This essay, while a bit dated, is still spot-on as a layout of what got us into this predicament. I have also posted this essay on my Resources Page under the category, Manifestos. A very clear description of exponential growth and the limits to growth… and how we have powered ahead oblivious to both.



Sid Smith All the Bunnies in the Meadow Die

All the Bunnies in the Meadow Die

Take a pinch of the spores of a certain fungus and drop them into a gallon jug, one whose walls are coated with nutrient so the fungus can grow as fast as it likes. Suppose you do this one morning, and that exactly 4 days later you find that the fungus has filled the jar and is just beginning to overflow it. How full was the jug the morning of the 3rd day?

    1. Three-quarters full.
    2. Half full.
    3. One-quarter full.
    4. Mostly empty.

This question is one I pose to students in general math courses at the start of my standard lecture on modeling natural growth, a lecture with the deliberately provocative title of this chapter. I find that if the lecture isn’t provocative students are unlikely to absorb the main point, because people tend to think of growth as something steady. Trees grow a few feet each year just as children grow by 5 to 7 pounds each year. Our lives grow with the steady accumulation of the years themselves.

Growth, we think, means adding a fixed amount for each fixed period of time. But most natural growth is nothing like that. In most kinds of natural growth it is not a fixed amount but a proportional amount that is added at regular intervals. This is why so many rates of growth, such as the growth of an investment or the growth of a population, are expressed in percentages-per-unit-time.

If natural growth matched our intuition, the jug would be three-quarters full the day before it overflows. But in fact the jug is still mostly empty the day before. The correct answer is D. One way of understanding how this could be so is to think in terms of doubling times. In natural growth the time it takes for whatever is growing to double in size is always the same regardless of how big or small it is. For the fungus to fill a gallon jug in 4 days starting from just a pinch means it has to double about once very 7 hours or so.* So 7 hours before the jug overflows it is half full, and 7 hours before that it is one-quarter full, and 7 hours before that it is one-eighth full. Three times 7 is 21, so when we check the jug 24 hours before it overflows it is less than one-eighth full—mostly empty.

Now to make it vivid.

Suppose we start with 10 breeding pairs of bunnies living in a fenced meadow. There is no such thing as an infinite meadow: every meadow produces just so much food and water, and has just so much space. Suppose for the sake of the example that this is a large meadow that continuously produces enough food and water and contains enough space to support a thousand rabbits. Now the doubling time for populations of rabbits under ideal conditions is about 3 months, or 1 season, so they double their number 4 times per year. Thus after the first year our starting population of 20 rabbits has doubled 4 times, from 20 to 40, then to 80, then to 160, then to 320. So the second year starts with 320 rabbits. How many rabbits are there at the start of the third year?

The correct answer is 0. At the start of the third year there are no rabbits in the meadow at all. Not only that, the meadow is gone too. This is because the rabbits’ population went into overshoot. In the first three months of the second year their numbers doubled from 320 to 640 and everything was going along fine. By midyear they had doubled again, to 1,280, and were eating up the available food faster than the meadow was regrowing it. Things were getting crowded too, but there was no panic; all the rabbits were still getting enough to eat, even though all the fresh, green shoots were gone and everybody had to eat the tough, older leaves and stems. But by the autumn the population had doubled again, to 2,560—far more than the meadow could support. Having eaten all the leaves, the rabbits ate the stems down to the ground. As they began to starve, in desperation they dug up the roots and ate those. In the end they ate each other. When the last rabbit died from starvation, it died in a desert.

This is a horrifying outcome, and fortunately nature has ways of preventing it. Coyotes, to begin with. Also, when food supplies become stressed the strongest and luckiest survive while others, weakened by hunger, succumb to predators and disease. In the real world populations of rabbits and of all animals fluctuate, sometimes wildly, from year to year, but over time tend to remain in rough balance.

Humans are an altogether different kind of animal. There is nothing to play the role of coyotes for us—we are the top of the food chain. We don’t have to depend any longer on natural processes alone for our food, because with machinery and fertilizers and science we can extract from Mother Nature far more than she would give on her own. Disease too, while not conquered, is nothing like the scourge it once was. With the aid of technology we can fit vast numbers of us into comparatively small spaces. There are few natural forces left to constrain our numbers, and because of this our rate of population growth increased dramatically in the 20th century while our doubling time decreased from half a millennium to half a century.*

Humans are a different kind of animal, but not so different that the laws of mathematics don’t apply to us. We live in a finite meadow, called the Earth. There is no other to which we might go. With all our knowledge and all our technology, there is still an absolute limit to how much this meadow can provide. We are not as prolific as rabbits, but our population is 7 billion and growing; our current doubling time is about 65 years.*

And we are in overshoot. It is not a question of if we will overshoot our environment, and it is not a question of when. We are in overshoot—right now. Sometime in the 1970s came the first year when the ecological footprint of humanity became so large that the impact on the planet was no longer sustainable.* Now, some 30 to 40 years later, we use up about 150% of the planet’s yearly supply of resources every year.* In effect, if we apply the rabbits-in-the-meadow analogy, we are at the point of eating the stems.

