The Arithmetic of Intent

Map of 2017 seasonal hypoxic zone late summer extent. Credit: NOAA and LSU/LUMCON

The annual dead zone at the mouth of the Mississippi – which spread out over an area roughly the size of New Jersey by the end of this past summer – appears to have mostly receded from its August peak. (Turns out NOAA runs an interactive map of dissolved oxygen measurements from monitoring stations in the area, linked here for those of you who like playing with data.)

Around the time of its largest spread this summer, the dead zone stretched offshore from the Mississippi delta toward the upper reaches of the Texas coast (as shown above). Now, however, the zone seems to have mostly recovered:

Screen capture of data from NOAA’s interactive hypoxia data viewer, taken 10/31/2017. 

[Side note: those red splotches visible above near Houston and Beaumont on NOAA’s current hypoxia map may be related to the enormous amount of rainfall runoff from Hurricane Harvey, which was still hanging out as a fairly cohesive blob in the Gulf as of last week.]

The cause of this seasonal oxygen loss, which researchers have been studying since the early 1970’s, isn’t a mystery at this point (though this year an unusually wide swath of ocean went temporarily dead). The Mississippi river catches rain and runoff from around 40% of the continental U.S., including the “breadbasket” farmland of the Midwest. A little bit of excess fertilizer washes away into a ditch near a cornfield in Iowa; a little bit is carried off the property from an apple orchard in southern Minnesota into a nearby stream. Add just a little more from the millions of acres of farmland in Kansas, Indiana, anywhere else the river’s fingers reach. The river gathers and gathers and gathers, stream by stream, ultimately carrying well over a million tons of nitrogen each year out past the levees of New Orleans. Not all of this nitrogen comes from agricultural sources (a large part comes from the ground itself), but by far the biggest new input in the system since the 1950s has come from increased fertilizer use (as the USGS chart to the left shows).

Once in the ocean, all that fertilizer keeps doing its job: causing plants to grow. Clouds of algae and photosynthetic bacteria blossom in the warm Gulf waters and live out short lives. Then the plant matter dies off, sinks and breaks down with the help of other organisms – driving a complex set of processes that ultimately sponge up oxygen and leave behind almost nothing for fish to breathe. This over-enrichment of the water is called eutrophication; the resulting lack of oxygen at the bottom of the water column is called hypoxia.

The animals than can swim away do; some of the bottom-dwelling sea life, like crabs, may have more trouble escaping (and the oysters glued in place to their reefs sure aren’t going anywhere). Other effects are less direct – the algae blooms can block out sunlight, potentially damaging or killing underwater plants that feed and house other species in the food chain. Young fish and other organisms may be hit hard, reducing future population growth. Coastal ecosystems can only hold their breath for so long.


The Mississippi is by no means the only place where these kinds of dead zones occur. Many lakes and coastal areas around the world, especially those fed by water from heavily agricultural zones, experience the same cycle to some degree. The entire Black Sea was thought to be on the brink of death around the time the Soviet Union collapsed in the early 1990’s; unchecked runoff from the Danube and other rivers, especially strong industrial fertilizers, lead to wide-spread ecosystem collapse. Closer to home, planners around the Chesapeake Bay area worry about the health of those famous Baltimore blue crabs.

When eutrophication happens only seasonally, a bay or estuary can recover once oxygen-rich waters mix back into the zone, though ecosystems (including those that support commercial fishing) may be seriously damaged and degraded over time. If the problem becomes more severe or more permanent, whole stretches of the water body could potentially be overrun by sulphur-producing bacteria and maybe jellyfish.

What’s interesting about the Mississippi dead zone is that it’s nobody’s fault, in a sense – no one person’s, anyway, and certainly not the result of anyone’s intentional actions. No farmer in Illinois woke up last spring and decided to try his hardest to damage an oyster reef off the Mississippi Delta. Instead, somebody woke up and thought about how to improve their harvest, how to stay in the black this season. Maybe someone stepped up their fertilizer schedule, hoping to boost the size and color of their tomatoes to make their crop more appealing to incredibly picky supermarket consumers in the city nearby. Maybe a farm owner is trying to pay medical bills, send their kids to school, buy more land to expand, take that trip they’ve always wanted to take. What do any of their day-to-day actions or worries – or those of their farm’s customers – have to do with a coastline thousands of miles away?

Economists call this kind of situation an externality: a cost somewhere in the economic system that isn’t directly paid by the actor that causes it.  The actor may not even be aware that they’re doing any damage; this means there’s no feedback loop built into the cost of doing business, so prices stay lower than they should be to reflect what any single unit of product really costs society to produce.

