For three billion years, the average temperature on the Earth’s surface has fluctuated between ten and twenty degrees Celsius.
It is thirteen degrees Celsius today. This variation may seem small, but it is sufficient for the glaciation periods to succeed in the tropical climate periods in Earth’s history.
18,000 years ago, when they were only a few degrees lower, the Sahara had a humid climate, and Scandinavia was covered by a layer of 2,000–3,000 meters of ice.
Knowledge of past climate
To know the past climate or paleoclimate, it must have left a trace, a “record” on material support.
The nature of this support varies: it can be a relief from, a rock, ice, or wood.
The coral reef fossils indicate warm paleoclimate and an increase in sea level due to the melting of the glaciers. The trunk of a tree shows rings that form as the tree grows. When the temperature is high and the rains are abundant, the rings are larger. This is an indication of past climate.
The warm climate during its history
The Earth has recorded periods with a warm climate, as it was during the dinosaur era (245–65 million years).
It is estimated that the average temperature of the globe was six degrees Celsius higher than today. Tropical plants and crocodile fossils were discovered in Mongolia.
One hundred million years ago, the level of the seas was three hundred to four hundred meters higher than that of the current ocean, covering 20% of the land.
The glaciations — the cold periods — also marked the history of the Earth. During these periods, the ice-covered the land as far as the north of Europe and America, that’s why these periods were called glaciations. The quaternary era was marked by seven glacial periods, separated by interglacial periods, milder, with a duration of ten thousand to twenty thousand years, such as the era we are crossing now and which began eleven thousand years ago.
At the maximum moment of the last glaciation, produced approximately 20,000 years ago, the volume of the glaciers that covered the continents was twice that of the ice caps of today.
The cap of the North Pole descended as far as New York and Berlin, and the ocean level was 120 meters lower than the current level.
Astronomical causes
The Earth revolves around the Sun on a trajectory that varies every 100,000 years, approaching or moving away from the Sun, which determines the temperature change on our planet.
At the same time, the Earth rotates around its own axis, which is inclined. The axis can be changed in its turn, leading to a change in the amount of heat received. Finally, even the activity of the Sun varies periodically. The sun has dark spots and their number changes cyclically, with a maximum value recorded every 11 years.
Geological and atmospheric causes
The movement of the continents due to the movement of the lithospheric plates changes their latitude, which periodically causes climate variations.
Thus, 450 million years ago, glaciers settled in the southern part of Africa, America, India, and Australia. These continents, at that time united, were located near the South Pole. The change in the composition of the Earth’s atmosphere compared to its origins is another important factor. Carbon dioxide, more abundant for example during the secondary era, can explain, through the greenhouse effect, the high temperature of that period. Volcanic eruptions throw enormous amounts of dust into the atmosphere that prevent part of the Sun’s rays from reaching the ground, leading to a decrease in the temperature on Earth.
Climate and the carbon cycle
The average temperature on the Earth’s surface depends on the amount of carbon dioxide (or carbon dioxide) that the atmosphere contains. Between the atmosphere, oceans, rocks, and living things there is a constant exchange of carbon dioxide, a gas made of carbon and oxygen. During these exchanges, the amount of gas contained in the atmosphere can change, and this change influences the climate.
The carbon cycle
Certain elements of our planet absorb carbon dioxide from the atmosphere. The most important of these is the ocean: every year, it receives 92 gigatons of carbon dioxide from the atmosphere and returns 90 gigatons (one gigaton is equal to one thousand million tons). The world ocean, therefore, pumps carbon dioxide into the atmosphere. Plants also play an important role in the carbon cycle because, thanks to photosynthesis, they absorb carbon dioxide, reducing its concentration in the atmosphere. Instead, beings emit an extremely large amount of carbon dioxide through breathing. Volcanic eruptions also produce carbon dioxide.
For a century, an increase in the amount of carbon dioxide in the atmosphere has been observed. It is considered that this increase is caused by a series of human activities, such as deforestation and the burning of wood, oil, or coal.
How does the Earth heat up?
As we advance in altitude, the temperature drops, even though we are getting closer to the Sun. The Sun’s radiation that crosses the atmosphere to reach the Earth’s surface only releases a small amount of heat. Reaching the surface of the soil, they are absorbed, then emitted again in a new form, called thermal radiation. These radiations release heat into the atmosphere. Thus, even if the Sun represents the Earth’s energy source, the heat is released especially from the soil, which explains the higher temperatures at low altitudes. If there were no system to retain thermal radiation, they would be eliminated in space, and the temperature on Earth would be much lower. But, when it goes up into the atmosphere, the thermal radiation is absorbed by certain gases, like carbon dioxide. These gases prevent the elimination of heat in space and direct it back to the ground. The phenomenon is called the greenhouse effect.
The higher the amount of carbon dioxide in the atmosphere, the higher the temperature. Thus, the carbon cycle plays a fundamental role in regulating the temperature on Earth.