- The creation of synthetic fertilizers in the early 20th century was a turning point in human history, enabling an increase in crop yields and causing a population boom.
- But the overuse of nitrogen and phosphorus from those fertilizers is causing an environmental crisis, as algae blooms and oceanic “dead zones” grow in scale and frequency.
- Of the nine “planetary boundaries” that scientists say we must not cross in order to sustain human life, the boundary associated with nitrogen and phosphorus waste has been far surpassed, putting Earth’s operating system at risk.
- Global policymakers are beginning to slowly recognize the scale of the problem, as climate change threatens to make it worse. Absent major reforms to agribusiness practices, scientists are aiming to convince the world to reduce waste.
On a sunny day, Guatemala’s Lake Atitlán is normally a sweeping vista of turquoise and sapphire, with the lake drinking the color of the sky until the two are nearly indistinguishable from each other. Ringed by mountains and towering volcanos, below Atitlán’s waves lie sunken ruins that were once used for worship and ceremony by the Mayan communities that have thrived here for thousands of years.
But in 2009, those communities awoke one morning to an unsettling sight. Vast swaths of the azure lake had instead turned sickly green. A film of foul-smelling scum was drifting on its surface, washing ashore, and swirling in the coves where people bathe and catch fish.
Atitlán had been overrun by cyanobacteria, or as it’s more commonly known, blue-green algae. The outbreak was massive, covering 40% of the lake’s surface; big enough to be seen from space. It was the first time a blue-green algae outbreak that size had been recorded at the lake, but it wouldn’t be the last. In 2015, another extensive bloom chased away tourists, a vital part of the local economy, and now, algal blooms are a semiconstant health threat for the lake’s Indigenous residents.
The explanation for the algal growth explosion at Atitlán, an affliction now shared by many of Earth’s freshwater lakes, is inseparable from the story of our species’ 20th-century population boom. In recent decades, the lake has been flooded with phosphorus and other nutrients, delivered largely via sewage and agricultural runoff. Along with its sister element nitrogen, phosphorus is almost entirely responsible for our ability to feed billions of people. The two elements are essential for plants and crops, and as the building blocks of synthetic fertilizers, vital to modern industrial food production.
They are also the cause of one of the world’s least recognized, and most severe, ecological crises: the human-caused destabilization of Earth’s natural nitrogen and phosphorous cycles.
Widespread use of synthetic fertilizers enabled crop yields to skyrocket during the past century. But there’s a catch: Excess nitrogen and phosphorus runoff from agriculture is leaking into lakes and flowing into estuaries, bays and seas planetwide, causing toxic algal blooms like the ones at Atitlán, as well as immense oxygen-free “dead zones” in the oceans, where most marine life can’t survive. Mega storms and higher temperatures caused by climate change are making the problem worse.
These algae blooms and dead zones are now on the rise, and experts say that unless governments start taking effective action, and fast, Lake Atitlán’s woes are just a taste of what’s to come. The imbalances caused by nitrogen and phosphorus overload are threatening biodiversity across the world, and according to some scientists, pushing us out of the “safe operating space for humanity.”
From guano to green revolution
The struggle to provide, or “fix,” enough nitrogen in soil to grow bountiful crops has been a constant of human history. Both nitrogen and phosphorus are necessary components of photosynthesis; without enough, plants turn sickly and stunted. Nitrogen is naturally abundant, making up 80% of the atmosphere, but most plants can’t use it until it’s converted into its “reactive” form, limiting where they can grow.
Early societies found a way to get around those limits by burning vegetation, then planting crops in the charred soil to boost growth. With this discovery, “slash-and-burn” agriculture was born, and while they didn’t know it, these Neolithic farmers were injecting reactive nitrogen and phosphorus, released from incinerated plant matter, into the soil.
Later civilizations learned that interspersing their crops with clover and legumes, such as soybeans (among the rare plants that can fix their own nitrogen), or applying animal manure (which contains digested reactive nitrogen and phosphorus) were even better solutions than slash and burn. These innovations allowed agriculture to support bigger populations.
