Canning the Round Numbers

Scientific American online just published the following article of mine, reposted here in full:

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Ever notice that we’ve got a thing for round numbers? We like our data neat and tidy.

The world of ocean pollution and litter prevention is filled with nice round numbers. Like those lists of how long various consumer goods take to go away once they escape into the environment...

Source: http://cmore.soest.hawaii.edu/
cruises/super/biodegradation.htm 

But recent finds on the beach have me asking: Are those numbers actually any good? Take aluminum.

An oft-repeated line says that aluminum takes 200 years to break down. Now I’ve found old pieces of aluminum -- like this top to a steel can from the early pulltab era, most likely used on a Coke product c. 1971-72:

Found by author March 12, 2012, Bay View beach, Saco, Maine 

This bit of aluminum, 40 years old, is on its way to disappearing. In something maybe not too far from the 200-year mark.

But you see, I’ve found other pulltab-era can tops that tell a very different story. This one, also about 40 years old, is still in remarkable shape:

Found by author April 10, 2012, Curtis Cove, Biddeford, Maine 

On the flip side, this one, probably more like 30 years old, is more than half gone:

Found by author February 29, 2012, Bay View

And this very modern can is already turning into Swiss cheese after perhaps a year of exposure:

Found by author April 25, 2012, Bay View

It turns out, the breakdown of aluminum isn’t a set event, it’s a system. One in which all the pieces have to fall into place for it to corrode back to dust.

When iron rusts, the new compound -- iron oxide, Fe2O3 -- takes up more physical space than the old. That’s why rust blisters & bubbles out. Those blisters expose more fresh iron underneath, which then rusts, and on and on until it’s all gone.

But when aluminum oxidizes, the aluminum oxide doesn’t take up any more space. It maintains its tight bond with the underlying aluminum. It’s actually a brilliantly weathertight seal. An undisturbed piece of aluminum can exist for... well, indefinitely long.

Now if you take that aluminum outside its comfort zone pH of 4.5 to 8.5, its protective oxide film will fail and true corrosion can set in. But such pH levels are rare in the ocean.

So what happened to the aluminum I’ve found? Corrosion got a boost from something more mechanical: abrasion. Get currents to drag aluminum back and forth through sand and gravel. Over & over & over. Each scrape wears a little surface aluminum oxide away, revealing fresh aluminum, which then transforms into more aluminum oxide. Tide rolls in, scrape scrape. Tide rolls out, scrape scrape. Maybe something acidic settles on it briefly, dissolve dissolve. Do it just right, and you can erode away an entire can in a matter of months -- not centuries.

Do it wrong, and you bury that aluminum under inert protective sediment.

Which brings me back to those photos. For the first year and a half at my beach, zero pulltab-era (30+ years old) can tops washed in. In the past six months six have washed in -- four within one month!

Why now?

Well, in recent months a sand bar has appeared at my beach at low tide.

March 12, 2012, Bay View

Never seen it before, but it’s there now. All that sand has come from further offshore. Where it perhaps once covered, buried, and protected those old bits of aluminum -- some for years, some for decades.

The study of how beached flotsam changes over time -- and what that can say about larger environmental change like seafloor shifts -- is interesting in its own right. But for the purpose at hand, it’s just a reminder: The world is not a static place. It’s ever-evolving. Things get moved, stuck, buried, freed, bashed. Each piece of debris has its own journey, and can tell a vastly different story.

Here’s one last photo.

Photo credit: Tim Wolter

Obviously, this isn’t aluminum. It’s a hewn log. This week a friend pulled it out of a ditch he was excavating at the Roman fort of Vindolanda, just south of Hadrian’s Wall in northern England. The ditch was in use around AD 200 and was sealed about AD 213, making this discarded chunk of wood ~1800 years old! It shouldn’t survive. But because of the soil conditions, it did.

If organics can do that, aluminum can do it that much more easily.

A blanket statement, like “Aluminum takes 200 years* to degrade,” denies the fact that the environment is a complicated thing. Worse, most often it just isn’t true (noted well on NOAA’s Marine Debris FAQ page).

One beer can lost today will be around in AD 4000. Another one will be gone by next year.

If the “facts” on aluminum are so far off, what does that say about the rest of these lists? 10 years for a polyethylene bag to completely go away? Where does that come from?

So a word of caution to environmental sites. Posting, as fact, nice round numbers that have no relation to reality (other than the metaphorical stopped clock being right twice a day) does a disservice. It misinforms -- and it risks discrediting the site when a person sees different results with their own eyes. We should avoid the pitfall of pretending there is any scientific truth behind something that’s just, well, a nice round number.

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* This number gets hedged sometimes, from “80 to 200 years” in one direction, to “200 to 500 years” in the opposite. More evidence that there’s little if any science backing it up.

Breaking It Down

"It doesn't go away." A phrase repeated a number of times already in the short life of this blog. Modern flotsam is a problem because so much of it is persistent -- it doesn't go away.

But everything rots, right? I mean, a plastic bag or flowerpot gets brittle and shreds if left out in the sun over a summer.

(photo courtesy flickr/wygd and licensed by Creative Commons)

So clearly, plastic does go away over time, right?

Wrong. With other trash, like, say, a napkin (which is just tree pulp) or nail or chicken bone, nature has had hundreds of millions -- in some cases billions -- of years of trial and error in figuring out how to recycle it.

Not so with plastics. The first artificial plastic was created in 1862, but plastic use only really exploded after World War II. For all intents and purposes, nature has only been exposed to plastics for a few generations.

And it has no idea what to do with them.

Plastics are synthetic polymers - long, large chains of (mostly) carbon and hydrogen molecules composed of repeating structures that are tightly bound together. The bonds that make them strong also make them incredibly resistant to microbes, weathering, rainwater, freeze/thaw, anything that can break a plastic back to its carbon and hydrogen. Science still knows of nothing that can be used in nature to break it up. (A high-schooler made big news in 2008 with a possibility, but there has been no effective follow-up since.) Estimates on how long plastic takes to break down in nature range from a century to thousands of years, for a simple reason: nobody knows.

Back to the shredded plastic bag. It's not biodegrading, it's photodegrading. Sunlight can make plastics brittle and break apart. But they're not breaking down -- they're just becoming smaller versions of themselves. Eventually they get small enough to swirl around the ocean as a "soup" of tiny colored fragments that can be ingested by plankton and then work their way up the food chain.

This picture, courtesy the Algalita Marine Research Foundation, shows the contents of a brief sample trawl in the North Pacific Gyre, a vortex in the ocean hundreds of miles from any island or shipping lane. That's all plastic, brought in by the currents.

Sometimes, plastic doesn't have to break down too small to have an effect.

This image, from CBC News, shows a dead albatross on Kure Atoll in the middle of the Pacific. This and other atolls around Midway are dotted with the carcasses of hundreds (thousands?) of albatross like this one. They ingest colorful plastic bits floating in the water, which (surprise!) don't break down in stomach acid. Eventually, the bird always feels full and stops eating altogether, ultimately dying of starvation.

Kure Atoll is 1200 miles from the nearest inhabited island.

When you see a piece of milk bottle floating in the harbor on a cruise, or see plastic wrappers on the beach at low tide that all get washed into the ocean at high tide, just remember: they don't go away.