There are several aspects that must be considered in measuring the degree and effects of human overshoot. They include the availability of food and water, disposal of waste, energy, biodiversity, and climate change. Each of these affects all the others.


Predictions of mass starvation owing to overpopulation have been made for 200 years, but have not come to pass.* This is because our capacity to produce food has increased far beyond what was predicted. In the last 45 years in particular food production globally has more than doubled, keeping pace with the growth in population, even though the amount of land used for agriculture has scarcely increased at all.* This “green revolution,” as it is called, is the result of four factors: Increased irrigation from underground aquifers, the increased use of manufactured fertilizers and pesticides, the use of machinery to more efficiently plant, harvest, process, and distribute food, and the development of higher-yielding, more disease and insect-resistant crops.** However, currently a billion people, 1 in 6, does not get sufficient nutrition, and an additional billion people are at risk.* Moreover, in order to keep pace with the population food production would have to double again in the next 35 years,* and this can’t happen. Instead, food production is going to decline soon, and before long decline drastically.

One reason is that three of the four factors that made the green revolution possible cannot be sustained.

First, water. The United States’ Great Plains, often called the breadbasket of the nation, produces enough grain for the country to be the world’s biggest exporter of food.* However, this is possible in large part because of irrigation with water pumped from the Ogallala aquifer, an immense underground body of fresh water that stretches from South Dakota to Texas. Because of the geology and weather patterns of this region the aquifer is not replenished, and it is being used up so fast that in many areas water can no longer be pumped from it. It will be largely unusable for agriculture within 25 years.* This situation is mirrored across the globe, as aquifers that have driven increased food production in Europe, Asia, India, Africa, Australia, and the Middle East are drying up.* There is no replacement for water. As these aquifers run dry, food production will decline sharply.

Both the use of manufactured pesticides and fertilizers and the use of agricultural machinery depend on petroleum, which is a finite, non-renewable resource. Every calorie of food grown in the developed world requires many calories of fossil fuels to produce.* Global production of petroleum has either peaked or will peak within the decade. This does not mean that we will run out of oil; it means that every new barrel of oil will cost more to produce than the one before.* This is occurring at a time when the global demand for oil is increasing, so the average price of a barrel of oil is now more than four times what it was just 10 years ago, and continues to rise ever more steeply.* The result is that the cost of producing food is increasing, and will soon increase dramatically.* The increased price of oil was a principle factor in the 2007–2008 food price crisis that led to world-wide food riots and political instability,* a harbinger of events to come.

Fossil fuels are necessary not just for the increased food production of the last 45 years, but even to obtain previous levels of food production. This is because the fertility of the soils in the major food-producing regions has been depleted by industrial agriculture,* so the only way necessary nutrients can be supplied to growing crops is through the use of massive quantities of chemical fertilizers. For this reason, the role of fossil fuels in high-production agriculture cannot readily be filled by some other energy source. All the wind, solar, geothermal, and nuclear energy we could produce will not fertilize the soil.

Not only has the soil in which crops must be grown been depleted of its nutrients, the soil itself is disappearing. In the Great Plains about half of the original topsoil is gone, lost to erosion from industrial farming practices and poor soil management.* Current erosion of topsoil in the United States is occurring at a rate 10 times faster than nature can replace it, and this symptom of overshoot too is mirrored around the globe.* At the current rate of loss, all the Earth’s topsoil will be gone in 70 years.*

A billion people rely on fish as their primary protein source, and fish accounts for about 20% of all protein in the human diet. However, the world’s supply of fish has begun to collapse.* Three quarters of the world’s fish stocks are in distress,* and populations of the largest food fish including tuna, swordfish, marlin, cod, halibut, and flounder have been decimated—literally, their numbers are a tenth of what they were in 1950.* The ocean ecosystem cannot recover unless there is a large decrease in human consumption of fish.

