Unfortunately, there seems to be no way to get those two giant forces into equilibrium in any gentle fashion, because in every year that has gone by for the last few thousand years — and every year that will arrive — the human population of Earth is automatically adjusted so that it is roughly equal to the planet’s carrying capacity. Like so many other animals, human beings always push themselves to the limits of that carrying capacity. The Age of Petroleum made us no wiser in that respect, and in fact dependence on fossil fuels has led us to a crisis far greater than any in the past.

For the average human being, life has always been a matter of bare survival, and the same is true now. Population growth is soaring, whereas oil production is declining. If, at the start of any year, the world’s population is greater than its carrying capacity, only basic arithmetic is needed to see that the difference between the two numbers means that mortality will be above the normal by the end of that year. In fact, a simple calculation shows that before the year 2050 there will be about 3 billion deaths above normal, with a grand total of about 4 billion by the end of the century. Whether there are any partial solutions to that crisis is something to be considered at the end of this argument.

Depletion of oil and other fossil fuels will greatly affect food production. In terms of its effects on daily human life, in fact, the most significant aspect of oil depletion will be the lack of food. “Peak oil” is basically “peak food.” There will be severe problems with transportation (e.g., shortages of diesel fuel and asphalt) and communication (e.g., sources of electrical power), as well as with more immediate aspects of getting crops to grow (e.g., the use of fertilizer and pesticides, and the availability of irrigation). Crop yield-per-hectare is far lower in societies that do not have fossil fuels or modern machinery. Maize production, for example, declines by about 80 percent in the absence of modern technology, as David Pimentel notes in “Food and Energy Resources.” We should have no illusions that several billion humans can be fed by “organic gardening” or anything else of that nature.

Over the next few decades, therefore, there will be famine on a scale several times larger than ever before in human history. Let us refer to those above-normal deaths as “famine deaths.” There will, of course, be famines for other reasons during those years. It is also true that warfare and plague will take their toll to a large extent before famine claims those same humans as its victims.

The increase in the world’s population has followed a simple curve: from about 1.6 billion in 1900 to about 6.1 billion in 2000 A quick glance at a chart of world population growth, on a broader time scale, shows a line that runs almost horizontally for thousands of years, and then makes an almost vertical ascent as it approaches the year 2000. Of all the humans who have ever lived on the Earth, most were born in the last 50 years. That is not just an amusing curiosity. It is a shocking fact that should have awakened humanity to the realization that something is dreadfully wrong.

Mankind is always prey to its own “exuberance,” to use William R. Catton’s term in “Overshoot.” That has certainly been true of population growth. “Do you have any children?” or, “How many children do you have?” is a form of greeting or civility almost equivalent to “How do you do?” or, “Nice to meet you.” World population growth, nevertheless, has always been ecologically hazardous. The destruction of the environment reaches back into the invisible past, and the ruination of land, sea, and sky has been well described if not well heeded. But what is even less frequently noted is that with every increase in human numbers we are only barely able to keep up with the demand: providing all those people with food and water has not been easy. We are always pushing ourselves to the limits of Earth’s carrying capacity.

Even that is an understatement. In the late twentieth century it could be said that we actually went beyond the carrying capacity. No matter how much environmental degradation we created, there was always the sense that we could somehow “get by.” But in the late twentieth century we stopped getting by. It is important to differentiate between production in an “absolute” sense and production “per person.” Although oil production, in “absolute” numbers, kept climbing — only to decline around 2000 or 2010 — what was ignored was that although that “absolute” production was climbing, the production “per person” was not. In the year 1990 there were 4.5 barrels of oil per person per year. By the year 2000 there were only about 4.3. As the FAO has discovered, the same sort of problem was occurring with world grain supplies: although government sources cheerfully tell us that grain production in absolute terms is still increasing every year, what they are not telling us is that because of overpopulation the amount of grain per person is actually declining. There is more grain, but there are more mouths to feed. The same problem of resources “per person” can be seen in the world’s fish catches. We were always scraping the edges of the earth’s carrying capacity, but we are now entering a far more dangerous era. The main point to keep in mind, however, is that, throughout the twentieth century, oil production and human population were so closely integrated that every barrel of oil had an effect on human numbers.

While population has been going up, so has oil production: from about 0.1 billion barrels in 1900 to about 4.2 in 1950, to about 27.0 in 2000. (The data are readily available in many publications, such as BP’s annual “Global Statistical Review of World Energy,” and John Gever et al., “Beyond Oil,” is still valuable.) According to most estimates, the peak was (or will be) around the year 2010. The rest is a steep drop: 20 billion barrels in 2020, 15 in 2030, 9 in 2040, 5 in 2050.

The relation between population and oil production is one of cause and effect. The skyrocketing of population is not merely coincident with the skyrocketing of oil production. It is the latter that actually causes the former: that is to say, oil is the main source of energy within industrial society. With abundant oil, a large population is possible — ignoring, of course, the many other things that might wipe out human numbers anyway. Without that abundant oil, on the other hand, a large population is not possible. It was industrialization, improved agriculture, improved medicine, the expansion of humanity into the Americas, and so on, that began the upward climb, but it was oil that allowed human numbers to triple in less than 100 years. Yes, there are other sources of energy, but these range from the inefficient to the irrational.

