Human-caused emissions of aerosols – tiny, light‑scattering particles produced mainly by burning fossil fuels – have long acted as an invisible brake on global warming.

Human-caused emissions of aerosols – tiny, light‑scattering particles produced mainly by burning fossil fuels – have long acted as an invisible brake on global warming. 

This is largely because they absorb or reflect incoming sunlight and influence the formation and brightness of clouds. 

These combined effects act to lower regional and global temperatures.

Aerosols also have a substantial impact on human health, with poor outdoor air quality from particulate matter contributing to millions of premature deaths per year.  

Efforts to improve air quality around the world in recent decades have reduced aerosol emissions, bringing widespread benefits for health.

However, while cutting aerosols clears the air, it also unmasks the warming caused by carbon dioxide (CO2) and other greenhouse gases (GHGs). 

In this explainer, Carbon Brief unpacks the climate effects of aerosols, how their emissions have changed over time and how they could impact the pace of future warming.

Key points include:

• Clean air rules are driving a rapid decline in sulphur emissions around the world. Global sulphur dioxide (SO2) emissions have fallen by around 40% since the mid‑2000s.
• There is around half a degree of warming today that is “hidden” by aerosols. Without the cooling from sulphate and other aerosols, today’s global temperature would already be close to 2C above pre‑industrial levels, rather than the approximately 1.4C the world is currently experiencing.
• Chinese SO2 emissions have fallen by more than 70% between 2006 and 2017 as the national government has brought in a series of air-pollution measures. These declines have added around 0.06C to global warming since 2006. 
• Shipping’s low‑sulphur fuel rules have added to recent warming. The International Maritime Organization’s (IMO’s) 2020 cap on marine‑fuel sulphur has already warmed the planet by an estimated 0.04C, albeit with a wide range of estimates across published studies.
• Roughly one‑quarter of the increase in global temperature over the past two decades stems from this unmasking of human-caused heat. Altogether, recent aerosol cuts may have contributed ~0.14C of the ~0.5C of warming the world has experienced since 2007.
• By unmasking warming from CO2 and other GHGs, aerosols have flipped from reducing the rate of decadal warming (as emissions increased) to increasing the rate of warming (as emissions decreased) after 2005.
• Sulphate and other aerosols are a major component of PM2.5 air pollution, which has been linked to millions of premature deaths each year.
• Most future‑emissions pathways project continued aerosol declines. Unless methane and other short-lived GHGs fall at the same time, the rate of warming could accelerate in the coming decades even if CO2 emissions plateau.