LIFE, MASS EXTINCTIONS, THE ANTHROPOCENE: 5th part (The Anthropocene, the sixth extinction?)

 

Post n. 44

1st part etichetta Zga, 2nd part etichetta Zha, 3rd part etichetta Zia, 4th part etichetta Zla

 

With the disappearance of the dinosaurs, 65 million years ago, the era of mammals begins. As the reptiles, to ensure their survival, from small animals some evolve slowly towards gigantism and around 30 million years ago appeared the mastodons. In the same period among the primates appeared the anthropomorphic monkeys, omnivorous animals that lived mainly in trees but also walked on four legs. When they were on the ground on four legs they saw more a two-dimensional world with the third dimension quite reduced and in the savannah they had a very limited vision. In the trees they certainly had a broader vision but it reduced the size of prey and predators, not exactly ideal for survival. But around five million years ago, from a primate offshoot, at least one lifted up on its two hind legs and saw and understood a different world, a real world, a 3D world more useful for survival. The first hominids, progenitors of man, appeared about 4.5 million years ago, the Australopithecus. Approximately 2 million years ago appeared Homo abilis, capable of producing stone tools and 1.8 million years ago appeared Homo erectus who learned to use fire. Homo Neanderthalensis, 200,000 years ago, developed social feelings and buried the dead. Around 80000 years ago appeared Homo sapiens who developed the artistic sense and the ability of abstraction, and soon after Homo sapiens sapiens, the modern man, Homo Sapiens 2.0. During the Pleistocene, the era of the glaciations where the sea level, up to 18000 years ago, was 120m lower than today's level, man lived by hunting and fishing. With the end of the glaciations begins the Holocene epoch, which includes the last 11700 years, with a pleasantly mild climate. The level of the seas slowly rose and about 5000-6000 years ago reached almost the current level. Around 10000 years ago, man switched to agriculture and animal husbandry, and 4500 years ago appeared the first large cities in the Middle East and Egypt. With phases of decadence and growth, cities were and are the protagonists of human history. But in recent times, population growth that has brought the world population to reach 7.5 billion, human activities and the dizzying expansion of cities, with a greater need for energy mainly from fossil fuels, food, and water, are rapidly altering the chemical and physical balance of our planet. These alterations leave indelible traces in geological time and for this reason according to many scientists a new era has begun, the era of man: the Anthropocene that is an era in the history of the earth characterized by the presence of man.

But are we really in an era that we can call Anthropocene?

Actually we should not be the ones to determine if our presence on the planet has given rise to a geological epoch, but geologists and paleontologists of the distant future. What we can do is to imagine that our civilization will disappear tomorrow, imagine what remains of us in sedimentary deposits and as fossils so that a new civilization, in 10000 or 100000 years or millions of years, analyzing sedimentary deposits and fossils, finding evidence of our presence can say: someone was already here.

On the effects of human activities on our planet there is a vast literature composed of essays but above all of scientific articles about climate change, air pollution, habitat loss, species extinction. It is a fragmentary literature that deals with single topics of modern events, centuries or millennia past. But in order not to lose the big picture, perhaps it is useful to trace the history of Homo sapiens 2.0, or Homo with all the characteristics of modern man. So, let's start from the data and the conclusions of this path taken from the essay "The Human Planet" by Simon L. Lewis and Mark A. Maslin convinced supporters of the Anthropocene.

