New observations of aerosol-polluted, low-lying clouds appear to refute the idea that the microscopic particulates increase the clouds’ overall water content, which would significantly boost their capacity to reflect sunlight back into space. While there’s no doubt that aerosols in low-level clouds can cause some cooling through an increase in the concentration of individual water droplets, researchers in the UK and Germany have found that this effect is moderated by a reduction in overall water content within clouds. The result, they say, should help to more accurately constrain future climate projections.

Scientists have known for many years that the release of microscopic aerosol particles into the atmosphere has the potential to change the properties of low-level clouds and, in turn, the amount of solar radiation that they reflect back into space. Two different factors are at play in this process. The first, which is backed up by strong observational evidence, is that the aerosols provide additional nuclei around which water can condense. This produces clouds that are made up of more numerous, but smaller, water droplets, and this increase in overall surface area typically causes the clouds to last longer and reflect more sunlight.

At the same time, the presence of aerosols in clouds is also believed to alter the total amount of water held within the cloud – but the exact nature of this effect has not been clear. Some researchers have suggested that aerosols act to suppress to formation of rain, which would significantly increase the total amount of water inside the clouds. Such an increase in water content would further boost the reflectivity and reinforce the cooling effect of aerosols, potentially mitigating the warming caused by the emission of greenhouse gases.

To test this hypothesis, meteorologist Velle Toll of the University of Reading and colleagues decided to directly compare the properties of aerosol-rich clouds with cleaner clouds nearby. They looked for localized sources of man-made aerosols, including industrial towns, coal-fired power stations and oil refineries, which generate aerosol-rich clouds in their downwind wake and form distinctive features known as pollution tracks that can be detected from space.

Similar phenomena are also created behind ships and downwind of volcanic eruptions, but these typically only reach heights of 1.5 and 2.5 km respectively. In addition, ship tracks in particular tend to be limited in their lateral extent, typically only being around 10 km wide. In contrast, pollution tracks from land-based, anthropogenic aerosol sources cover a wider range of clouds at different heights.

The team analysed 15 years’ worth of high-resolution satellite data recorded by NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) satellite, covering a total of 2400 tracks from different climate zones across the Earth. In common with earlier observations, the researchers found that the droplets were on average 30% smaller in pollution tracks than in unpolluted clouds.

Despite some variability in water content, their observations show that the polluted clouds had a slightly lower water content overall – and not the increase that some researchers had expected. This, they report, is because the increases in water content by lower levels of rain formation are counteracted by an increased evaporation of water from aerosol-rich clouds.

The team conclude that the cooling effect of aerosols as a result of the increased surface area is moderated by the effect the particles have on cloud water content. “We estimate that the observed decrease in cloud water offsets 23% of the global climate-cooling effect caused by aerosol-induced increases in the concentration of cloud droplets,” Toll and colleagues said. “These findings invalidate the hypothesis that increases in cloud water cause a substantial climate cooling effect and translate into reduced uncertainty in projections of future climate.”

Writing in an associated comment piece, atmospheric physicist Anna Possner of Goethe University – who was not involved in the present study – called the extent of the overall cooling effect of aerosol emissions “one of the key uncertainties in climate science”.

The researchers’ work, she concludes, “strongly suggests that the sensitivity of cloud water content to changes in the concentration of human-made aerosols might not be accurate in many current global climate models, and that large cooling effects caused by variations in cloud water content are unlikely.”

• The research is described in the journal Nature.