NEW YORK -- What would the world look like with 7 billion human beings in the mix, vying for resources? Pretty much what it looks like now.

That's because the planet is about to pass the 7 billion mark any day now. Or maybe it already has, according to academics at Columbia University.

Joel Cohen, a professor of populations at Columbia, briefed attendees here this week during an Earth Institute gathering on the subject. The conference was meant to celebrate, raise awareness and sound a few alarms regarding a U.N. estimate that the 7 billionth human is due to join the party Oct. 31.

To Cohen, whether the Earth will be able to sustain 7 billion humans, 9 billion or 10 billion will "depend on choices we and future generations make." He added that we may have already crested over 7 billion for the first time, though pinpointing an exact date on the threshold is next to impossible.

"The truth is we have never censused the entire Earth," Cohen said. "The U.S. Census, which costs billions, has an error of 1 to 2 percent."

What is clear is the Earth is poised to pass the mark, making it 12 years since we passed 6 billion in 1999 and 12 more years since 5 billion in 1987. More broadly, the 7 billionth addition to Homo sapiens represents a spurt of 4 billion people in five decades.

Cohen calls the phenomenon an "extreme explosion [that] ... has no precedent" in the history of human evolution. The growth rate prior to the mid-20th century was much slower and had effectively held steady for thousands of years until the 19th century's Industrial Revolution.

"That is an exceptional event and will probably never be repeated within human history," he said during a lecture here. "Not within the next few centuries, anyway."

'5 sub-Saharan Africans for every European'
Looking ahead, Cohen expects total fertility rates to drop almost everywhere by the end of the century. This pattern is already in evidence in Europe and the United States and eventually will trickle into less-developed nations as they raise their economic profile and education levels, he said.

Barring an apocalypse, Cohen predicts the Earth will hit 8 billion humans in another 12 years, then 9.3 billion by 2050 and 10.1 billion by 2100.

More remarkable than the numbers perhaps is how the demographics within that growth rate will shift. Cohen says that by 2100, the African continent will have overwhelmed a historic balance among continents, with "five sub-Saharan Africans for every European."

"You can only imagine that might have an impact geopolitically," he said.

The good news is that with the steady increase over the past half-century has come improved life expectancy to a global average of 70 years. The bad news is dwindling natural resources, food and water could mean 1 billion starving people across Africa and South Asia's "hunger belt" sooner than many think.

Whether the planet's supply of energy, water, food and other resources can handle the demand is an open question. Cohen says the Earth is finite, and fresh water in particular could be in short supply for about 10 percent of the planet by 2100.

The same dynamic is evident for agriculture, as farming already consumes about 40 percent of ice-free land. If that pattern continues to hold or increase, less forests to meet the demand for food could mean a corresponding uptick in carbon emissions worldwide.

Cities are likely to feel the brunt of the growth as humans continue their migration from rural areas to urban. Cohen says the small city (of about 1 million) is the future of urban life, and he feels civilization needs to do better at paying attention to their design.

"We are going to need to construct a city of a million people every five days for the next 40 years," he said.

Elderly to outnumber the young
The residents of those cities will also see a generational shift, he said, as the aging start to outpace the young. By 2065, Cohen expects aging to have "gone global," with people over the age of 65 outnumbering children under the age of 15.

"The slowing population growth will not solve all of humanity's problems," he said. "But it will make it much easier to solve many of them by slowing growing demands for more teachers, jobs, schools, appliances, energy, land, water and food."

Cohen's lecture was the prologue here to a roundtable of Columbia academics brought together to discuss "the 7 billion challenge." In a video on the subject, Earth Institute Director Jeffrey Sachs celebrated the upthrust in population as "a pivotal global moment" while calling it a threat to human existence as we know it.

Feeding 7 billion already requires so much fertilizer that fresh water around the globe is in danger, Sachs said. He also pointed to "huge dead zones" in estuaries of more than 100 rivers, the carbon costs of massive energy consumption and ongoing deforestation to feed the masses.

Sachs said he hopes the 7 billion moment will help bring attention to the "unprecedented, extremely dangerous and unsolved problems of human impact." On climate change, for instance, Sachs sees a planet "barreling forward" without much sense for "how objectively unsustainable we already are."

Fertility rate drops; consumption doesn't
"We have absolutely no evidence over the last 20 years that we've accomplished anything on climate change," he told the conference. "We're in a pretty serious state of affairs."

Others saw the moment in similarly mixed terms. Ruth DeFries, a professor of sustainable development at Columbia, said she chooses to view the population mark "as a success," because it is "the goal of any species to increase its numbers." But she also sees no end in sight for consumption increases.

Not even the lower fertility rate is expected to halt human consumption, because the output of a single human being continues to grow, she said.

"While we see a decline in fertility, we don't see a decline in the consumption of resources," she said. "We don't really have a clear pathway to deal with this."

Others were a touch less dire. Jean-Marie Guéhenno, director of Columbia's Center for International Conflict Resolution, said he would expect ongoing population growth to "trigger a lot of mobility" between nations, which could correspond to economic opportunity. Between Europe and Africa, for instance, Guéhenno senses "an enormous potential from a European standpoint."

"Here is next door a continent that can become a huge market," he said.

Klaus Lackner, a geophysics professor at Columbia, struck an equally evenhanded note. With population growth showing signs of slowing, Lackner said, it may actually come to a stop, at which point technology -- especially in energy -- might start to handle higher demand.

"Better wealth, better education ultimately solves the problem," he said. "We just might solve the resource question, in time."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500