Atmospheric greenhouse gas concentrations

The total concentration of all greenhouse gases and other forcing agents, including cooling aerosols, reached 460 parts per million CO2 equivalents in 2019. This is already within the range of the peak level that the Intergovernmental Panel on Climate Change states should not be exceeded if — with a 67% likelihood — the global temperature increase is to be limited to 1.5oC above pre-industrial levels by 2100. The peak concentrations corresponding to a temperature increase of 2oC by 2100 could be exceeded around 2030.

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This indicator assesses the global atmospheric concentration of all greenhouse gases and forcing agents, and checks 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 recent agreement in Glasgow has been to ‘drive efforts to limit the temperature increase even further 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), total CO2e reached 460 ppm in 2019, which is about 52 ppm more than 10 years ago (Figure 1), and about 180 ppm more than in pre-industrial times. 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), especially CO2, the annual concentration of which reached 410 and 412 ppm in 2019 and 2020, respectively, or more than 130 ppm (+147%) above pre-industrial levels . As a group, the gases covered by the Montreal Protocol (MPGs) contributed about 31 ppm to climate forcing in 2019. The non-protocol gases (NPGs) have a net cooling effect overall. In 2019, this effect amounted to nearly 55 ppm CO2e, and as such, compensated for about 23% of the forcing induced by other greenhouse gases. The forcing (cooling) trend of NPGs is decreasing 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 the chances of staying below specific increases in temperature compared with pre-industrial levels. 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 conservative peak and 2100 concentration levels — those corresponding to a 67% chance of staying below target values — 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, at the present decadal growth rate of about 5 ppm per year, the peak concentration for limiting the increase to 1.5°C was exceeded around 2020. In the case of the 2°C limit, the peak concentration will be reached around 2028. Taking into account uncertainty ranges (see supporting information), peak concentrations will be reached within 0-5 years (for +1.5°C) or from 2-15 years (for +2°C) (compared to 2019).

Supporting information

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