Source:
The World At Seven Billion
BBC, October 26, 2011

Crowded Planet: A conversation with Alan Weisman (Orion Magazine):
Andrew: The human population stayed relatively stable, or grew at a manageable rate, for tens of thousands of years but exploded in the past century. What happened? How did we humans come to dominate the planet so quickly?

Alan: The explosion began during the Industrial Revolution. Jobs were suddenly in cities rather than on farms. People were living in tight quarters, and that became an incentive for doctors to begin dealing with diseases that were starting to spread much more easily. Beginning with the nineteenth century, medical advances, such as the smallpox vaccination, were either eradicating diseases or controlling the pests that spread diseases. Suddenly, people were living longer, fewer infants were dying.

Andrew: Before that, we were basically at a replacement rate?

Alan: Pretty much. Women would have seven or eight kids, and if they were lucky, two survived. Two is replacement rate. If a male and female have two kids, then they have essentially replaced themselves. Population remained stable because as many people were dying as were being born.

The other thing was that suddenly we learned how to produce far more food than nature could ever do on its own. Nature’s ability to produce plant life has always been limited by the amount of nitrogen that bacteria could pull out of the air and provide as food for plants. In the twentieth century, we discovered how to pull nitrogen out of the air artificially.

Andrew: You’re speaking of the Haber-Bosch process.

Alan: Yes. As a result, we suddenly came up with artificial fertilizer that could produce much more plant life on this planet than had ever existed before. We were at about 2 billion in 1930 when we started using artificial nitrogen extensively. Today we’re at 7 billion. Between 40 and 50 percent of us would not be alive without artificial nitrogen fertilizer. It nearly doubled the food supply.

Andrew: They say that, in some ways, too much abundance isn’t actually good for a population, that it can actually stress it because it leads to overpopulation. For example, if you overfeed city pigeons, they have more babies and the population starts maxing out, whereas if you don’t overfeed them, the population keeps itself in check.

Alan: That’s the paradox of food production—it can ultimately undermine the viability of a population. At a certain point, it expands beyond its resource base, and then it crashes. Wildlife managers, for example, well know that if we don’t keep population in balance with food, a species can run into serious problems. They know that they can either relax controls on natural predators, or issue more permits to hunters—that is, human predators.


Is population self-correcting?

Some argue that population is in fact self-correcting, and that the correction is already underway. But it’s a little like saying a house fire is self-correcting, because it will eventually put itself out. Unfortunately the damage is done. One way or another, when a species exceeds its resource base, the population will come down. Nature does that in 100 percent of the cases in the history of biology. The question that I keep coming back to is how soon is that going to happen?... If our population is coming down because nature is going to do it for us, well, it’s going to be, frankly, unpleasant to watch. When nature does in a horde of locusts because they eat themselves out of sustenance, it’s interesting for us to observe. When it happens to our own species, it’s not going to be very pretty.

Is the problem sheer numbers or consumption?

I’m always curious about what people are thinking when they say, “It’s not population; it’s consumption.” Who do they think is doing all the consuming? The more consumers there are, consuming too much, the more consumption.
According to Tom Murphy, surplus energy equals surplus people: The Real Population Problem (Do the Math).
For many thousands of years following the end of the last Ice Age, human population rose steadily and slowly, at a rate of about 0.032% per year—translating into a leisurely doubling time of some 2000 years. About 3000 years ago, spreading agricultural practices led to a modest boost in growth rates. But the wild ride did not start in earnest until modern times.

Even on a logarithmic plot—on which exponentials are tamed into straight lines—our population trend resembles the infamous “hockey stick” curve seen in so many domains (atmospheric CO2, global surface temperature, and practically any measure associated with human activity). A logarithmic hockey stick is truly scary. Because human impacts on the planet scale with population, it is not terribly surprising that a hockey-stick population curve should translate into hockey sticks everywhere. It is in this sense that population underlies nearly every issue and challenge of our times.

But what can we understand about population? What governs its rate? What accounts for the discontinuity in slope? Why did we leap into 1% growth and a 70 year doubling time in recent centuries?