In the mid-1600s, a German alchemist inadvertently discovered that phosphorus was part of the makeup of the human body when he boiled down 50 buckets of urine while searching for the mythical “philosopher’s stone.” As a result, crushed human bones (along with copious amounts of manure) were subsequently used as fertilizer in Europe.
Despite these discoveries, famine remained a constant danger, and when the colonial powers of Europe and the United States learned of a technique used by Indigenous groups in Peru, who fertilized their crops with nutrient-rich seabird guano, a hotly contested cross-Atlantic extractive trade was born. By the mid-1800s, newly independent Peru, Chile and Bolivia were fighting a bloody war over control of the islands where the bird droppings fell. Around the same time, U.S. scientists discovered that natural rock deposits of soluble phosphorus existed and could be mined.
Then, in the first decade of the 20th century, two German chemists, Fritz Haber and Carl Bosch, changed the course of history, for good and ill. The pair developed the Haber-Bosch process, a means of directly converting atmospheric nitrogen and hydrogen into ammonia (a reactive nitrogen compound) through intense pressure and heat. This breakthrough led to the production of synthetic fertilizers — and the deadly explosives used in World War I.
The twin developments of phosphorus mining and artificial nitrogen fixation marked an underappreciated turning point in the story of humanity. Coupled with new high-yield crops and chemical pesticides, people were suddenly able to produce food on a scale previously inconceivable, and our population was off to the races. Between 1900 and 2000, the number of human beings on the planet rose from 1.6 billion to 6 billion, while the total land mass used for agriculture increased by only 30% — an impossible feat without synthetic fertilizer.
The downside was that reactive nitrogen and phosphorus were introduced into the biosphere in staggering volumes. In 1890, humanity annually produced 15 million tons of reactive nitrogen (almost entirely by growing legumes and rice), and used about 2 million tons of phosphorus for agriculture. Today, that figure tops 200 million tons of reactive nitrogen per year and 47 million tons of phosphorus.
The growth of industrial agriculture was one of many technological miracles that arose during the 20th century. But as with many others, there was a hidden price.
By land, by air, and by sea
Before the advent of synthetic fertilizers and fossil fuels, the movement of nitrogen through the biosphere was relatively stable. In what’s known as the “nitrogen cycle,” the element’s atoms traveled through flora and fauna, being released via excretion and death back into the ground, with some escaping through bacterial conversion to the atmosphere or trickling into waterways. The nitrogen cycle was a foundation of life on Earth, helping to sustain and nourish flora and fauna alike in a harmonious balance of atomic movement.
That balance was shattered by industrialization and technology. While nitrogen-based fertilizers, which also contain phosphorus mined from the earth, today produce food that sustains around half of the world’s population, their liberal application has created a nitrogen “cascade.” Governments, eager to expand national economies, subsidized fertilizer purchases, allowing farmers to grow more food and faster, while chemical companies used the Haber-Bosch process to saturate the market with relatively cheap nutrients.
In turn, the massive influx of nitrogen and phosphorus became a form of pollution, spilling into Earth’s ecosystems. Now, nearly 80% of the nitrogen used in synthetic fertilizer is lost into the environment through soil erosion, runoff, atmospheric conversion and other forms of waste. A 2002 study estimated that for every 100 nitrogen molecules converted by the Haber-Bosch process into fertilizer, only 14 end up consumed as food.
This “overload” of nitrogen and phosphorus is wreaking environmental havoc across the world. Runoff from fertilizers and sewage, which contains nutrients consumed by humans, seeps into groundwater and enters waterways. Just as commercial crops love these two elements, so too do cyanobacteria and algae found naturally in water bodies like Lake Atitlán.
This nutrient feast leads to “blooms” of astronomical growth, sometimes dubbed “red tides.” The abundance of algae uses up the oxygen in the water, then dies, floating to the surface as a rotting green or red scum that is often toxic and which further depletes water oxygen levels. This process, called eutrophication, is now a common sight from Michigan to Shanghai, along with dead zones that began in river mouths and in bays but which have even occurred in the middle of the ocean.
