Last updated: September 19, 2024

Earth's Paleoclimates and Carbon Dioxide

Understanding the Importance of Greenhouse Gases

Earth formed 4.543 billion years ago, and all long-term climate changes, over all time scales, involve greenhouse gases. Understanding climate evolution from this perspective offers deep insight into the mechanisms that regulate Earth's climate on geological time scales.

Greenhouse gases in the atmosphere are transparent to most of the incoming solar radiation, allowing sunlight to heat Earth's surface. However, Earth re-emits this energy as infrared radiation due to its lower temperature, and this radiation is trapped by these greenhouse gases.

Solar Luminosity Variation and Greenhouse Gases

Solar luminosity plays a fundamental role in Earth's energy balance. Since the formation of the solar system, solar luminosity has gradually increased by 7% per billion years.

According to the theory of the "Faint Young Sun" solar luminosity was about 30% weaker 4.6 billion years ago. Yet, geological evidence shows that early Earth had liquid oceans and a high enough temperature to allow liquid water to exist. This apparent contradiction between weaker solar luminosity and the presence of liquid water is called the Faint Young Sun Paradox.

Thus, other factors must have counterbalanced the effects of a faint young Sun.
Among these factors are greenhouse gases such as carbon dioxide (CO2) and methane (CH4). One of the main sources of CO2 in early Earth's atmosphere was volcanic activity.
Young Earth experienced intense tectonic activity, with numerous volcanic eruptions. These eruptions released large amounts of gases, including carbon dioxide, as well as water (H2O), sulfur dioxide (SO2), methane (CH4), and other gases.

Without sufficient greenhouse effects, Earth would have been a frozen planet. The presence of high concentrations of greenhouse gases compensated for the faint solar energy by increasing heat retention. This prevented Earth from cooling too much, allowing temperatures suitable for liquid water and early biological development.

Plate Tectonics and Greenhouse Gases

The grouping and separation of continents follow geological cycles known as supercontinent cycles. These cycles describe periods when continents come together to form a single supercontinent, then break apart again due to plate tectonics. The complete cycle of formation, fragmentation, and reformation of supercontinents, linked to the opening and closing of oceans, typically lasts between 300 and 500 million years.
Examples: Rodinia (1.3 to 0.9 billion years ago), Pannotia (600 million years ago), Pangaea (335 to 175 million years ago).

Over geological ages, continents drifted, merged, and fragmented, thus altering oceanic and atmospheric currents that redistribute heat around the planet.

When continents were mainly located in equatorial zones, particularly during geological periods like the Carboniferous and Permian (around 300 to 250 million years ago), carbon dioxide (CO2) played a key role in climate and in regulating Earth's temperature.

When continents are concentrated in these regions, they are subjected to warm and humid climatic conditions, which promote erosion, i.e., the chemical weathering of silicate rocks. This chemical process consumes atmospheric CO2. Indeed, rocks react with carbon dioxide to form carbonates, which deposit in oceans, trapping carbon. In other words, chemical weathering acts as a natural carbon sink mechanism, regulating atmospheric CO2 concentrations and thus the global climate.

Conversely, when continents are located at the poles, they are more vulnerable to ice accumulation due to lower temperatures and reduced solar radiation.
The high albedo of ice (its ability to reflect sunlight) amplifies this cooling, creating a positive feedback loop: the more ice there is, the more Earth's surface reflects sunlight, intensifying global cooling.
However, periods of high volcanic activity persist, releasing CO2 and increasing global temperatures. As polar regions are very arid, rainfall is minimal.
When the chemical weathering of rocks slows down or ceases due to ice cover, the long-term carbon cycle is disrupted.
Chemical weathering of rocks is one of the most effective natural mechanisms for removing CO2 from the atmosphere and regulating levels of this greenhouse gas. In the absence of this process, atmospheric CO2 is no longer significantly consumed, allowing other processes, such as volcanic emissions, to continue increasing the concentration of CO2 in the atmosphere.

Glacial and Interglacial Periods and Greenhouse Gases

Glacial and interglacial cycles are primarily governed by the Milankovitch cycles, which describe periodic variations in Earth's orbit and axial tilt. These cycles include three main parameters:
• Eccentricity (variations in the shape of Earth's elliptical orbit around the Sun, with a period of about 100,000 years)
• Obliquity (variations in the tilt of Earth's axis relative to its orbital plane, on a cycle of 41,000 years)
• Precession (change in the direction of Earth's rotational axis, with a cycle of 23,000 years)

These variations influence the distribution of solar energy received by Earth at different latitudes and seasons, favoring the alternation between glacial periods (ice accumulation at the poles) and interglacial periods (ice melting and milder temperatures).

However, the climate's response to these orbital cycles is amplified by feedback mechanisms, such as changes in ice cover and CO2 levels.

In glacial and interglacial cycles, CO2 levels follow a complex dynamic of positive and negative feedbacks.

During glacial periods, lower temperatures and extensive ice caps reduce biological activity (photosynthesis) and continental erosion processes. Less CO2 is captured by the terrestrial biosphere and oceans. The reduced solar radiation caused by Milankovitch cycles leads to cooling. This cooling causes the oceans to capture more CO2, lowering atmospheric CO2 concentration and amplifying the cooling.

On the other hand, during interglacial periods, increased sunlight in certain regions causes warming, which triggers oceanic CO2 degassing. The increase in atmospheric CO2 strengthens the greenhouse effect and amplifies the initial warming.

The Importance of Carbon Dioxide

CO2 has played a fundamental role in maintaining temperatures compatible with life on Earth throughout geological eras. Through its interactions with geophysical and biological processes, it has allowed the planet to adjust to internal variations (tectonics, volcanoes) and external ones (solar evolution, orbital cycles). This balance, which has lasted for billions of years, highlights how CO2 is a key regulator of Earth's climate, a role it continues to play, though its current disruption by human activities presents a major challenge for humanity.