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Last updated: September 23, 2025

Global Warming in Figures: What the Scientific Data Says

Proportion of gases in the atmosphere

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 and the NOAA 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:

Anthropogenic Sources of CO₂ and Their Contribution
SourceAnnual ContributionEvolution since 1990Current Trends
Fossil fuel combustion~36 billion tonnes+60%Stagnation in some developed countries
Deforestation and land use change~5-10 billion tonnesStable to slight increaseConcerning in Amazonia and Southeast Asia
Cement production~2.5 billion tonnes+200%Strong growth with urbanization
Intensive agriculture~1-2 billion tonnes+30%Possible stabilization with new practices

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:

This Average Masks Significant Geographical Disparities

Regional Variations in Warming Relative to the Global Average
RegionObserved WarmingAmplification FactorExplanations
Arctic+3 to +4°C× 3Polar amplification due to ice melt
Continents+1.5 to +2°C× 1.5Land warms faster than oceans
Oceans+0.8 to +1°C× 0.8High heat capacity of water
Tropical regions+0.8 to +1.2°C× 1Warming close to the global average

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.

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 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

Global Warming Projections According to IPCC Scenarios
ScenarioDescriptionWarming in 2100Major Consequences
SSP1-1.9Ambitious climate actions
Carbon neutrality around 2050
1.4°C to 1.8°CLimited impacts, adaptation possible
SSP1-2.6Moderate sustainable development
Net zero emissions after 2050
1.7°C to 2.8°CModerate risks, ecosystems under pressure
SSP2-4.5Continuation of current trends
Stabilization of emissions around 2050
2.1°C to 3.5°CModerate to high risks
SSP3-7.0Uneven development and competition
Continuous emissions until 2100
2.8°C to 4.6°CHigh to very high risks
SSP4-6.0Pronounced inequalities
High-emission technologies
2.5°C to 4.2°CHigh risks, uneven adaptation
SSP5-8.5Strong development of fossil fuels
Intensive economic growth
3.3°C to 5.7°CCatastrophic consequences

Source: IPCC, AR6 Report, 2021; CMIP6 Scenario Database; NASA Climate Change.

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:

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, 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.

Imminent Climate Tipping Points (2025-2050) According to the IPCC
Climate SystemTriggering ThresholdMajor ConsequencesCurrent Status (2025)
Melting of the Greenland ice sheet1.1°C - 1.5°CSea level rise of 7 meters (over several centuries), disruption of ocean currentsAccelerated mass loss: 5,000 Gt/year of melted ice
Weakening of the AMOC (Atlantic current)1.4°C - 2°CWinters 5 to 10°C colder in Europe, disruption of monsoons, sea level rise on the US East CoastSlowing of 15% since 1950
Disappearance of summer Arctic sea ice1.5°C - 2°CAcceleration of warming (reduction of albedo), disruption of polar ecosystems, methane release40% reduction in area since 1979
Thawing of permafrost1.5°C - 2°CRelease of 200 to 400 Gt of carbon (CO₂ and CH₄) by 2100, amplifying warmingAlready observed in Siberia and Alaska with increasing methane emissions
Transformation of the Amazon rainforest into savanna2°C (locally +4°C)Release of 200 Gt of CO₂, loss of biodiversity, disruption of the water cycle17% deforested (critical threshold estimated at 20-25%)
Collapse of the West Antarctic Ice Sheet1.5°C - 2°CSea level rise of 3 to 5 meters (over several centuries)Accelerated melting, notably of the Thwaites Glacier ("Doomsday Glacier")

Where Are We Today?

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?

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

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