It started from Africa around 50000 years ago and 40000 thousand years ago had invaded the whole planet. At that time in all the continents lived a population of big animals, called megafauna: giant beavers of more than 40 kg and giant sloths, mammoths and American mastodon. Man immediately started to hunt these animals and became a world superpredator because he had by now the ability to plan, coordinate and adapt strategies according to the prey. This megafauna became extinct and from the fossil record we know that the extinction started 50000 years ago coinciding with the spread of our ancestors. According to Paul Martin of the University of Arizona, who developed the "Pleistocene Overhunting Hypothesis," it was humans that caused the extinction of the megafauna. Since at that time the end of the last glaciation was beginning, many researchers blamed the disappearance of the megafauna on the climatic changes of the deglaciation. But between 15,000 and 10,000 years ago, the ice age had ended, the climate had stabilized, and the megafauna had survived in both North and South America. Man arrives in the Americas 15,000 years ago and after a few thousand years the megafauna disappears. "Take the example of the woolly mammoth, which by the end of the last glaciation, or ice age, about 10000 years ago, was nearly extinct. The exception was a population of a few hundred mammoths on Wrangel Island, about 140 km northeast of the East Siberian coast. Again, humans were absent and mammoths present. Rising sea levels created the island and protected the mammoths from human hunters for about 6000 years. When humans landed on the island 4,000 years ago, the woolly mammoth became extinct.

The tusks found on the island provided genetic material and evidence that the extinction was not caused by either small population size or inbreeding.

 

wikimedia

Almost certainly, newly arrived humans were the culprits. Ultimately for what concerns the megafauna, using approximations of the number of animals that lived in each of these habitats, we can estimate that the few million people that existed at the end of the Pleistocene, incredibly, killed a billion large animals."  As evidence to Martin's over-hunting hypothesis we can add the disappearance of the tame white bird the "Rodrigues loner" and the dodo, unable to fly, and the Caribbean monk seal which coincided with the arrival of European sailors who appreciated their meat.

The disappearance of megafauna also disrupted the ecosystem. "Being made up of large animals, megafauna shape ecosystems. These animals modify vegetation by breaking it up and trampling it and consuming it in large quantities. This promotes the growth of grasslands. [...] The presence of herbivorous megafauna usually prevents forests or densely wooded areas from predominating, producing an overall increase in local and regional biodiversity. [...] The scarcity of megafauna during the current interglacial means that the landscape is dominated by mossy, low diversity, shrub tundra and forests. The absence of megafauna can restructure entire ecosystems."

Homo sapiens brought to extinction the megafauna on land, changed the environment but did not give rise to any mass extinction, nothing comparable to the great mass extinctions of the earth. Meanwhile that the megafauna decreased man had to invent new ways to survive: agriculture was born.

"The Earth has gone through more than fifty glacial-interglacial cycles over the past 2.6 million years, each of which has produced a profound effect on the Earth system, including on climate. At the peak of the last ice age, just 21,000 years ago, North America was crisscrossed by a nearly unbroken layer of ice from the Pacific to the Atlantic coasts. In the region where it was deepest, above Hudson Bay, the ice was more than 2 miles thick and extended southward to New York and Cincinnati."

 

DigiLands

  

One of the reasons of these glacial-interglacial cycles was proposed in 1941 by the mathematician and climatologist Milutin Milankovic, the oscillation of the earth's orbit modify the insolation of the earth's surface making the earth enter and exit from an ice age. The theory was later verified by several studies, according to which the earth today has an orbital configuration similar to that of 21000 years ago, that is the period of maximum expansion of the ice layers. So we should be today in full glacial period with all the northern Europe covered with ice almost up to touch the Alps and the Urals and instead we are in full interglacial period.

So where is the ice?

"Air bubbles trapped in the ice sheets of Greenland and Antarctica give us information about the variation of greenhouse gases in the past. Analysis of the bubbles shows that greenhouse gas levels were lowest during cold glacial periods and highest during warm interglacial periods: carbon dioxide varied between 180 and 240 ppm (parts per million) and methane varied between 350 and 700 ppb (parts per billion). Greenhouse gas levels are an essential part of the self-reinforcing positive feedback loops that bring the Earth system in or out of an ice age. 