Meanwhile the production and consumption of meat is increasing worldwide—tripling in the last 40 years**—and this is contributing to a net decline of food production in two ways. First, raising meat, either by grazing on land that could be producing food crops or in feedlots where the animals are fed grain and fodder, is a very inefficient way to produce food because the animals consume many times the value in food that they produce, and require many times the petroleum and water resources too.* Second, increased grazing in the 40% of global agricultural land that is arid has led to a process of desertification that is accelerating. At least 10–20% of drylands have turned into deserts. The Sahara desert is expanding southward at a rate of nearly thirty miles per year.* In China, half a million square miles have turned to desert in the last 10 years.** This land will not recover within time-frames that are relevant to our civilization.*

Only 10% of the Earth’s land can support agriculture.* The United Nations Food and Agriculture Organization reports that 25% of this land is highly degraded and that an additional 44% is slightly to moderately degraded.* There are no new frontiers, no fertile lands waiting to be put to use. Every part of the planet that can be exploited for human use is already being overexploited: humans are rapidly turning enormous regions that once supported agriculture into environments that cannot even support life. The meadow-to-desert conversion is well underway.


Humans need fresh water. Without it, we die in a week. We need water that is uncontaminated by pollutants, disease organisms, and parasites. To sustain a civilized existence, we need extra water for sanitation, both for bodily cleanliness and to safely process human bodily wastes. At present about a billion people do not have adequate drinking water. They and an additional billion and a half people do not have adequate water for sanitation, a number that is increasing rapidly.* Half of the world’s sick are sick because of inadequate water, and it is the leading cause of death for young children.* This is no longer just a third-world problem: 80% of the global population is “exposed to high levels of threat to water security.”* That’s 5.6 billion people.

Much of the current water crisis is owing to mismanagement, ignorance, and inadequate infrastructure in poorer parts of the world. However, human overshoot is becoming the dominating factor, with the needs of agriculture driving the shortages. About 70% of available fresh water goes to agriculture, and as noted above that resource is being depleted far faster than it can be renewed.* The depletion of aquifers results in reduced amounts of water flowing at the surface. Rivers are disappearing. The Yellow River in China, the Colorado River, and the Indus River in Pakistan no longer reach the ocean. This not only affects water availability for human consumption and use, but has grave implications for the ecosystems and biodiversity that depend on these waters.*


Each of 315 million Americans generates, on average, nearly 4 1/2 pounds of waste every day. Together we produce about 250 million tons every year. This is three times the amount we produced 50 years ago.* Although Americans produce more waste per person than in any other country, the rest of the world is catching up. Precise estimates are difficult, but total solid waste production world-wide is in the billions of tons. The World Bank recently warned that this portends an economic and environmental burden as catastrophic as climate change.*

This waste is mostly just dumped, although some is incinerated or buried. It is a primary contributor to environmental degradation including loss of arable land, air and water pollution (including poisoning of groundwater from leaching), disruption of local ecosystems, and global warming.* Millions of tons are dumped into the world’s oceans each year, destroying marine habitats such as coral reefs. Dozens of large pieces and many thousands of tiny pieces of plastic litter are floating on every square mile of ocean.* This plastic absorbs non-water-soluble poisons such as DDT, so when it is ingested by fish these poisons enter the food chain.* Recently vast islands of floating trash have been discovered around the globe, some larger than the state of Texas.*

Even greater damage to fresh and ocean-water systems is caused by agricultural pollution, especially excess nitrogen. Areas like the 8,000 square mile dead-zone in the Gulf of Mexico at the mouth of the Mississippi River, caused by fertilizer runoff from as far away as Minnesota, are becoming commonplace.* The Mississippi River itself, one of the great rivers of the world, has been destroyed as a functioning ecosystem*—it is now just the biggest of several national sewer pipes emptying into the sea. It is difficult to find any stream or other body of water in the world that is unaffected by waste, a fact closely related to the threat to water security for the majority of the world’s population.


There are just a few sources of energy on Earth: nuclear, geothermal, tidal, and everything else—and the everything else is solar. Wind power is really solar energy, as is hydroelectric. Most importantly, all fossil fuels are solar energy, banked by the planet over vast geological time frames.

Between 100 and 135 billion metric tons of petroleum have been extracted and used since 1850.* About 190 billion tons remains, which at current rates of extraction is about 60 years’ worth. However, only 30% of the remaining oil is conventional light crude. The remainder consists of much-harder-to-extract-and-process heavy crude and tar sands.* Although there are many ways to replace both the energy and materials that crude oil provides, there is no way to replace petroleum that will allow us to consume resources at the rate we now do. This is because petroleum is a uniquely concentrated form of chemical energy with many important properties:

  • It is lightweight,
  • It is easy to store and transport,
  • It is comparatively safe to handle,
  • It releases its energy as slowly or as quickly as almost any application requires, and
  • It costs (while the light oil lasts) little to bring it to market.

