China saw a decline in overall levels of health-harmful particulate pollution (PM 2.5) in 2024 as compared to 2023 thanks to ardent mitigation efforts of leading pollution sources.

But India remained a global air pollution hotspot, while wildfire activity led to above average PM 2.5 levels in Canada, Siberia and central Africa, according to the latest Air Quality and Climate Bulletin of the World Meteorological Organization (WMO), released on Friday.

Highest rise in the Amazon  

The biggest anomaly, however, was in the Amazon basin where dramatic increases in air pollution, as compared to 2023 levels, were driven by record wildfires and drought-fuelled fires in northern Latin America. Both wildfires and droughts are being worsened by climate change.

Wildfires are a big contributor to particle pollution and the problem is expected to increase as the climate warms, posing growing risks for infrastructure, ecosystems and human health, warns the new WMO bulletin. It also underlines the “vicious cycle” that global warming is exacerbating.

As its title suggests, the report traces the complex interplay between air quality and climate, highlighting the role of tiny particles called aerosols in wildfires, winter fog, shipping emissions and urban pollution in climate trends – mainly warming, but some cooling as well. It stresses the need for improved atmospheric monitoring and more integrated policies to safeguard human and environmental health and reduce agricultural and economic losses.

“Climate change and air quality cannot be addressed in isolation. They go hand-in-hand and must be tackled together in order to protect the health of our planet, our communities, and our economies,” WMO Deputy Secretary-General Ko Barrett said.

A ‘complicated’ picture

WMO experts described the bulletin as presenting a “complicated” picture as there were both reasons for cheer and gloom.

The bulletin highlights the leading sources of PM2.5 releases as transport, industry, agriculture, wildfires and wind-blown desert dust. While not the only dangerous air pollutant, PM2.5 is a leading health hazard as the tiny particles, that are 1/28 of the width of a human hair, or smaller, penetrate deep into the lungs, the blood stream, and even the brain, increasing risks of heart attack, stroke, dementia and pre-natal conditions, as well as lung disease and cancers.

Both the burning fossil fuels and biomass also lead to large releases of black carbon, methane and nitrous oxide, the latter a precursor of  ground-level ozone. These “super pollutants” accelerate the effects of climate change causing the planet to even warm faster than CO2 emissions alone. The bulletin terms this as a “vicious cycle” when combined with climate change pressures.

“Climate impacts and air pollution respect no national borders – as exemplified by intense heat and drought which fuels wildfires, worsening air quality for millions of people. We need improved international monitoring and collaboration to meet this global challenge,” Barrett said.

In the Indo-Gangetic plain where nearly 900 million people live, air pollution is also worsening winter fog both in intensity and in length.

“Persistence of fog is no longer a simple, seasonal weather event – it is a symptom of escalating human impact on the environment. Addressing this requires comprehensive strategies, such as enforcing regulations on agricultural residue burning, and promoting cleaner energy for cooking, heating, lighting and public transport systems,” the WMO stated.

Spotlight on aerosols 

The report placed special emphasis on aerosols, another term for tiny airborne particles of solids or liquids. Aerosols can have both a warming or a cooling affect depending on their composition.

Darker ones, such as the black carbon that is released from incomplete combustion of diesel fuel or biomass, can warm the atmosphere and accelerate ice and glacier melt by absorbing more radiant heat from sunlight. But the brighter aerosols such as sulphates tend to have a temporary cooling effect as they reflect solar radiation back to space before returning to the earth’s surface in the form of acid rain and snow.

In 2020, UN agency International Maritime Organization (IMO) put regulations in place capping the use of sulphur in shipping fuel. Reduced sulphur dioxide (SO2) emissions by vessels plying the world’s oceans has translated into lower atmospheric PM2.5 levels and a comparative decline in related impacts such as premature mortality as well as childhood asthma cases, particularly in South Asia and Africa, said Lorenzo Labrador, WMO’s scientific officer.

It also has had an unintended environmental consequence.

SO2 and other sulphur-containing aerosols were previously making cloud cover brighter and thus helped the clouds reflect more light into space, cooling off temperatures, Labrador explained.

“So that [reduction in SO2 emissions] results, or translates into a very slight increase in the temperature of 0.04 degrees in 2025 so what we have here, and this is very important to emphasize, is not an increase in temperature due to aerosols, but rather an unmasking of the true warming of greenhouse gasses as a result of the offset that these aerosols were having,” said Lorenzo Labrador, WMO’s scientific officer.

Bulletin underscores the importance of monitoring

The models draw on data from the European Union’s Copernicus Atmosphere Monitoring Service (CAMS), NASA’s Global Modeling and Assimilation Office (GMAO), and the Finnish-based System for Integrated Modelling of Atmospheric Composition (SILAM).

This is the first time that the WMO experts used estimates from three different amospheric models for its reporting, and while there were minor differences, all models had the same conclusion.

The Bulletin also underscored the importance of ramping up the atmospheric monitoring infrastructure, especially in developing regions. While satellites do provide critical insights for the globe, ground-based monitoring networks are also essential to validate that data. In developing countries, and particularly Africa, such infrastructure remains sparsely distributed, WMO experts said.

WMO experts also drew attention towards the positive finding of the report. “When we see that countries or regions or cities are taking measures to fight against bad air quality, it works, and we see in many areas, an improvement of the air quality,” said Paolo Laj who is the Chief of Global Atmosphere. “In regions where these measures have been taken, there is a great improvement of the air quality,” he said.

Image Credits: Mike Newbry/ Unsplash, WMO.