When Pakistan faced appalling floods in June this year, global attention focused on climate change as the culprit. The country had three times the usual rainfall in its summer monsoon, exacerbated by short spikes of extremely heavy rain. Riverbanks burst and more than 1,600 people died. Formal attribution studies and politicians alike blamed global warming for making such an event much more likely. Something else should have been mentioned, too: aerosols.

Aerosols are the miasma of soot (black carbon), sulfur dioxide, organic carbon and other compounds that drives poor air quality over many of the world’s most-populated regions. Studies show that aerosols strongly affect the likelihood of extreme precipitation events¹, such as those that contributed to Pakistan’s floods, and many other climate hazards.

Worse, it is not clear whether aerosols are set to rise, fall or stabilize. The amount of uncertainty about aerosol levels by 2050 is as large as the total increase since pre-industrial times (see ‘Drastic uncertainty’). Over the next 20–30 years, we might — or might not — see aerosol-driven climate changes as large as those that have played out over the past 170 years, adding as much as 0.5 °C to global warming. That could rapidly change the likelihood of extreme events occurring in many regions.

Yet the impacts of aerosols on climate risk are often ignored. The issue was not on the official agenda of the 27th United Nations climate conference (COP27) in Sharm-El-Shaikh, Egypt. This neglect must end.