The total concentration of greenhouse gases and other forcing agents, including cooling aerosols, reached 472 parts per million CO2 equivalents in 2021. This is within the range of the peak level 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 before 2026. The peak concentrations corresponding to a temperature increase of 2oC by 2100 could be exceeded before 2032.

Figure 1. Observed trends in total greenhouse gas concentration levels between 1860 and 2021, considering all greenhouse gases and other forcing agents (including aerosols)
YearAll greenhouse gases including aerosols67% of probability staying below 1.5ºC67% probability of staying below 2ºC
1860285.71465505
1865285.39465505
1870285.06465505
1875285.66465505
1880286.25465505
1885287.05465505
1890287.84465505
1895287.74465505
1900287.65465505
1905285.81465505
1910283.98465505
1915286.67465505
1920289.38465505
1925291.48465505
1930293.6465505
1935293.71465505
1940293.83465505
1945295.36465505
1950296.89465505
1955294.51465505
1960294.21465505
1965293.28465505
1970295.15465505
1975304.02465505
1978310.5465505
1979313.01465505
1980315.63465505
1981318.36465505
1982320.96465505
1983324.07465505
1984327.07465505
1985330.14465505
1986333.28465505
1987336.61465505
1988341.02465505
1989344.31465505
1990346.87465505
1991351.27465505
1992354.38465505
1993357.41465505
1994361.32465505
1995365.56465505
1996369.73465505
1997373.21465505
1998378.4465505
1999383.15465505
2000386.76465505
2001388.68465505
2002391.04465505
2003394.13465505
2004396.3465505
2005398.65465505
2006401.22465505
2007403.58465505
2008406.5465505
2009408.36465505
2010411.29465505
2011416.62465505
2012422.22465505
2013428.57465505
2014434.3465505
2015440.46465505
2016446.25465505
2017450.76465505
2018455.67465505
2019460.6465505
2020468.32465505
2021472.35465505

This indicator assesses the combined 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), Sharm el-Sheikh (2022) and Dubai (2023) 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 information). 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 472ppm in 2021, which is about 50ppm more than 10 years ago (Figure 1), and about 192ppm more than in pre-industrial times. The increase was 4.0ppm, the lowest in the last 10 years. 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 414ppm and 417ppm in 2021 and 2022 respectively (+137ppm or +148% above pre-industrial levels , while the average concentration of CH4 reached 1,893ppb in 2021 (plus 1,173ppb or +265%). As a group, the gases covered by the Montreal Protocol (MPGs) contributed about 31ppm to climate forcing in 2021. The non-protocol gases (NPGs) have a net cooling effect overall. In 2021, this effect amounted to nearly 51ppm CO2e, and as such, compensated for about 21% of the forcing induced by other greenhouse gases. Note that the forcing (cooling) trend of NPGs has been decreasing since 2010, especially due to lower concentration of sulphur dioxide and its cooling effect . Further reading available here.

Figure 2. Peak and 2100 concentrations of total greenhouse gases in the atmosphere consistent with a 67% probability of keeping the average global temperature increase below 1.5°C (left) and 2°C (right)
Peak and 2100 concentrations of total greenhouse gases in the atmosphere consistent with a 67% probability of keeping the average global temperature increase below 1.5°C (left) and 2°C (right)

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.

As such the concentration in 2021 of 472ppm CO2e is close to the upper limit of the mentioned range of (most-precautional) peak concentration that should not be exceeded to limit the global temperature increase to 1.5°C above pre-industrial times (445-485 ppm CO2e) (figure 2). When allowing a temporary temperature overshoot and considering the present decadal growth rate, the peak concentration threshold could be exceeded at maximum around 2028. So, there are few years left to stabilise the concentration, but concentrations must reduce even more afterwards. In the case of the 2°C limit, the peak concentration will be reached around 2028-2031. Taking into account uncertainty ranges (see supporting information), peak concentrations will be reached within 0-8 years (for +1.5°C) or from 3-13 years (for +2°C) (after to 2021).