The Earth, forever...

"Nothing that we can do to our planet can really harm her. But we can certainly harm ourselves," writes Ted Nield (1). For this British geologist, active member of the International Year of Planet Earth 2008, this event is a chance to step back and try to better understand and better "treat" the Earth.

Ted Nield © Richard H.Smith
Ted Nield © Richard H. Smith
Empreintes de dinosaures, datant de 155 millions d’années, retrouvées dans
le Jura (CH). La Terre a connu cinq extinctions massives des espèces vivantes. La dernière,
celle du jurassique, a valeur de symbole. © CNRS Photothèque/Jean-Michel Mazin
Dinosaur footprints, dating from 155 million years ago, found in the Jura Mountains (CH). Earth has known five large-scale extinctions of living species. The last, during the Jurassic period, is of symbolic value. ©CNRS Photothèque/Jean-Michel Mazin

Many themes of the International Year of Planet Earth have already been given prominent media attention. Is one theme particularly important to you?

Speaking of the Earth, I think we should not succumb to the temptation of highlighting any single theme. In some ways, this reductionism has been the way of science for hundreds of years and it has to change. The fact that our Science Committee - made up of Earth scientists - did not include "life" as a theme - which was added in extremis - is indicative of how far even the best-intentioned of us has to go before we fully realise the need to be "multi-disciplinary" when considering the Earth system. The Year is intended to highlight all the knowledge available today on the way the planet works. The Earth does not have departments of biology overseeing the biosphere, or geology departments administering plate tectonics, or meteorologists making weather...

Climatologist Paul Crutzen believes that the current impact of humans on the fate of the planet is bringing the start of a new age of the Earth's history, which he has dubbed the Anthropocene. But some suggest very old changes for which humans are certainly not responsible, such as the rising of the sea level more than 100 metres between 13 000 and 8 000 BC.

The Earth's climate, when looked at through the eyes of a geologist, is a bewildering and fascinating mixture of the two sorts of changes that characterise all Earth processes - secular and cyclical. As the atmosphere has evolved through geological time it has experienced some massive revolutions - the appearance of photosynthesis, for example, putting oxygen into the atmosphere for the first time about three billion years ago, led to profound and irreversible chemical changes to our planet, possibly even affecting the deep Earth, and led to the possibility of much lower temperatures at the surface.

The positions of continents as they move across the globe - whether they are dispersed, as now, or united; and if united, whether they sit near the poles or straddle the tropics - also profoundly affect weather systems on timescales of hundreds of millions of years.

The orbital cycles of the Earth around the Sun, combined with the inclination of our rotational axis relative to the plane of the ecliptic, all interfere with one another and create an ever-changing scenario.Climate is not constant, never has been, and never will be.

Whatever we manage to do to the climate, unless we cause a runaway greenhouse effect that ends up boiling off the oceans and returning all the Earth's carbon to the atmosphere, like on Venus, the Earth will have seen it before. The question facing us is one of our own convenience. If we create a new geological age by our own foolish unsustainable practices, we are beginning an uncontrolled experiment whose outcome could be very inconvenient indeed. What rational being would voluntarily do such a thing? The Anthropocene could be the period when humans get to prove just how well they deserve the specific name sapiens!

Science fiction and film, notably the work of Michael Crichton and Spielberg, have contributed to the popularisation of Jurassic era dinosaurs, and from there, created an awareness of the history of the Earth. But what cataclysms caused the extinction of dinosaurs? The two hypotheses so far are extraordinary volcanic activity or a meteorite crashing into the Gulf of Mexico.

This is a fascinating subject. It also illustrates the difference between different types of scientific outlook. When Luis and Walter Alvarez and their co-workers identified the iridium-rich layer that marked the end of the Cretaceous period 67 million years ago, they proved conclusively that this worldwide horizon must have been created by a kilometresized object colliding with the Earth. After a while they realised that in the Earth's long history, impacts are relatively common, and that occasional rare catastrophes must form part of the "uniformitarian" approach that makes geology scientific.

