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SOURCE: The Atlantic
DATE: February 8, 2021
SNIP: Life as we know it is carbon based. But every organism requires other elements, too, including nitrogen and phosphorus. Nitrogen is the basis of all proteins, from enzymes to muscles, and the nucleic acids that encode our genes. Phosphorus forms the scaffolding of DNA, cell membranes, and our skeletons; it’s a key element in tooth and bone minerals.

Too little of either nutrient will limit the productivity of organisms, and, by extension, entire ecosystems. On short timescales, nitrogen often runs out first. But that scarcity never lasts long, geologically speaking: The atmosphere—which is about 80 percent nitrogen—represents an almost infinite reservoir. And early in the course of evolution, certain microbes developed ways to convert atmospheric nitrogen into biologically available compounds.

Alas, there is no analogous trick for phosphorus, which comes primarily from the Earth’s crust. Organisms have generally had to wait for geologic forces to crush, dissolve, or otherwise abuse the planet’s until it weeps phosphorus. This process of weathering can take thousands, even millions, of years. And once phosphorus finally enters the ocean or the soil, where organisms might make use of it, a large fraction reacts into inaccessible chemical forms.

That we breathe oxygen today—and exist at all—might be thanks to a series of climatic cataclysms that temporarily freed the planet from phosphorus limitation. About 700 million years ago, the oceans repeatedly froze over and glaciers swallowed the continents, chewing up the rock beneath them. When the ice finally thawed, vast quantities of glacial sediment washed into the seas, delivering unprecedented amounts of phosphorus to the simple marine life forms that then populated the planet.

Planavsky and his colleagues propose that this influx of nutrients gave evolution an opening. Over the next 100 million years or so, the first multicellular animals appeared and oxygen concentrations finally began to climb toward modern levels. Scientists still debate exactly what happened, but phosphorus likely played a part. (To Planavsky, it’s “one of the most fascinating unresolved questions about our planet’s history.”)

Another group of scientists, led by Jim Elser of Arizona State University, speculate that such a pulse of phosphorus could have had other evolutionary consequences: Since too much phosphorus can be harmful, animals might have started building bones as a way of tying up excess nutrients.

What’s clear is that after this explosion of life, the phosphorus vise clamped down again. Geologic weathering kept doling out meager rations of the nutrient, and ecosystems developed ways to conserve and recycle it. (In lakes, for instance, a phosphorus atom might get used thousands of times before reaching the sediment, Elser says.) Together, these geologic and biologic phosphorus cycles set the pace and productivity of life. Until modern humans came along.

Long before phosphorus was discovered, however, humans had invented clever ways of managing their local supplies. [I]n the Americas, for example, Indigenous people managed hunting and foraging grounds with fire, which effectively fertilized the landscape with the biologically available phosphorus in ash, among other benefits.

But human waste was perhaps the most prized fertilizer of all. Though we too need phosphorus (it accounts for about 1 percent of our body mass), most of the phosphorus we eat passes through us untouched. Depending on diet, about two-thirds of it winds up in urine and the rest in feces. For millennia, people collected these precious substances—often in the wee hours, giving rise to the term night soil—and used them to grow food.

The so-called Sanitation Revolution followed close on the heels of the Industrial Revolution. In the 1700s and 1800s, Europeans and Americans moved to cities in unprecedented numbers, robbing the land of their waste and the phosphorus therein. This waste soon became an urban scourge, unleashing tides of infectious disease that compelled leaders in places like London to devise ways to shunt away the copious excretions of their residents.

[T]he volumes involved posed logistical challenges, and critics raised concerns about the safety of sewage farms—as well as their smell. Thus, waste ultimately was sent to rudimentary treatment centers for disposal or, more often, dumped into rivers, lakes, and oceans.

This created what Karl Marx described as the “metabolic rift”—a dangerous disconnect between humans and the soils on which they depend—and effectively sundered the human phosphorus cycle, reshaping its loop into a one-way pipe.

That single disruption has caused global chaos, you could argue,” Cordell says. For one thing, it forced farmers to find new sources of phosphorus to replace the nutrients lost every year to city sewers.

[G]eologists discovered [large deposits of phosphorus] in Florida. (To this day, most of the phosphorus on American fields and plates comes from the southeastern U.S.) Other massive formations of phosphate rock have since been identified in the American West, China, the Middle East, and northern Africa.

These deposits became increasingly important in the 20th century, during the Green Revolution (the third revolution in agriculture, if you’re keeping track). Plant breeders developed more productive crops to feed the world and farmers nourished them with nitrogen fertilizer, which became readily available after scientists discovered a way of making it from the nitrogen in air. Now, the main limit to crop growth was phosphorus—and as long as the phosphate mines hummed, that was no limit at all. Between 1950 and 2000, global phosphate-rock production increased sixfold, and helped the human population more than double.

But for as long as scientists have understood the importance of phosphorus, people have worried about running out of it. These fears sparked the fertilizer races of the 19th century as well as a series of anxious reports in the 20th century, including one as early as 1939, after President Franklin D. Roosevelt asked Congress to assess the country’s phosphate resources so that “continuous and adequate supplies be insured.”

These events raised a terrifying possibility: What if the phosphorus floodgates were to suddenly slam shut, relegating humanity once more to the confines of their parochial phosphorus loops? What if our liberation from the geologic phosphorus cycle is only temporary?

In recent years, Cordell has voiced concerns that we are fast consuming our richest and most accessible reserves. U.S. phosphate production has fallen by about 50 percent since 1980, and the country—once the world’s largest exporter—has become a net importer. According to some estimates, China, now the leading producer, might have only a few decades of supply left. And under current projections, global production of phosphate rock could start to decline well before the end of the century. This represents an existential threat, Cordell says: “We now have a massive population that is dependent on those phosphorus supplies.”

Simply extracting more phosphate rock might not solve all of our problems, Cordell says. Already, one in six farmers worldwide can’t afford fertilizer, and phosphate prices have started to rise. Due to a tragic quirk of geology, many tropical soils also lock away phosphorus efficiently, forcing farmers to apply more fertilizer than their counterparts in other areas of the world.

The grossly unequal distribution of phosphate-rock resources adds an additional layer of geopolitical complexity.

We have already glimpsed how the phosphorus supply chain can go haywire. In 2008, at the height of a global food crisis, the cost of phosphate rock spiked by almost 800 percent before dropping again over the next several months. The causes were numerous: a collapsing global economy, increased imports of phosphorus by India, and decreased exports by China. But the lesson was clear: Practically speaking, phosphorus is an undeniably finite resource.

Phosphorus is a classic natural-resource parable: Humans strain against some kind of scarcity for centuries, then finally find a way to overcome it. We extract more and more of what we need—often in the name of improving the human condition, sometimes transforming society through celebrated revolutions. But eventually, and usually too late, we discover the cost of overextraction. And the cost of breaking the phosphorus cycle is not just looming scarcity, but also rampant pollution.

At nearly every stage of its journey from mine to field to toilet, phosphorus seeps into the environment. This leakage has more than doubled the pace of the global phosphorus cycle, devastating water quality around the world. One 2017 study estimated that high phosphorus levels impair watersheds covering roughly 40 percent of Earth’s land surface and housing about 90 percent of its people. In more concrete terms, this pollution has a tendency to fill water bodies with slimy, stinking scum.

[See also: Phosphate Mining Brings a Mosaic of Destruction to Florida including Sinkholes and Radioactive Waste]