The global sea surface temperatures recorded since the beginning of June 2023 have reach unprecedented levels for the season. The rise in temperatures is especially worrying in the northeastern Atlantic Ocean, where the temperatures peaked around 1.6°C above average, on June 21. Many factors are involved in this global heatwave outburst and this does not account for the recurring weather phenomenon, known as El Nino, which usually affects the tropical Pacific whose effects are predicted to become stronger by the end of 2023 and leading into 2024.
Current explanation for the recorded temperatures involves weaker surface wind speeds, reduction in the movement of Saharan dust westward over the north Atlantic or the multi-decadal fluctuation of north Atlantic atmospheric circulation and the transport of heat.
The global warming, induced by the worldwide carbon dioxide (CO2) emissions, is a long-term driving factor in the registered sea temperatures change but recent studies claim that the dramatic rise, witnessed this year may be due to the fact that other pollution’s sources were shielding the oceans from the effect of climate change.
An accidental planet size geoengineering experiment:
To understand the registered rise in sea temperatures, we must focus on the fuel used by container ships.
Those ships often used poor quality fuel oil or heavy fuel oil, which derives as a residue from crude-oil distillation, containing a lot of sulfur. When the fuel is burned in the engine, it releases not only carbon dioxide but also sulfur oxide in the atmosphere. The sulfur dioxide released combines with moisture in the atmosphere to form droplets which in turn leads to the formation of clouds and/or create brighter clouds, also called ship tracks in this case.
Those clouds due to their composition produce acid rain, which is bad for the environment, resulting in an acidification of seas, lakes and soils. In this case, even though the clouds are composed of sulfur, they remained clouds acting as a barrier between the sun light and the ocean’s surface water, having a cooling effect on the planet.
However, in 2020, the International Maritime Organization (IMO) Marine Environment Protection Committee (MEPC) implemented fuel sulfur regulations, limiting sulfur in marine fuels from 3.5% by mass to 0.5% or requiring the usage of exhaust gas cleaning systems to achieve an equivalent reduction in sulfur oxide (SOx) pollution. This reduction in sulfur oxide (SOx) emissions from ships is expected to have major health and environmental benefits, providing better air quality, leading to less respiratory, cardiovascular and lung diseases and limiting acid rain (produced by the release of SOx in the atmosphere) and its effects on crops, forests and the acidification of the oceans.
This resulted in ships gradually using cleaner fuels, which in turn led to less ships tracks formation. Less cloud formation, led to more light reaching the surface of the ocean hence the global rise in temperatures recorded in the northeastern Atlantic Ocean.
This “experiment” could lead to a better understanding of anthropogenic activities on climate and actually act as a basis for future planned geoengineering actions to mitigate the effect of climate change.
 Idem 1
 Diamond, M. S.: Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations, Atmos. Chem. Phys., 23, 8259–8269, https://doi.org/10.5194/acp-23-8259-2023, 2023
 Source (same as n°3)