Geologists are a staid, often conservative bunch, not known for making political waves. These are the people for whom the idea of plate tectonics was so heretical that it took decades to achieve scientific consensus within the field, because continental drift threatened the idea that the Earth was static and and unchanging in structure. Now another controversy has erupted among geologists. It concerns the so-called Anthropocene, the idea that humans have altered Earth in such a way as to cause human activity to appear globally in the geological record, which constitutes a boundary between epochs in the history of our planet. This idea was presented by a working group delegated by the International Geological Congress, after seven years of deliberation. But opinions in the field remain sharply divided. So let’s take this apart, shall we?
Here’s the argument presented by the IGC working group:
Geological time moves slowly, and in large increments of sidereal time: a geological age takes millions of years to pass, an epoch tens of millions. Sometimes in the history of our planet, Something Big has happened that leaves a global mark, like the pattern of growth rings in a tree: differing layers in the strata show a clear, unambiguous signal that there has been a change in the pattern. That’s what we use to divide up the history of our planet into manageable temporal chunks like epochs and ages. Like the K-T impact event that left a fine dusting of meteorite iridium in the global stratigraphic record, the advent of the nuclear age left a fine dusting of manmade radioisotopes in the sedimentary layers around the world, and in the polar ice caps too. Since sedimentary rock is made from silt and sand and other such sediment, the activities of man sometimes represent a literal, actual line in the sand. And since it’s humans who drew the radioisotopic line, the working group recommended naming the ensuing time after humans, in the way that we named the Cambrian era after the Cambrian explosion visible in the fossil strata. Enter the Anthropocene Epoch: the “new age of man.” To put a finer point on it, the working group proposed a formal start date of 16 July 1945 – the day of the first atomic bomb blast.
But there’s still dissent in the ranks. A rebuttal published by the Geological Society of America dismisses the idea that humans have changed the planet as “pop culture” – claiming that no such trace on the planet’s record exists, and that people who like the idea of an Anthropocene era can have it, but should keep their noses out of geology entirely. Other commenters confuse the Anthropocene epoch with the idea of anthropogenic climate change. Accusations of political bias have been lobbed at the working group. One member of the working group actually went on record saying that we should stop thinking about this problem for a thousand years because to do otherwise would be “premature.”
That’s the opposite of science.
Let me be clear: nobody is accusing dissenting scientists of malice or bad faith. But whatever any individual’s politics might be, the definition of a geological epoch is a matter of observation. The International Stratigraphic Commission says it looks for a “golden spike”: an event that’s sudden and sharply defined, a boundary after which the whole world was different. And “we’re spoiled for choice,” explains Jan Zalasiewicz, head of the working group. Nature points out that through mining activities alone, humans move more sediment than all the world’s rivers combined. Starting about the middle of the twentieth century, we started using things like plastics, fertilizers, coal power plants, concrete and leaded gasoline, all of which left an imprint in the sedimentary record.
The rebuttal letter from the GSA reminds us that “Time stratigraphic units represent layers of rock containing lithologic, fossil, mineral, chemical, or geophysical signatures that allow for the recognition and measurement of geologic time.” But there’s evidence of all these things, up to and including the bones left by the “global proliferation of the domestic chicken.” On the deep seafloor, the layer of sediment representing the past 70 years would be less than a millimeter thick, but that millimeter could contain heavy elements like manmade radioisotopes or lead from airline fuel. Nobody will know until we go down there and dig up some silt and check.
In any event, a lot more questions have to be answered before the Quaternary commission will accept the proposed Anthropocene epoch. Does the “golden spike” have to be a single signal, present everywhere? There’s supposed to be a defined place in the rocks that marks the physical beginning of the new epoch. The pastiche of human chemical changes to the sedimentary record is present everywhere we go, but it’s not one single thing: it’s patches of radiocontamination, heavy metals, plastics, and the like. If we haven’t leaded the ocean floor yet, it’s only because nobody has done mass commercial submarine trips. What of the great dams we’ve built? What of the mountaintops we’ve leveled, the mines we’ve dug, and the roads we’ve carved out through solid rock? Everything humans touch, we change. Our presence on this planet is written in stone, and we should recognize it.
The beginning of the Anthropocene isn’t a political argument. It’s just a thing that happened. Humans have made an impact on the Earth that’s visible in the planetary geological record. Apart entirely from anyone’s views on pop culture, we’re talking about rocks here. Everyone agrees that in terms of geological time, if you can’t find it in the rocks, it doesn’t count. Invoking the climate change nontroversy is nothing but a straw man.