The Earth's climate

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The sun and the greenhouse effect

The Sun is the central star of our solar system, which consists of 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.The "surface" of the Sun is very hot, with a temperature of about 6,000 degrees Celsius. Due to its position, neither too near nor too far from the Sun, the Earth is the only planet in the solar system that can host abundant life, in particular because its average surface temperature of 15 degrees Celsius enables the presence of liquid water. Solar energy and the presence of an atmosphere are the two main elements that condition the Earth's temperature. Like the glass panels of a greenhouse, certain gases that are naturally present in the atmosphere, notably water vapor and carbon dioxide, trap heat from the sun, maintaining this mild average temperature on the planet's surface. This natural phenomenon is called the greenhouse effect. Without it, the surface temperature of the Earth would be –18 degrees Celsius and life, if any life could be sustained, would be quite different. The Sun emits radiation consisting of a small amount of ultraviolet rays and large quantities of visible light and near infrared. 30% of this radiation is immediately reflected by clouds, the atmosphere and the Earth's surface. The remaining 70% is absorbed: 20% by gases naturally present in the atmosphere and 50% by the oceans and land. Consequently, only half of the initial solar radiation reaches the Earth's surface. The absorption of solar energy warms the atmosphere and, most importantly, the surface of our planet. Converted into heat, this energy is then released back into space by the atmosphere and the Earth's surface in the form of middle infrared radiation. 10% of this terrestrial infrared radiation escapes directly into space, while the remaining 90% is trapped by greenhouse gases that are naturally present in the atmosphere. After trapping the heat from the sun as well as the heat rising from the Earth's surface, these gases, mainly water vapor and carbon dioxide, re-release this energy in the form of infrared rays. Two-thirds of these rays return to the surface, warming it a second time with the same solar energy, and the remaining one-third escapes into space.

This is a simplified explanation of the greenhouse effect disregarding certain factors, for example the phenomenon of water evaporation on the Earth's surface, which leads to the formation of clouds. This evaporation contributes to the warming of the atmosphere, and also plays a key role in the water cycle.

This constant exchange of energy between the Earth's surface and the greenhouse gases is what enables the planet to maintain its average temperature of 15 degrees Celsius. Our atmosphere's natural greenhouse effect is thus a beneficial phenomenon, without which life as we know it would not be possible.

The greenhouse effect is often mentioned in the news as a dangerous phenomenon, but what really is dangerous is the intensification of the natural greenhouse effect. This is the result of an increase in greenhouse gases due to human activity, in particular the combustion of fossil fuels. It is thought that this intensification of the greenhouse effect is a major cause of the global warming trend that has been observed over the past 50 years. Today, scientists are trying to predict more accurately how this trend is likely to develop between now and the end of the 21st century.