[...] ice core data cover the last eight warm interglacial periods. In each, greenhouse gases begin at very high levels and then slowly decline. Studying them. Paleoclimatologist Bill Ruddiman realized that the current interglacial period, the Holocene, is different: there, after several thousand years of decline, about 7000 years ago carbon dioxide levels began to rise, and about 5000 years ago methane levels began to rise again. Ruddiman's idea is that early farmers caused a reversal of the usual downward trend in atmospheric carbon dioxide by deforestation for agricultural purposes and a reversal of the downward trend in atmospheric methane by growing rice. This idea has caused a lot of controversy, but it has been tested over and over again, as one should do with all promising hypotheses, and it has emerged even stronger. Other data collected in the last decade have corroborated the hypothesis that humans influenced Earth's climate thousands of years ago".

Ultimately from the beginning of the Holocene 11700 years ago, the 280 ppm concentration of CO2 should have decreased and initiated a new ice age. But glaciation did not occur because of the greenhouse gases released by early farmers who transformed huge forest-covered areas that stored CO2,  into agricultural land with low CO2 storage. And Lewis and Maslin conclude, "In a slow and imperceptible way, and without humans being aware of it, the new way of life that emerged 10500 years ago managed to postpone a new glaciation event, producing a truly global environmental impact."

With the appearance and spread of agriculture the climate, instead of proceeding towards glaciation, remained mild for several thousand years, trade began, the world population from about 10 million initial passes to 500 million, empires and large cities arise.

The supercontinent Pangaea separated 200 million years ago. The continents that formed drifted apart and formed the continents as we know them today. Along with the continents drifted all the living things that were on those lands. The living things trapped on each of the continents followed different evolutionary paths. Through the Bering Strait, 15000 years ago, humans reached the current Americas and within 3000 years spread throughout the continent. In 1492 Columbus landed in America. Since then intercontinental navigation began. So, after 12000 years the Europeans meet the Native Americans and after a century the Native Americans estimated in about 60 million, decimated by the diseases transmitted by Europeans and by famine were reduced to about 5 million. The great empires collapsed and with them agriculture. Huge expanses of agricultural land, in about a century, were invaded by forests that stored huge amounts of carbon dioxide subtracting it from the atmosphere. Data obtained from Antarctic cores of that period indicate a decrease of carbon dioxide began from 1520 until 1610 causing a cooling of the planet detectable in geological deposits around the world. Actually as Lewis and Muslin point out, around 1350 began the so-called "little ice age" maybe caused by the internal variability among the interacting parts of the Earth system. The decrease of carbon dioxide caused by the collapse of agriculture in the Americas caused a drop in temperature that added up and amplified a phenomenon already in place.

With the intercontinental journeys that began in the sixteenth century in addition to humans and their pathogens also traveled plants and animals to and from the Americas and the evolutionary processes of many species changed radically. 

 

Limes

 

Through these exchanges, that Lewis and Muslin call "Globalization 1.0", changed not only human history but also the history of the Earth. The acceleration of trade from the twentieth century to the present day, always according to the authors has led to a "Globalization 2.0" reunifying in fact, after 200 million years, the continents in a new Pangaea. A century later, with the advent of the industrial revolution, prosperity increased and the population reached one billion at the beginning of the nineteenth century and at the beginning of the twentieth century was already 2 billion. With the progress of the industrial revolution and the increase in population, today at 7.5 billion, the need for energy increases. Starts the massive exploitation of fossil fuels that releases into the atmosphere huge amounts of carbon dioxide that increases the greenhouse effect and with it the temperature creating a superintergalacial period.

In conclusion, the Holocene interglacial period, started 11700 years ago would have already ended and we should be under a blanket of ice. Between 7000 and 5000 years ago the advent of agriculture, with the transformation of forests into agricultural areas and livestock farming (which produce methane, a greenhouse gas 20 times more powerful than carbon dioxide) were released into the atmosphere large amounts of greenhouse gases that blocked the path of the Holocene stabilized the temperature of the planet giving rise to a mild climate. The advent of the industrial age with the massive use of fossil fuels and the resulting increase in greenhouse gases reversed the course of the Holocene pushing it to a deglaciation and giving rise to what Lewis and Muslin call the superinterglacial period. (Incidentally, Lewis and Maslin write, "However far-fetched in today's political scenario, in theory we could reduce atmospheric carbon dioxide and then maintain a constant interglacial climate. [...] the chemical composition of the atmosphere, the acidity of the oceans, and the energy balance in our hands.").