“The overloading of nutrients into fresh waterways feeds more algae blooms, and as the algae duplicates and grows very quickly, they will consume oxygen and create a very dangerous living environment for fish and other types of animals in the water system,” said Xin Zhang, associate professor at the University of Maryland Center for Environmental Science. “It’s the same case with ocean fronts and in estuaries.”
While most people have never heard the word “eutrophication,” growing numbers are becoming familiar with its effects. In the U.S., lakes and coastlines from Florida to Michigan are struggling with algae, and in China, which has some of the highest concentrations of nutrient overloads on Earth, a massive bloom forced the competitive sailing event to be postponed during the 2008 Olympics. In India, the sacred Ganges River has turned green with algae.
“Human disturbance to the nitrogen and phosphorus cycles has already exceeded the planetary boundary,” Zhang said. Scientists have so far recognized nine planetary boundaries that they say represent the safe limits of human activity. Beyond them, we risk disrupting natural Earth systems and putting our very survival. (The Stockholm Resilience Center, which helped develop and popularize the concept, maintains a description of the full nine boundaries, which include: climate change; biodiversity loss; ocean acidification; ozone depletion; atmospheric aerosol pollution; freshwater use; land-system change; release of novel chemicals; and biogeochemical flows of nitrogen and phosphorus.)
Nutrient pollution “dead zones”
Across the planet, people whose livelihoods depend on lakes and oceans are bearing the brunt of the worsening crisis. For decades now, shrimpers who fish the Gulf of Mexico have borne the cost of one of the biggest marine “dead zones” in the world, with agricultural runoff traveling to the Gulf from the Midwest via the Mississippi River causing an estimated $2.4 billion in damages per year. Similar dead zones exist off the coast of Oregon and in Chesapeake Bay, in northern Europe and East Asia.
As much as 10% or more of the ocean is now a dead zone, according to some estimates.
Thanks to global warming, they are also getting bigger and arriving earlier. This summer’s dead zone in the Gulf of Mexico, for example, was the largest ever recorded, covering an area the size of Connecticut, a 16,400-square-kilometer (6,334-square-mile) graveyard where fish and other aquatic life can’t find enough oxygen to survive.
“It’s not going away. It’s the size of a state, and it’s been there for 30 years,” said Rebecca Boehm, an economist with the Union of Concerned Scientists who wrote a paper on the Gulf dead zone’s impact on fishing livelihoods. “It’s like what are we doing? The definition of insanity is letting this happen over and over again.”
The threat of nutrient overload isn’t new, and the risk has long been recognized. Since the 1970s, regulations have been put in place to limit nitrogen and phosphorus pollution in some regions, and where there’s been political will to enforce them, they’ve shown results.
But scientists say those piecemeal successes aren’t anywhere close to the scale of what will be necessary to reverse the damage.
For nitrogen, the planetary boundary hasn’t just been crossed — it’s been smashed, researchers say. Climate change is the boundary that grabs most headlines, but nutrient runoff, what scientists dub “biogeochemical flows,” is an unsung crisis that is already damaging ecosystems as well as the people who rely on them around the world, and which is almost certain to get worse.
“The science is a bit like climate 20 years ago, where the scientists are mobilizing and highlighting an issue, and the policy is coming afterwards,” said Mark Sutton, an environmental physicist and chair of the International Nitrogen Initiative. “I think in order to define the policy response, one has to be very clear about the problem in the first place.”
No easy answers, for farmers, people or the planet
Sutton says a key problem blocking global action is what he calls “fragmentation” of efforts to address nitrogen pollution by policymakers.
Agricultural runoff isn’t the only way that nitrogen is being pumped into the biosphere. It’s also released into the atmosphere as nitric oxide when fossil fuels are burned, and is also converted into another gas, nitrous oxide, by bacteria on agricultural lands. Both are greenhouse gases, with the latter being 300 times more powerful than carbon dioxide as a climate change-causing greenhouse gas.
Nitrous oxide is also caustic to the ozone layer, but despite this threat, it isn’t covered by the Montreal Protocol, which Sutton says is emblematic of the incoherent and ineffective control of nitrogen and phosphorus pollution by global environmental policy.
