Earth's environment is often portrayed as generally stable, with the proverbial balance of nature ensuring an inexorable return to past equilibria. The distant past offers much evidence to the contrary. Earth has been so warm that forests extended to the poles and so cold that glaciers extended to the equator. Even the planet's rotation is changing: billions of years ago, Earth spun so quickly that a day only lasted two hours. In fact, our environment is so precariously situated that living creatures can profoundly affect it. Earth's biosphere has already been irrevocably changed by the planet's inhabitants. In particular, one group of organisms dramatically altered the composition of the atmosphere by pumping a toxic gas into the environment; in doing so, they initiated one of the greatest mass extinctions in Earth's history. However, we would not be here if it were otherwise: the culprits in this ecological annihilation were cyanobacteria and the toxin they produced was oxygen.
Gaseous oxygen was not present in the atmosphere when early life arose. It is a highly reactive element; fires burn and iron rusts due to the action of oxygen. This reactivity also made oxygen intensely harmful to the ancient unicellular organisms that evolved in its absence. When cyanobacteria developed the capacity for oxygenic photosynthesis, they harnessed the power of the Sun; they also began to produce enormous quantities of oxygen gas, which would eventually eradicate many of the species alive at the time. The microscopic victims of this event may not engender much compassion from a species such as ourselves, but to a world inhabited only by microbes, the Great Oxygenation was a cataclysm. Yet the vast majority of life now requires the presence of oxygen to survive; a deadly toxin has been rendered a fundamental constituent. In the words of Jurassic Park's Ian Malcolm, "Life, uh, finds a way."
A few billion years after the Great Oxygenation, a hairless ape named Thomas Malthus published An Essay on the Principle of Population. At the dawn of the 19th century, human population growth was rapidly surpassing growth in food production. Malthus extrapolated this trend forward and predicted that humanity would eventually outgrow its capacity to feed itself. He was nearly proven correct. Though Malthus did not know it, the problem was nitrogen. In addition to sunlight and water, plants need substantial quantities of nitrogen, phosphorous, and potassium to grow. Nitrogen is one of the most abundant elements on Earth; it constitutes 78% of each breath we take. However, in contrast to oxygen, nitrogen gas is entirely unreactive. Its molecular structure is exceptionally stable. A molecule of nitrogen gas must be broken apart and incorporated into a molecule of nitrate or ammonia in order to be utilized. This process is known as nitrogen fixation. The only beings capable of unlocking the nitrogen in the atmosphere are symbiotic bacteria that live on the roots of certain plants. In the wild, plants release nitrogen back into the soil when they die and decompose. In agriculture, we eat the plants, taking the nitrogen out of the system. This gradually depletes even the most productive agricultural land. While our excrement contains a great deal of nitrogen, spreading human waste onto fields can lead to disease outbreaks and the transmission of parasites. This is why farmers use the composted manure of other animals when fertilizing their fields. As populations exploded through the 19th century, so did the need for greater agricultural productivity. Fertilizer became an important geopolitical asset. In fact, it was so coveted that several countries went to war over islands covered in bird shit.
The Chincha Islands sit 21 kilometers off the coast of Peru. They are small and rocky, offering little to attract human visitors. However, they have long been popular with seabirds. Huge breeding colonies thrived on the islands for millennia. The birds' excrement—known as guano—slowly accumulated over the centuries. By the time that humans began searching for fertilizer, the Chincha Islands were covered by heaps of preserved guano over 60 meters deep. During the mid-19th century, the extraction of these deposits accounted for 60% of the Peruvian government's annual revenue. The states of the age relied on fertilizer, much as they rely on petroleum today. In an effort to reassert its colonial authority in South America, Spain occupied the islands in 1864, which led to war with Peru and Chile. Though Peru was successful in defending its territorial claim, the guano deposits on the Chincha Islands rapidly depleted following a few decades of further exploitation. By the end of the century, the world's natural nitrate deposits were diminishing, yet population growth was accelerating. Unlocking the endless nitrogen in the air around us was becoming a pressing concern.
In 1898, Sir William Crookes delivered a speech to the British Association for the Advancement of Science, stating:
[A]ll civilised nations stand in deadly peril of not having enough to eat. As mouths multiply, food resources dwindle. Land is a limited quantity, and the land that will grow wheat is absolutely dependent on difficult and capricious natural phenomena . . . I hope to point a way out of the colossal dilemma. It is the chemist who must come to the rescue of the threatened communities. It is through the laboratory that starvation may ultimately be turned into plenty . . . The fixation of atmospheric nitrogen is one of the great discoveries awaiting the genius of chemists.
The pressure to develop synthetic fertilizers was most profound in densely populated countries such as Britain and Germany. Much of the world's grain was grown in expansive countries like Russia and the United States, but smaller countries were beginning to feel the pinch of overcrowding. There were some early successes in fixing atmospheric nitrogen, but none of the methods were practical for use outside of the laboratory. In 1908, a German chemist named Fritz Haber became the first to succeed in harnessing the air. Haber collaborated with an engineer named Carl Bosch to successfully industrialize his process. By heating nitrogen gas under intense pressures and utilizing a series of catalysts, they were finally able to produce ammonia. The sky was quite literally the limit. To say that the process was successful would be an understatement: nitrogen is a fundamental constituent of every protein and strand of DNA in our bodies, and half of the nitrogen inside us today was conjured from the air by an industrial furnace. To phrase it another way, half of the world's current population would not be alive without the Haber-Bosch process.
