Only due to their scale, there seem to be very little 'carbon reducing' technologies employed in the shipping trade.
I don't know why some sort of modern Sail power hasn't been looked at. If you're sailing with the wind why would you not save "$40 - $80k" a day?
Of wind turbines for that matter, as far as I know, most ships still derive electricity from diesel generators. Why not wind or solar?
The size of the sails / solar / wind turbine to replace this would be...? You get the idea.
As for carbon reduction, the engines already run at ~50% efficiency so there is little waste. The amount of carbon produced is the reality of pushing such a huge weight through the water.
I should have made it clearer in my post but I wasn't suggesting replacing diesel engines with Sail power, just augmenting it in the same manner as the SkySails (see response from jk4930 and markdown).
I also don't see why covering the deck in solar panels, or installing some form of wind turbine, wouldn't go some way to reducing the demand from diesel power generators.
In either case, I don't think you understand just how brutal the maritime environment is. A typical oceangoing ship has steel panels at main deck level dented in from the impact of the sea. I don't think solar panels are going to hold up so well.
Unless you can find a way for that wind turbine to both generate electricity and work as a "sail" so it propels the ship, it is just going to add drag that is counterproductive to the problem you're trying to solve.
Oceangoing cargo ships maintain speeds of up to about 25 knots. In recent years, many have practiced "slow steaming", cutting their speed in half to save 30% of their fuel needs (and doubling transit times).
By contrast, sailing ships averaged around 8 knots -- though one of the characteristics of sail is that your speed is much less predictable (and hence, shipping schedules). The ships were also much smaller than today's freighters, for various reasons. While it's possible that we could build larger sail-powered or hybrid ships today, they'd probably still be much smaller than fossil-fuel driven ships. Other alternatives include biofuels (most likely pelletized wood and/or cellulose).
I wrote about this a few months back on G+: Is solar-powered ocean freight in your future? Wait for it .... https://plus.google.com/104092656004159577193/posts/eRygnNMp...
Not true. Take a look at what Maersk does: http://www.maersk.com/sustainability/environmentclimate/page...