Causes of climate change

The natural greenhouse effect and its disruption by human activities

Current energy flows in W/m²

Note: the Earth constantly receives energy from the sun. The part of this energy that is not reflected by the atmosphere, such as clouds or the earth's surface (oceans and continents), is absorbed by the earth's surface, which heats up by absorbing it. In return,surfaces and the atmosphere emit infrared radiation, the hotter the surface, the more intense the radiation. Some ofthis radiation is absorbed by certain gases and clouds, then re-emitted towards the surface, helping to warm it. This phenomenon is known as the greenhouse effect.
Sources : from Météo-France; IPCC, 1^st working group, 2021

The increase in the atmospheric concentration of GHGs through anthropogenic emissions (see glossary) increases the emission of energy to the ground, leading to an imbalance in the Earth's energy balance and a rise in its surface temperature. The change, relative to a reference year, in the radiation induced by an element is called radiative forcing. A positive radiative forcing indicates a positive contribution to global warming. Net anthropogenic radiative forcing was zero in 1750, +0.6 W/m² in 1950 and +2.3 W/m² in 2011. By 2024, it was +3 W/m² (Indicators of Global Climate Change, 2025).

Greenhouse gases (GHG)

The main greenhouse gas is water vapor, which fluctuates between 0.4% and 4% of atmospheric volume. Human activities have very little direct impact on fluctuations in water vapor concentration. On the other hand, they do have a strong impact on concentrations of other GHGs, which occupy less than 0.1% of atmospheric volume. This is enough to destabilize the climate.

	CO2 Carbondioxide	CH4 Methane	N2O Nitrous oxide	HFC Hydrofluorocarbons	PFC Perfluorocarbons	SF6 Sulfur hexafluoride	NF3 Nitrogen trifluoride
Atmospheric concentration 2025 (in 2005 in brackets)	426  ppm (379  ppm)	1,934 ppb (1,774 ppb)	338 ppb (319 ppb)	321 ppt (> 49 ppt)	117 ppt (> 4.1 ppt)	12 ppt (5.7 ppt)	3.4 ppt (0 ppt)
Global warming potential (cumulative over 100 years)	1	28	273	[< 1 ;12,400] depending on gases	[< 1 ; 11,100] depending on gases	24,300	17,400
Source of anthropogenic emissions	Fossil fuel combustion, industrial processes, deforestation	Agriculture (livestock), energy processes, landfills	Agriculture (fertilizers), industrial processes	Sprays, refrigeration, industrial processes			Manufacture of electronic components
Change in radiative forcing in 2024 since 1750 due to anthropogenic emissions (W/m2) (in 2011 in brackets)*	+ 2.33 (+ 1.8282)	+ 0.57 (+ 0.4848)	+ 0.23 (+ 0.177)	+ 0.05 (+ 0.02)

* In 2019 (and 2005) for HFC, PFC, SF₆ and NF₃ gases.
Note : ppm = parts per million; ppb = parts per billion; ppt = parts per thousand billion.
Sources: IPCC, 2021; Indicators of Global Climate Change, 2025; NOAA, 2025

The global warming potential (GWP, see glossary) is the ratio between the energy returned to the ground by 1 kg of gas and that which would be returned by 1 kg of CO₂ over a given period. It depends on the radiative properties and lifetimes of gases in the atmosphere. For example, 1 kg of methane (CH₄) will warm the atmosphere as much as 28 kg of CO₂ in the century following its emission, and as much as 84 kg of CO₂ in the first twenty years. Although CO₂ is the gas with the lowest global warming potential, it has contributed the most to global warming since 1750, due to the large quantities emitted.

Atmospheric carbon dioxide (CO₂) concentration

Source : US National Oceanic and Atmospheric Administration (NOAA), 2025

CO₂ is the main anthropogenic greenhouse gas. Its atmospheric concentration has been rising steadily for several decades, reaching 426 ppm in early 2025. To limit global warming to 2°C compared with the pre-industrial era, the average concentration must not exceed 450 ppm.

Atmospheric methane (CH₄) concentration

Note: annual averages of air samples taken from the world's marine surfaces.
Source: US National Oceanic and Atmospheric Administration (NOAA), 2025

The average atmospheric concentration of methane in 2025 is 1.94 ppm (1,935 ppb), some 217 times less than that of CO₂. However, its global warming potential (GWP, see glossary and p. 13) is much higher than that of CO₂. More than a quarter of global warming since pre-industrial times is attributable to methane. The increase in methane emissions has accelerated in recent years, including during the Covid-19 pandemic.

Average annual net flows of anthropogenic CO₂ since 2014

Emissions to the atmosphere and uptake by terrestrial and oceanic reservoirs

Note: the uncertainty for the increase in atmospheric CO₂ concentration is very small (± 0.02 Gt CO₂/year) and has not been represented on the graph. The sum of the sources is not equal to the sum of the sinks, leaving a budget imbalance that reflects the imperfection of the data and the research still needed to better understand the carbon cycle.
Source: Global Carbon Budget, 2025

Over the last decade (2014-2023), of the 40 Gt CO₂ generated on average per year by human activities, the atmosphere has absorbed almost half, the terrestrial reservoirs (vegetation and soils) a quarter and the oceans a quarter. The atmosphere is therefore the reservoir most affected by human activity, contributing to the greenhouse effect.

On a global scale, forest land is a major carbon sink. By integrating deforestation and, to a lesser extent, forest fires and degradation, the forestry sector becomes a source of carbon. These phenomena generate emissions linked to the loss of forest carbon stocks through the combustion and decomposition of organic matter. Emissions due to deforestation represented 6.2 Gt CO₂ per year on average in 2014-2023, or 15% of annual anthropogenic emissions (see p. 63 for information on forest carbon sinks in France).