For people too, tipping points are the rule rather than the exception. In fact, if you think about what happens to you and your loved ones, tipping points are the defining moments. A woman is pregnant for nine months, then in the space of minutes a baby is nursing at her breast. Tipping point. The baby grows, gradually learns to talk, walk, play, reason, and then all of a sudden hits puberty. Big tipping point. More gradual change through the teen years, young adulthood, and then that special someone comes along and two previously separate lives join together in marriage – once again, a tipping point. Gradually the two grow old together, accumulating their respective aches and pains as middle age gives way to the retirement years, then a fall breaks a hip. Another tipping point. And finally, of course, the biggest state-change of all, from life to death.
The ubiquity of tipping points has prompted a great deal of research among theoreticians of late, which has made it very clear why living things tend to experience major changes in such fits and starts. Basically, it has to do with how many parts something is built of, and how those parts are connected to one another. The more parts there are, and the more intricate their connections, the greater the likelihood that the resulting system will remain stable for long periods of time. But by the same token in such systems, the greater the likelihood that when change does come, either by tweaking lots of parts simultaneously, or by damaging just one super-critical part, it will come fast, and it will hit hard, flipping the system from its ‘old normal’ to a ‘new normal’. These sorts of systems – with many intricately connected parts that influence each other – are called, not surprisingly, ‘complex systems’ in the jargon of science.
Living things are extremely complex systems, composed as they are of millions of mutually interacting parts that are connected to each other in spaghetti-tangle ways. The scale of the complexity begins to boggle your mind when you realise that anything alive is actually built of many smaller-scale complex systems, separate entities in themselves, but connected together to form ever-larger and even more complex systems. Starting at the molecular level, for instance, DNA replication itself is a complex system that we are just barely beginning to get a working knowledge of. That, of course, is intimately dependent on the workings of, and at the same time influences, the slightly larger-scale complex systems that we call cells. And so it goes, with connections between cells, organs, individuals, groups of individuals, species, communities, and entire ecosystems. The human body has more cells of microbes than human cells; the complexity of our own body’s ecosystem is not remotely well understood. The most complex system of all is the global ecosystem, which is composed of all life on Earth, and the myriad ways that life forms interact with each other and with the inanimate environment around them (like air, water, soil, and so on).
Which means that humans, being life forms ourselves, are not at all separate from the rest of the global ecosystem; on the contrary, we’re intimately embedded in it, just like every other animal (and plant, and microbe – it’s a long list). We count on it for such essentials as a place to live, air to breathe, food to eat, water to drink, and for comfort and solace. But there is no denying that, unlike other animals, our place in the global ecosystem has taken on an unusual role – because we now dominate it. So much so that, just like the flame heating that pot of water towards a boil, we have been inexorably pushing key pieces of our planetary life-support systems towards a tipping point. The tipping point we are pushing towards, however, differs from the boiling of water in an important respect. Once you cool steam, it returns to its previous state, liquid. By contrast, there is no going back once we cross the sort of threshold we’re marching towards, which is more like the one an egg crosses when it tips off the edge of the counter.
People who study tipping points for a living have a name for crossing those thresholds of no return – the system is said to exhibit ‘hysteresis’. The resulting irreversible kinds of state-changes become more and more likely as the complex system gets, well, more and more complex. Intuitively, that makes a certain amount of sense. The more parts to a system, and the more interdependencies between those parts, the harder it is to get all the pieces back in the same order if it happens to fall apart – say because a critical part wears out, or because you inadvertently broke it. Think of the watch you took apart as a kid, or the cars we drive today.
At the huge scale of the global ecosystem, the number of parts, their diversity of function, and the number of connections between them are so enormous that it is little wonder that hitting a tipping point means big, irreversible changes. That isn’t just theory: the geological and palaeontological record is replete with evidence of past threshold crossings that changed the planet forever. One of the most famous is dinosaur extinction, which happened about sixty-six million years ago. In that case, the global ecosystem was almost literally pushed past a critical threshold by an asteroid slamming into it, with cascading impacts throughout the planet. For tens of millions of years prior to that, the Earth had maintained a supersized version of the food chain, where Tyrannosaurus rex and its cousins hunted prey that in some cases stood as high as a two-storey house and weighed thirteen tons or so. In the course of what may have amounted to a bad weekend, all that was over, and the new state of the world was one where puny mammals, and eventually us, began to rule the Earth.
Not all planetary-scale tipping points are caused by something as dramatic as an asteroid strike, but even so, they still show up unmistakably in the geological record. The most recent global state-shift was when the last ice age gave way to the interglacial warm time in which we still live. The glacial state featured ice, miles thick, covering much of the northern hemisphere and mountain glaciers throughout the world, a condition that had prevailed for about a hundred thousand years. Then, over the course of a few millennia, beginning about fourteen thousand years ago, the climate gradually got warmer, without much overall effect, until suddenly, with a last flicker from cold to warm between eleven and thirteen thousand years ago, the new interglacial state arrived, and it was a whole new world, one without massive continental glaciers.
That was an important global tipping point for humanity, because it set the stage for our domination of the planet. The cause was a complex interplay between three features of the Earth’s orbit around the sun that vary regularly, but at different paces: how elliptical the orbit is, how much the Earth’s axis tilts, and how the Earth wobbles as it rotates. As those three orbital features came into alignment over thousands of years, not much happened to the glaciers, until finally the Earth was in just the right position to maximise the amount of sunlight striking at critical seasons. With that, a warming threshold was crossed for the planet, the glaciers disappeared rapidly, and new ecosystems assembled virtually everywhere, as plants and animals formerly separated by ice came together. At the same time, humans finally arrived on every continent (except Antarctica), and began to grow dramatically in number, while other large animals died out rapidly. Then, around eleven thousand years ago, the global ecosystem stabilised into its interglacial new normal, where it has been right up until the last couple of centuries. This new normal caused formerly continuous landmasses to become separated by high sea-level stands, and their plants and animals set off on new independent ecological and evolutionary journeys. Now, though, the world has once again begun to change in a way, and at a speed, that signals a new planetary tipping point is just ahead.
This time it’s not something from outer space or Earth’s orbit that’s pushing the planet towards a point of no return. It’s us, pushing relentlessly towards thresholds on several different interconnected fronts: population growth, overconsumption of natural resources, climate disruption, pollution, disease, and killing anything that gets in our way. Some scientists think that going too far in any one of those arenas could push Earth past a planetary boundary that would have devastating consequences. Think, then, what would happen if we exceeded critical thresholds in more than one of them at once. Again, both intuition and data predict some very bad effects. Intuitively, two bad things hitting at the same time is clearly going to have much more impact than only one. We can again turn to the past to see that this is indeed the case when it comes to global tipping points. The kind of climatic shift that eleven thousand years ago turned an ice-age Earth into the warmer planet we’re used to had in fact happened many times over the past couple of million years.