why the earth won’t remember us fondly

Scientists are debating how humanity will be remembered millions of years from now and it won’t be by our sprawling cities and historical sites. It will be by our pollution, garbage, and weapons.

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Humans don’t have fangs, claws, poison, or the muscle mass to defend ourselves against the vast majority of predators on our planet. What we do have, however, is science and technology, and they allowed us to ascend to the top of the food chain and impose our order on nature. No one seems to debate that save for major natural disasters, we as a species are no longer at our planet’s mercy. But where there is debate is at what point in our history that truly started to happen. Was it when agriculture began? Was it the industrial revolution? Or was it when we harnessed the power of the atom and detonated the first nuclear weapons? At stake is an official designation for a new epoch in our planet’s history: the Anthropocene.

Our impact on Earth is no laughing matter. We built towns and cities across 15% of the world’s surface and dedicated over 38% to growing our food and biofuels. Virtually every major river is now fragmented by dams for hydropower generation and as reservoirs for population centers. We paved or carved out almost 21 million miles of roads. We’ve domesticated entire species to serve our needs, creating a new one intentionally, and many by proxy. Some 19 billion chickens, 1.4 billion cows, and a million goats and sheep exist because they either produce our food or become our food. And who can forget that our cities light up the planet when darkness falls on a hemisphere, visible so prominently they could theoretically be detected from light years away by other intelligent life in the galaxy.

And, of course, we’re currently starting to change our planet’s atmosphere after more than a century of heavy industrial activity. Denials about whether this is happening has, at this point, veered into the conspiratorial and absurd. According to decades of studies, we’re on our way to creating what’s called a Hothouse Earth unless we radically change course by 2050. Hundreds of millions are suffering from ever stronger and more persistent heat waves while our cities are beginning to scorch from heat reaching 50 °C (or 122 °F) too often for comfort, with 60 °C (or 140 °F) by 2070 not out of the realm of possibility. Climate change caused by global warming is already beginning to reshape geopolitics and giving us hints at what life will look like once the resources we too often take for granted start to disappear.

At this point, we’ve even started to change the seasons, which makes concerns about the future of our species not a matter of high minded abstract idealism, but very real issues to be tackled sooner rather than later. So, perhaps this, the time when nature is being changed by our daily lives is where we should mark the start of the Anthropocene, the age when we began to shape the planet to our whims, ask some geologists. What else shows our influence on Earth as much as altering cycles that have come and gone for millions of years now? How will scientists in the far future, or another intelligent species that may one day replace us know where to dig for artifacts from our era?

Scientists are having a very heated argument about this. Some groups proposing that the dawn of the Anthropocene be set to 1945 because that’s when we invented nuclear weapons which would leave clear geologic signals. Other groups vehemently disagree, arguing that our impact began a lot earlier, when we cultivated the first crops 11,300 years ago and laid the foundations for the first empires. Other still say industrialization, which started in 1800, is when we truly started to reshape the Earth. And yet others want to draw lines at a brief global cooling (8,300 years ago), a global drought (4,300 years ago), or the age of global exploration by Europeans (500 years ago).

anthropocene start illustration
Proposed beginnings of the Anthropocene. Illustration by Greg Fish

But while an argument can be made for all these chronological markers, the bottom line is what will be obvious in the geologic record and also reflective of human activity. Droughts and a cold snap are well outside our control, and surviving traces of invasive species brought by explorers across the world would be difficult to blame on humans a million years from now. Farming will absolutely leave a clear record, but again, attribution and scale could be complicated over the next million years. This leaves us with two obvious, leading candidates which would leave very little room for natural explanations: industrialization and nukes.

In an email exchange, climatologist Michael Mann says that sediment “should preserve evidence of a dramatic spike in the concentrations of anthropogenic greenhouse gases [like] carbon dioxide, methane, and nitrous oxide as well as isotopic signatures related to the ratio of carbon-13 to carbon-12 and radiocarbon.” The last two are important because the ratio of carbon isotopes from burnt fossil fuels and natural emissions are dramatically different, and the carbon from fossil fuels is less radioactive than natural carbon.

Even if you were an alien or an intelligent squid 100 million years from now, you’d be able to tell that something really weird happened after a time period we call industrialization. And the same goes for traces of nuclear weapons. Humans detonated over 2,000 warheads of various yields and according to Dr. Mann, they would instantly stand out to future scientists because “the human input [in these events] is even more profound relative to natural (e.g. cosmogenic) sources.” In other words, if you looked at the aftermath of a meteor or comet impact and the scars of a nuclear blast, you’d be able to tell the difference by how many more radioactive isotopes the detonations would leave behind.

Now, hold on a minute, you might say. Why did you breeze through farming, global droughts, and cold snaps? Why wouldn’t traces of globetrotting Europeans bringing species that don’t belong get a shrug? Well, part of the problem of determining when the Anthropocene starts is having to fight a very anthropocentric perspective. Yes, the global cold snap and drought may have been important points for many civilizations, but those were natural events and we can’t assume that we’ll have historical records showing their impact. Clearing out forests to make space for farms would be significant, but how do you rule out a natural explanation? It’s not like the evidence will show up in neat, farm-shaped blocks and without humans around to tell you about ancient farming practices, you’d just have a strange mystery on your hands.

Conversely, not only will unnatural traces of carbon isotopes point to an artificial origin future paleontologists will also have to deal with even more unnatural fossilized plastic, and traces of what might look like cosmic impacts at first but would require meteors made of plutonium to fully explain. Again, remember that we’re talking about deep time, when humans could’ve left the planet or went extinct. There would be no traces of cities and anything we left behind would’ve either fossilized or turned to dust. Questionable distributions of species or what will seem like arbitrary natural events a million years won’t be nearly as impactful as evidence that something very unnatural and widespread happened at a certain point in time.

And there’s another important element when it comes to mapping the Anthropocene in the geological record. In the far future, it’s extremely unlikely that anyone will know we called it that. Our taxonomy is constantly changing and could be very different in the future. But etching our outsized impact on this planet by debating what traces we’ll leave millions of years from now really brings what we’re doing into focus. If our pollution and nuclear detritus are how the far future will mark our presence, maybe it’s time to really think about leaving a better legacy.

# science // geology / global warming / nuclear weapons / pollution


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