Incidentally, carrying capacity does not increase in direct proportion to the number of barrels of oil per person, because as the population goes up there is more strain on the environment. As a result, we were comfortable enough with 1 barrel per person in 1940, but less comfortable with 4 barrels per person in 1990.

Because oil production is the determining factor in population growth, we now have a useful set of numbers: the “existing population” for any given year in the past is roughly the same thing as the “carrying capacity” for that year. We can thereby deduce another useful set of numbers: the “existing population” at the start of any given year in the future must decrease to become the “carrying capacity” for that year. “For any industrial society, fossil-fuel production determines carrying capacity”: that is an immutable law.

Human population will collapse in any year in which there is a difference between the initial population and the carrying capacity. The equation is not complex: (A) the previous year’s population (in billions) can be subtracted from (B) the carrying capacity (in billions) to give us (C) the number of deaths (in billions) by famine. The data for the carrying capacity of any future year can be inserted by looking at similar data for oil production and population in the years 1900 to 2000. Some samples of future years are as follows.

2031 (oil 13.8G bbl): (A) 3.5000 minus (B) 3.4465 equals (C) 0.0535

2032 (oil 13.2G bbl): (A) 3.4465 minus (B) 3.3937 equals (C) 0.0527

2033 (oil 12.6G bbl): (A) 3.3937 minus (B) 3.3418 equals (C) 0.0519

The “normal,” non-famine-related, birth and death rates are not included in those 50 million annual deaths, since for most of pre-industrial human history the sum of the two — i.e. the “growth rate” — has been nearly zero. And there is no question that the future will mean a return to the “pre-industrial.”

Of course, it will often be hard to separate “famine deaths” from “normal deaths.” War, disease, global warming, topsoil deterioration, and other factors will have unforeseeable effects of their own. It is probably safe to say, however, that an unusually large decline in the population of a country will be the most significant indicator of famine.

These equations obliterate all previous estimates of future population growth. Instead of a steady rise over the course of this century (as generally predicted), there will be a clash of the two giant forces of overpopulation and oil depletion, followed by a precipitous ride into the unknown future.

The term “carrying capacity,” as used here, incidentally, is not entirely synonymous with the term as used by William R. Catton. A more-appropriate but lengthier and clumsier term might be something like “temporary feeding capacity.” For the last 50 years or so, human population has always expanded to press against the very limits of what is possible for the oil-production level of any particular year — although in recent years even that tight correspondence has started to collapse.

The figures in the above list can only be approximate, of course, and even the most elaborate mathematics will not entirely help us to deal with the great number of interacting factors. We need to swing toward a more pessimistic figure for humanity’s future if we include the effects of war, disease, and so on.

The most serious pessimistic factor will be largely sociological: To what extent can the oil industry maintain the advanced technology required for drilling ever-deeper wells in ever-more-remote places, when that industry will be struggling to survive in a milieu of social chaos? The future will be anarchy. Intricate division of labor, large-scale government, and high-level education will no longer exist.

If the above figures are correct, we are ill-prepared for the next few years. The problem of oil depletion turns out to be something other than a bit of macabre speculation for people of the distant future to deal with, but rather a sudden catastrophe that will only be studied dispassionately long after the event itself has occurred. Doomsday will probably be upon us before we have time to look at it carefully.

The world has certainly known some terrible famines in the past, of course. In recent centuries, one of the worst was that of North China in 1876-79, when between 9 and 13 million died, but India had a famine at the same time, with perhaps 5 million deaths. The Soviet Union had famine deaths of about 5 million in 1932-34, purely because of misguided political policies. The worst famine in history was that of China’s Great Leap Forward, 1958-61, when at least 20 million died, but probably far more than that.

A closer analogy to “petroleum famine” may be Ireland’s potato famine of the 1840s, since — like petroleum — it was a single commodity that caused such devastation. Cecil Woodham-Smith describes the Irish tragedy in “The Great Hunger.” The response of the British government at the time can be summarized as a jumble of incompetence, frustration, and indecision.

On the other hand, there are elements of optimism that may need to be plugged in. For one thing, there is what might be called the “inertia factor”: the planet Earth is so big that even the most catastrophic events take time for their ripples to finish spreading. An asteroid fragment 10 kilometers wide hit eastern Mexico 65 million years ago, but we are alive today to tell the story.

Somewhat related, among optimistic factors, is the sheer tenacity of the human species: we are intelligent social creatures living at the top of the food chain, in the manner of wolves, yet we outnumber wolves worldwide by about a million to one; we are as populous as rats or mice. We can outrace a horse over long distances. Even with Stone-Age technology, we can inhabit almost every environment on Earth, although most of the required survival skills have been forgotten.

Specifically, we must consider the fact that neither geography nor population is homogeneous. All over the world, there are forgotten pockets of habitable land, much of it abandoned for the ironic reason that urbanites regarded rural life as too difficult, as they traded their peasant smocks for factory overalls. There are still areas of the planet’s surface that are sparsely occupied although they are habitable or could be made so, to the extent that some rural areas have had a decline in population that is absolute, i.e. not merely relative to another place or time. By careful calculation, therefore, there will be survivors. Over the next few years, human ingenuity must be devoted to refining our understanding of these geographic and demographic matters, so that at least a few can escape the tribulation. Neither the present nor future generations should have to say, “We were never warned.”