Every day, tens of thousands of ships are on the move to transport goods worldwide. These ships pollute the air and leave traces in the clouds. Using satellites, we can analyze these traces and investigate how they affect the climate.

Low-lying Clouds and Ships

Large areas of the ocean are covered by low-lying clouds. These stratus and stratocumulus clouds reflect a large part of the incoming solar radiation, while the outgoing thermal radiation is not strongly affected. Therefore, they have a net cooling effect on the climate.

Ships emit not only CO₂ but also aerosols, fine particulate matter released during combustion in the engines. These aerosols form additional condensation nuclei for the water vapor in the air. This results in more and smaller droplets, which reflect more sunlight and cool the climate. However, there are other processes that can thicken or thin the clouds through effects on precipitation and evaporation.

Shipping Corridors

Movements of ships are typically concentrated in so-called corridors. Most shipping corridors run over the Northern Hemisphere (see Figure 1). Depending on the location and season, ships sail under low-lying clouds and leave behind extra reflective traces that can be detected by satellites. Long-term observations clearly show this influence in corridors where prolonged cloud cover is present and where the prevailing wind direction keeps the pollution concentrated within the corridor. In the southeastern Atlantic Ocean (see the gray box in Figure 1), there is a corridor that particularly meets these characteristics.

Effect on Clouds

In measurements from the geostationary Meteosat satellites we compared the properties of clouds that were and were not influenced by shipping. The mentioned shipping corridor is well recognizable in those measurements, with more and smaller droplets in the corridor compared to adjacent areas (see Figure 2). The difference with the background level not disturbed by shipping emissions shows the effect of shipping emissions on the clouds (see Figure 3).