Next Monday will be a big day for humanity, and not just because it's Halloween. The U.N. predicts Earth's human population will reach 7 billion on Oct. 31, a historic milestone for our species — yet one that also feels hauntingly familiar.

It took about 200,000 years for modern humans to reach the 1 billion mark in 1805, but then suddenly there were 2 billion of us in 1927, 3 billion in 1960, 4 billion in 1974 and 5 billion in 1987. The 6 billion mark came in 1999, and 8 billion is due around 2025. We're widely expected to hit 10 billion before the end of this century.

Since Earth has a limited supply of important things like freshwater, phosphorus and farmland, it seems unlikely this trend can go on forever. But there's little agreement on how many people is too many, and even less agreement on what — if anything — should be done.

This is a delicate subject, and one that will be further examined here and elsewhere during the next week. For now, though, as the U.N. kicks off its "7 Days to 7 Billion People" countdown, let's just take a moment to reflect on how explosive our rise to 7 billion has been.

(Quick side note: The U.N. admits the Oct. 31 estimate should be taken "with a grain of salt," since no one knows exactly when we'll hit 7 billion. The U.S. Census Bureau says it will happen next spring, but the exact date doesn't really matter. Seven billion is an arbitrary point on a line; the line itself is what's important.)

Homo sapiens grew relatively slowly for the first 95 percent of its existence, not even hitting 1 million until about 10,000 years ago. The pace sped up as we adopted farming and city life, and then the Industrial Revolution put us in overdrive. After finally breaking 1 billion in the early 1800s, we grew by 270 percent in the 1900s — compared with an average of 22 percent during each of the previous nine centuries. Someone born in 1850 would likely have seen just two "X billion people" milestones in his or her lifetime, but baby boomers from the 1950s are about to see their fifth.

Words can only go so far in conveying the scale of humanity's population growth, especially over the last 200 years. The story needs visual aids, so I made the following bar graphs in Google Charts, using two data sets (here and here) that include population records and forecasts from 10,000 BC to 2050 AD:

Global human population (in billions), 1 AD - 2011 AD

In this first graph, notice how the time intervals change. Most of the bars skip 200-year gaps, but after 1800 the gaps shrink to 100, 50, 30, 20 and finally 11 years (I used 11 for the last one just so it would end on 2011). If humans were reproducing at a more linear rate, messing with the time intervals could distort the data, making our growth seem slow or stagnant. But that doesn't happen. Compare the gaps between 1800-1900 and 2000-2011, for example: It took 100 years for us to grow by 736 million people in the 1800s, but now we've added 906 million in just 11 years.

For the same data with equal time intervals, see the alternate version below:

Global human population (in billions), 1 AD - 2000 AD

This is what the last two centuries' growth looks like in a head-to-head comparison. But there is one key detail that gets lost in the graph above: Even though we're reaching unprecedented overall numbers, the rate of population growth is already slowing down. The annual growth rate always jumps around a bit, but after passing 1.5 percent in the 1950s and peaking at 2.23 percent in 1963, it has been generally declining ever since. It stayed above 2 percent until 1972, then dropped to 1.86 percent in 1980, 1.57 percent in 1990 and 1.27 percent in 2000. The U.S. Census Bureau predicts it will fall below 1 percent by 2020, and down to 0.5 percent by 2050.

It's hard to link specific causes to this trend, or to be sure how long it will continue, but it does hint at a shift from the population dynamics of recent centuries. Still, with such huge numbers of people already on the planet, even dropping below 1 percent annual growth means adding tens of millions of people every year. To illustrate how big humanity is becoming even as its growth rate slows, here's a graph combining data from 1900 to 2010 with Census Bureau projections for the next 40 years:

Global human population (in billions), 1900 AD - 2050 AD

Update: For ideas on how the global population can better sustain itself, check out this post on how to make overpopulation OK.

And lastly, here are two line graphs that show growth rates for the 10 countries forecast to be the most populous in 2050, using observed and projected data from 1950 to 2050. (The first graph shows China and India, since their populations dwarf all others. The second graph shows the eight other countries in the top 10 for 2050).

Population by country (in billions), 1950-2050

Population by country (in millions), 1950-2050

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