One gallon of gasoline will do about the work of 13 kilowatt hours of electricity, taking into account typical efficiencies.* However, the gallon of gas weighs 7 pounds and can be held in a tank costing a few bucks, and the bank of batteries to hold 13 kilowatt hours ready for immediate use weighs 435 pounds and costs $8,000.** Also, those batteries require several hours to charge fully. If you wanted to create those 13 kilowatt hours yourself using a typical household solar installation rated at 5 kilowatts, it would take your system several hours on a sunny day—or several cloudy days—to do so, longer if you live in a northern state.*

Even on a very large scale, electricity is two to three times more expensive to produce with renewable sources than with fossil fuels.* In theory, nuclear fission can be used to produce electricity as cheaply as fossil fuels can, and without the greenhouse gas emissions. In practice however, avoiding the risk of catastrophic accidents such as happened at Chernobyl and Fukushima makes the costs of constructing, maintaining, and eventually decommissioning nuclear energy plants prohibitive***—even leaving aside the horrific long-term risk to people and the environment they represent—which is why countries around the world are abandoning their nuclear energy plans.

But even supposing the cost of generating electricity could be held low, perhaps by some yet undiscovered or unproven technology like nuclear fusion, we still face an unavoidable crisis. Using electricity to create the kinds of fuels and materials necessary to sustain intensive agriculture would still require far greater time, effort, and money than simply pumping oil out of the ground and refining it. Moreover, such fuels and materials themselves can only be created—absent fossil fuels—from agricultural products. Replacing even a small fraction of our current fuel and fertilizer requirements with renewable sources diverts a significant percentage of agricultural output away from food production; diverting 25% of the United States’ corn crop in 2007 to ethanol production resulted in 6 billion gallons of ethanol, an amount equal to less than 5% of all the gasoline we used that year.* Worse, when the petroleum products used to grow the corn to make the ethanol are figured in, the net gain was at best 2 billion gallons of ethanol, and some analyses suggest there was no net gain at all, but a net loss.* It isn’t possible to grow enough to create enough fuel and fertilizer to allow us to grow enough.

A century or so ago humanity stumbled upon a great prize: the accumulated store of eons-worth of solar energy in the form of petroleum and other fossil fuels. This discovery is the primary reason for the sudden and rapid rise in our numbers. But we didn’t understand soon enough that it was not just a prize but a trap; that once the store was spent there could be no replacement. There is now no escape. Within the lifetimes of those now living humanity will revert to a level of consumption that has not been seen in the developed world for 100 years.


Comparatively few of the millions of species of plants and animals are, by themselves, of obvious importance to humans and their survival. However, every species of plant or animal is part of a complex web of relationships that sustains a stable ecosystem. Changing those relationships by reducing or removing any one or more species can have far-reaching consequences, and it is impossible to predict at what point such interference will cause the ecosystem to break down.* The destruction of any ecosystem is a direct threat to humans because we depend on healthy ecosystems for processes that are essential to our survival. Some of the most important roles that ecosystems play are:*

  • To replenish and fertilize the soil,
  • To filter and purify water and recharge aquifers,
  • To absorb carbon and regulate the climate,
  • To pollinate food crops,
  • To sustain fisheries, and
  • To absorb and recycle waste.

From 2001 to 2005 the United Nations sponsored the Millennium Ecosystem Assessment, a wide-ranging study involving more than a thousand scientists. The assessment concluded that 60% of ecosystems are degraded or unsustainably managed.* As a result, the current rate of species extinction is 1,000 times higher than normal.* This rate is consistent with those of the five previous “Great Extinctions” in the fossil record, brief periods when diversity was reduced by more than 75%, the most recent of which 60 million years ago wiped out the dinosaurs. Absent a dramatic turnaround in the impact of humanity on the planet, we are witnessing the sixth Great Extinction.* The unavoidable result is the collapse of the ecosystems on which human civilization depends.

Climate Change

The Earth is not a stable environment but a profoundly complex system undergoing continual change. The present temperate climate in particular is neither permanent nor even typical; even the recent geological past has seen periods of both warmer and much colder weather. The constant changes in climate are owing to many factors, from the behavior of the sun to the influence of physical and biological processes on Earth.

Presently the Earth is getting warmer. What is different this time is that it is getting warmer much faster than can be explained by any known natural process. Each of the last 11 years has been one of the warmest 12 on record, and 2012 has thus far (this was written in August) been the hottest year ever recorded. This can only be explained by the influence of humanity** and is therefore also a feature of overshoot.

