Force of nature
What are we leaving for future scientists to discover? (10 minute read)
Tuesday is rubbish day, and it’s my dog Donna’s favourite day for a morning walk. Every few metres, there’s a yellow rubbish bag or a recycling bin. So many smells! I can see her nose twitching as she gives each bag or bin a good sniff, then rushes on, trying to hurry me along to the next one.
Sometimes a bag has been left out since the day before, and an animal has torn it open. This can mean food scraps scattered over the footpath, so I need to be alert – she’ll eat any discarded morsel she can reach. If I spot something before she does, I can tempt her away with one of the treats I carry. Every now and again, though, she lunges for something and I’m too slow to stop her. Sometimes it’s crusts of bread, sometimes it’s something I can’t identify, but most often it seems to be chicken bones.
If there are paleontologists a million years from now, the bones that they excavate from the early 21st century will tell a startling story. Starlings and sparrows are almost worldwide in distribution, and there are estimated to be more than a billion of each species on Earth right now. So they are sure to find a good number of their fossils. There are also two northern hemisphere seagull species with around a billion birds each, so they are likely to find fossilised seagulls too. But most of the fossils they find will not be from wild birds. They will be from domestic chickens.
In sheer numbers, the chicken swamps any wild bird species. The house sparrow is the most abundant wild bird, but there are only around 1.6 billion of them. A 2021 paper estimated that there were around 50 billion wild birds in the world. In contrast, there were around 33 billion chickens in 2020.
While the numbers are staggering, the weight is even more so. If all the birds in the world were weighed, those billions of seagulls, sparrows and starlings, with trifling contributions made by many others, would make up less than a third of that weight. Farmed birds would make up the rest, over 70% of the total weight of birds. Three quarters of that weight would be chickens. That means that nearly 60% of the total weight of birds in the world is made up of domestic chickens.
The figures for mammals are even more extreme. Of the total weight of mammals in the world, wild mammals make up only around 5%. Of the remaining 95%, just under two thirds of the weight is domestic animals, particularly cattle, and just over one third is humans. But while a future palaeontologist will certainly see many human and cow bones, the average human life expectancy is 71 years. Figures for cattle vary, depending on whether they are bred for meat or dairy, but they generally live 1-4 years. Chickens, on the other hand, live a very brief life indeed.
It's hard to find data from an unbiased source on how long farmed chickens live. Industry websites are often silent on the subject. Data from Germany indicates an average lifespan for meat chickens of 37 days. The US Department of Agriculture suggests meat chickens can live for 7 weeks to 1 ½ years. It depends on the kind of chicken, but it’s clear from other US Department of Agriculture data that chickens known as broilers, which are the ones slaughtered at 7 weeks, are the vast majority of the flock.
All of this adds up to an astounding quantity of chicken bones for a future palaeontologist. What will they think of this planet apparently dominated by chickens? Will they, perhaps, name this era the age of poultry?
While we will never know what a future palaeontologist might think of our world, they might notice that our biodiversity crisis is about more than just extinctions. It’s also about the way we have altered the balance between different species and their global distribution. Our domestic animals overwhelmingly outnumber wild birds and mammals. Even among the wild animals, in many areas the same few species, such as starlings, sparrows and rats dominate. If they find ancient pollen or fossilised leaves, they will realise that we’ve done the same with plants. Corn from Mexico, wheat from the Middle East, soybean from central China and radiata pine from California cover vast areas around the world.
But we’ve changed much more than just the plants and animals. Future archaeologists, studying the artifacts we left behind, will find materials made from entirely new molecules. They’ll discover buildings on one continent containing timber from another, and steel from another again. Future scientists will discover marked changes in the atmosphere, changes in the ocean’s chemistry and the signatures of new radioactive elements.
The changes we are seeing now – changes that will leave a mark hundreds of thousands of years into the future – are profound and pervasive. They are of such magnitude that chemist Paul Crutzen suggested that the world had entered a new geological epoch – the Anthropocene.
Crutzen was a hugely influential figure in science from the 1970s onwards, working in a number of countries including his native Netherlands, Sweden, Germany and the USA. He was the first to show that certain gases could damage the ozone layer and the first to recognise the risk of nuclear winter. He laid the foundations for the international agreement phasing out ozone-damaging chemicals and he clarified the role of deforestation in climate change. So, when he made his suggestion at an international meeting in 2000, geologists took him seriously.
Geologists look at time in a different way from the rest of us. We observe the passage of time in the rising and setting of the sun, the change of seasons and the cycles of life. Geologists observe the passage of time in layers formed in rock. We divide our activities into minutes and hours, and our lives into months and years. They divide time into epochs, periods, eras and eons.
