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...

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:

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:

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

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

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.

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.

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.


* 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.


Previously in this series:

Cigarette butts to the left, fishing rope to the right, plastic everywhere: what I found on the beach over a year.

Too Good to Be True: Sea Mammals, Plastic Pollution and a Modern Chimera

Tsunami Debris and North America: Is the Tail Wagging the Dog?