This is the 8th episode in a series recounting the history of measurements, data, and projections related to global climate change. If you’re just joining, you can catch up on the previous episodes:
- Episode 1: Beginnings (or two British scientists’ adventures with leaves and CO2 measurements)
- Episode 2: First measurement of anthropogenic global warming
- Episode 3: Our “large scale geophysical experiment” (1940-1960)
- Episode 4: Dave Keeling persists in a great idea
- Episode 5: Icy time capsules
- Episode 6: The “geologic eons of time”
- Episode 7: Our global thermometer since 1850
Episode 8
Though the details are not known for certain, most who have studied it believe the first inhabitants of the island, numbering not more than 150 people, arrived between 400 and 1200 AD in wooden canoes from previously settled Pacific islands that may have been as far as 2,000 miles away. They were highly skilled seagoing navigators, having over previous generations employed navigational instruments and charts, detailed observations of the sun, stars, seabird behavior, wave formations, winds, and weather, and extensive accumulated knowledge maintained in oral tradition and songs to discover nearly every island in the vast Polynesian Triangle of the Pacific. They had arrived at a faraway corner of that explored territory, to this day among the most remote inhabited locations on Earth. The first people off the boats encountered an isolated tropical paradise, forested with multiple species of up to 50-foot trees, including possibly the largest palm trees in the world, and populated by six species of indigenous land birds.
Undaunted by their isolation, they set about rapidly building a complex, vibrant, and thriving agricultural civilization on the island; it would eventually reach a population of 10-15,000 people. Oral tradition, later recorded by European missionaries, held that nine separate clans, each with its own chief, were ruled over by a high chief, the eldest of the first-born descendants of Hotu Matu’a, the island’s legendary founder. Over generations, the clans paid homage to their ancestors by erecting over a hundred giant stone monuments, unique in the world, up to 32 feet tall and weighing as much as 90 tons. The precise methods by which these Stone Agers accomplished that impressive feat, testifying to their ingenuity and artistry, a deep spirituality, and a cooperative society with an apparent luxury of time and resources, is still a matter of controversy and wonder in today’s digital age.
But, by just a century after the island civilization’s peak, things had gone terribly awry.
When Dutch explorers happened upon the island on April 5 (Easter Sunday), 1722, they found a largely bald landscape, with no tree over 10 feet tall and a population of 2,000-3,000 inhabitants living in a radically diminished condition. With no wood left capable of making a seaworthy vessel, they were stranded and had lost much of their former fishing range. Twenty one tree species and all of the birds were extinct.
What happened on Easter Island during that tragic 100 years?
Again, the details are debated, but the broad strokes are fairly clear. Some combination of intensified agriculture to support the expanding population, rats, the monument construction, and possibly climate change caused the rapid and near complete deforestation of the island. Aside from the clearing of forests for fuel and agriculture, large trees may have been felled as rollers to transport the heavy monuments. Polynesian rats, stowaways on the boats that had carried the original settlers, had no real predators on the island and ate the tree seeds. Some speculate that the Little Ice Age, beginning around 1650, may have additionally stressed the large palm trees. Without protection from the trees, the fertile topsoil began to dry up and blow away. Over-hunting of the land birds by humans, or rats, or both drove them to extinction while, at the same time, access to fish protein was dramatically reduced by the loss of wood for large boats. As resource depletion continued, the systematic class system of the society gave way to loosely organized, warrior-led bands that frequently fought fiercely and took to toppling each other’s statues in anger.
The absence of written records on the island leaves much room to speculate about the state of mind of the people who inflicted on themselves such a seemingly predictable fate. The island is some 15 miles across at its widest point; a single person could survey the state of its entirety in a matter of a few days. Yet, someone cut down the very last of its remaining trees, thus eliminating the possibility of escape from a deteriorating environment. What were they thinking as they did that?
- Were they our proverbial frog in a soup pot, unaware of their developing crisis because it appeared to occur slowly? Had each succeeding generation become accustomed to a “new normal” with fewer trees, less productive farms, and less cultural emphasis on fishing, until it seemed like no big deal to fell the last few remaining trees on the island?
- Were they engaged in internal conflict, such that the folks who felled the last few trees believed if they didn’t someone else surely would?
- Had the tree-felling folks successfully convinced the chiefs that the alternatives were too costly, or that the rumored decline of farmland productivity was a hoax?
- Did they believe they would be supernaturally delivered from their declining state by the deified ancestors to whom their statues payed homage, such that they may have felled the very last trees as rollers to transport statues they hoped would hasten their deliverance?
- Did they simply place more value on the now than on the future?
