It looks like a heartbeat.
A jagged line, all rhythmic and vibrant and pulsing with energy – what you’d hope to see if you were hooked up to a heart monitor.
Only it isn’t a heartbeat, at least not in any literal sense. It is in fact the earth’s climate, expressed in a graph, a record of our planet’s cold snaps and warm flushes over the last 450,000 years or so – based largely on evidence gathered from core samples of polar ice.
The time-span covered by the graph is a little brief to tell us much about ice ages (the planet has been going through one of those for around 40 million years!).
What it does reveal are movements in and out of a series of cooler spells – known as glacials - and warmer periods - or interglacials - that have occurred within the broader sweep of the current ice age. Scientists have learned a great deal about our planet’s climate in the last few decades.
It is now widely accepted that human activities are transforming global climate in its entirety. That has come as a shock for many of us, prompting a profound awakening as to just what a potent physical force our species has become. It also means that what people do in one part of the world can influence weather and life on the far side of the earth, driving home just how deeply, inescapably, interdependent we have all become at a global scale.
But human-induced climate change is not the only eye-opener. Through their attempts to pin down the extent to which human beings are altering climate, scientists have also discovered some quite breathtaking things about the way that climate changes of its own accord. What they have found may well turn out to transform the way we think about geological time and our very sense of the continuity of our world.
Glacial used to be a word for slow. For inching, crawling, imperceptibly slow change. Not any more. Many climate scientists now believe that shifts in and out of glacials takes place not over millennia, or even centuries, but in the matter of just a few short years.
That’s what the sharp peaks and troughs on the graph are all about: dramatic shifts in climate occurring in less than the term of a government, less time than it takes to get a university degree or to finish the payments on a new car. It’s all about part of the climate system reaching a tipping point.
One way this can happen is that changes in ice sheets cause a lot of fresh water to be dumped in the ocean, such as when a large river dammed up behind ice suddenly breaks free, or when glaciers do a lot of melting. This, in turn, can change the circulation of the ocean currents that convey warm salty water from the tropics to the northern oceans, sometimes switching this `conveyor’ off entirely.
When this occurs, the northern latitudes suddenly get a lot colder – cold enough, perhaps, to trigger a new glacial epoch. Now look back at the graph. Though it is still rather spikey, the section to the far left – the last 10,000 years or so - stays within a fairly narrow limit, up there at the warmer end of the scale.
By comparison with most of the rest of the graph, this is a remarkably stable and balmy time. So stable that some commentators describe it as `a fluke in earth history’. This pleasant little interval, a kind of geological happy hour, encompasses the whole time that humankind has settled into village and town life, domesticated plants and animals, and got ourselves `civilised’ - as we like to put it.
How most of us humans live today, and just how many of can live anywhere like this, depends in a big way on this stable spell persisting. Theorists of abrupt climate change caution against complacency. Human-induced changes, they warn us, could tip us out of this stable phase, with a small warming quite possibly triggering a runaway return to extensive glaciation.
What is more, scientists argue, even without our own contribution, we should expect this fortuitous stability to come to an end, sooner or later.
Some recent evidence, however, is reassuring, suggesting we might even have another 16,000 years of relatively stable weather, if we don’t mess it up. In a more general sense, what the record from the polar ice cores seems to be saying is that abrupt climate change is a normal part of our planet’s history.
The implication is our species has had to endure sudden, dramatic climate change many times in the past, and that these changes have helped shape who and what we are, just as they have shaped most of the rest of life on earth.
A lot of things get shaken up when we start to think about the possibility of very sudden climatic change. We know it’s important to try and work out which people and which countries are contributing most to climate change right now. Just as it’s important to try and calculate what kind of impacts a slight warming might have on the different ways we now live.
But one of the lessons of looking at abrupt climate change is that sometimes quite small acts can have effects way out of proportion to the scale of the acts themselves. This means that we might have to start thinking rather differently about causes and effects, and about the way we apportion blame and responsibility. It also means we might have to think about impacts in a whole new way.
As one geologist put it "try writing an environmental impact statement for a continental glacier". In other words, we are tangled up in some vast and dynamic systems upon which we are very much dependent. Or rather, interdependent, as we too are an important part of these systems.
What the sudden climate change story reveals is just how much the interdependence which sustains us also makes us vulnerable, how it leaves us open to sudden change and the shock that may go with it. Now, we might take all this to be so complicated and so frightening that we just block it all out, and carry on as we are.
And in this way, greatly increase the risk of triggering events we are not at all prepared for. Or, we might see it as awe-inspiring and life-changing, something that puts everything else into perspective.
In the words of writer Salman Rushdie, as he ponders some of the more momentous events that take place in our world: "these are the moments when the bolts of the universe fly open".
What if we were to let the bolts fly open for a while, let the story of abrupt climate change put a new twist on all the other stories we live by? We might start getting our heads around the idea that we live on a very lively and very volatile planet.
Scary, yes. Sobering, yes, indeed. But also intriguing. Perhaps even a little thrilling.
And that in itself could be a trigger for thinking about our interdependence with others and with our physical world in new and different ways. It might even turn out to be a 'tipping point' for some of our own rather momentous changes in what we do and how we do it.
Nigel Clark is Lecturer in Geography at the Open University.
This article was used to support the OU / New Economics Foundation event - Interdependence Day - held in July 2006.