A verdant witness

How often, I wonder, does political tribalism meet geology? Not very, I'd imagine.

Yet as I write this geologists are being forced to ask themselves whether they are offering up a objective ­scientific concept or playing politics when it comes to the otherwise sedate world of geological time scales. And this is the seed of a rollicking debate – one that could all hinge, amazingly, on a single tree.

Many now consider the Earth to have transitioned into the Anthropocene – an epoch entirely defined by our impact on the world. But not all agree ­– there are critics of a formalised Anthropocene who say that the idea did not arise from geology and that it is more a part of human history than the immensely long history of the Earth. Most damming, they also suggest this is a political statement, rather than a scientific one ­– the evidence, they say, just isn’t there.

But the proponents of the Anthropocene – the international Anthropocene Working Group – argue that, in fact, there is an embarrassment of riches when it comes to such evidence – evidence that is distinctive and rich in geological detail. Artificial radionuclides, plastics, metals such as aluminium, pesticides and concrete – it all suggests an indelible human mark on the very bedrock of the planet.

One stumbling block is that there needs to be a consensus over a clear ‘golden spike’ of evidence to define the start of the Anthropocene Epoch. Enter our all-important tree. Chris Turney, from the University of New South Wales, Australia, and colleagues, say a Sitka spruce located on Campbell Island in the Southern Ocean, records in its wood a clear radioactive trace from the A-bomb tests of the 1950s and 60s. It is this he wants to put forward as the all important golden spike to mark the start of the Anthropocene. And it is a powerful proposition – mainly by virtue of the sheer isolation of the 117-year old tree. Dubbed the ‘loneliest tree on the planet’, it is situated hundreds of miles south of New Zealand, which in essence means that any record of human-generated atomic isotopes suggests a genuinely global impact.

By quietly recording environmental changes in the fixing of their tissues, these incredible organisms have, in their own way, borne witness to human existence

My guess is that by using evidence like this the Working Group will manage to enshrine the Anthropocene in the Geological Time Scale. We have singlehandedly altered the planet to such an extent it is becoming impossible to keep this off the books, as it were. There will be no loopholes for us – it will, almost certainly, be reflected in the official classification of the Earth’s geological history.

And it isn’t simply geological golden spikes that can be attested to by our arboreal neighbours. By quietly recording environmental changes in the fixing of their tissues, these incredible organisms have, in their own way, borne witness to human existence.

And ­– when we decode these documents from our past – what they have to say doesn't always cast us is a good light. This this isn’t politicisation – this is simple objective truth.

Phil Prime Managing Editor Laboratory News