The United Nations (UN) projects that the global population will zoom from approximately 7.1 billion people today to 9.6 billion people by 2050. The World Bank and U.S. Census Bureau expect it to reach 9.4 billion. Other models suggest anywhere from 8.3 billion to 10.9 billion souls on the planet by 2050.

These models "assume a no-surprise future and focus on three main drivers of population -- births, deaths and migration. Life expectancy is kept on a consistent upward trend, and unpredictable events like epidemics and wars are ruled out," National Geographic explained in its population update this year.

In other words, the population forecasts that all policymakers around the world use are based on existing trends, not resources or surprise events.

But those who look at population through the lens of resources come to very different conclusions about population growth.

One example is geophysicist Jean Laherrère, whose work I have featured previously in this column. Laherrère compiled from various sources this chart of forecasts showing global oil production per capita:

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Oil production per capita peaked in 1979 at 5.5 barrels of oil per person per year, Laherrère calculates. That rate that can never be matched again, he says, because population continues to grow even as global oil production has hit a plateau, soon to be followed by an inevitable decline. By 2050, Laherrère calculates that the global per-capita oil consumption will have to fall to around half the 2012 level.

What does this mean? It means that those countries where oil consumption is well above the per-person average will experience intense pressure -- presumably in the form of high prices, at first -- to reduce consumption. Unless, of course, they can somehow compel the countries that already use the least oil to consume even less.

And who are those people? Another Laherrère chart shows per-capita oil consumption of some of the world's the top consumers (countries that consume at least 0.5 million barrels of oil per day):

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While the world average petroleum consumption per capita was 4.7 barrels in 2012, Laherrère found a huge variation from country to country: 81 barrels for Singapore, 43 barrels for the United Arab Emirates, 39 for Saudi Arabia, 24 for Canada, 22 for the United States and Belgium, 17 for Australia, 9.7 for France, 8.8 for the United Kingdom, 2.8 for China, 1.2 for Africa, and 1 barrel per person for India.

Compelling the nations that use the least oil to use even less will be difficult. China and India are right near the bottom of the per-capita consumption list, but they're also the most populous nations in the world. As I wrote in 2012 , they are able to tolerate much higher prices than the oil-guzzlers because they derive so much more economic value out of a gallon of fuel than their developed-world counterparts.

Fortunately, the competing pressures for oil will be assisted by demographic trends already in place. As fund manager Jeremy Grantham detailed earlier this year, fertility rates have been falling sharply across most the world. In most of the developed world (and China) birth rates are already below the replacement rates. Almost all of the countries with fertility rates still far in excess of the replacement rate are in Africa.

Grantham calls this decline in fertility "our last best hope" to avoid the worst effects of basic resource overshoot, in which the world consumes resources to the breaking point, then suffers catastrophic collapse.

But even under what Grantham calls the U.N.'s "optimistic" scenario, the world’s population peaks around 2050 at slightly more than 8 billion, then declines to around 6 billion by the end of the century.

Even that population scenario may be well in excess of what the remaining oil can support.

John Howe, an engineer, author, farmer, manufacturer of bicycle-powered generators, and (perhaps most famously) inventor of solar-powered tractors, has just self-published a spiral-bound book (his fourth) titled The End of Fossil Energy and Per-Capita Oil. It examines a series of scenarios based on various estimates for the world's remaining oil and population replacement rates.

The simplified chart of Howe's analysis shows that world per-capita oil consumption will fall to zero by the end of the century in all scenarios:

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In Howe's "Most Optimistic Case," global fertility rates start immediately at 1 child per female, and there are 1.8 trillion barrels of oil yet to produce. In this scenario, per-capita world oil consumption falls to zero around 2080.

In his "Less Optimistic Case," birth rates hold firm at 1.5 children per female, and there are 1.3 trillion barrels of oil yet to produce (this is roughly the remaining oil endowment assumed by most peak oil models). In this scenario, per-capita world oil consumption falls to zero around 2075.

In his "Most Probable Case," birth rates are equal to the replacement rate at 2 children per female, there are 1.3 trillion barrels of oil yet to produce, and the energy returned on energy invested (EROEI) ratio falls by 1 percent each year, reflecting the increasing difficulty of producing oil from ever-more-marginal sources. (For more on EROEI, also known as EROI, see " What EROI tells us about ROI .") In this scenario, per-capita world oil consumption falls to zero not long after 2060.

Here we can begin to see the huge gap between the trend-based population forecasts and the oil-based forecasts. As Howe's latter scenario is closest to the default path the world is on, it should be clear that the world cannot support 9.6 billion people by 2050, as the UN and other agencies expect, and have oil consumption per capita fall to zero a decade later. At a per-capita oil consumption of zero, for example, all commercial agriculture -- which runs on diesel -- would cease, along with all trucking and most of what we recognize as modern civilization.

