# K: Calculating Earth’s limits

Published in

Emily Boring | Oct. 27, 2015

Most of my fears about climate change can be summarized by a single letter: K. K is the ecological term for carrying capacity, the maximum population of a species that a given habitat can sustain. The letter has the power to tell us whether or not our way of life on Earth can continue — and the numbers don’t look good.

The story of K is simple. Initially, most species grow at an exponential rate. When the population reaches a certain number of individuals, the habitat’s resources — food, water, or living space — start to run low. Without going into the math of the model, here’s the gist of what can happen next:

Option 1: The population slowly approaches K and stays there, balanced by natural feedback loops.

Option 2: The population overshoots K by a little, declines, rebounds, and eventually oscillates around K.

Option 3: The population shoots past K, ignores environmental feedback, dies off, and declines to extinction.

All populations, from plants to animals to people, are limited by the number of resources available in their habitats. Humans are no exception. Of the three scenarios above, which one sounds most likely for the human population? You guessed it: Case 3. Our growth rate is “super-exponential.” We’ll likely exceed 9 billion people by 2050. All those people will put an enormous strain on Earth’s resources, which, thanks to climate change, have never been more unstable. Put together a huge population with fast-disappearing resources, and the outlook is grim. What can we expect for the future of the human species?

K offers a simple ecological lens to examine this complex question. The model is built on three key concepts: limits, resources, and feedback loops. It’s not a perfect forecast or a perfect fit; ecology is messy, and human beings are a particularly messy species. But by looking to K, we can begin to ask some crucial questions. What resources set our population’s upper boundaries? How many have we crossed already? Are we capable of avoiding the worst of the collapse?

First and foremost, K invites us to examine our own limits. My environmental wake-up call came a few years ago when I read Bill McKibben’s article, Global Warming’s Terrifying New Math. McKibben argues that to stay below 2 degrees Celsius of warming (the “safe” upper limit), we can emit 565 gigatons more CO2 in the form of fossil fuels. There are 2,795 gigatons of CO2 left in the ground. His conclusion: to salvage a livable future for our planet, we need to leave 80% of our fuel reserves untouched. In other words, we need to do the virtually unprecedented: impose our own limits before reaching those set by finite natural resources. Instead of retroactively responding to shortage after we have passed K, as most species do, we need to anticipate at what point we’ll run out of resources and choose to stop short.

This choice is not particularly popular among the giants of the fossil fuel industry. It’s also radical within our culture of consumption and economy of unquestioned growth. This kind of self-regulation seems so unprecedented, in fact, that it’s earned some strong titles. Biologists call it an evolutionary leap. Historians call it a turning point for civilization. Do we have the values — altruism, ability to look ahead, desire for collective wellbeing — that would enable us to stop short of K?

Unfortunately, fossil fuels are only one type of resource we need to worry about using up. What other factors might set the limit on human growth? Unlike many species in nature, the value of K for humans can’t be determined by baseline factors like food, water, and shelter. Humans aren’t satisfied with mere survival. We want comforts, luxuries, and freedoms. We have infrastructure — in production, technology, transport, and communication — that transforms our pursuit of basic resources to a complex and dynamic global web.

Clearly, we need a model of human population limits that includes the wide array of resources on which humans depend. The Nine Planetary Boundaries, a concept developed by the Stockholm Resilience Center, offers just such an approach. The model proposes nine interacting thresholds, from ocean acidification to ozone depletion to freshwater use, that act as warning flags of the planet’s resilience. We’ve already crossed three of the nine boundaries. To avoid collapse, we need to stay beneath the rest.

The Nine Planetary Boundaries model gives us a clear idea of how and when we’re likely to exceed our resources, and what factors might be the tipping points. It also offers clear suggestions about the areas in which we need to limit our growth. We cannot use all of our water; we’re almost at the “Freshwater” boundary. We cannot harvest all our rainforests; we’ll max out in “Land-system Change.” Unfortunately, naming our limits does not mean we will choose to respect them. We’ve passed three boundaries, and we’re well on our way to crossing more. Why have we not yet taken strong measures to slow our growth? How many more boundaries can we cross before we’re forced to act?

