Global Warming in Figures: What the Scientific Data Says

The Impact of Human Activities: The Exponential Increase of CO₂

The work of Svante Arrhenius (1859-1927) had already established the link between CO₂ and temperature as early as 1896. Today, attribution studies show that human activities are responsible for more than 95% of the warming observed since 1950.

The concentration of carbon dioxide in the atmosphere has experienced an unprecedented increase since the beginning of the industrial era, rising from 280 ppm (parts per million) in the mid-19th century to 420 ppm today.

Understanding Climate Figures: Beyond the Global Average

Global warming represents one of the major challenges of the 21st century. Behind media and political discourses lie complex scientific data that deserve to be deciphered.

Data from NASA (national Aeronautics and Space Administration) and the NOAA (national Oceanic and Atmospheric Administration) show an average increase of 1.2°C since the pre-industrial era (1850-1900). This value may seem modest, but it hides significant regional variations: the Arctic is warming 2 to 3 times faster than the global average.

N.B.:
1850–1900 is the period when reliable instrumental measurements began to be available globally, before industrial emissions became massive. Estimated average temperature: About 13.7°C in 1900 (vs ~14.9°C in 2023–2025, i.e., a +1.2°C increase).

Main Anthropogenic Sources of CO₂

Human activities responsible for this increase can be classified in order of importance:

What Does an "Average Increase of 1.2°C" Mean?

This value represents the increase in the average temperature at the Earth's surface but this global average hides complex realities. It is not a uniform increase that would be felt everywhere on the planet. This value of 1.2°C is a global average calculated over the entire globe:

• It integrates temperatures from continents and oceans
• It combines daytime and nighttime measurements
• It represents an average over several years (generally over 30 years)

This Average Masks Significant Geographical Disparities

How Can CO₂, Present in Such Small Amounts in the Air, Affect the Climate So Much?

How can a gas, CO₂, present in such a small proportion (0.04% of the atmosphere) have such a determining influence on the Earth's climate? The answer lies in the specific physical properties of carbon dioxide and its role in the greenhouse effect (phenomenon by which certain gases retain heat in the atmosphere).

The Molecular Structure of CO₂ and Its Interaction with Infrared Radiation

Carbon dioxide (CO₂) has a linear and asymmetric structure (O=C=O) that gives it unique infrared absorption properties. Unlike oxygen (O₂) or nitrogen (N₂), CO₂ can vibrate in a way that efficiently absorbs the thermal radiation emitted by the Earth, and then re-emit it in all directions – including back towards the Earth's surface.

Infrared Absorption Mechanism

The CO₂ molecule mainly absorbs infrared in the 15 µm (micrometer) band, a wavelength characteristic of Earth's thermal radiation. This absorption results from its three vibration modes:

1. Symmetric stretching (slightly active in IR), the oxygen atoms move away from and closer to the carbon at the same time → not very effective at capturing heat.
2. Asymmetric stretching (highly active, responsible for absorption at 15 µm), one oxygen pulls the carbon one way while the other pushes it → this movement traps infrared best.
3. Bending (absorption at ~4.3 µm, less intense but significant), the molecule bends slightly → captures another part of the heat.

These vibrations change the molecule's dipole moment, a necessary condition to interact with infrared electromagnetic waves. CO₂ absorbs IR photons whose energy corresponds to its vibrational transitions (like asymmetric stretching), then re-emits this energy as heat or new photons, thus contributing to the greenhouse effect.

Ground state (CO₂) ─────[Absorption of an IR photon]─────► Excited vibrational state (E = hν, λ ≈ 15 µm) Excited state ─────[Re-emission of an IR photon or collision]─────► Ground state + heat

Consequences on the Greenhouse Effect

By re-emitting part of the infrared radiation towards the surface, CO₂ contributes to trapping part of the heat in the atmosphere. Unlike water vapor (another major greenhouse gas), its concentration is less sensitive to local temperature variations, making it a long-term climate regulator.

Comparison with Other Atmospheric Gases

Oxygen (O₂) and nitrogen (N₂), which are majority in the atmosphere (99%), do not absorb infrared because their bonds are symmetric and non-polar. In contrast, methane (CH₄) or nitrous oxide (N₂O) have structures even more effective than CO₂ at absorbing IR, but their concentration is much lower. CO₂ therefore plays a central role in the Earth's radiative balance.

N.B.:
Normally, CO₂ is electrically neutral. But when it vibrates, it develops a small unbalanced charge. This property allows it to intercept infrared – the heat that the Earth tries to evacuate into space. Without this mechanism, the average temperature on Earth would be 30°C colder!

IPCC Scenarios: From Optimistic to Pessimistic

The IPCC (intergovernmental Panel on Climate Change) has developed several scenarios, from the most optimistic (SSP1-1.9) to the most pessimistic (SSP5-8.5). These projections take into account greenhouse gas emissions, global demographics and climate policies.

Global Warming According to the IPCC SSP (Shared Socioeconomic Pathways) Scenarios

• Source: IPCC, AR6 Report, 2021 (new window); CMIP6 Scenario Database (new window); NASA Climate Change (new window).

Climate Tipping Points: When the Earth Loses its Balance

The Earth is a thermodynamic system constantly seeking stability. The climate functions in the same way: certain key elements of our planet could, beyond a certain threshold, change radically and irreversibly. These critical thresholds are called tipping points. A tipping point is a threshold beyond which a climate system:

• Undergoes an abrupt and often irreversible change
• Shifts to a new state of equilibrium, very different from the previous one
• Can self-sustain, even without further perturbation

N.B.:
These phenomena are not distant predictions: some could be triggered with a warming of 1.5 to 2°C. Once crossed, their effects could propagate like dominoes through the climate system.

Imminent Climate Tipping Points: Has the Countdown Already Begun?

Experts from the IPCC (intergovernmental Panel on Climate Change), NASA, and recent studies published in Nature and Science (2020-2024) identify several vulnerable climate systems. These tipping points could be triggered with a warming of 1.5°C to 2°C, a threshold we are rapidly approaching (1.2°C reached in 2025). Their crossing would lead to changes irreversible on a human timescale and cascade effects on the entire Earth system.

Where Are We Today?

• Some points (like the melting of Greenland or the weakening of the AMOC) could be crossed as early as 1.5°C of warming.
• Others (like the disappearance of the Amazon) become likely at 2°C.
• We are still emitting 40 billion tonnes of CO₂ per year (2025), bringing us dangerously close to these thresholds.

N.B.:
A study published in Science (2022) estimates that we have already crossed 5 of the 16 identified tipping points, including partial melting of Greenland and West Antarctica, and the slowing of the AMOC.

What Can We Do?

• Drastically reduce emissions to stay below 1.5°C.
• Protect ecosystems (forests, peatlands) that act as climate buffers.
• Develop technologies for carbon capture to offset residual emissions.
• Prepare for inevitable changes (coastal adaptation, resilient agriculture).

Every tenth of a degree counts: limiting warming to 1.5°C rather than 2°C could avoid crossing several tipping points.