So, to return to the original question: are we in an era that we can call the Anthropocene?

We are subjecting the Earth to colossal environmental upheavals, from the change of the carbon cycle to microplastics, from the residues of metals due to mining, to the residues of manufactured goods. To all this we have to add the radioactive full out released by the nuclear explosions that followed from 1945 until 1960, in particular Carbon 14, that will last at least for 50000 years and Cesium 137and plutonium 239 and 240, that will last for millions of years. The result of human activities will be preserved in the ice of glaciers, and in marine sediments and after some million years in sedimentary rocks. Homo sapiens gave rise to a new global economy and led the Earth into a new evolutionary trajectory. Future geologists will find obvious evidence of our presence in sedimentary rocks and fossils and will surely conclude that someone was there before them. Most scientists admit that we are living in an era where human activities are disrupting our planet and as per Lewis and Maslin: "We can conclude with certainty that we are living in the Anthropocene".

Some scientists while being supporters of the Anthropocene have raised doubts on specific points. For example in 2002 on Science on line in: "The Role of Greenhouse Gases in Ice Ages" a study by scientists at the University of Sheffield published in the journal Proceedings of the National Academy of Sciences is highlighted. The researchers, led by David Beerling, used fossil leaves to determine how much carbon dioxide was present in the air during the various periods of the ice age. The leaves have pores on their surfaces, called stomata, that open and close depending on carbon dioxide levels. "Much to our surprise," Beerling comments, "we found that carbon dioxide levels did not rise significantly when the polar ice caps began to melt. This suggests that another factor besides global warming was responsible for the end of the ice age. Perhaps the South Pole simply shifted to a slightly warmer climate."

In 2021, a study of paleoclimate records was published in the journal "Science" by Alan Cooper of the South Australian Museum in Adelaide, Australia, and colleagues. According to the data collected by these scientists, 41,000 years ago there was a reversal of the Earth's magnetic field that caused a profound change in the concentration and circulation of ozone in the atmosphere, affecting the global climate coinciding with the extinction of the megafauna and the disappearance of the Neanderthals. These studies would also show that Earth's magnetic field strength has been fading by about 9 percent over the past 170 years, with rapid movement of the magnetic north pole, fueling speculation that a pole reversal is imminent. This prediction has caused much concern, because a new pole reversal event could result in increased exposure to solar storms, with damage estimated at many billions of dollars per day.

But is the Anthropocene driving the sixth mass extinction?

Most scientists agree that the Anthropocene is having a devastating impact on the Biosphere and many even speak of a sixth mass extinction, the Anthropocene extinction.

It is estimated that at least 10 species normally disappear each year due to natural causes. The cause of these disappearances is continental drift, local climatic variations, volcanic explosions and the natural disappearance of their habitat.

According to many scientists, the rate of extinctions has been increasing for more than a century, with about 1000 species going extinct every year. Responsible for this acceleration of species extinctions, which many have called the sixth extinction, are human activities that are constantly changing the habitat'.

Although several intellectuals in previous centuries had already pointed out the damage caused to the environment and animals by human activities, the alarm bells rang for the first time around the year 2000.

