From roads and buildings to cars and plastic, human civilization is built on lots of stuff.
But roughly how much stuff have we actually created? And in the process, how much of the natural world have we consumed or destroyed?
A new analysis finds that on both counts, it's a lot ... so much, in fact, that these materials may now outweigh all of the living things left on Earth.
The year 2020 could be the year when human-made mass surpasses the overall weight of biomass - estimated to be roughly 1,100,000,000,000 tons, or 1.1 teratons - a milestone scientists say speaks to the enormous impact that humans have had on the planet.
The analysis was published Wednesday in the scientific journal Nature, and was conducted by a group of researchers from Israel's Weizmann Institute of Science.
To make their calculations, the researchers divided human-made objects into six main categories: concrete, aggregates (including materials like gravel), bricks, asphalt, metals and “other” materials, which includes plastic, wood used for construction and paper, and glass.
This mass is now overwhelmingly dominated by concrete, aggregates, bricks and asphalt, which today are the foundation for modern buildings, roads and other infrastructure.
The researchers did not consider waste in making their calculation, though if it were factored in, it would have likely tipped the scales in favor of human-made materials as early as 2013, the study found.
Looking at biomass, the authors found that plants represent the overwhelming majority of living things -approximately 90% - followed by bacteria, fungi, single-celled archaea, protists, and animals. This also includes humans themselves, as well as crops and livestock raised for food.
The study finds that humans have changed the planet with staggering speed.
Since the first agricultural revolution began roughly 12,000 years ago, humans have cut global biomass nearly in half, from 2 teratons to around 1.1 teratons today.
Though an ever-growing amount of Earth's land is being used to grow crops, their total mass is dwarfed by losses elsewhere in the biosphere, where deforestation and other shifts in land use driven by humans have dramatically shrunk plant mass. The study finds that hunting, overfishing and the raising of farm animals have also cut into the overall biomass.
While the mass of organisms has continued to shrink over the last 120 years, humans have dramatically ramped up material production, making it the key driver of the shift in balance between human-made materials and nature.
At the start of the 20th century, human-produced objects were equal to just 3% of global biomass.
But explosive growth has continued almost unabated since World War II. Today, the authors estimate that human-made mass is being produced at a rate of roughly 30,000,000,000 tons (30 gigatons) per year.
This means that on average, materials outweighing the body weight of every person on the planet are being produced each week.
If this pace continues, human-made mass - including waste - is expected to exceed 3 teratons by 2040, tripling the weight of all living things.
Given the challenges of accounting for such huge global stocks of human-made mass and biomass- and how you define the two - the authors acknowledge there is uncertainty about exactly when we will reach the crossover point.
It is possible that the point of transition already occurred in the past decade, and if not, it will occur in the next 20 years, the study says.
And though the scientists say the finding is symbolic, they say this milepost does provide humanity with a chance to take stock of how we got here - and what the future looks like.
“The study provides a symbolic and mass-based quantitative characterization of the Anthropocene - the geological age of 'the era of humanity,’” two of the study's authors, Emily Elhacham and Ron Milo, said in response to questions via email. “Given the empirical evidence on the accumulated mass of human artifacts, we can no longer deny our central role in the natural world. We are already a major player and with that comes a shared responsibility.”
Fridolin Krausmann, a professor at the University of Natural Resources and Life Sciences in Vienna who has studied interactions between nature and society but was not involved with this study, said the researchers’ findings show how large society has become compared to the natural world.
And though he said the findings were not particularly surprising, he does hope they send a strong message to people, and bring more attention to how modern societies can grow sustainably.
“It’s the two highly problematic trends, that the study relates here, that are important: The comparatively slow, but long-term, continuous human-induced reduction of the global biomass stock vis-à-vis the exponentially growing anthropogenic (human-made) mass,” Krausmann said by email. “Better knowledge about the dynamics and patterns of anthropogenic mass, and how it is linked to service provision and resource flows is key for sustainable development. The big question is how much anthropogenic mass do we need for a good life.”