In Germany, cars run on relatively clean fuel that is highly processed to reduce the pollutants produced in the combustion process. Gasoline is amber in color, a distillate hardly recognizable from its original fossil fuel -- crude oil.

The fuel that cargo ships around the world use to fire their powerful engines comes from what are literally the dregs of the oil refining process. The asphalt-colored substance is so thick that it must be mixed with heating oil so that its viscosity is low enough to be pumped. In the syrup-like fuel, there is a large amount of sulfur -- up to 4.5 percent.

When burnt, much of this sulfur in the form of sulfuric dioxide is emitted from ships' smokestacks. It then dissolves in the water, creating sulfuric acid. This in turn strongly increases the water's acidity levels.

"Sulfuric acid is a strong acid," said Karin Lochte, a professor at the Leibniz Institute for Marine Sciences in Kiel. "For one, when you compare it with carbonic acid, the effect of sulfuric acid is stronger. Secondly, the sulfuric dioxide creates little condensation particles in the atmosphere, thus building more clouds."

The product of that process is acid rain.

Baltic Sea suffers more than other large bodies of water

The Baltic Sea is comparable to a large lake. The narrow bay between Denmark and Sweden, called the Kattegat, prevents an exchange of water in a way similar to what happens in the world's oceans or even the North Sea.

To prevent further acidification in the Baltic, ships navigating the sea must burn fuel with sulfur amounts of 1.5 percent or less. Many ships, however, do not remain solely in the Baltic Sea, but are exiting it through the Kattegat to the North Sea and beyond, where the higher sulfur fuel is not forbidden. Thus, the question for much of the shipping industry on the Baltic is whether or not to refit ships and bunker stations, the name for marine fuelling stations, with an additional tank specifically for low-sulfur fuel.

A second tank means additional costs according to bunker station owner Werner Schlüter. He is talking with ship owners to see what they think and is holding off installing another tank for the time being.

New regulation drives fuel price higher

The environmental rule has an economical price to it. In Rotterdam, where fuel futures are traded, the price of low-sulfur fuel costs $30 (23.3 euros) per ton more than its high-sulfur counterpart, and the gap between the two is increasing. Considering that a container ship can take up to 12,000 tons on board, the price makes a difference.

The operator of the Stena line that runs five ferry routes on the Baltic Sea had already switched to low-sulfur fuel. Despite rising costs, passengers will not pay more according to Ulrich Kock, head of the ferry line's operations in Germany. He said that freight customers will have to pay a fuel surcharge.

In one year, shipping traffic on the North Sea and the English Canal will also have to make the switch to low-sulfur fuel.