One hint comes from a closer look at the recent history. Plotting global population in the last thousand years (below), we see a few breaks in the slope. For most of this period, we saw a modest 0.12% growth rate, amounting to a 600 year doubling time. Around 1700, the rate stepped up to 0.41%, doubling every 170 years. The next break happens around 1870, jumping to 0.82% and 85 years to double. Then around 1950, we see another factor-of-two rate jump to 1.7% and an impressively short 40 year doubling time.

We can perhaps attribute the 1700 jump to the Renaissance and scientific progress. We learned to wash our hands after wrestling with our pigs, and that diseases were not caused by bad vapors conjured by impure thoughts. The jump around 1870 corresponds to the Industrial Revolution, in which coal transformed the production of steel (providing agricultural tools), rail transport of commodities, and began to mechanize agriculture in a limited way. 1950 marks the Green Revolution: full-scale petrolification of agriculture, accompanied by massive fertilizer campaigns using natural gas as the chemical feedstock.

This leads to a rather simple thesis: the surplus energy presented to us by fossil fuels allowed us to feed people more easily the world over. The bounty of fossil-fuel-turned-food encouraged an explosion in birth rates, as happens for virtually all organisms given similar circumstances. It’s so blindingly obvious that I am embarrassed to have belabored the point as long as I have.

Surplus energy grows babies. That was my statement to the audience, based on the dots I connected above. So what about the astute comment that countries with the largest excesses display the weakest—and even negative—population growth rates? This statement also rings true, so how can we hold both thoughts in the head?
A surprising conclusion:
We have a clear correlation at the left edge: poorer countries have high growth rates, but rapidly dive to lower—and even negative—rates by about the time average earnings reach a quarter of those in the U.S. Then a funny thing happens: the cluster turns up again! This narrative-busting truth made me sit up straight, I’ll tell you. I fully expected a more-or-less smooth and gradual transition: rich means fewer offspring, according to the demographic transition idea. Granted, immigration plays a role here. It certainly is responsible for growth in the U.S. population. But it would be hard to convince me that the wholly lopsided statistics at higher incomes are all about immigration. Where are the rich nations with negative growth? After Germany, the negative growth countries are not to be found.
Some scary numbers:
India adds about 15 million people per year (wow). China adds a little over 6 million. Nigeria is next, at about 4 million. Then we have the U.S., adding 3 million per year. But of those, the U.S. is the hungriest in energy terms...What we see is that population growth in the U.S. is adding energy demand faster than any nation on Earth. China and India are also important (and in absolute terms they are certainly more important energy growers, due to a rapidly changing standard of living). But the answer to the question: who’s population growth is having the largest effect on global energy demand?—it’s the U.S.
Similarly, from the Weisman interview:
I did some long division to make it more understandable. It came down to every four to four-and-a-half days, there’s a million more of us on the planet. That just doesn’t seem like a sustainable figure, and that’s pretty much where we are unless we start to do something about it...There’s also no question that the most overpopulated country on earth is actually the United States, because we consume at such a ferocious rate. We may not be as numerous as China or as India, but our total impact is huge.
Only half a child more or less is critical (and it's hard to have just half a child). From the Weisman interview:
If family planning does not keep up with our population growth, or, if suddenly, for whatever reason, the supply lines break down and birth control pills or whatever contraception they’re using is not available to women in a lot of places around the world, a half a child more per fertile woman means that by the end of the century we’re going to increase to 16 billion people. A half a child less per woman means that we’re going to be back down to 6 billion really quickly. Then we can decide at that point if we want to bring it down further. But the difference is, on average, half a child either way.
But what about those industrialized countries that are desperate to boost their populations for "economic" reasons?
Maybe this shouldn’t be too surprising from the perspective that economic powerhouses are dependent on growth, and a flagging population is a recipe for recession. A key ingredient for guaranteeing a bigger future is more working young people: in addition to providing labor, the growing youth enables pension systems and health insurance to function. Indeed, Germany is trying to battle its population decline, which imperils economic competitiveness. When I saw this article in the NYT, I lamented that the policy of protecting economic growth enslaves us to produce more children. I would rather humankind be the master of the economy, rather than the other way around.
From the Weisman interview regarding economic growth and population:
It turns out that population growth and economic growth are inextricable. For an economy to keep growing, you have to have growing populations, because you need more laborers to produce more products, and then you need more consumers for those products.

