Ship tracks are pollution-seeded clouds caused by the exhaust gases from commercial shipping vessels. They were first discovered in NASA satellite images in the 1960s. They usually follow broad linear tracks similar to the 'contrails' from aircraft. They can be hundreds of kilometers long and sometimes persist for several days. (photos from NASA)
The contribution to climate change from shipping is very poorly quantified, even though it's quite straightforward to calculate the amount of CO2 per voyage - simply by measuring the fuel use.The global total is thought to be in the region of 900 million tonnes per year. (around 3% of the human-caused total).
But the overall contribution to climate change is currently unknown because of the lack of data on the ship-track clouds and the ways in which they might affect the climate. The clouds can reflect sunlight back into space and also reflect infra-red radiation back to the Earth's surface - a process known as cloud forcingplugin-autotooltip__plain plugin-autotooltip_bigCloud forcing
Cloud Forcing - a.k.a. Cloud Radiative Forcing (CRF) or Cloud Radiative Effect (CRE) - refers to the complex effects that cloud cover has on the Earth's 'radiation budget' - in other words how much clouds affect global warming by the Sun. (which is itself poorly understood).
In the case of aviation 'contrails', it's now generally agreed that the effect of the clouding has more influence on climate change than do the CO2 emissions.
International shipping remains a challenging topic in global climate change abatement discussions and was left out from the Paris Agreement, something considered as a major hindrance to keeping a temperature increase under two degrees Celsius (Meinhard, 2016).
Source : Journal of Cleaner Production, Volume 205, Pages 895-908
Note: Although the prevalence of 'dirty' ship-tracks has declined due to globally agreed exhaust pollution controls introduced in 2020, the total number of commercial ship voyages is currently increasing.
Also see : Aviation contrails (climate impact)plugin-autotooltip__plain plugin-autotooltip_bigAviation contrails (climate impact)
Jet engines used in commercial and military aircraft typically burn carbon-based fuels (e.g. kerosene). When the fuel burns, a by-product is CO2, which is a potent greenhouse gas and contributes towards global warming.