Just as importantly, no single actor causes the dead zone. No single farm or company on its own added enough fertilizer to the water to make the difference, when diluted out over the nearly 5 million gallons or so that flow past New Orleans every second. In general, everybody who added something to the river added just a little. But just a little, over and over again, adds up.

Visualization highlighting the Mississippi River watershed and Gulf Coast eutrophication. Urban areas are shown in pink; farmland is shown in green. Courtesy of NOAA Environmental Visualization Laboratory


Some 12 weeks later, the dead zone is mostly gone again now, and my first semester of grad school is racing by. One of our first lectures in my Resource Economics class after the fall break touched on a fact near to the heart of my reasons for coming back to school this year: It is very hard to even think about the diffuse impacts of many people’s actions, much less measure and quantify them. Our understanding of seemingly-distant, interwoven environmental feedback loops is still growing as we tease them apart scientifically, from ecosystem dynamics to human health to climate change. How does a splash of pesticide here or factory waste there, a million times over, link to birth defect rates in the Rio Grande valley? What does an “insignificant” amount of benzene leaking from a pipeline joint really mean, when multiplied over hundreds of pipelines converging toward the neighborhoods and schools around the Houston ship channel?

On the other hand, it’s pretty easy to think about the immediate cost from a regulation meant to protect a natural resource. The cost a company pays to install a sulfur scrubber at a power plant is much more immediate and tangible than the cost of the sulphur itself on any particular day. (Is it some slightly acidic rain falling, somewhere, and changing the chemistry of a lake by just a smidge? A handful of people in a nearby city having a little more trouble breathing than usual? We can’t follow it like an inventoried product to find out.) It’s hard to compare the two without the tangible hit to the plant’s finances feeling more important, even if the overall problem the plant is contributing to is much more costly in the long run. (That could be potential collapse of a fishing industry, loss of tourist revenue to a lake, and/or millions of dollars in healthcare costs linked to air pollution, not to mention early deaths).

A planner or economist looking at the big picture of Mississippi hypoxia may not be able to tell you, precisely, what the value of a tiny bit less fertilizer in the Gulf is. A landowner can definitely tell you what it might cost them not to be allowed to use it, or to suddenly have to pay a new fee for letting it run off into the watershed.

This kind of problem – the startling arithmetic of tiny impacts that accumulate to large ones – shows up in pretty much every sort of shared resources. It doesn’t matter whether the resource we’re discussing is fish in a lake, or patches of wetlands that builders want to sell as a gated community; it could be the ability of a river to take in pollution before its water starts to occasionally catch fire; it could be the atmosphere’s ability to absorb greenhouse gas emissions before reaching any number of ecological tipping points.

How do we fight environmental harms done unintentionally, by no one in particular, that any one person has good reason to (perhaps unknowingly) help commit? Can we get better at highlighting a dollar value of potential costs that won’t be paid directly by any one person, or maybe won’t become fully clear for generations? There are plenty of people working on exactly that question, here at Duke and beyond, with some success.

There are other ways to help balance the equation, however. [Warning: Way-Easier-Said-Than-Done Alert.] Perhaps we should be working just as hard on learning to shift how we collectively view these diffuse, hard-to-quantify impacts. Can we learn to embrace an understanding of our own actions as part of a broader system, which create ripples through the lives and spaces of others? In the long term, our success as a species on an increasingly-crowded planet may depend on our ability to make such a shift. At some point, cumulative impacts add up; they become too large to ignore.

2 thoughts on “The Arithmetic of Intent

  1. Finally. Someone who can translate complex science into language that is both understandable and compelling. Rachel Carson’s writings come to mind as I followed the stories you’ve told. It is my hope that your work will help readers connect with their “own backyards” to discover those ripple effects you mention. My church is struggling to organize a Green Team with guidance from an organization called Faith in Place, whose purpose is to support congregations as they learn to protect common air, land, and water resources. Your perspective and writings may serve to bridge the gap between our pastor’s goal to join this action and individuals within the church community who might be interested in leading the way. Your second article is perfect for this purpose since our backyards in Illinois are part of the watershed for the Mississippi.
    Please continue to write and share your work with me and I’ll take it from here!

  2. Glad you’ve found the post useful, Emily! I’m hoping to write a lot more about these sort of cumulative problems, and what kinds of solutions might be possible in addressing them. Good luck with your organization!

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