There is an unfortunate addendum to the tale of Fritz Haber. He was instrumental in the development of chemical weapons during the First World War and personally attended the Battle of Ypres to observe the first use of his chlorine gas. After the war, he continued to work to develop chemicals for the German military. Haber was fervently patriotic but he was also Jewish; he fled Germany as the tide of anti-Semitism surged in the lead up to the Second World War. One of the chemicals developed in his lab would become Zyklon-B, the gas used to murder millions during the Holocaust. Members of his own extended family were among the Nazi's victims. Haber's work resonated through the 20th century, saving billions from starvation and condemning millions to death. Yet even his greatest achievement was not without consequences. When Haber unlocked the sky, he opened Pandora's box.
In nature, nitrogen is commonly the limiting compound in the growth of plants and algae. The rate of biological fixation is slow, so most nitrogen is derived from the decay of organic waste. When the leaves fall in autumn, the nutrients they contain are destined to eventually return to the tree from whence they came. Fungi and bacteria in the soil break the complex molecules into forms that are palatable to plant life. That is, unless we rake the leaves up; then we have to add a little something extra. And add we do: the synthetic nitrogen that we contribute to the environment will exceed 120 million tons in 2018. For comparison, the annual sum of biological nitrogen fixation is approximately 60 million tons. We have tripled the amount of new nitrogen entering the global system each year. This may seem to be a blessing: more nitrogen means more growth potential. However, once the nitrogen is in the system, we can't control what grows.
Any good villain will rise again. Billions of years after the Great Oxygenation, cyanobacteria are wreaking havoc. It is we that have fueled them. The fertilizer in our fields is washed into waterways, eventually finding its way into lakes and oceans. Cyanobacteria and other plankton are better equipped to utilize this excess nitrogen than aquatic plants, which results in toxic blooms. During a bloom, free-floating plankton—such as cyanobacteria, dinoflagellates, and diatoms—rapidly reproduce near the water's surface. The danger of such blooms is paradoxical: though cyanobacteria were responsible for oxygenating the world billions of years ago, they now kill by depriving systems of oxygen. Photosynthesis can only occur in the presence of sunlight. In darkness, plants and algae are unable to produce chemical energy, though they continue to consume it. In doing so, they use oxygen. At night, clouds of cyanobacteria suck dissolved oxygen out of the water they inhabit. In addition, at the end of their short lives, they fall through the water column. Upon landing at the bottom, they begin to decompose. Decomposition also consumes oxygen. When large numbers of phytoplankton die off, this process rapidly depletes oxygen levels. If levels are sufficiently reduced, things begin to die. Around the world, agricultural runoff and human waste initiate massive phytoplankton blooms which asphyxiate surrounding life. These are the dead zones.
There are hundreds of dead zones across the globe, ranging in size and persistence. Many grow large in the summer, when conditions for phytoplankton growth are optimal. The Gulf of Mexico now has a dead zone that is the size of Belize. The mighty Mississippi drains much of the United States' mainland, and it carries effluent from countless farms and cities into the Gulf. Massive fish kills are common and make for dramatic imagery, but the problem with dead zones goes deeper: even short periods of hypoxia—low oxygen—can alter the chemistry of the seafloor for years or decades. When hypoxic conditions persist, sulfur bacteria are one of the few groups that thrive. They produce hydrogen sulfide, which is highly toxic to oxygen-dependent life. When present in large numbers, sulfur bacteria render their surroundings uninhabitable to other species. Though they are now relegated to the sea floor, in the past they dominated Earth's ocean and spewed their toxic gas into the atmosphere. In fact, sulfur bacteria contributed to the apocalyptic extinction event known as the Great Dying.
There have been five major extinction events in the world's history. We are most familiar with the Chicxulub meteor impact that doomed the non-avian dinosaurs. In on way or another, all of the mass extinctions seem to have been the result of rapid changes to the chemical composition of the oceans and atmosphere. During the Great Dying, which occurred at the end of the Permian period 252 million years ago, more than 90% of marine species and 70% of terrestrial species went extinct. Many that persisted barely did so. Unlike the victims of the Great Oxygenation, these were not microbes: reptiles, amphibians, fish, plants, and insects were all present by this time. The ultimate cause may have been greenhouse gas emissions from enormous volcanic eruptions, which resulted in rapid global warming. As the climate warmed, hypoxic ocean conditions allowed sulfur bacteria to dominate. They belched hydrogen sulfide into the atmosphere, killing off plants and animals. The sulfuric gas also eroded the ozone layer, which increased the rate of warming and exposed the Earth to high-energy radiation. It took 10 million years for life to recover from the Great Dying.
