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The total concentration of greenhouse gases and other forcing agents, including cooling aerosols, reached 465 parts per million CO2 equivalents in 2020. This is around the peak level that the International Panel on Climate Change states 'should not be exceeded if — with a 67% likelihood and not allowing a temperature overshoot — the global temperature increase is to be limited to 1.5oC above pre-industrial levels'. When allowing for a temperature overshoot, the peak level could be exceeded in 2024. The peak concentrations corresponding to a temperature increase of 2oC by 2100 could be exceeded between 2027 and 2030.
This indicator assesses the total global atmospheric concentration of all greenhouse gases and forcing agents, and evaluates how the status of and trend in that concentration relate to scientific knowledge and policy ambitions for limiting global temperature increase at the end of the century. The objective of the 2015 Paris Climate Agreement is ‘to hold the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels’ . An outcome of the agreement in Glasgow (2021) and Sharm el-Sheikh (2022) has been to ‘pursue efforts to limit the temperature increase to 1.5oC’. It is important to consider all gases and other forcing agents using the so-called ‘CO2 equivalent’ (CO2e); (see supporting material for details). Note that some of the gases, such as sulphate aerosols, have a negative forcing (i.e. a cooling effect).
Considering all greenhouse gases and other forcing agents (including aerosols), the total CO2e concentration reached 465 ppm in 2020, which is about 49 ppm more than 10 years ago (Figure 1), and about 185 ppm more than in pre-industrial times. The rate of increase has stabilised over the last 5 years at 4.7ppm per year. Assessing the contribution of the various groups of greenhouse gases has shown that by far the most forcing is caused by gases covered by the Kyoto Protocol (KPGs). The annual average concentration of CO2 reached 412 and 414ppm in 2020 and 2021, respectively (+130ppm or +147% above pre-industrial levels , while the average concentration of CH4 reached 1,874ppb in 2020 (plus 1,138ppb ot +248%). As a group, the gases covered by the Montreal Protocol (MPGs) contributed about 31ppm to climate forcing in 2020. The non-protocol gases (NPGs) have a net cooling effect overall. In 2020, this effect amounted to nearly 54ppm CO2e, and as such, compensated for about 22% of the forcing induced by other greenhouse gases. Note that the forcing (cooling) trend of NPGs has been decreasing since 2010, especially due to the declining indirect effect of sulphur dioxide (through its cloud interaction) .
Pathways developed by the IPCC show concentrations of atmospheric greenhouse gases in relation to specific temperature increases. These pathways show (1) peak concentrations that should not be exceeded to ensure that (2) CO2e concentrations in 2100 remain compatible with limiting the temperature increase to 1.5°C or 2°C above pre-industrial levels. According to the IPCC’s most precautionary peak and 2100 concentration levels — those corresponding to a 67% chance of staying below target values without allowing a temperature overshoot in that period — global greenhouse gas concentrations most not exceed 465 (range 445-485) ppm CO2e and should return to 411 (390-430) ppm by 2100 to limit the increase to 1.5°C; for the 2°C limit, the corresponding values are 505 (470-540)ppm and 480 (460-500) ppm CO2e, respectively.
Given these numbers and a 2020 concentration of 465ppm CO2e, the peak concentration threshold for limiting the increase to 1.5°C was exceeded in 2020 (Figure 2). When allowing a temporary temperature overshoot and considering the present decadal growth rate, the peak concentration threshold could be exceeded around
2024. So, there are few years left to stabilise the concentration, but concentrations must reduce even more after 2024. In the case of the 2°C limit, the peak concentration will be reached around 2027-2030. Taking into account uncertainty ranges (see supporting information), peak concentrations will be reached within 0-7 years (for +1.5°C) or from 1-14 years (for +2°C) (compared to 2020).