As reported by Le Scienze, 'A new mass extinction'. Six large datasets on plants, birds and butterflies, collected in Britain over the last 20-40 years, have been used to compare the fate of the three groups. The information on birds is summarised in two publications ('The Atlas of Breedings Birds in Britain and ireland', for the period 1968-1972, and 'The New Atlas of Breeding Birds of Britain and Ireland 1', for the period 1988-1991) edited by the British Trust for Ornithology (BTO). «We have excellent information on changes in the distribution and numbers of birds in Britain and Ireland» explains Jeremy Greenwood of the BTO, «and the information overall is better than for any other group of animals or plants. Analysis of comprehensive data on all 201 native bird species in Britain and Ireland shows that, over twenty years, the distribution of 56 per cent of species has declined, which compares with a decline of 71 per cent of butterfly species (over twenty years) and 28 per cent of plant species (over forty years). The fact that the losses are also seen in butterflies and plants, as well as birds, shows that human activities are having a 360-degree impact on wildlife». The study, presented in two papers by Jeremy Thomas of the Natural Environment Research Council (NERC) Centre for Ecology and Hydrology in Dorchester published in the journal Science, supports the hypothesis that the world is experiencing a mass extinction comparable to the previous five great extinctions.

In 2002 Mick Frogley, of the University of Sussex in 'Humans most destructive of the ice ages' in Le Scienze 2002, explored a site near Lake Ioannina in north-western Greece. Frogley highlighted how deforestation and wild grazing over the last 5000 years had destroyed important tree species that had survived the last ice age. The results of this research were published in the journal 'Science'.  Frogley says: «During the Ice Age, plants and animals from temperate climates took refuge in protected areas, especially in the southern regions of Europe, where climatic conditions were less extreme. When the ice receded, these species were able to recolonise the northern regions. We have analysed fossil pollen from the lake bottom sediments, we can clearly see which tree species were able to survive long glacial periods in this area, and which were later destroyed by humans». In 2017 Science online in "On the brink of a sixth mass extinction" Two ecologists from Stanford University and the University of Mexico Paul R. Ehrligh and Gerardo Ceballos, In a paper published in "Proceedings of the National Academy of Sciences" presented a study on extinctions of populations of medium and large mammals. «Because of the diversity and variety of habitats used by mammals, they can serve as an indicator of what is happening globally to animals and plants» Ehrlich says. The researchers compared the historical geographical distribution of 177 mammal species between 1990 and 2015 with their current distribution. The results showed that these species have collectively lost more than 50 per cent of their historical range. It was also seen that population extinctions were concentrated where human activities are densest. According to the authors, even the most conservative estimates from their study indicate that about 2 percent of the world's mammal populations have already been lost. The danger of a mass extinction, perhaps worse than those of the past, could therefore be real.

One could go on and on listing the impressive number of papers published in the last twenty years concerning the decline or disappearance of species of animals, insects and plants. In most cases, these papers have remained a matter of debate among experts. A few years ago, it emerged that the risk of extinction also involved bees and other pollinators with catastrophic consequences for our food supply.

 

Animali volanti

  

 At that time, alarm about the fate of bees quickly spread with great concern throughout the world. In other words, we are not much affected if some species go extinct, if our survival is not affected.

Of course, some scientists are not worried about the disappearance of species because, as the great extinctions of the past teach us, the niches left vacant will be colonised by other species. Besides they add, what right do we have to decide which species should survive and which species have no right to emerge.

And yet, a reflection is necessary because, as Massimo Sandal writes in "La malinconia del Mammut" 2019 (The Melancholy of the Mammoth), when the last dinosaur disappeared, not just one dinosaur disappeared, but the only dinosaur in the universe disappeared.

And so we can end this long journey through "life, mass extinctions, the Anthropocene" with Lewis Dartnell's words from the "Apocalypse" chapter of Jim Al-Khalili's essay (cited above): "Making accurate predictions about the rate of climate change, and its local effects, is extremely difficult with a system as complex as the Earth's atmosphere, oceans and continental masses, with all the feedback loops involved. [...] The risk is that climate change may occur so rapidly that our infrastructure proves incapable of adapting, leading to the collapse of modern civilisation'.

But this is a history that has yet to be written.

                                              

                                                                                  Giovanni Occhipinti

 

The next article in the fall: (probable date end of October)

Where are we with Darwin: from the hot pool to the microbiome