Clouds reflect sunlight back to space, which cools temperatures locally and across the planet. When aerosol particles in the atmosphere from both natural sources (such as biological emissions and sea spray) and human activities (such as from burning fossil fuels, wood and vegetation) mix into low clouds, particularly over the ocean, they can cause the clouds to brighten and thus reflect even more sunlight back into space. This has a cooling effect on the Earth’s climate.

At the same time that humans are continuing to release climate-warming greenhouse gases into the atmosphere, expanding air quality regulations are leading to declines in the concentration of atmospheric aerosol particles. This is beneficial for public health and local ecosystems — at the same time, because most of these particles have a cooling effect on our climate, removing them generally adds to climate warming.

Atmospheric scientists are still not certain just how much of a cooling effect these aerosol particles have on the climate overall. Scientists estimate that aerosols from human emissions are offsetting 0.5℃ of global warming from greenhouse gases, but they could be cooling as little as 0.2 ℃ or as much as 1.0 ℃ (Forster et al., 2021). The uncertainty in this number leads to uncertainty about how much warming to expect in the coming decades. As humans consider making changes to our aerosol emissions — either reducing them via regulation and technology, or pursuing intentional marine cloud brightening as a climate intervention — we need a stronger scientific understanding of these processes in order to both advance positive effects and limit unintended effects.