Since the start of the 21st century temperatures on land each year have been between 1 1/4 and 1 3/4 degrees fahrenheit above the average for the 20th century. The same period has seen a marked increase in destructive weather events such as floods, droughts, and storms, and these are now confirmed to be a result of the higher temperatures.** Additional visible consequences are record melts of polar ice, the vanishing of glaciers, shifting habitats and behaviors of many animal species, changes in seasonal vegetation growth and habitat, and rising sea levels.*

Every climate model predicts that the temperature will continue to increase rapidly at least in the medium term, and there is no indication that any effective program to prevent or even mitigate continued forcing of the climate by human activity is politically possible.* The probable consequences for those now living include:**

  • Greater extremes of both hot and cold weather, as seasonal oscillations increase in amplitude.
  • More severe flooding, more severe droughts, and larger and more intense forest fires.
  • More intense storms, in particular more intense tropical cyclones, including Atlantic hurricanes.
  • Reduced water resources in semi-arid areas such as the western United States.
  • Shifting wildlife habitats resulting in accelerated extinctions.
  • Movement of tropical diseases such as malaria into temperate regions.
  • Frequent and widespread crop loss.
  • A rise in average sea levels of at least several inches, up to over a foot.
  • Mass displacements and migrations of human populations.

These effects are all, for practical purposes, permanent and irreversible. Other more abrupt and catastrophic effects such as are known to have happened in the past are also possible. Although no single one appears probable, they all are consistent with the expected shift in climate.*

  • Runaway climate change, in which higher temperatures cause a sudden release of large amounts of greenhouse gases which cause yet higher temperatures, leading to a large amplification of all of the above effects.
  • A large-scale shift in ocean currents, completely and permanently disrupting all established weather patterns and leading to climatic and ecological chaos.
  • Runaway forest fires in the tropical zones, resulting in a severe loss of carbon-sequestering, oxygen-releasing plant-life and dramatically increasing the greenhouse effect.

The one certain consequence of climate change is increased stress to already overburdened natural and agricultural systems. The very recent past provides a number of examples, such as the severe world-wide droughts in most of the last 10 years (causing major food shortages) and an unrelenting series of severe floods, storms, and forest fires in the United States and elsewhere around the world during the same period. This year has seen record-setting droughts, with serious implications for the global food supply.* Global warming is an overshoot accelerant: it magnifies, exacerbates, and hastens all the other effects of human overpopulation.

* * * * *

It is difficult to summarize the consequences of human overshoot, and even more difficult to draw specific conclusions about its final outcome. As the physicist Niels Bohr remarked, prediction is difficult, especially of the future. However, if you see someone stacking blocks one atop another into a high tower, and if that tower gets many times higher than the base that supports it, you can say with certainty that if blocks keep being added it will topple over. You can’t say exactly when it will topple. You can’t say in what direction. You can’t say whether the collapse will begin at the bottom or somewhere in the middle. But—you can still know with a certainty that it must fall, and fall soon.

It is also true that the sheer number of people on Earth is not by itself what determines the severity of our overshoot; it is the average rate of consumption multiplied by the population. In this respect Americans have much to answer for, because our ecological footprint is several times larger than most of the rest of humanity’s.* If everyone on Earth consumed as much as the average American, all would have perished long before now.

So there are two things we can say with certainty. First, the size of the human population will drop very substantially. Estimates of the carrying capacity of the planet vary widely, but those estimates that take account of the facts presented here are much lower than the current population.* When we also take into account the severe degradation of the planet’s ecosystems that is likely yet to occur, we must be even more pessimistic.* How the drop in population will occur, when it will begin to occur, how long it will last, and what the population will be afterward is impossible to know. It may be disease, or war, or (the default) famine. It is likely to be a combination of things. But it cannot be avoided.

The second thing we can say with certainty is that the rate of human consumption must decrease to a small fraction of the current average in the developed world. In particular, anything like the current consumer culture will be quite impossible.

Human ecological overshoot is the defining fact of life on Earth for the foreseeable future. This issue doesn’t just dwarf other issues; it absorbs every issue—political, cultural, economic, and environmental—into itself. Far too many people are still asking how we can avoid the consequences of overshoot. This is like someone who is already falling asking how to avoid hitting the ground. The right question is, how can we best cope with and mitigate the consequences? How do we shape our lives to fit the future we have made for ourselves? Or rather, not our lives, for that die is cast. What can we do now so that our children’s children’s children may have a world to live in, in freedom, dignity, and peace?