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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.”
Geologic impact
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.”
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