At the beginning of this year, we reflect on a significant event of 2024: for the first time, the global average temperature exceeded 1.5°C above pre-industrial levels (1850-1900). This symbolic threshold was previously considered the lower limit of the 2015 Paris Agreement and the most ambitious target for 2100. Although this exceedance does not yet mean that the climate has permanently crossed this threshold (a 20-year average would be required for that), this observation—just 10 years after COP 21—is alarming.

According to the World Meteorological Organization (WMO), 2024 was the hottest year ever recorded, with a global average temperature exceeding the pre-industrial period’s mean by 1.55°C (±0.13°C).

We have often discussed energy-related issues, but rarely those concerning climate. This series of articles aims to explain fundamental climate concepts. Today, we begin with the notion of radiative forcing and the parameters that influence it.

1. What is Climate?

Climate refers to the statistical distribution of atmospheric conditions over a specific region and period. It differs from meteorology, which focuses on short-term weather conditions in specific locations. Here, we will discuss only global average temperatures calculated over 30 years.

Over long timescales, Earth’s temperature is determined by a delicate balance between the energy received from the Sun and the energy radiated back into space. This system is based on a fundamental physical phenomenon: the radiative balance.

2. The Concept of Radiative Balance

The Earth’s radiative balance is the difference between incoming solar energy and outgoing energy. It determines whether the planet is warming, cooling, or maintaining a stable temperature. Below are its key components:

2.1 Incoming Energy

The Sun emits radiation primarily in the visible spectrum, along with ultraviolet (UV) and infrared (IR) waves.

• About 30% of this energy is directly reflected back into space by the atmosphere, clouds, and reflective surfaces such as snow, ice, and aerosols (a phenomenon known as albedo).
• The remaining 70% penetrates the atmosphere: some is absorbed (e.g., ozone absorbs UV radiation), but the majority reaches the Earth’s surface and is converted into heat.

2.2 Outgoing Energy

The Earth’s surface, heated by solar energy, emits infrared (IR) radiation back into space. However, part of this radiation is trapped by gases in the atmosphere, a phenomenon known as the greenhouse effect.

2.3 Global Energy Balance

• If incoming energy equals outgoing energy, the Earth’s average temperature remains stable.
• If incoming energy exceeds outgoing energy (positive radiative forcing), the Earth warms.
• If outgoing energy exceeds incoming energy (negative radiative forcing), the Earth cools.

Thus, radiative forcing caused by changes in greenhouse gas concentrations or alterations in albedo directly leads to climate changes.

The graph below illustrates energy flows with their values in W/m².

3. Factors Influencing Radiative Forcing

3.1 The Greenhouse Effect: The Role of Gases in Heat Retention

The greenhouse effect is a natural process in which certain atmospheric gases absorb and reemit infrared radiation. The main greenhouse gases include:

• Carbon dioxide (CO₂)
• Methane (CH₄)
• Water vapor (H₂O)
• Nitrous oxide (N₂O)

These molecules have a unique ability to absorb infrared radiation due to their molecular structure. When they interact with this radiation, they vibrate and reemit energy in all directions, including back toward the Earth’s surface. This process helps maintain an average global temperature of 15°C, compared to the -18°C the Earth would have without these gases.

3.2 Albedo: The Reflection of Solar Energy

Albedo refers to the ability of certain surfaces or aerosols (airborne particles) to reflect solar energy. In general, light-colored surfaces (such as ice and snow) have a high albedo, reflecting a large portion of solar energy back into space. This concept is easy to experience in daily life—on a sunny day, wearing white clothing keeps you cooler, whereas dark clothing absorbs more heat, making you feel much warmer.

A crucial factor in climate change is that when ice melts, it is replaced by darker surfaces (such as oceans and land), which absorb more heat. This creates a positive feedback loop, further amplifying global warming.

Albedo-related feedback works both ways. During glaciation periods, the expansion of ice sheets increases solar reflection, leading to further cooling. A striking example is the Huronian glaciation around 3 billion years ago, when a high albedo helped sustain a global ice age for 300 million years. The Earth only emerged from this deep freeze after volcanic activity released greenhouse gases, warming the atmosphere.

4. Other Factors with Local Influence

Greenhouse gases and albedo are the two main factors affecting global temperatures on Earth. However, several other factors have an impact, though their role is either minor or localized. These include:

• Ocean Currents and Regional Climates:
  The oceans store an immense amount of thermal energy (about two-thirds of the total) and redistribute it through ocean currents, such as the Gulf Stream. This process influences regional climates (for example, Western Europe’s mild climate is largely due to the Gulf Stream), but its effect on the global energy balance remains secondary.
• The Marginal Influence of Solar Cycles:
  The Sun’s intensity varies slightly due to 11-year sunspot cycles, which cause small fluctuations in the amount of energy Earth receives. However, these variations are too minor to account for current global warming trends.

Conclusion

The current value of radiative forcing is estimated at 2.7 W/m², composed of 3.8 W/m² from the increase in greenhouse gas concentrations and -1.1 W/m² due to the rise in aerosols (albedo effect). By 2100, radiative forcing could reach up to 8.5 W/m² in the most pessimistic scenario. The IPCC has also proposed five Shared Socioeconomic Pathways (SSPs), each associated with a different level of radiative forcing by 2100.

In this article, we have explored the concept of radiative forcing, its magnitude, and the factors influencing it. Understanding these mechanisms is crucial for grasping current and future climate dynamics. In the next article, we will focus on greenhouse gases, their role in this process, and their degradation cycles.

Source :

1. Organisation météorologique mondiale (OMM) : https://wmo.int/fr/news/media-centre/lomm-confirme-que-2024-est-lannee-la-plus-chaude-jamais-enregistree-avec-une-temperature-superieure
2. Assistance scolaire personnalisée : https://www.assistancescolaire.com/eleve/1re/enseignement-scientifique/reviser-le-cours/1_sci_18
3. Rapport du GIEC 2021 (AR6) : https://www.ipcc.ch/report/ar6/wg1/
4. NASA – Earth Observatory : https://earthobservatory.nasa.gov/features/EnergyBalance
5. https://actugeologique.fr/2022/07/le-bilan-radiatif-de-la-terre/
6. https://www.maxicours.com/se/cours/le-bilan-radiatif-terrestre/
7. Illustration fresque du climat
8. Illustration rapport du GIEC AR6