Share of non-CO2 pollutants contributing to global warming almost as much as carbon dioxide: Study

Global temperatures are likely to exceed 1.5 degrees Celsius over preindustrial levels by 2035 and 2°C by 2050 if the focus is merely on decarbonisation efforts, according to a new study.

But this fate can be averted by simultaneously devising strategies targeting CO2 and non-CO2 pollutants like methane, black carbon, hydrofluorocarbons (HFC), tropospheric ozone and nitrous oxide, according to the study published in the journal PNAS. 

Currently, climate mitigation efforts target only CO2 emissions although the ill impacts of the non-CO2 counterparts are well-known, the study argued.

The Working Group III report of the Intergovernmental Panel for Climate Change (IPCC), which deals with mitigating climate change, focuses on CO2 and a few greenhouse gases, but excludes other non-CO2 pollutants, the study highlighted.

“We should continue to decarbonise the economy, but that by itself is not enough to reduce the dangers of climate change in the near term,” Veerabhadran Ramanathan, a Distinguished Professor at Scripps Institution of Oceanography at UC San Diego and UNESCO Professor of Climate and Policy at TERI University, New Delhi, told Down To Earth (DTE). He is one of the authors of the study.

“We need to urgently bend the emission curve of methane, HFCs, black carbon soot and a few other precursor gases that increase lower atmosphere ozone,” he added.

The team reviewed IPCC Working Group I (WGI) 2013 and 2021 reports, which evaluate the physical science behind climate change over the years as well as that projected for the future. 

The team also analysed historical emissions from 1750-2019.

The share of non-CO2 pollutants contributing to global warming is almost as much as carbon dioxide, according to their analysis.

This agrees with previous IPCC WGI reports, which have shown that the contribution of CO2 and non-CO2 greenhouse gases to global heating was 52-57 per cent and 43-48 per cent, respectively, the study stated.

The researchers also considered aerosols — small, solid particles or liquid droplets that float in the air — released from fossil fuel sources.

Some aerosols are known to warm the planets, while others have the opposite effect. Cooling aerosols include primarily sulfates, nitrates and organic carbon, according to other studies.

The expert added:

What was surprising is that when we account for co-emitted air pollution aerosols by fossil fuels, warming from non-CO2 greenhouse gases and black carbon soot was close to 80 per cent.

Relying on decarbonisation alone could take global temperatures beyond the 2°C mark in 2045, their analysis showed.

Though decarbonisation efforts will lower the levels of CO2, the warming caused will not go down in the near term next 25 years, Ramanathan explained. 

Cooling aerosols from fossil fuel sources will also decline due to decarbonisation, the study noted.

Without tackling non-CO2 pollutants, Ramanathan warned, these gases will continue to trap heat and keep the warming above 1.5°C, as there are not many cooling aerosols to mask the warming.

Combining decarbonisation measures with targets can provide net cooling by 2030, bring down the rate of warming from 2030 to 2050 by roughly 50 per cent, and avoid the 2 degrees Celsius level, the study concluded.

Avantika Goswami, programme manager, climate change at the Centre for Science and Environment, a Delhi-based non-profit, told Down to Earth:

The study’s statement that mitigating both CO2 and non-CO2 pollutants reduces near-term warming and helps limit temperatures to 2C more easily is a vital finding.

She was not involved in the study.

Though CO2 is still the main focus due to its abundance and long life, mitigation of non-CO2 pollutants is gaining attention, she added, citing the recent Global Methane Pledge as an example. 

The Glasgow Climate Pact, an agreement signed during the 2021 United Nations Climate Change Conference (CoP26), recognised the need to consider further actions to reduce non-carbon dioxide greenhouse gas emissions, including methane, by 2030.

Goswami also pointed out a few concerns. Uncertainties around measuring non-CO2 pollutants such as methane and aerosols need to be addressed, she said.

The knowledge about their sources and mitigation methods needs to become mainstream, she explained.

The study also raises other questions. Speaking of methane alone, there is also the political question of who reduces methane: Coal-consuming and rice-producing countries in the Global South, or oil and gas-consuming countries in the North, she pointed out.

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