Causes of climate change

The natural greenhouse effect and its perturbations by human activities
Current energy flows in W/m2

Note: the Earth is constantly receiving energy from the sun. The part of this energy which is not reflected by the atmosphere, particularly the clouds or the earth's surface (oceans and continents), is absorbed by the earth's surface which warms up by absorbing it. On the other hand, the surfaces and the atmosphere emit infrared radiation that intensifies as the surfaces grow warmer. Some of this radiation is absorbed by certain gases and by clouds and then re-emitted towards the surface, which contributes to warming it. This phenomenon is called the greenhouse effect.
Sources: from Météo-France; IPCC, 1st working group, 2021

The growth in the atmospheric concentration of GHG due to anthropogenic emissions (see glossary) increases the radiation of energy towards the ground, leading to an imbalance in the Earth’s radiative balance and causing a rise in the Earth’s surface temperature. The change, relative to a reference year, in radiation due to a climatic factor is known as radiative forcing. A positive radiative forcing value indicates a contribution to global warming. The total anthropogenic radiative forcing amounts to + 3.8 W/m2 for GHGs and - 1.1 W/m2 for aerosols in 2019 compared to 1750, for a net total of + 2.7 W/m2.

Greenhouse gases (GHG)

Excluding water vapor, GHGs occupy less than 0.1% of the atmospheric volume. Water vapor, which fluctuates between 0.4 and 4%, is the main greenhouse gas. Human activities have very little direct impact on fluctuations in its concentration, but have a strong impact on the concentrations of other GHGs.

	CO2 Carbondioxide	CH4 Methane	N2O Nitrous oxide	HFC Hydrofluorocarbons	PFC Perfluorocarbons	SF6 Sulfur hexafluoride	NF3 Nitrogen trifluoride
Atmospheric concentration 2021 (in 2005 in brackets)	415 ppm (379 ppm)	1 896 ppb (1 774 ppb)	334 ppb (319 ppb)	25 ppt (>49 ppt)	92.8 ppt (>4.1 ppt)	10.6 ppt (5.6 ppt)	2.5 ppt (0 ppt)
Global warming potential (cumulative over 100 years)	1	27-30	273	[1.5; 14 590] depending on the gas	[7 380; 12 400] depending on the gas	25 184	17 423
Source of anthropogenic emissions	Fossil fuel combustion, industrial processes and tropical deforestation	Landfills, agriculture, livestock and industrial processes	Agriculture, industrial processes, fertilizer use	Sprays, refrigeration, industrial processes			Manufacture of electronic components
Change in radiative forcing in 2021* since 1750 by anthropogenic emissions ( W/m2) (in 2005 in brackets)	+2.14 (+1.66)	+0.53 (+0.48)	+0.21 (+0.16)	+0.05 (+0.02)

* In 2019 for HFC, PFC, SF6, NF3 gases.
Note: ppm= part per million; ppb= part per billion; ppt= part per thousand billion.
Sources: IPCC, 1st Working Group, 2013, 2021; Agage, 2021; NOAA, 2022

The global warming potential (GWP, see glossary) is the ratio between the energy returned to the ground in 100 years by 1 kg of gas and that which would be returned by 1 kg of CO2 . 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 27 to 30 kg of CO2 in the century following its emission. While CO2 has the smallest global warming potential, it has contributed the most to global warming since 1750, due to the large amounts emitted.

GHG reservoirs and fluxes: example of CO2 over the years 1750-2019 and 2011-2020

Note: this graph shows: (i) in square brackets, the size of the reservoirs in pre-industrial times in billions of tons of CO2 in black and their cumulative variation over the period 1750-2019 in red; (ii) in the form of arrows, the carbon fluxes between the reservoirs in billions of tons of CO2 per year(see glossary) Pre-industrial fluxes are in black. Those linked to anthropogenic activities between 2011 and 2020 are in red.
Sources: from IPCC, 1st Working Group, 2021; Friedlingstein et al, Global Carbon Budget 2021, 2022

Four major reservoirs store carbon in different forms:

• atmosphere: CO2 gas;
• terrestrial biosphere: soils and vegetation of forest, agricultural and peatland ecosystems...
• ocean: limestone, dissolved CO2 ; marine fauna and flora (plankton);
• subsoil: rocks, sediments, fossil fuels.

The carbon flows between these reservoirs come from the natural carbon cycle to which are added the disturbances linked to anthropogenic CO2 emissions (notably the combustion of fossil organic carbon reserves) which modify the exchanged flows or create new ones.

Imbalance between emissions and CO2 storage capacity
Net annual flows of anthropogenic CO2 averaged over the period 2011-2020 (emissions to the atmosphere and removals 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 plotted.
Source: Friedlingstein et al, Global Carbon Budget 2021, 2022

Over the last ten years (2011-2020), of the 39 Gt CO2 generated on average per year by human activities, the atmosphere has absorbed 19 Gt CO2, the terrestrial reservoirs (vegetation and soils) 11 Gt CO2 and the oceans 10 Gt CO2. The atmosphere is the reservoir most affected by anthropogenic activities: it has absorbed nearly 50% of the amount of carbon emitted over the past 60 years.

Role of the forest cycle on a global scale

On a global scale, forest lands are a carbon sink. The gross sink attributed to the terrestrial biosphere – that is, primarily forests – offsets 29% of annual anthropogenic carbon emissions, or about 11 Gt CO2 (Friedlingstein et al., 2022). Forests that remain forests are therefore carbon sinks. By integrating deforestation (forest land converted to other uses), the forestry sector becomes a source of carbon. Indeed, deforestation leads to emissions related to the loss of forest carbon stocks through the combustion and decomposition of organic matter. These net emissions (from forested lands in particular) represent about 14% of annual anthropogenic carbon emissions worldwide (IPCC, 2022).

In France, the net carbon sequestration in forest biomass is estimated at about 30.4 Mt CO2 eqfor the year 2020, while the sink in wood products is 0.8 Mt CO2 eq. In total, forests and wood products sequestered 8% of national GHG emissions (excluding land use, land use change and forestry, LULUCF, see glossary) - (Citepa, 2022).

Atmospheric CO2 concentration

Source: CMDGS, under the aegis of WMO, 2020

Since the development of industrial activities, land and ocean reservoirs have absorbed more than half of anthropogenic emissions. The remaining emissions persist in the atmosphere, causing GHG concentrations to increase.