- Did they fell the last trees in deep sorrow, having realized their fate but, after concerted effort, having failed to come up with any social or technological solutions to the problem of the rats and their need for fuel?
We will almost certainly never know the answers to these questions but, as we shall see, reflecting on them will be important to our own future.
It’s difficult to get inside the heads of ancient people as they faced intensifying degradation of their environment’s ability to support them. But it turns out the process can be understood, modeled, and even predicted using cold, hard math. In the graph below, Bill Basener, an Applied Mathematician, and coworkers modeled the interaction between a human population (top equation) and resources on which the humans depend (bottom equation). The human population, P, grows at a growth rate, a, but its survivability is constrained by access to resources, R (in the case of Easter Island, primarily arable farmland and trees). The resources self-replenish at growth rate, c, have a maximum carrying capacity on the island, K, and are harvested at a per-person rate, h. Solving this pair of equations using assumptions appropriate to Easter Island (see details in figure caption below) yields a population curve, represented by the solid line, which closely matches actual population values estimated from the archaeological record on the island (x’s on the graph).
There it is, in stark, mathematical detail. The math is relatively simple and based on common-sense assumptions about how a population of people and resources might interact. It can’t be far off — it matches the archaeological record quite closely at key times (when it was large, and after it rapidly became small again). The simplified math doesn’t perfectly capture the ultimate fate of the Easter Island people; in fact, the population didn’t fall to zero. The archaeological record shows that people shifted to resources they hadn’t used earlier (including rats), enabling the survival of a fraction of the population, in a state of relative poverty, into the 1700’s. After that, the population was significantly affected, both negatively and positively, by contact with foreigners, and the present population of around 6,000 people relies significantly on resources from outside the island.
Still the complex native civilization on the island, built over a millennium, ended abruptly during about a 100-year period. It’s rather sobering to consider the human experiences that must have attended the rapid, downward sweep at the right side of the graph. An intensifying scarcity of fuel and food. A cooperative, seagoing civilization giving way to increasingly separate, competing bands. (Isolationism.) Proud farmers and fishers learning to survive on native grasses and rats. Fierce internal conflicts. Starvation.
One wonders if, at the height of the civilization’s powers in the 1600’s, thriving but surely confronted with mounting and ever more visible evidence of their environmental impacts, the people of Easter Island sensed the coming catastrophe.
It wasn’t the only mathematically possible outcome.
Indeed, according to some mathematically possible scenarios, the original Easter Island population would still be going strong to this day, even indefinitely. Below are four mathematically possible scenarios according to the equations above. In each, the island’s resources appear as a green line, while the human population is represented by the black line.
The historically-representative scenario is shown in (a), where the root of the problem can easily be seen as the green curve that exponentially falls as the human population exponentially rises. Before 1800, the resources are depleted, and the human civilization is rapidly decimated.
In (b), the people manage to control their population growth rate to only 1/4 of the growth rate in (a). This might be done, for example, by instituting some version of China’s “One Child” law. Inevitable collapse still occurs, but it takes a lot longer — until after the year 4400.
In (c), the population growth is the same as in (a), but the people manage to sustain themselves using a resource that self-replenishes more quickly than it’s harvested. After an initial growth phase, the human population comes into balance with the island’s resources, and both live on indefinitely.
A key learning of scenarios (b) and (c) is that controlling population growth, which we often tend to think about when considering Earth’s limits, is not in itself sufficient to avoid eventual civilization collapse. Rather, the key to indefinite survival is finding a way to live on resources that self-replenish faster than they are consumed.
Controlling population growth can give people time to think, however. Scenario (d) starts the same way as (b), with a slow-growing population that is consuming resources much faster than they are replenished. Around the year 4400, however, a technologically advanced civilization comes to understand its predicament and finds a way to consume resources that self-replenish faster than they are harvested. (Perhaps the humans learn to practice irrigation and discover a tree species that grows very rapidly.) This transition ensures the civilization’s indefinite survival on the island.
But why are we talking about ancient islanders on a blog about modern global climate change? What do we have to learn from mathematically modeling the fate of these archaic people?
Well, of course, we live on a finite, isolated paradise. An island in space. In a project that has taken several thousand years, we have built a complex, thriving human civilization. The only intelligent civilization we know of, capable of pondering its origins and meaning and sending spacecraft to take photos of itself from 898 million miles away.
But we have to imagine our civilization is constrained by the limits of our island’s responses to our population, which has rapidly filled it up. And, as we have seen in previous episodes of this series, there are troubling signs we are nearing those limits. Can the learnings and math of Easter Island be applied to our bigger island?
Can we discover math to avoid the Easter Islanders’ fate?
We look at that in Episode 9. Stay tuned…