At first, geological time was only relative – they recognised that lower layers of rock were older, but they had no way to measure the age of each layer. It was only in the first decades of the 20th century, when they began to study radioactivity, that they had a way of measuring absolute time. This is why geological time is not divided into more-or-less equal segments, as our time is. While an epoch is the shortest unit and an eon the longest, some epochs are a couple of million years long while others are tens of millions.
The geological time period most of us know is the Jurassic – thanks to the book and film Jurassic Park. The age of the dinosaurs began in the Jurassic Period, which began 201 million years ago. However, some dinosaurs, such as Tyrannosaurus rex, and Velociraptor, didn’t appear until the next period, known as the Cretaceous. This period began 150 million years ago and ended abruptly 66 million years ago with the giant meteor strike I wrote about last week. But “Cretaceous Park” doesn’t have the same ring.
Today, we live in the Quaternary Period, which began only 2.5 million years ago, and Crutzen’s suggestion wouldn’t change this. Instead, he has suggested that the second epoch of the Quaternary Period has ended, and that we have now entered a third.
Have humans really made such vast and irreversible changes to the Earth that the evidence will be visible in layers of rock millions of years in the future?
This is still being debated, but there is considerable evidence to say that we have. Among the physical changes are the marked increase in carbon dioxide in the atmosphere, a warming climate with now-irreversible melting of ice-sheets and rising sea levels, increased erosion, and the appearance of new, highly durable materials. These sit alongside the marked changes in abundance and distribution of animals.
But if geologists were to use this new term they would need to properly define it. To do that, they would need to decide when it began.
As a species, humans have been around for 300,000 years, but we haven’t been such a force of nature for all of that time. Global changes caused by humans begin only with the development of agriculture, from 12,000-8,000 years ago. Agriculture was not an innovation which began in one place and then spread elsewhere. It developed independently in at least 14 different locations, perhaps as many as 23, on every continent except Europe and arguably Australia, but including New Guinea.
Although there were fewer than ten million of us when agriculture began, we were much less efficient at producing food, so we needed to farm more land. This led to deforestation on a massive scale. By 2000 years ago, carbon dioxide and methane levels were detectably higher, and the earth had warmed by 0.8oC, and 2oC at higher latitudes.
While substantial, these changes were not significant enough to warrant the beginning of a new geological epoch.
The next possible beginning of the Anthropocene is linked to our use of fossil fuels. Usually, people date this to the 18th century, with the start of the industrial revolution in Britain. But it actually began at least 1000 years ago, in China, where coal was used in iron production. Fossil fuel use became widespread in the 19th century, and humans escaped one of the key constraints on development – the amount of available energy. It took time for the impact of fossil fuel use to show in the atmosphere, however. Only around 1850 was there a detectable rise in carbon dioxide. This left various possibilities for the start of the Anthropocene – one proposal was 1800.
With the help of abundant energy from fossil fuels, we escaped another environmental constraint – the amount of nitrogen available for plant growth. The Haber-Bosch process converted atmospheric nitrogen into a form which could be used by plants, albeit at a huge energy cost. The sudden abundance of nitrogen fertiliser allowed us to grow more food than ever. This led to an increasing population, more deforestation and more water diversion for irrigation.
Although the Haber-Bosch process was developed in the early 20th century, its use accelerated after 1945. So too did our use of petroleum. Our economic activity, our urbanisation, our water use – all of these accelerated after World War Two. Our life-spans extended and population growth rose sharply. So, too, did deforestation, greenhouse gas emissions, temperature rise and species extinctions. This period has been called the Great Acceleration because of its scale and impact, and it emerged as the leading candidate for the official start of the Anthropocene.
But how would it be measured in the rocks?
There’s something else which happened in this time period – something which would leave its signature in the sediment of lakes around the world. We began exploding nuclear devices in the atmosphere. As a result, there’s the abrupt appearance of radioactive plutonium. This plutonium was proposed as the sign that we had entered the Anthropocene.
The proposal was put to the scientific body which oversees the naming and definition of geological time periods and given serious consideration. Last year, they decided against classifying our current era as the Anthropocene. Despite some searching, I haven’t been able to find out precisely why they made this decision, only that it was subject to a vote by the committee established to look at the question.
Nonetheless, the troubling facts remain. We are now irreversibly changing the planet on a scale which will be visible to the hypothetical future palaeontologist I mentioned at the start of this article. Whether that palaeontologist will be human – whether we survive long enough for that to be so – is debatable. Can we turn our extraordinary ability to adapt and innovate in a direction which allows us to coexist with nature, instead of simply consuming it?