Earth’s nine thresholds. (Image from Wikimedia)

The issue of planetary limits brings up a final, crucial aspect of K: our problematic lack of feedback and accountability for resource use. In the U.S., our economic system is notorious for masking the true cost of consumption. Five-dollar t-shirts hide the price of production, disposal, and labor; the seemingly endless fuel from our gas station pumps belies an impending oil crisis. Similarly, in the developed world culture of unquestioned growth, climate change can seem comfortably abstract. Rising sea levels, mounting atmospheric CO2, and retreating glaciers don’t yet impact our daily lives. The t-shirts keep coming and the gas pumps flow. Our distance from the problem, and society’s ability to mask the crisis, is a consequence of humanity’s rare ecological position: only humans have the social structure and technology to stretch nature’s limits and ignore the response. Can we acknowledge the cost of our growth within a system built to hide it?

Increasingly, however, climate change is making itself known. Global warming is imposing itself on people’s daily lives; it’s getting more and more difficult to dismiss the problem as abstract. Last summer, the consequences of climate change arrived in my backyard. Oregon’s lakes are dry. Our ski mountains are closed for lack of snow. Our blueberry season came four weeks early and ended in July. Much more drastic feedback is apparent in the developing world. In the Philippines, mass typhoons kill thousands of people. In Bangladesh, rising sea levels threaten agriculture. As close as the South Bronx, fossil fuel emissions cause triple asthma rates for children of color. It shouldn’t come as news that the costs of climate change fall disproportionately on the people least responsible for pushing us toward K. In this light, K cannot remain an abstract ecological concept. K becomes an issue of environmental justice and ethics, demanding a swift and strong response.

By looking to K, we have constructed a simple model for a complex and weighted dilemma. We are approaching our limits, and approaching them fast. We’ve already crossed many boundaries — so many, in fact, that some measure of decline may no longer be avoidable. Ecology offers several clear predictions for our situation. If we continue our current trajectory, we are on course to hit K, overshoot, and live — or not — with the consequences. To avoid K at this point would take a kind of foresight and altruism rare in ecological systems. Will we be the exception to the rule?

As much as it fills me with fear, ecology’s outlook is not entirely bleak. Examples in nature suggest that maybe the traits we need to overcome K — cooperation, forward-looking, collective interest — aren’t so unheard-of after all. Chimpanzees in the wild form kinship bonds, save food for the future, and sacrifice their own needs for a group. This network is built on an “I’ll help you because you’re my kind” mentality. The question is not whether we’re capable of these things. The question is how we can extend our capacity for “kin cooperation ” to include all 9 billion members of Earth’s community.

Alternatively, one could argue that humans are simply not like other species. Perhaps we are so fundamentally different — in our culture, our moral systems, and our ability to pass down knowledge and equate cause and effect — that population ecology is not a helpful approach. Instead, we might turn to history, or literature, or social science for a more humanistic approach. How do people respond to scarcity of resources, systems of injustice, and huge structural change? Historians point to the American Abolitionist movement as an example of acting beyond one’s own immediate needs to take a moral stance for societal change. Activists compare the current movement for environmental justice to the mobilization, solidarity, and collective vision of Civil Rights. These frameworks have a key thing in common. They involve searching for narratives of the human capacity for innovation, cohesion, and sacrifice. We need stories that prove that these values have precedence — that it’s truly possible for humans to anticipate K and respond to it before we get there.

More than an ecological tool, I see K as a valuable lens to examine what is deeply human: our dual capacity for adaptation and altruism, but also for over-consumption, ignorance, and greed. History and biology offer many uplifting narratives, but I’m equally aware of the alternate endings: collapse of civilization, total extinctions, and the suffering of the many at the hands of the few. When I hear about climate deniers, or the government’s decision to go forward with fossil fuel campaigns, or the dumping of nitrates in coral reefs, these alternatives feel real, terrifying, and close. I don’t know which side will win out. At a basic level, we are a species of 7 billion individuals facing the limits of a finite habitat called Earth. The ecologist in me is terrified. The person in me has to believe we will rise and respond. How will we act? What will it look like? And when?

Emily Boring is a sophomore in Pierson College. Contact her at emily.boring@yale.edu.

(Featured image from here.)