Steel-blue skies and a "balmy" 14 degrees celsius (57 degrees Fahrenheit): As far as I can tell after eight days at sea, that's August at its finest in Cuxhaven, a port town just west of where the Elbe river meets the sea.

The strong tides set the pace in this part of the North Sea. If you want to get into port or out onto the open water, there's no point fighting the currents - especially if you're on a traditional sailing vessel such as ours.

Today we're heading out to measure particulate emissions from ships - and we don't have to go far. To look out at the horizon from Cuxhaven is to witness an endless caravan of ships traveling to and from Hamburg, Germany's largest port.

Many of them are maritime giants whose massive engines can be heard roaring in the distance, day and night.

The waters just off the coast of Cuxhaven are like a maritime highway with massive cargo ships and other vessels making their way to Hamburg day and night.

"Look at that spike," says Sönke Diesener as the Ryvar, our 101-year old lugger, crosses the shipping lane behind a huge tanker. Diesener, who works on transportation policy issues at the Nature and Biodiversity Conservation Union (NABU) is holding a metal wand over the gunwale of our vessel.

This is attached to a device that measures particulate matter in the air. Within seconds, the figure on his display shoots from 800 particles per cubic centimeter to over 50,000. It finally tops out at 73,000.

"At a busy intersection of a major traffic artery in a big city, you might get around 16,000," Diesener says - with the reserved calm for which northern Germans are known. The figures speak for themselves.

Playing dirty

The real issue isn't that these ships consume a lot of fuel - which they do - it's that the fuel they burn is often dirty, as is the way it's burned.

Sönke Diesener (NABU) on board the Ryvar holding a device that measures particulate emissions

"Even now, most ships burn highly toxic heavy fuel oil when they are out on the high seas," Diesener explains. Heavy fuel oil is essentially a residual product from fuel production. "You produce gasoline, you produce diesel - and what is left over is heavy fuel oil."

And it burns 100 times dirtier than marine diesel - and an incredible 3,500 times dirtier than regular car diesel, when it comes to sulfur dioxide he adds. Aside from plenty of carbon dioxide, the ships blow massive amounts of the gas into the air - and sulfur dioxide causes acid rain.

Ships also emit nitrogen oxides - the main culprit in theDieselgate scandal still coursing through Germany. Nitrogen dioxides (also known as NOx) make soil more acidic, and over-fertilize lakes and coastal areas, destroying the balance in those ecosystems.

Last but not least, ships release large amounts of particulate matter and black carbon into the atmosphere - both of which are known to have both climate and health impacts.

Enjoy your cruise

But why should we care about ship emissions? After all, big ships spend much of their time out at sea and most of us don't.

Well for starters, there is climate change. A recent study published in the Journal of Geophysical Research ranked black carbon as the second-most-significant greenhouse gas emission after carbon dioxide.

Black carbon absorbs sunlight, heating the atmosphere. When this gets deposited on snow, in the Arctic, for example, this reduces the snow's whiteness, or albedo. The snow consequently reflects less of the sun's energy, which in turn contributes to global warming - an unpleasant feedback loop.

A fairly easy first step that would dramatically reduce ship emissions would be to stop burning heavy fuel oil altogether, and use diesel instead.

That would also make it possible to install diesel particulate filters (DPFs), which reduce particulate matter and black carbon emissions by up to 99.9 percent. Selective catalytic reduction (SCR) systems, in turn, could eliminate 70 to 80 percent of NOx emissions. Both systems are already in use on several ferries and cruise ships.

The huge ships criss-crossing our oceans not only burn a lot of fuel, they often also burn fuel that is particularly dirty.

Heavy fuel oil could still be used elsewhere. "The solids that are eventually left could be used in road construction or burned in a fossil fuel power plant," Diesener says.

What's more, suchbig polluters may be closer than you think.

In harbor cities, cruise ships often anchor in central locations - and leave their engines running while they are at port, because they are essentially huge floating hotels that have to keep the pool heated and the air-conditioning running.

A modern cruise ship requires about as much electricity as a city of 20,000 people - so it's not easy to just plug in an extension cord and turn off the engine - which is what we do on the Ryvar when at berth.

Hybrids, plugs and sails

It's not easy, but it is possible. After long negotiations, an international standard for onshore power supply (OPS) was finally established in 2012. One challenge was that different ships use varying voltages, depending on their age and country of origin.

Although progress is slow, a growing number of ports now offer OPS, and several ferry lines and cruise ship operators are either in the process of fitting their ships with the necessary connectors or are already using them.

For a greener ride between ports, there is also liquefied natural gas (LNG), which burns much cleaner than oil or diesel. Several ferries and even a container carrier already use this technology. Much like hybrid cars, they combine combustion and electric engines.

Scandlines, which operates ferry lines between Denmark, Germany and Sweden uses hybrid ships.

Cruise ships often come right into the center of major port towns, bringing significant emissions with them.

"This allows the ferry to adjust its fuel consumption to the workload - which results in a 15 percent reduction of CO2 emissions," says Anette Ustrup Svendsen, Head of Corporate Communications at Scandlines. "Our long-term goal is zero-emissions."

One fully-electric ferry in Norway has already achieved just that. The 80-meter catamaran has been operating in the country's largest fjord since 2014 and the electricity to charge its batteries comes from carbon-neutral hydropower. And while significantly smaller, there are even solar-powered ferry boats. Berlin's public transport system operates four of them.

Back on the Ryvar, Diesener is interrupted by a sudden loud call from the stern. The captain has started to turn the ship, and the massive boom of our main mast comes flying across over our heads.

Time to man our stations at the fore-mast and main-mast - and start pulling. Sometimes traveling on a zero-emissions ship requires a little effort.