Air quality is improving worldwide. This is good for health, but particulate matter also acts as a cooling sunscreen. Less particulate matter means we notice more warming from greenhouse gases.

Effect of Particulate Matter on Climate 

Particulate matter has a cooling effect on weather and climate. This is because particulate matter particles (also known as aerosols) reflect sunlight and create whiter -- and thus more reflective -- clouds. The more aerosols, the more sunlight is reflected back into space, and thus the cooler the Earths surface. Particulate matter, as polluting as it is, acts somewhat like a sunscreen. 

Aerosols Mask a Part of Warming 

When fossil fuels are burned, sulfur dioxide (SO2) is released alongside carbon dioxide (CO2). While CO2 has a warming effect as a greenhouse gas, SO2 has a cooling effect because it forms sulfate aerosols in the atmosphere. 

The cooling effect of aerosols grew significantly after World War II when industrialization increased sharply. This is the main reason that during those decades, the globally averaged temperature hardly changed: the cooling effect of the increase in particulate matter roughly matched the warming effect of more greenhouse gases. 

The warming effect of CO2 is ultimately stronger, especially since CO2 remains in the atmosphere much longer than aerosols. Therefore, the amount of CO2 in the air continues to accumulate due to our ongoing emissions, while the sulfur emitted a month ago has already been absorbed by the Earths surface. Since the late 1970s, warming has clearly taken precedence (see image 1). 

However, the cooling effect of aerosols is still substantial: this has estimated that the Earths surface is half a degree cooler than it would otherwise be. In other words, without the aerosol effect, it would not be 1.5°C warmer than in pre-industrial times, but 2°C warmer. Aerosols have thus masked part of the warming. 

Cleaner Air = Warmer Climate 

Due to the harmful effects of air pollution, measures were taken in Europe and North America starting in the 1970s to reduce emissions. Power plants had to filter sulfur from their flue gases, automotive fuel had to contain less sulfur, and cars had to be equipped with a catalyst to remove nitrogen oxides (NOx) from exhaust gases. Meanwhile, air quality is also noticeably improving in China. 

As every disadvantage has its advantage, every advantage has its disadvantage: Cleaner air leads to less reflected sunlight (fewer aerosols and fewer clouds) and thus less cooling. This allows a larger part of the warming from greenhouse gases to emerge. The decrease in particulate matter over the past ~20 years has revealed approximately 0.14 degrees of warming (see image 1). Recently, there may have been additional warming due to the reduction of sulfur emissions from international shipping.