Environmentalists often say that humanity needs to shrink its planetary footprint. Now, a new study has demonstrated how literally massive that footprint is.
While the mass of Earth’s life forms stands at about 1.1 trillion metric tons (1.2 trillion U.S. tons) and has not changed much in recent years, the so-called “anthropogenic mass” of artificial materials is growing exponentially. The mass of everything people have built and made, from concrete pavements and glass-and-metal skyscrapers to plastic bottles, clothes, and computers, is now roughly equal to the mass of living things on Earth and could surpass that this year, according to research published today in Nature.
The finding may bolster the argument that Earth has entered the Anthropocene, a proposed geologic epoch in which humans are the dominant force shaping the planet. As senior study author Ron Milo of the Weizmann Institute of Science in Rehovot, Israel puts it, the world is undergoing a material transition that “happens not just once in a lifetime, but once in an era.”
While that insight is more symbolic than scientifically meaningful, the material scale of the human enterprise helps explain how we’ve managed to transform global nutrient cycles, alter the climate, and drive myriad species to the brink of extinction.
This isn’t the first attempt to weigh humanity’s impact on the planet. In 2016, a team of scientists estimated the weight of the “technosphere”—including not just wholly artificial buildings and products, but also the approximate weight of the land and seafloor that we’ve excavated, modified or trawled to build cities, plant crops, raise livestock, and catch fish. They came up with a figure of 30 trillion tons. Other recent studies have tracked changes just in the biological world, such as the amount of carbon stored in plants or the number of chickens on the planet.
But to the authors’ knowledge, there hasn't been a comprehensive analysis looking at changes in the weight of the artificial and biological worlds simultaneously but separately. That has made it difficult for scientists to draw an apples to apples—or apples to iPhones—comparison.
An explosion of stuff
To fill that knowledge gap, Milo and his colleagues pulled together several previously published datasets on the mass of artificial materials and life forms and constructed a timeline of how the two have changed from 1900 to the present day. The team obtained anthropogenic mass estimates for the past 120 years from recent work in the field of industrial ecology; satellite data and global vegetation models provided historical information on global biomass shifts. The findings were dramatic.
At the start of the 20th century, the mass of human-created stuff weighed in at 35 billion tons, or roughly 3 percent of global biomass. Since then, anthropogenic mass has grown exponentially to approximately 1.1 trillion tons today. It’s now accumulating at a rate of 30 billion tons a year, which corresponds to each person on Earth generating more than his or her own weight in manufactured stuff every week.
Most of that stuff is concrete—humanity’s favorite building material—followed by gravel, bricks, asphalt, and metals. If current trends continue, these manufactured materials will weigh more than twice as much as all life on Earth by 2040, or about 2.2 trillion tons.
Roughly 90 percent of the living world by weight, meanwhile, is composed of plants, mostly trees and shrubs. But while humans manufacture ever more materials each year, the weight of Earth’s plants has held relatively steady, due to what the authors describe as a “complex interplay” of deforestation, forest regrowth, and vegetation growth stimulated by rising atmospheric carbon dioxide levels.
The study’s results offer a striking illustration of humanity’s impact in a way we haven’t seen it before, says Jan Zalasiewicz, an emeritus professor of paleobiology at the University of Leicester who wasn’t involved with the paper.
“It gives another perspective on the speed and the scale of transformation” of Earth’s surface by humanity, Zalasiewicz says. “It’s a kind of bird’s-eye view of change.”
That bird’s-eye view may inform the debate over whether human activity has pushed Earth into the Anthropocene, a question currently being investigated by the Anthropocene Working Group at the International Commission on Stratigraphy, the body of experts who oversee Earth’s geologic time scale.
Zalasiewicz, who chaired the working group for many years, says the new paper supports the concept that the Anthropocene is “real in a physical sense.” Physical evidence of the Anthropocene is now “quite extensive,” he says, and it’s likely to leave a clear mark in the fossil record.
While the comparison of biological and human-made mass is a clear-cut indicator of our impact, it’s important to note that Earth’s biomass has been profoundly altered by humanity as well. As the study notes, there may have been twice as much plant biomass on Earth at the onset of the agricultural revolution about 12,000 years ago, before people started clearing vast swaths of forest for land cultivation. Humans and their livestock, meanwhile, now outweigh all of Earth’s wild mammals and birds by a factor of nearly 20.
And at 4 billion tons, the mass of all of Earth’s animals combined now sits at just half the mass of the amount of plastic that has ever been produced (over 8 billion tons).
Milo says that these shifts in the mass and makeup of Earth’s biosphere are “another aspect of the impacts of humanity” that shows our “dramatic effect.”
Erle Ellis, an environmental scientist at the University of Maryland, Baltimore County, says that the new paper provides a “really nice set of illustrations” of humanity’s impact. But he doesn’t think we can precisely date when Earth’s living and human-made mass will reach a crossover point as the study attempts to do.
“This is not a question of scientific precision, as there are so many ways to compute numbers like this, and so many uncertainties in so many of these numbers,” Ellis wrote in an email.
The authors also acknowledge data uncertainties that make it hard to say when, exactly, Earth will be more artificial than biological by mass. Lead author Emily Elhacham says the biggest uncertainty lies in the current estimate of plant biomass. The study also assumes that the smaller animal and microbial biomass fraction has remained constant through time, but that assumption might not be borne out by future investigations.
However, the overall picture is unlikely to change even if the final numbers shift slightly, Zalasiewicz says.
“I’m sure the numbers can be shifted by different statistics,” he says. “But given the scale of the difference between the early 20th century, mid- and late-20th century, and now, early 21st century, it’s hard to see how the pattern can be shifted.”