“Depending on what country you’re in, let’s say that nitrous oxide is about 5% of your total greenhouse gas emissions, or maybe more. But is [that even] 5% of the discussion? From what I can see, it’s more like zero — it’s completely forgotten,” he told Mongabay.
Until recently, the problem of nitrogen runoff into waterways and its emission into the atmosphere were treated as separate problems. And without a coordinated global plan, policies to deal with them have been scattershot, or in many cases actively counterproductive. One analysis of national laws and regulations covering nitrogen, for example, found that one-quarter were written to spur greater use of fertilizers to boost crop yields.
Depending on the region, that may not be a problem. Africa, for example, suffers from a shortage of phosphorus and nitrogen, whereas the U.S., India and China are in heavy surplus. But in the absence of an effective, global policy on nutrient usage, distribution, and waste, the consequences of fertilizer overuse are being almost entirely overshadowed by the need to grow economies and feed people.
“The challenge for national policymakers is to balance the concerns of food security with increasing concerns over the environment,” Zhang said.
One thing consumers can do to help, scientists say, is reduce food waste and meat consumption. Producing feed for livestock often requires large amounts of fertilizer, and the amount of nutrients that reach the plate is lower for meat than for vegetables and cereals. “Only a small portion of nitrogen in the feed is converted to meat from livestock,” Zhang said. “It’s a highly inefficient system.”
But while consumers can play a minor role, in the long run the nutrient pollution crisis won’t be solved without confronting the way we grow and produce food. Easy answers, however, will not be forthcoming. Our World in Data estimates that without synthetic fertilizers, humanity could only sustain around half its current population. Those fertilizers might be poisoning the world’s lakes and oceans, but billions of people also need them to survive.
A slow awakening
As with the other planetary boundaries, policymakers have been slow to grasp the potentially catastrophic impacts of nitrogen and phosphorus pollution. Those who have begun to recognize the scale of the problem are finding there are few palatable approaches to it. As with fossil fuel companies, industrial agribusiness wields immense political and economic power, and more crucially, there is no easy path to an immediate sharp reduction in fertilizer use without creating a risk of food insecurity.
Instead, Sutton said, a more realistic strategy is to focus on waste instead of overuse. Aiming for reduced nitrogen and phosphorus leakage via adaptive agricultural techniques and better waste management may be an easier sell politically, and research shows that a sizeable portion of nutrients can be recycled instead of carelessly released into ecosystems.
Some examples of existing practices that could be scaled up and implemented planetwide include the planting of “cover crops” that hold nutrients in soils after harvests, instituting the inter-cropping of nitrogen-fixing legumes, tightening up storage of manure, and introducing agroforestry buffer zones, along with other proposals under discussion. Research shows that by applying some of these solutions, inputs of fertilizer could even be reduced without sacrificing yields.
“Our message is that it’s good financially, so that if you look at the total amount of nitrogen pollution in the world, added up just in terms of nitrogen price and not even valuing the health and ecosystem costs, you get something like $200 billion of wasted nitrogen,” Sutton said. “If you halve that nitrogen waste you save yourself $100 billion for the circular economy.”
For phosphorus, figuring out ways to reduce waste is crucial for another reason. Unlike nitrogen, there’s no way to manufacture phosphorus; nearly all of it is mined in North Africa, with Morocco controlling nearly three-quarters of the world’s reserves, some of which lie in occupied and disputed Western Sahara. While there’s no immediate threat of it running out, the supply is finite and will at some point be depleted.
“We have a situation where 85% of the supply is controlled by just five countries, so my perspective is that we should be thinking hard about it,” said Elena Bennett, the Canada Research Chair in Sustainability Science at McGill University.
Reducing nutrient waste could save billions of dollars, and protect the planet, but implementing new measures will cost money. For farmers, often in debt and barely surviving economically, making those changes will require capital many of them don’t have, especially in low-income countries. Governments will likely have to step in to cover much of those costs.
“Farmers are quite vulnerable. It’s not like they have super high profit margins. A lot of them are working off-farm jobs to make enough income to keep things running, and so whatever we do to solve this problem has to have in it some solution that also protects farmers,” Bennett said.
