in the past:

• There could have been more volcanic activity. More volcanoes would be expected if the rate at which continental plates were moving, driven by sea-floor spreading, was higher. Geologic records indicate that the rate of sea-floor spreading was indeed higher in the past, so probably volcanic emissions were higher.
• Less carbon dioxide might have been removed into sea-floor sediments. The rate of carbon dioxide removal is determined by chemical weathering of rocks. The minerals in exposed rocks (such as silicon, calcium, potassium) gradually react in contact with rainwater that contains carbon dioxide, making carbonate compounds that are carried into rivers and to the ocean. There, the carbonates can be precipitated to the sea floor. The rate of chemical weathering seems to have increased towards the present, compared to 50 million years ago, in part because of the collision of India with Asia which has caused more uplifting of high mountains, with greater rates of exposure of fresh rocks containing minerals that are good for removing carbon dioxide by chemical weathering.

The reason why carbon dioxide has been gradually declining over the last 50 million years is not yet quite clear, but both these factors (decreasing emissions from volcanoes and increasing rates of chemical weathering) were important.

The processes of volcanism and chemical weathering can change the total carbon in the ocean and atmosphere only over millions of years. They are far too slow to affect the anthropogenic carbon dioxide increase at present, which is being mitigated only by absorption of carbon dioxide into the ocean as the ocean comes to equilibrium with a higher carbon dioxide abundance in the atmosphere, over a timescale of centuries.

The climate in the primitive Earth

The Earth is about 4.5 billion years old. During this time, the Sun has been gradually increasing its luminosity as a result of the normal evolution of a star. According to astrophysical theory, a normal star will slowly increase its luminosity as it consumes more of its hydrogen to make helium in its core by nuclear fusion reactions which generate the energy of the star. At a time of 4 billion years ago, the luminosity of the Sun was only 70% of the present one. This suggests that the Earth should have been much colder.

In fact, climate models show that with such a faint Sun, the Earth should have been completely frozen (meaning covered by a thick layer of ice even in the equator) during its first 3 billion years of history, if the greenhouse effect were similar to the present one.

The reason the Earth was not frozen in its early history is believed to be a much greater greenhouse effect from a very high carbon dioxide concentration (nearly all the atmosphere might have been carbon dioxide in the primitive Earth). This may have been due to a higher rate of volcanism and lower rates of chemical weathering in the primitive Earth.

The Gaia hypothesis for the evolution of the Earth postulates that the presence of life has provided mechanisms to regulate the climate, among them regulating the rate at which chemical weathering occurs, so that the Earth does not get too hot or too cold to maintain life. This hypothesis has generated a lot of debate among scientists and the public.

Why is Venus so hot today?

The planet Venus has a temperature on its surface of 460 degrees Celsius, or about 800 degrees Fahrenheit.

Venus is closer to the Sun than Earth, so you would think that would make it hotter. However, Venus also has a higher albedo than Earth, meaning that it reflects a lot more of the sunlight. Because of this higher albedo, Venus should actually be cooler than Earth if it had the same greenhouse effect. The reason Venus is so hot is because of its very large greenhouse effect.

Almost all the atmosphere of Venus is carbon dioxide (96% of it). In addition, the total mass of Venus atmosphere is 100 times the mass of the atmosphere of Earth. At the surface of Venus, the pressure is 100 times the pressure at the Earth surface.

The reason there is so much carbon dioxide in Venus' atmosphere is because that is where all the carbon in Venus is. Venus has no ocean, and no chemical weathering, and no subduction of carbonates into its mantle. Over its history, all the carbon in Venus's interior has been emitted by volcanoes and has stayed in the atmosphere.

Earth and Venus actually have about the same amount of carbon, but they have it in different places. If one could take all the carbonate rocks in Earth's interior, vaporize them and emit all the carbon dioxide to the atmosphere, then Earth would become like Venus. We are lucky that over many millions of years, carbon dioxide has been constantly removed by dissolving in rainwater, combining with minerals in rocks, running through rivers to the ocean, sedimenting in the sea-floor, and then being subducted to the mantle!