Aerosols – suspended particles of soot and other pollutants – affect the climate both directly and indirectly. They directly reflect or absorb solar radiation themselves. But their indirect effects on cloud properties may have a greater impact on global climate.

One indirect aerosol effect, called the cloud albedo effect or the Twomey effect after the scientist who first described it, increases the amount of solar energy reflected from a cloud. It happens because aerosols act as cloud condensation nuclei, so clouds with added aerosols develop a greater number of smaller water droplets. Increasing the albedo (or diffuse reflectivity) of clouds in this way causes a cooling effect, or in other words, a negative radiative forcing.

While the cloud albedo effect is characterised in many global climate models, it is very challenging to measure directly. The physical properties of clouds are complex and constantly changing. Different types of cloud respond differently to aerosols and it is extremely hard to be sure whether an observed difference in albedo is caused by aerosols, by other factors such as temperature and humidity change, or by different initial cloud properties.

Now, atmospheric physicists Lazaros Oreopoulos and Steve Platnick from the NASA Goddard Space Flight Centre, US, have taken a new approach to finding out how the albedo of the world's clouds can be expected to respond to added aerosols. They used cloud observations taken by MODIS (Moderate Resolution Imaging Spectroradiometer), an imaging system aboard two NASA Earth Observation satellites launched at the start of this century.

Data from MODIS provided global maps of actual cloud albedo for four months in 2005, calculated from measurements of each cloud’s opacity (known as optical thickness) and water droplet distribution. Oreopoulos and Platnick then superimposed a uniform change in the concentration of water droplets onto all the clouds and re-calculated their albedo. "Essentially, we mimic the effect of adding more aerosols," says Oreopoulos.

Their results showed two general patterns. First, clouds over oceans are more sensitive to aerosol addition, increasing their reflectance more than those over land. "This is partly because of the properties of marine clouds before they were perturbed, and partly because they are sitting over a dark surface – the ocean," says Oreopoulos. The other pattern is that clouds at high latitudes are less sensitive to aerosols than those nearer the equator.

The study found that if you increase the number of water droplets in the clouds by 10%, it leads to a global change in radiative forcing of 1–2 Wm–2. Happily, this is similar to the numbers emerging from climate models that have not incorporated observational cloud data.

The team now plans to collaborate with climate modellers and predict the effect of a more realistic, variable distribution of aerosols on their cloud data. They are also starting to investigate the potential effect of aerosols on ice clouds, which are far less well understood. "The data coming out of MODIS are relatively new, and there is still a lot of science we can do with them," says Platnick.

This study was published in the Journal of Geophysical Research.