A tale of more cities: Are we reducing our impact on the Earth by sharing it?

Humanity used up a year's worth of the Earth's resources in just seven months this year, calculate scientists at the Global Footprint Network. And, with the human population projected to hit 9.7 billion by 2050, that impact may only get worse.

But there's hope, according to another group of scientists. Even as global populations continue to rise, the growth of our collective "human footprint" on our planet may be slowing down.

While the human population grew by 23 percent between 1993 and 2009, the overall amount of land impacted by human activity grew by just 9 percent during the same period, according to a study published Tuesday in the journal Nature Communications. This suggests that humans are becoming more efficient in our use of the Earth, and this may largely be due to urbanization.

The new study draws on satellite imagery, census records, and other data to examine just how much of the Earth's land surface is experiencing pressure from human activity. The result is a set of maps revealing that 75 percent of our planet's land is in some way feeling our species' impact.

"Overall, things haven't gotten better in the last two decades. Almost everywhere across the planet, the environment has degraded," says study author Oscar Venter, a quantitative forest scientist at the University of Northern British Columbia, in an interview with The Christian Science Monitor. "But at the same time, if you look at it in terms of impacts per person, things have gotten better."

The Global Footprint Network takes a darker view, calculating that we are consuming more of the Earth than ever, with Earth Overshoot Day falling the earliest it has since the organization began calculating it: August 8, this year. In seven months and one week, humans used all the resources that the planet generates in a year, they said.

So why are Venter and his colleagues still hopeful?

Because during the same 16-year period that the footprint grew by 9 percent, the global economy exploded 153 percent. 

The researchers had expected the numbers to keep pace with each other, as economic growth is often linked to expanding infrastructure. But that wasn't the case for all thriving nations. 

While some countries' footprints worsened as their economy strengthened, most did not. The secret: Strong governments and high rates of urbanization.

When a nation's central government minimizes the environmental impact of growth, it pays off, Venter explains. In addition, "concentrating people into cities – what it effectively does is concentrate impacts."

Cities allow people to share land and other infrastructure, thus relieving other regions of the population burden. People can ride public transportation, live in tall apartment buildings, walk the same streets, and share the same streetlights. With people concentrated in cities, Venter says, "We're not spreading our infrastructure needs and our houses across the entire landscape, we're sparing some for nature."

But this isn't the whole picture, says Mathis Wackernagel, founder and chief executive officer of the Global Footprint Network. Venter and his colleagues missed the most significant human impact on the planet, he says: fossil fuels.

Venter's maps do not factor in fossil fuel-induced climate change, instead focusing on the threat to biodiversity and truly wild places. But, Dr. Wackernagel points out, "Climate change could be a very significant biodiversity threat."

Cities can help reduce fossil fuel use in some ways, with shared transit and more opportunities to commute by foot or bicycle. But urbanization might not be a long-term solution for other reasons, says William Schlesinger, president emeritus of the Cary Institute of Ecosystem Studies and former dean of the Nicolas School of the Environment and Earth Sciences at Duke University. 

The challenge lies in the enormous resources that support the booming urban populations, he says. Fossil fuels still produce most of the planet's energy, despite a growing shift toward renewable resources like solar- or wind-generated power.

Another challenge is food. In order to feed the growing human population without expanding croplands extensively, agricultural productivity will need to increase, Dr. Schlesinger says. Humans will need to get more sustenance from less land.

But making agriculture more efficient isn't going to be a straightforward technological advance. Right now, Schlesinger says, "We're growing more food on less land, but it comes from fertilizer, herbicides, genetically modified crops, irrigation water."

"That's an increase in efficiency, and it allows people to move to cities and therefore we have a lower footprint on nature," Schlesinger tells the Monitor, "But all of that, with the exception of the genetic engineering, is coming from the use of fossil fuels" and is increasing our impact on the environment.

"So yes," he says, "we've become more efficient. But it's because we've put these fossil fuels to work for us globally." And that's just not sustainable. 

The solution may come back to Venter's original question: Is a growing population increasing our collective impact on nature?

Even though we are getting more efficient with how we share the Earth's resources, the answer is yes. More people need more resources, and we may not be able to innovate enough to support a growing population, particularly when three-quarters of the Earth's land is already feeling human pressures.

So in addition to changing how we eat, sharing more resources, and powering our communities with renewable resources, Schlesinger and Wackernagel both say, the ballooning population growth has to stop, a call that has been echoed by other environmentalists. 

"This is an interesting and careful analysis," writes Peter Raven, president emeritus of the Missouri Botanical Garden, in an email to the Monitor. "But it should not distract us from the fact that we’re adding 250,000 people net every day and heading for an estimated 9.8 billion in mid-century, or that global warming is proceeding rapidly while our lust for consumption seems to increase."

He adds, "We may be becoming more efficient, but we already are well over the top!"

William Rees, a bio-ecologist who served as Wackernagel's PhD adviser at the University of British Columbia, where they developed their eco-footprint analysis, says that he sees this report as "remarkably low key" and lacking the necessary sense of urgency around humanity's impact on the Earth.

"My goodness, the extent of existing environmental degradation is so great, that most of the ecosystems occupied or exploited by humans are properly called ‘remnant ecosystems’ that have only a fraction of the species diversity and abundance they exhibited prior to modern human occupation," Dr. Rees writes in an email to the Monitor.

"If the human population increases to the expected 11 billion (by about 50 percent), and consumption more than doubles (as is expected in the next 35 years or so with rising incomes)," he writes, "then the human eco-footprint as we calculate it will double and it will be all over for wild nature and the functional integrity of the ecosphere."