The current climate change is a reality that cannot be denied. Numerous observations testify to a global warming trend. Glacier melting, decreasing polar ice caps, rising sea levels… these rapid changes observed over the past few decades are directly linked to human activities, which have been causing an unprecedented increase in greenhouse gas emissions into the atmosphere for over a century now.
Earth’s History Marked by Regular Climate Oscillation
Let’s set aside the current period for once and look at the long climatic history of Earth. And what a history it is! By studying sedimentary and glacial records, it becomes evident that the Earth’s climate has continuously varied over millions of years. The past is punctuated by major climatic events (both ice ages and warm periods), often associated with mass extinctions, overlaying a regular climate oscillation that alternates between glacial and interglacial periods following cycles of remarkable regularity ranging from tens to hundreds of thousands of years. These oscillations are particularly clear when examining the last 800,000 years for which we have detailed and precise data.
Cyclical Variations in Sunlight Influenced by Earth’s Movement in Space
Since the 19th century, scientists have been intrigued by this regularity and sought to identify its cause. Starting from the premise that the amount of sunlight, i.e., solar energy received by Earth, is the primary factor controlling temperature variations, scientists like Adhémar, Croll, and Milankovitch quickly identified the – or rather, the culprits. Such regularity can only be linked to variations in Earth’s orbital parameters that cyclically modify the amount of sunlight received. This astronomical forcing is experienced on a very short timescale: the alternation of seasons is tied to the fact that Earth is tilted on its axis of rotation, which alters the amount of sunlight received by the Northern and Southern Hemispheres during the year.
Notably, Milankovitch determined that three orbital parameters primarily control climate evolution over relatively short periods (geologically speaking) of tens of thousands of years. These parameters are the eccentricity of Earth’s orbit, the obliquity of its axis of rotation, and precession.
Eccentricity
Eccentricity seems to have the most influence. It signifies that Earth’s orbit is not a perfect circle around the Sun but rather an ellipse, whose shape is not stable over hundreds of thousands of years. The Earth’s orbit is influenced by other bodies in the Solar System or those that pass nearby. These planetary interactions gradually change Earth’s orbit from nearly circular to highly elongated. In the latter case, Earth ends up very close to the Sun annually.
Currently, Earth’s eccentricity is gradually decreasing, meaning the orbit is becoming more circular. However, the eccentricity variation is too small within human timescales to have a visible impact on climate. Its influence is only noticeable over much longer timescales.
Obliquity
As mentioned earlier, Earth’s axis of rotation is slightly tilted compared to the orbital plane. This tilt is what causes the seasonal changes. Similar to eccentricity, this tilt is not stable over time. There is a slight variation in the tilt angle, oscillating between 22.1 and 24.5 degrees. This variation occurs in cycles of 41,000 years and influences the intensity of seasons, which are more extreme in high latitudes (colder winters and hotter summers) when the tilt angle is strongest. Earth’s obliquity oscillation is also linked to gravitational interactions with other planets.
Currently, Earth’s axial tilt is 23.4 degrees relative to the “vertical” (in a frame of reference where the orbital plane is seen as the equator).
The Earth’s Climate: The Milankovitch Cycles and Human Influence
The Earth’s climate is influenced by various factors, including the Milankovitch cycles, which are astronomical variations in the Earth’s orbit and orientation. These cycles consist of changes in eccentricity, obliquity, and precession.
Eccentricity
Eccentricity refers to the shape of the Earth’s orbit around the Sun. It oscillates over a period of about 100,000 years from more circular to more elliptical. Currently, the Earth’s orbit is moderately elliptical, which affects the amount of solar radiation received by the planet. This variation plays a role in shaping the climate patterns on Earth.
Obliquity
Obliquity is the tilt of the Earth’s axis relative to its orbit around the Sun. It varies between 22.1 and 24.5 degrees over a cycle of about 41,000 years. Currently, the Earth’s obliquity is approximately 23.5 degrees (where 0 degrees represents a vertical axis and 90 degrees represents a horizontal axis). This angle is decreasing and will reach its minimum in about 10,000 years, leading to less pronounced seasons. A minimal obliquity promotes the formation and growth of polar ice caps, which in turn intensify climate cooling by reflecting solar energy. Conversely, a maximum obliquity favors deglaciation and global warming. This is a situation that Earth experienced 10,000 years ago.
Precession
Obliquity variation is not the only change that the Earth’s rotation axis undergoes. The gravitational influence of the Moon and the Sun, through tidal forces and the resulting deformation of the Earth, also perturbs the axis in another way. The Earth’s axis describes a small circle, like a spinning top about to stop. This oscillation, known as the precession of the axis, follows cycles of about 25,771 years. Again, this affects the seasons by either attenuating or enhancing the seasonal contrasts between the two hemispheres (North and South). Currently, the Southern Hemisphere is expected to experience stronger contrasts between summer and winter, while the seasonal differences should be less pronounced in the Northern Hemisphere. In 13,000 years, this situation is expected to reverse.
However, human activities have an impact on the climate as well.
Earth’s Climate: Milankovitch Cycles are not the Only Factor
Several cyclic patterns influence Earth’s climate, and it is their interactions that modulate the terrestrial climate on a large scale. The duration of each cycle depends on the relative position of the planets in the solar system and variations in their orbital parameters over time. From this perspective, the behavior of Earth and Mars appears rather chaotic, leading to significant variations in Milankovitch cycles over time, occasionally resulting in extreme climatic events, such as the Paleocene-Eocene Thermal Maximum 56 million years ago, as explained by Dutkiewicz and co-authors in a recent scientific publication.
Astronomical parameters are not the sole influencers of Earth’s climate, which responds to a multitude of factors and interactions that are not yet fully understood. Plate tectonics, through the rearrangement of continental masses, can influence Earth’s climate by controlling major ocean currents. These currents play a significant role in climate by affecting the moisture and temperature of the air and water. Factors such as the albedo effect of ice caps and chemical processes associated with soil weathering play a part in the carbon cycle, thus influencing the climate. All these factors tend to modulate the climate response to Milankovitch cycles and large astronomical cycles.
In a scenario without human presence, Earth would slowly move towards a new ice age, although this is not expected to occur for another 50,000 years.
However, with humanity’s presence, the situation is different.
Current Warming: What is the Impact of Milankovitch Cycles?
While some theories suggest that the current global warming could be due to astronomical forcing, most experts agree that this is not the case. The timescale is a crucial argument. The current warming is happening too rapidly to fit within the Milankovitch cycles, which operate over tens of thousands of years. Furthermore, there is evidence against warming being linked to increased solar radiation. Over the last 150 years, the amount of solar energy absorbed by Earth has remained relatively stable, with satellites even recording a decrease in radiation over recent decades.
On the other hand, the atmospheric CO2 levels are at record highs. Natural fluctuations in CO2 during past glacial cycles ranged between 180 ppm (parts per million) and 280 ppm. However, in just 150 years, CO2 levels have risen from 280 to 421 ppm (2023 value). This increase is attributed to massive CO2 emissions from fossil fuel combustion. The result is an enhanced greenhouse effect, leading to increased surface temperature and lower atmosphere temperature, despite the current astronomical forcing indicating cooling of the stratosphere.
Through human activities, mankind has effectively masked the natural climate signal linked to Milankovitch cycles. While astronomical forcing still plays a role, its impact is no longer discernible since the atmospheric CO2 concentration exceeded 350 ppm, as indicated by NASA.