One of the particularities of this new IPCC AR6 report released on the 9^th of August is to analyse the effect of air quality improvement policies in climate projections and that of climate change mitigation policies on air quality. The former will act directly on aerosols, their precursors and ozone precursors, while the latter will aim at reducing methane and, by tackling activities emitting greenhouse gas and primarily CO₂, will also lead to an indirect reduction in pollutants co-emitted by these activities. The five illustrative scenarios used in this report cover a wide range of warming by the end of this century from a warming centred around 1.4°C to 4.4°C. These trajectories are mainly determined by the evolution of future CO₂ emissions. The evolution of other greenhouse gases and aerosols will nevertheless modulate these trajectories. In all 5 scenarios, future changes in emissions of short-lived drivers are expected to increase global warming. This additional warming would be caused either by the reduction of aerosols – whose net effect is cooling – to improve air quality, or in the case of scenarios based on more lax assumptions, by an increase in methane, ozone or hydrofluorocarbons (HFCs).

“We state at the end of the Summary for Policymakers that decarbonising our economies and thus moving away from fossil fuels implies benefits for air quality. This is mechanical and it is an established fact. But it is not enough in many parts of the world to reach a level of air quality corresponding to the WHO recommendations, and pollution reduction policies must be added to this,” says Sophie Szopa. Policies that are implemented or considered to reduce air pollution, and thus aerosol concentrations, are causing additional warming. However, the climate is not limited to temperature and aerosols also have an effect on precipitation. Reducing their quantity reduces their impact on the hydrological cycle, in addition to the enormous public health benefit of reducing them in anthropised regions. “The methane lever can partially compensate for this,” points out Sophie Szopa. Because of the short life span of methane and aerosols, their respective impacts on temperature can be partially balanced. Methane is a short-lived greenhouse gas but it is also involved in the formation of ground-level ozone and thus linked to air quality issues. Strong, rapid and sustained reductions in methane emissions will have a beneficial impact on both global warming and air quality.

“From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO₂ emissions, reaching at least net zero CO₂ emissions, along with strong reductions in other greenhouse gas emissions. Strong, rapid and sustained reductions in CH4 emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.” – IPCC, Summary for Policy Makers

“It is important to bear in mind that we can be in scenarios with high greenhouse gas emissions but low emissions of polluting compounds, because these hypotheses are taken almost independently from one another. Countries can continue to develop economically through fossil fuels but choose to improve the health of their citizens by emphasising policies to reduce these pollutants,” adds the researcher. This is why the scenarios assessed by the IPCC take into account a wide range of possibilities regarding the evolution of emissions of these short-lived pollutants.

The impact of these pollutants on human health is undeniable. But regarding the evolution of their concentration in the air, there are still several possible scenarios depending on the extent of air pollution reduction policies. “It depends on the narratives behind each of the scenarios, which is why on this issue of pollution there is an obvious interest in not having RCP-type scenarios as previously, but SSP-type scenarios, taking into account the socio-economic issues and the choices made by countries,” concludes Sophie Szopa. The report also highlights that scenarios combining a reduction in greenhouse gases and pollution show a better improvement in air quality in the medium term (2040 and beyond).

What is the IPCC ?

Created in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), the objective of the IPCC is to provide governments at all levels with scientific information that they can use to develop climate policies. IPCC reports are also a key input into international climate change negotiations. The IPCC is an organization of governments that are members of the United Nations or WMO. The first working group (WG1) of the six assessment report (AR6) published its assessment on the physical science basis of our climate on the 9^th of August 2021. It is the work of 234 international researchers that have analysed the entirety of science literature on the subject. The two following working groups on the AR6 will render their assessment in 2022 respectively on impacts, adaptation and vulnerability and finally mitigation.

To go further:

Find out more on volatile organic compounds with Juliette Lathière: Biogenic VOC, (mysterious) ingredient of the atmosphere’s recipe

What are short-lived climate drivers? Learn all about it with Didier Hauglustaine: Cocktail of short-lived gases

Access all articles of the series Détour vers le futur – L’IPSL à l’heure du GIEC