To be clear, our entire ocean is not going to become hypoxic, nor will clouds of hydrogen sulfide gas dissolve the ozone layer in the foreseeable future. At least, one hopes not. It is exceptionally difficult to make long-term predictions involving the complex and interdependent biogeochemical cycles that govern our planet. The chemicals we add into the environment conspire together in unanticipated ways, and their combined impact is often greater than the sum of their parts. We know with certainty that atmospheric carbon dioxide levels are now the highest they have been in 800,000 years; in fact, it is likely that levels are higher than they have been in over 2 million years. Carbon warms the climate, and contributes to hypoxia and acidification in the ocean. These are profound issues unto themselves. Yet we have also disjoined nitrogen, phosphorous, and sulfur from their natural cycles. We rival the disruptive impact of the aforementioned ancient microbes. Unlike them, we should know better.
Throughout human history, Earth's environment has existed in a relatively stable state. We should not take that for granted: when a system is subjected to sufficient forcing, it may pass a tipping point, as described in the diagram above. Such a change can be effectively irrevocable. Many of Earth's deserts were once grasslands or forests; now that they are not, there is little hope that they will be again. Ecological stability is reinforced by negative feedback mechanisms. For example, as the ocean warms, more water evaporates, causing more clouds to form. Clouds can block sunlight from reaching the ocean, thereby preventing further warming. In this case, the feedback diminishes the effect of the perturbation. Conversely, positive feedback mechanisms reduce stability by enhancing the effects of a perturbation. The melting of sea ice is an example. Liquid water tends to absorb the Sun's heat, while ice tends to reflect it. As sea ice melts, the ocean retains more heat, causing more ice to melt. Positive feedback mechanisms are self-perpetuating. The closer we get to the edge, the more they will push us.
If this seems unnerving, perhaps it should. Our impact on the planet is such that we may already stand at the edge. One recent study found a 5% chance that the current concentration of carbon in the atmosphere poses an existential risk to humanity. Take a moment to appreciate the significance of that point: even if we completely and permanently eliminate greenhouse gas production at this very moment, this analysis found a 1 in 20 chance that the existing warming trend will be catastrophic. Such probabilities can seem abstract, but imagine you were told there was a similar chance that your house would someday burn down; how well would you sleep? We should avoid undue alarmism, but when objective scientific inquiry identifies a potential existential threat to life on Earth—even if the odds were 1 in 1000—there is legitimate cause for consternation.
The Intergovernmental Panel on Climate Change is an international organization that produces reports detailing the current understanding of climate science. Its most recent report describes four potential trajectories based on differing greenhouse gas concentrations. The choices we make in the coming years will decide which trajectory we follow. Some paths lead to salvation, others to perdition. The roads to redemption have a little-known catch: in order to avoid catastrophe, it is not enough to reduce emissions. We actually need to remove enormous quantities of carbon from the atmosphere. And we need to start soon. Some carbon removal techniques already exist, but they would need to be further developed, then implemented with unprecedented scale and rapidity. In the present moment, the gargantuan financial cost of rising to the occasion would likely be suicidal for the government of any democratic state; the danger of climate change remains less tangible to the public than other everyday concerns. Perhaps we should have gone with philosopher-kings.
The entirety of recorded history has occurred in the Holocene; the epoch's name means "wholly recent". It began at the end of the last glacial period approximately 11,700 years ago. There was only a few million of us then. Our global population trundled along in the millions for millennia; only around 1800 did we arrive at 1 billion. By 2100, we may surpass 11 billion. Much like the inadvertently genocidal bacteria of the past, we didn't become problematic until there were too many of us. When Malthus proclaimed the dangers of overpopulation, he foresaw starvation. But a century ago, as we stood at the brink of famine, we learned to conjure food from the air. We must act as the agents of our own deliverance again. For the first time in our history, the danger comes not from absence, but from plenty.
Global poverty ranks alongside climate change as the great challenge of our era. Yet the last century has been profoundly successful on that front: in 1990, 35% of the world's population lived in extreme poverty; by 2012, that number was 12%. This change represent 1.1 billion of the poorest people making an advancement up the economic ladder. There is still much work to be done, but recent advances have been truly remarkable. Unfortunately, as we eradicate poverty it exacerbates climate change. No sane person would recommend foregoing poverty reduction to prevent global warming, but income is unquestionably the primary determinant of a individual's ecological footprint. The world's wealthiest 10% generate 50% of greenhouse gas emissions. All our reusable shopping bags and hybrid cars amount to very little: studies have shown that most of our green behaviours do not substantively alter our footprint, because the structure of our society makes excess unavoidable. As the world's poor increasingly and deservedly improve their economic standing, their contribution to warming will increase. Even if they only approach a fraction of our prosperity, their sheer numbers will exacerbate the world's environmental issues. We cannot fight the demographics. We must innovate. We must move beyond empty virtue signalling and ask more of our governments, our industries, and ourselves. We must commit to deep, structural reforms, rather than mere window dressing. It will not be painless, but redemption rarely is.
Geological epochs are named for their defining characteristic. Some argue that our impact has been so profound that Earth has entered the Anthropocene, the age of humanity. Given our current standing, Shelley's Ozymandias seems a fitting epilogue:
Thanks for reading.