Well of course per passenger-mile shipping is less efficient than air travel, because shipping is primarily transportation for goods, not for people. A big container ship has what, 20 people? Nonetheless, you're right that shipping should receive more attention. I think that it's less visible, less profitable, and more economically critical than air travel, which is often seen as a luxury. This would make taxing it more of a tax on economic activity.

Air travel produces a factor of ~100 more C02 per unit mass transported than maritime transport [1].

It seems that shipping is a little understudied, but apparently a leaked UN IPCC report (from 2008, so pre-crash, peak globalization) states that shipping produced 1.1 billion tonnes† [2] of CO2 annually. This number is ~4x higher than other studies so maybe it's more accurate, maybe it's not.

Air travel on the other hand produces 780 million tonnes of CO2 per year [3]. The major issue is that, as stated in TFA, it's supposed to triple in coming decades. I am not sure that global shipping is.

†I am quoting 'tonnes' in both which I think refers to metric tons, not imperial measurements, so I believe the units are comparable.

Shipping may be much easier to improve technologically than air travel, though. I'm sure many of the ships burn low-quality diesel. However, they're of the scale where very small nuclear reactors are available, and I have heard talk of ships being powered by natural gas. Ships can also deal with heavy batteries, so maybe in some instances offshore wind or solar farms could provide electricity? I have also heard of ships actually being powered directly by wind, but maybe this is a rumor, some far off Silicon Valley pipe dream.

[1]: http://www.davidsuzuki.org/issues/climate-change/science/cli...

[2]: https://www.theguardian.com/environment/2008/feb/13/climatec...

[3]: http://www.atag.org/facts-and-figures.html

Shipping burns tar and sludge. The worst smokey emissions possible. Its appalling.

I believe there are exactly two direct-wind-powered cargo ships in the world. http://www.skysails.info/

You have that reversed, shipping is vastly more energy efficient than air travel. ~30% of most airlines budget is for fuel.

Not the first time this confusion has come up on HN. https://news.ycombinator.com/item?id=10716102 CO2 emissions are low (which is what this article is about), but particulate and other emissions can be pretty extreme.

Agreed, it should be noted that most shipping related pollution has a short half life in the atmosphere and are released far from people limiting it's impact. However, the impact near ports can be rather significant and it will travel over a large area.

Because only people need clean air? This is the armchair ecologist viewpoint I hear all the time. If it doesn't bother me, its not a problem for the environment right?

Scale is important.

Not a lot of stuff lives over the middle of the ocean or even under it past the continental shelf's. Further, the ocean really does dilute this stuff to a rather extreme degree. Granted, it's still an issue, but it's simply a smaller one than CO2.

Almost all life in the ocean is in the top two meters. That's the part that would be most affected by air pollution. Its the part doing the 'diluting'.

Again, this is a case of "its not bothering me so its ok" ecology.

> Its the part doing the 'diluting'.

This is really far from true.

Acid rain is one of the primary issues with SOx pollution. It concentrates in rivers for example because the volume of rain water to overall water is very high. In the ocean new rain water never makes up a significant fraction of the top two meters of water.

PS: There is no need for gut reactions here, it's not hard to test for sulfur compounds.

what about ocean acidification that i keep hearing about?

This is mostly caused by CO2.

PS: First line "When carbon dioxide (CO2) is absorbed by seawater, chemical reactions occur that reduce seawater pH, carbonate ion concentration, and saturation states of biologically important calcium carbonate minerals." http://www.pmel.noaa.gov/co2/story/What+is+Ocean+Acidificati...

Compare jet travel to other forms of hauling humans around, and they start to look pretty good. Of course we're not going to be stacking people into ship cargo holds; that comparison is pointless.

Joe, you're the one who brought up shipping.

The article says air travel already accounts for 2% of global CO2 emissions, and the absolute amount is set to triple between now and 2050. Hopefully other sources of emissions decrease over that time, so air travel will be >6% of global CO2 emissions. It needs to be addressed quickly and globally, even if it is relatively clean.

And as air travel increases, does other forms of surface travel decrease? Surface travel is terrible in comparison. This trend could be the best news, emissions-wise.

Of course as it becomes a major mode of travel, its worth addressing. But for now, air travel is actually the preferable mode.

Surface travel can be great, depending on what you're comparing against.

Against single-occupant cars, airliners win big. Airliners typically see 80-100 passenger-miles per gallon, versus about 36MPG for the average American car.

But if you get a more efficient car and add more people to it, airliners don't win anymore. Put four people in a new Prius and you're looking at about 200 passenger-miles per gallon. Long-distance busses and trains do really well too. And of course you can power surface transportation with alternative energy sources much more easily. Electric trains are common and electric cars can make long trips now, while nobody really even knows how to build a useful electric airliner.

I don't understand the emphasis put on air travel when it comes to emissions, and it's definitely better than typical car habits, but surface travel in general can be a lot better than air travel.

> And as air travel increases, does other forms of surface travel decrease?

No.

Air travel is generally a separate category of travel that's mostly independent from other forms of travel. It directly competes with long range high speed rail, but it's far worse than rail travel. http://www.seat61.com/CO2flights.htm

Air travel sit's between single passenger cars and buses in terms of pollution. The break-even is generally ~3 people in a car depending on MPG and how out of the way the airport is.

6000 ships produce what, 25% of all the air pollution in the world? How can that possibly be true?

You are thinking of a very specific type of pollution such as sulphur oxide. Ships have zero emission standards and burn very low grade fuel. Cars/jets etc have the sulphur removed from their fuel. However, this stuff tends to go away fairly quickly, in terms of long term pollution such as CO2 they are very low.

Don't forget Jet fuel takes a lot of processing and produces a lot of pollution before it's added to the aircraft.

Or more likely, the original source for that claim was thinking of a very specific type of pollution, but it then got badly reported and mangled so that a surprising number of people now think that a small number of ships produce a major chunk of all emissions.

As I recall, the article which really made a splash on this topic was clearly written about sulphur dioxide, but the headline just said "pollution" and everybody ran with it.