The long summer of the last 8000 years is without doubt the crucial event in human history. Although agriculture commenced earlier (around 10,500 years ago in the Fertile Crescent in the Middle East), it was during this period that we acquired most of our major crops and domestic animals, the first cities came into being, the first irrigation ditches were dug, the first words written down, and the first coins minted.
And these changes happened not once, but independently many times in different parts of the world. Before our long summer was 5000 years old, cities had sprung up in Western Asia, East Asia, Africa and central America. The similarities in their temples, homes and fortifications are astonishing.
It is as if the human mind contained a plan for the city all along, and was just waiting until conditions permitted to build it.
These human settlements were ruled by an elite who relied on artisans. In a few societies writing developed, and in even the earliest of these jottings—clay tablets from ancient Mesopotamia—we recognise life as it is lived in a great metropolis.
Did this long summer result from a cosmic fluke? Were Milankovich’s cycles, the Sun and Earth all ‘just right’ to create a warm period of unprecedented stability? During every warm period we know about over the last million years Milankovich’s cycles caused a sudden spike in temperature followed by a long, unstable cooling. There is nothing unique about the current Milankovich cycle that can account for the long summer. Indeed, were Milankovich cycles still controlling Earth’s climate, we should be feeling a distinct chill by now.
As he tried to explain the long summer, Bill Ruddiman, an environmental scientist at the University of Virginia, began to look for a unique factor—something that was operating only in this last cycle, but in none of the earlier ones. That unique factor, he decided, was us.
The Nobel laureate Paul Crutzen (awarded the prize for research into the ozone hole) and his colleagues had already recognised and named a new geological Period in honour of our species. They called it the Anthropocene—meaning the age of humanity—and they marked its dawn at 1800 when methane and CO2, brewed up by the gargantuan machines of the Industrial Revolution, first began to influence Earth’s climate.
Ruddiman added a revolutionary twist to this argument: he detected what he believes to be human influences on Earth’s climate that occurred long before 1800.
Charting the levels of two critical greenhouse gases— methane and CO2—in air bubbles trapped in the Greenland and Antarctic ice sheets, Ruddiman discovered that, beginning 8000 years ago, the Milankovich cycles could not explain what actually happened. Methane should have commenced declining at that time, and gone into a rapid decline by 5000 years ago. Instead, after taking a shallow dip, methane concentrations begin a slow but emphatic rise.
This, Ruddiman argues, is evidence that humans had wrested control of methane emissions from nature, and so we should mark the dawn of the Anthropocene at 8000 years ago rather than 200.
It was the beginnings of agriculture—particularly wet agriculture of the kind practised in flooded rice paddies in eastern Asia—that tipped the balance. These agricultural systems can be prodigious producers of methane. Farmers of other crops that require swampy conditions were making their own contributions at around this time. Taro agriculture (which involves the creation and maintenance of water-controlling structures), for example, was well under way in New Guinea by 8000 years ago.
Even hunter-gatherers may have had a role. The construction of weirs transformed vast areas of southeastern Australia into seasonal swamps. These structures were perhaps the most extensive ever created by non-agricultural people, and were used to regulate swamps for the production of eels. Harvested en masse at great gatherings of the tribes, the eels were then dried and smoked to be traded over large distances.
Ruddiman also found evidence in the ice bubbles that the concentration of CO2 in the atmosphere was being influenced by humans far earlier than first imagined. CO2 levels rise rapidly as the glacial stage ends, then typically begin a slow decline towards the next cold period. But in this cycle they kept rising. By 1800 atmospheric CO2 had risen to 280 parts per million. If natural cycles were still solely in control of Earth’s carbon budget, Ruddiman states, CO2 should have then stood at only 240 parts per million, and be declining.
At first glance his argument looks flimsy. After all, early humans would have needed to emit twice as much carbon as our industrial age did between 1850 and 1990—an output only made possible by an unprecedented population using coal-burning machines.
The key, notes Ruddiman, is time. Eight thousand years is a long span, and as humans cut and burned forests around the globe their activities acted like a hand casting feathers on a set of scales: eventually enough feathers piled up to tip the balance.
The delicate climatic stability created by humanity over the past 8000 years, Ruddiman argues, was still vulnerable to the great cycles of Milankovich. The archaeologist Brian Fagan argues that these cycles could be amplified into truly monumental impacts on human societies. The slight shift in Earth’s orbit between 10,000 and 4000 BC brought between 7 and 8 per cent more sunlight to the Northern Hemisphere.
This changed atmospheric circulation, which resulted in increased rainfall in Mesopotamia by 25 to 30 per cent. What was once a desert was transformed into a verdant plain that supported dense farming communities. After 3800 BC, however, Earth’s orbit reverted to its former pattern and rainfall dropped off, forcing many farmers to abandon their fields and wander in search of food.
Fagan believes that the famine-driven wanderers found refuge in a few strategic locations such as Uruk (now in southern Iraq), where irrigation canals branched off the main rivers. Here the starving migrants were put to work by a central authority in construction projects such as the maintenance of canals.
Reduced rainfall, Fagan argues, also forced Uruk’s farmers to innovate, and so they used, for the first time, ploughs and animals to till fields in a rotation that involved producing two crops per year.
With grain production localised around strategic towns, surrounding settlements began to specialise in producing goods such as pottery, metals or fish, which were traded at Uruk’s markets for the ever-scarcer grain.
Each of these changes led to the development of a more centralised authority, which in turn employed the world’s first bureaucrats, whose job it was to tally and distribute the vital grain.
The sum of all of this change was a shift in human organisation, and by 3100 BC Mesopotamia’s southern cities had become the world’s first civilisations. Indeed the city, Fagan argues, is a key human adaptation to drier climatic conditions.
Let’s return now to Bill Ruddiman’s analysis, because it contains several twists in its tail. He sees a clear correlation with times of low atmospheric CO2 and several plagues caused by the bacterium Yersinia pestis—the black plague of medieval times. These epidemics were global in their reach and killed so many people that forests were able to grow back on deserted farmland. In the process they absorbed CO2, lowering atmospheric concentrations by 5 to 10 parts per million. Global temperatures then fell and periods of relative cold ensued in places such as Europe.
Ruddiman’s thesis implies that, by adding sufficient greenhouse gases to delay another ice age, yet not overheating the planet, the ancients performed an act of chemical wizardry. Today, however, the changes scientists are detecting in our atmosphere are so great that time’s gates appear once again to be opening.
There are unmistakable signs that the Anthropocene is turning ugly. Will it become the shortest geological Period on record?