Halloween, it may seem as though there’s just maybe a few more trick-or-treaters than last year.

A new report documents the prodigious rate at which the world’s population is growing. It was just 1999 when we reached 6 billion. As of July 1 of this year we numbered 6.89 billion. By the year’s end we will have surpassed 7 billion. According to Hania Zlotnik, head of the population division of the United Nation’s economic department, that mark will be reached on October 31st.

The exact date, says Zlotnik, should be taken with “a grain of salt.”

The exact number, however, should be taken by the spoonful no matter how hard it is to swallow. There are plenty of reasons to feel queasy while looking at the bacteria-like rate of growth on world population charts. The most obvious question in my mind: where are the walls of our petri dish?

The exponential increase that we all take as a given is actually a relatively recent phenomenon – evolutionarily speaking. It was only around the year 1800 that the global population reached 1 billion. It took over a century to reach, in 1930, 2 billion. Then it was only 44 years before we doubled to 4 billion, and only 25 years until we reached 6 billion in 1999. A review by David Bloom, chair of Harvard’s Department of Global Health and Population, was recently published in Science with the title “7 Billion and Counting.” In it Dr. Bloom takes a close look at what he calls “the greatest demographic upheaval in human history.”

There are two major forces behind this great upheaval we are currently in the midst of: substantial reductions in mortality and a lagging reduction in fertility rates. In a world population that will go from 3 billion to 7 billion in only half a century, longer lives for everyone and reduced fertility in “more developed regions” – as the UN Population Division terms them – means the bulk of humanity is becoming increasingly “less developed.” In 1950, 68 percent of the world population was accounted for by less developed regions. Today they account for 82 percent and by 2050, the UN projects, will make up 86 percent of the world population. It is expected that between now and 2050 the world population will increase by 2.3 billion. Nearly all (97 percent) of that increase will occur in the less developed regions.

The more developed regions, for their part, are actually reversing the trend. Right now China tops the population charts at 1.35 billion people. India comes in second with 1.24 billion. The U.S. is a distant third at 311 million. By 2050 China’s population is projected to fall to 1.30 billion while India’s will rise to 1.69 billion, making it the hardest country in which to hail a taxi cab. Right now Japan and Russia are both in the top 10, but they’re not expected to be by 2050. 

The U.S. population is expected to climb to 423 million in that time. It’s still an increase, but a feeble one compared to the expected meteoric rise of Nigeria. Their current population of 166 million is projected to explode to 402 million by 2050, making it the world’s fourth most populated country right behind the U.S. At about 923,000 square kilometers, Nigeria occupies one tenth the land that the U.S. does.

Of course, these projections are guesses, and they are dependent on assumptions about the future such as how many children a woman will have 20 or 30 years from now. So, I suppose they should also be taken with a grain of salt.

But whether or not we reach the projected 9.3 billion by 2050 or the 10 billion the UN dares to venture by 2100, we’ve actually passed a tipping point in our skyrocketing growth. The most rapid population growth rate took place between 1965 and 1970 when it peaked at just over 2 percent per year. Right now overall growth is about 1.2 percent. 

The global fertility rate – number of births per woman – has decreased from 5 to 2.5 in the past 50 years but significant regional differences still remain. When asked how many children are ideal, the average answer from women in Austria is 1.6. In the UK it’s 2.4. Women in Uganda answer 5.3, and the tireless women of Niger think 9.1 children are ideal – statistically speaking. That explains their world’s highest 7.1 births per woman.

But while less developed countries like Niger are seeing their fertility rates drop – Niger’s rate in 1981 was 8.1 children per woman – they still remain higher on average (3) than rates in more developed countries (1.8). And that gap has only recently been narrowed. In 1950 women in less developed countries were having more than 6 children on average while women in developed countries were having just under three.

So what’s to become of the 2.3 billion newborns between now and 2050, 97 percent of which will be born in less developed countries? This question has been the matter of much scholarly debate for a long time. One noted example began in the early 20th century when a British bureaucrat warned of the impending environmental doom at the Kenyan colony of Machakos that would result from the explosive “multiplication” of the “natives.” He described unchecked misuse of the land and the consequentially miserable and impoverished inhabitants. He wrote his memo in 1937.