That was a valuable lesson. Nevertheless, physicists have a mindset that likes "simple" explanations, contrary to that of geologists. They know that in Earth's long history, nothing ever has simple, singular, instantaneous explanations. It is much more likely that all those many cycles - geological, climatological, astronomical - suddenly coincided and together mounted a concerted attack on the edifice of life. Yes, there was an impact 67 million years ago; but the late Cretaceous period was a truly terrible time to be a living being on Earth for all kinds of reasons. And the prime reason for that was the massive volcanic outpouring that created the Deccan Traps, as has been pointed out by the great French geologist Vincent Courtillot of the Institute of Geophysics of Paris (Institut de Physique du Globe de Paris). But the impact could well have been the last straw that broke T-Rex's back.

Finally, even though the end-Cretaceous impact was quickly associated with the crater at Chicxulub in the Gulf of Mexico, recent research has demonstrated conclusively that it cannot be the "smoking gun". Although it is the biggest-impact crater so far identified on Earth, it was made 300 000 years too soon - in an event that did not even kill off a single microfossil species. No other mass extinction in the history of life (there are generally agreed to have been five big ones) is known to be associated with impacts.

One of the even more dramatic eras seems to be the Permian era, 250 million years ago, which saw the extinction of 90% of all living species. How can this catastrophe, which first appeared close to three billion years ago, happen? And what new conditions were produced in the wake of this "reboot"?

The end-Permian extinction was perhaps the greatest of the five great mass extinctions, and it happened during another truly terrible time for living things. The continents of the Earth had fused into the most recent supercontinent to have formed, Alfred Wegener's Pangea. Having a single continent means that there is much less coastline, hence potentially much less shallow sea, where much marine life lives. The deep ocean is often poorly oxygenated because the aerating global conveyor that moves water around cannot cope. And the continental interiors, so far from any source of moisture, are unremittingly arid. With little plant life able to survive, oxygen levels plummet; with low erosion, carbon dioxide (CO2) builds up in the atmosphere - and it gets even hotter.

In the nick of time several things happened. First, the eruptions slowed down and Pangaea began to break up. As the Americas unzipped from Eurasia and Africa, erosion was able to start wearing away the former heart of the supercontinent, taking away CO2 from the atmosphere; life returned to the lengthening coastlines, pumping out more oxygen. After the long misery of the Permo-Triassic, life could breathe, literally, a sigh of relief.

In terms of knowledge, what lessons can be drawn from the once existence of supercontinents?

For me, the main message of the supercontinent cycle is humility before nature, and to the depth of time. Our species evolved a mere six or so million years ago. We have come this far and that's already quite good going when you consider that the average duration of a species is perhaps one million years. But when humans first stood upright, the continents were at most a few tens or hundreds of kilometres from where they are today. The process began while there were still dinosaurs roaming the land, and we are now halfway to the creation of the next supercontinent, in perhaps another 250 million years or so.

We will be fossils too; and otherwise not a single trace of us will survive. Yet the Earth will still be here, still alive, still breathing, her plates still forming at mid-ocean ridges and subducting at ocean trenches, dragging their passenger continents to collision, forming mountains and eroding them away, just as they have done for four billion years before we got here, and will do for perhaps another four billion years or so before the sun goes nova and destroys Earth forever.

Nothing that we can do to our planet can really harm her. But we can certainly harm ourselves. As I conclude in my book, we need to throw out ignorance and superstition and throw in all the knowledge we have if we are to solve this problem. And I hope that the International Year of Planet Earth is one way in which we can inject some scientific rationalism into our dealings with Earth, our unsuitable Mother. She does not care about us. We have to care about her, because we cannot fight her; and without her help, we will be fossils a lot sooner than we think.

Interview by Didier Buysse

  1. Ted Nield, Supercontinent, Ten billion years in the life of our planet, Granta Books, London, 2007.