Tipping points, asteroids and mass extinctions

Today, I want to connect a few dots that bring a new perspective to my reflections on the future of humanity in my last entry.

We hear a lot these days about ‘tipping points’ from climate scientists and activists. The idea is that as the planet warms, so processes are being set in motion that cause further warming, thus creating a vicious cycle, or positive feedback loop. Several such processes have been identified already. These include the release of the powerful greenhouse gas, methane, by thawing of the Siberian permafrost or melting of methane hydrates on the ocean floor, accelerating collapse of the polar ice caps, weakening of ocean currents such as the Gulf Stream, and desiccation of the Amazon rainforests. And there is growing evidence that such feedbacks have been associated with dramatic shifts in global climate in the distant past.

Last week, I read in New Scientist about a new strand of evidence that has emerged. Over the last decade or so, we’ve got used to the idea that the dinosaurs were wiped out by an asteroid impact. Many scientists have come to believe that other mass extinctions had similar causes, but the evidence was patchy. Now, a new research tool called biomarkers is revealing that the dinosaurs may have been an exception.

Biomarkers are like molecular fossils. They are stable, long-lived organic molecules found in rock pores. Some of them are made by just one or a few types of organism, so that their presence is a sure sign that these organisms were there when the rock was formed. Unlike ordinary fossils, these biomarkers enable tiny, single-celled organisms such as bacteria to be identified.

250 million years ago, in the greatest mass extinction Earth has witnessed, 95% of marine organisms and 85% of land plants and animals disappeared from the fossil record. One biomarker molecule from this period has now been found in several places around the globe. It is made only by green and purple sulphur bacteria which use sunlight in photosynthesis, but cannot tolerate oxygen. Instead, they thrive on hydrogen sulphide (H2S) which is toxic to most plants and animals. Their presence in sediments suggests that the water was shallow enough for light to penetrate, but had no dissolved oxygen and was saturated with H2S. In other words, the oceans were poisonous, and enough gas may have escaped into the atmosphere to kill land organisms too!

So where did all this H2S come from? The organisms that produced it left no biomarkers, but they were probably bacteria that live in stagnant water. But how could the vast oceans possibly become stagnant? One hypothesis is that it was the result of global warming triggered by the release of greenhouse gases in the massive volcanic eruptions that formed the Siberian Traps basalt.

One of the effects of greenhouse warming is that temperatures increase more near the poles than at the equator. This reduces the temperature differences that drive planetary weather systems and ocean currents. And it is the winds and currents that put oxygen into the water. So a strong greenhouse effect could result in the oceans stagnating. In a nutshell: release of greenhouse gases ended up poisoning the oceans and atmosphere for most forms of life.

As we know, life wasn’t extinguished, but it was a close call for multi-cellular plants and animals. For a while, it looked as if single-celled organisms would take over and rule the biosphere, as they did for the first 3 billion years of life on Earth. There is accumulating evidence that other major extinction events, with the exception of the dinosaurs, may have been the result of similar processes.

Our existence today, and the nature of modern life, tell us that Gaia can recover from such shocks. But there is no guarantee that Gaia will always succeed in pulling herself out of such positive feedback loops. Some scientists fear a runaway scenario in which Earth ends up like Venus. At the very least, recovery may take many millions of years – far too long for human civilization, or perhaps even humanity, to survive.

But all is not lost. There is still hope that we can avert such a catastrophe. Paradoxically, it is a hope that rests on the power of positive feedback, just as the apocalyptic scenario does. Research on complex, chaotic, self-organising systems shows that, at certain times, they are extremely sensitive to small disturbances. Just a small nudge may be amplified by positive feedback to totally transform the system. There are many millions of people and tens of thousands of organisations around the world working to change human behaviour. Gaia is not yet beyond any irreversible tipping point. Our individual and collective efforts may be just what is needed to turn the tide; the last straw that tips the balance back again.

To repeat: no action is futile or worthless. Every action, however small, is potentially significant. What if we all followed Rapunzel’s example as expressed in On the Wisdom Trail:
“I'm going to make a list of tools and skills that I have to share and offer them to the four neighbors I just slightly know right now. What if we all only needed one lawnmower for every 4 or 5 houses, one circular saw, maybe just one vegetable garden? What if we all knew someone who could do carpentry, or plumbing, or who could paint, and we could all take on more projects to make our world better a little at a time, right where we are?”

What if we all found our own small, sensible ways to contribute? Let’s go for it.