Even if there were an additional half-trillion barrels of oil yet to produce, as some oil optimists believe, oil extraction would fall to zero by 2080. The following summary chart shows what effect this would have on global population. If all reproduction were to cease now -- a fertility rate of zero children per female -- global population would follow the oil curve down, until population reached zero at the end of the century. In 2050, the population would be around 5.5 billion at a fertility rate of zero, or anywhere from around 6.7 billion to 9.8 billion at higher fertility rates.

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For a final perspective, Howe offers the following chart showing the large "food gap" that opens between now and 2060, with the UN's population projection soaring into the future as various oil production curves fall. Again, the H curve assumes around 1.3 trillion barrels of oil remain to be produced; the NO curve makes the same assumption but adjusts for EROI falling at 1 percent per year; and the R curve assumes that fertility is fixed at 1 child per female and that oil demand falls by 1.5 percent per year.

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The R curve would represent a towering policy achievement for humanity, in which we deliberately curbed our reproduction and oil consumption consistently for decades. But even under that scenario, global population would fall to around 4.2 billion people by 2050, and still be on its way to zero by the end of the century.

Models vs. reality

The implications of these models are fairly terrifying, but we should bear in mind that they are models, not firm forecasts. The population models of the UN and others are blind to resources, and the oil-based models are blind to policy. The ultimate shape of global population curves will depend on innumerable factors both expected and unexpected, and billions of individual decisions that people make between now and the end of the century. Our fate is formally unknowable.

So what will the actual global population be at the midpoint and the end of this century?

My educated guess is that population will respond to the oil pulse, and top out at around 7.5 billion people by 2050. Producing enough oil -- not to mention food, water, and all the other critical raw materials required by modern life -- to support 9 billion people or more will simply be too expensive, too difficult, and too risky. I believe the population models on which the world's policymakers depend are wrong.

If oil production does indeed fall to zero (or nearly zero) by the end of the century, then one must ask what the global carrying capacity is in the absence of modern civilization.

One easy answer to that is to look at what the global population was before we started exploiting fossil fuels in earnest two centuries ago: about 1.5 billion people. From that basis, we must make a number of adjustments. For one, the global population is now dispersed over much of the planet; two centuries ago, for example, the population of the United States was about 7.2 million and most of the rich land from coast to coast was very nearly empty. Now we have around 310 million people distributed over every desirable spot in the country. The same dynamic applies to much of the rest of the world. Globally, there were only around 1 billion people on the planet in 1800. So we might adjust the carrying capacity upward to 2 billion or 3 billion based on geographic distribution.

If we were able to replace oil (and ultimately, the rest of the fossil fuels) with renewable power, such that we could substitute at least a modest amount of mechanized work for human and animal labor, then the sustainable population might be even higher. Grantham speculates that if we "retune our agricultural system," improve our behavior, aggressively build renewable capacity, crack the storage problem, and do our very best, then by the end of the century population could hit a "probably sustainable level of 4 billion, particularly if we sensibly encourage its decline."

If we do not do those things, and simply follow the default path of overshoot, then any number of Mad Max scenarios too awful to contemplate would obtain, leaving the world weak, small, and desperately impoverished by the end of the century.

Any way you slice it, the global population is bound to fall, gracefully or catastrophically. Growth along the existing trend lines would lead to far worse outcomes than any alternatives.

How will we respond?

A straight reading of history suggests that the overshoot path is our default. Historical examples of humanity recognizing resource limits and deliberately curbing its appetites and reproduction rates are vanishingly rare.

Yet, what choice do we have, but to do something? And what might that something be?

This is where we come up hard against taboos. Nobody wants to be told whether or not they can reproduce, or what to eat, or how to live. To merely suggest that population might be limited intentionally raises sci-fi specters like Logan's Run and Soylent Green. Researchers who simply calculated the carbon footprint of having children two years ago were called eugenicists and Nazis. Deliberate population control is probably out of the question.

Howe believes that choosing a different lifestyle would take America a long way. He calculates that each household in the United States could get by with a 4,000-Watt (W) solar PV system (which is within the realm of possibility for most single-family homes), assuming "minimal transportation and agriculture." With a solar-powered tractor, he says, a farmer could farm three or more acres and feed 12 to 15 non-farmers, presumably on a primarily vegetarian diet. Globally, he advocates restricting fertility rates to one child per female, and decreasing "energy consumption, specifically oil, by at least one and one-half percent per year for the next sixty years, starting now!"

Global policy action along those lines seems unlikely, to say the least. But it is not out of the question that an informed populace, disabused of its fantasies about endless fossil fuels and endless economic growth, could choose to have fewer (or no) children, adopt lower-energy lifestyles, and try, for once, to act like they're smarter than yeast.

(Photo: James Cridland/Flickr)

This post was originally published on Smartplanet.com