Some countries are now waking up to the problem’s scale, along with the need for a more serious global response, but so far there hasn’t been much tangible progress. Early in 2019, the United Nations passed a resolution on sustainable nitrogen management in Nairobi. But the measure is both vague and voluntary, pledging to “consider the options for facilitating improved coordination of policies across the global nitrogen cycle at the national, regional and global levels.”
Later that year, the Colombo Declaration was signed by 29 countries, pledging to halve nitrogen waste by 2030. The non-binding declaration was celebrated with a new “nitrogen song,” recorded by Ricky Kej, a Grammy Award-winning musician from India. For now, it does not look like a Paris Agreement for nitrogen and phosphorus is on the near horizon.
What will have a definitive impact on the nitrogen crisis, though, is a more familiar catastrophe: climate change.
A harmony of disasters
In 2018, a group of scientists released a study analyzing satellite images for 71 of the world’s lakes. The results were consistent across regions: More than half showed evidence of algae blooms, and they were getting worse. The few lakes that showed signs of recovery were primarily those that had also experienced a reduction in atmospheric temperatures.
“One of the only things that we saw consistently across all the lakes is that the only ones that were able to sustain an improvement of water quality were those that had warmed less,” said Anna Michalak, director of the department of global ecology at the Carnegie Institute of Science and the report’s co-author.
The relationship between higher temperatures and algae blooms isn’t entirely straightforward. In hotter climates, there’s more rainfall evaporation and water runoff, which can reduce the amount of fertilizer nutrients delivered to lakes and coastlines. But once those nutrients do arrive, warmer lakes typically have worse algal outbreaks.
“Once they’re there, and this is true for coasts as well, higher temperatures tend to accelerate the growth of phytoplankton,” Michalak told Mongabay.
The climate change-induced severe storms that have become a familiar feature of life in recent years will also make the problem worse. Heavy rainfall dislodges fertilizer runoff from soils and transports it to waterways. If models forecasting that warming oceans will generate more intense storms are accurate, this could be very bad news.
“There’s huge swaths of the world that are already relatively wet and are projected to get wetter, both in terms of total and extreme precipitation, and those are areas where rainfall changes would lead to increased nutrient loading,” Michalak said.
Heavier downpours in the U.S. Midwest, for example, would likely worsen the Gulf Coast’s dead zone. “If you have these extreme rain events on barren farm fields that have manure or nitrogen, then that just washes into the river and ends up in the Gulf,” said Boehm of the Union of Concerned Scientists.
If extreme weather events worsen in the years ahead, that will hamper efforts to reduce nutrient runoff even if they are implemented at scale. Through many decades of overuse, the legacy of synthetic fertilizers will be here to stay for years or decades, even in the best-case scenario where the world takes action.
“We’re headed there whether we like it or not, simply because we’ve built up so much phosphorus in the agricultural soils over the last 70 years,” Bennett said. “That’s a very slow ship to turn around … There’s tons and tons just sitting in the soil waiting to be knocked downstream by one of these big summer storms.”
The convergence of climate change with the nitrogen and phosphorous crisis — the threat of globally warmed algal blooms and hypoxia in the planet’s lakes and oceans — is another blinking warning light for the environment. It’s also a reminder of an inescapable truth: Damage done to one of the biosphere’s life support systems will inevitably, and unpredictably, be amplified by damage done elsewhere.
As worrying as the links between the two crises are, Sutton said he’s hopeful that policymakers are waking up to the scale of the threat, citing the Colombo Declaration as an example of progress. But as of now, the needle is still pointing in the wrong direction, and time is running short.
If history is any guide, some unease is in order. Scientists today note that many of Earth’s worst biodiversity extinction events — including the Late Ordovician extinction event and End-Permian event that wiped out 90% of all species — were preceded by widespread ocean anoxia. In contrast, the Cambrian explosion of new life on Earth was catalyzed by increased ocean oxygen levels.
The inhabitants of the Late Ordovician and End-Permian eras had no control over their fate; we do.
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