If we have to start limiting our population, then we’re going to have to come up with a way to redefine prosperity that doesn’t involve perpetual growth. A shrinking population or a stable population can’t be a perpetual-growth society....

It doesn’t take a huge population to become economically viable. Countries that have smaller populations combined with education are more economically viable, so that’s a further incentive, at least at the governmental level. Look at a country like Singapore. It’s a small country. It’s on an island. They’ve had a terrific family-planning program that’s become very effective and very, very ingrained. They also have one of the higher per capita incomes of any country on earth.

It’s an oft-repeated fear that circulates in the business and economic world out there that an aging population is terrible for the world, because there’ll be all these unproductive people and there won’t be enough productive young people paying into the social welfare coffers to take care of them.

Yes, some countries have shrinking populations. But they’re not looking at a situation that goes on into perpetuity, in which they have far more older people than younger people. They’re looking at a generation or so of a bubble where they’re going to have more older people, and then, as that generation dies off, the number of older people and younger people are going to balance out again, and it’s not going to be a problem.

How do they economically get through those bubble years? As an American, I can think of an awful lot of things that my government is spending money on right now that if it dedicated those monies to taking care of a generation of older people until our population evened out, we’d be a much better society.
I'd quibble with one point here - it's total productivity that's important, not just the sheer number of people. Of course, we're so productive that we have a massive labor surplus. Tom Murphy:
We are in the midst of an unplanned experiment of unprecedented scale. We have 7 billion people on the planet, growing at almost three new (net) people per second. It’s an uncontrolled mad dash into the future. One could imagine metaphorical scenarios of crashing into a brick wall or running off a cliff, exhausting ourselves and stopping to catch our breath, or leaping into space to leave the planet. I certainly have my guesses, but I can’t spell out an unwritten future.
And from the Weisman interview:
I was in Niger, which has the highest fertility rate on the planet now. Its average is around eight children per fertile female. In every village, I heard, “Had you been here twenty-five years ago, you couldn’t have seen that house over there for all the trees that we used to have.” Where did the trees go? Well, they needed them for firewood, and then the climate began changing on them and there’s less rain now. They’re not responsible for the industrial pollution that has gunked up the atmosphere, but when you take down trees, things change. You graze too many animals, and things really change. They’re now in chronic drought. In every village, hundreds of children had died.
One of the ways that I like to think of this is looking back to my own boyhood. There was a lot more space. An awful lot of us can still remember when the traffic was not as bad, when you could get out of a city much faster, when there was a whole lot more wildlife around. We could go back to that. At the beginning of the twentieth century, there were only 1.6 billion people on this planet, a quarter of today’s population. That isn’t to say that humans weren’t already having an impact. But still, any of us who love nature, we would give a lot to go back to a time when that much of the world was still wild and still producing a lot of the things that we count on nature for—trees that hold our watersheds in place, insects to pollinate or to serve as a food source for all the birds that also pollinate or spread seeds. There are many things that nature does for us.

The corollary to the question of how many people could the world hold is: How much nature do we have to preserve in order to keep our species viable? How much of the habitat do we need? What other species on this planet are absolutely essential to our livelihood?
And now for some graphics:

Visualizing Global Population Density (The Big Picture)

Here's the world map rearranged by population:


One of the reasons we don't see the destruction of our planet is that urbanization means most of us are crowded into very small areas (I'm in the Milwaukee-Madison-Chicago triangle in the Upper Midwest):

Half of US Populaton Lives In < 10% of the land mass (The Big Picture) Similarly, Population Distribution of USA (ibid):

And global population expecations:




About that part about less deaths, see this informative article: Why Are You Not Dead Yet? Life expectancy doubled in the past 150 years. Here’s why. (Slate)