US geologists have identified the moment of the dinosaurs’ death in the Earth’s deep past as the time when the climate changed, even faster and more severely than it is changing as a consequence of human action.
That fateful moment occurred on the day around 65 million years ago when a vast comet or asteroid smashed into Earth over what is now Chicxulub in the Yucatan peninsula of Mexico and brought the Cretaceous era to a close.
The scientists used tiny bits of fish scales, teeth and bones to compose a temperature chart for the last 50,000 years of the Cretaceous, and the first 100,000 years of the Palaeogene, when planet Earth changed forever.
The planetary average temperatures rose around 5°C and stayed perilously hotter for at least another 100,000 years, and in the course of this the last dinosaurs disappeared, as if violently wiped out in one short episode.
There are many more questions to be settled before the dying convulsions of the dinosaurs become a model for what might happen to humanity in the coming century
Theorists predict that an impact with something 10kms or so across arriving at a minimum of 20 kms a second would have delivered a ferocious blast of heat, a huge ejection of rock and dust into the upper atmosphere, a darkening of the skies, an all-year-round winter that might have endured for a decade, and then dramatic warming as the air filled with carbon dioxide from blazing forests around the planet.
The researchers report in the journal Science that they see this fateful celestial traffic accident as “an unusually relevant natural experiment to compare to modern climatic and environmental changes.”
The evidence comes from a series of shallow marine marls deposited 65 million years ago in what is now Tunisia: these strata contain fragments of fish, and the phosphate compounds in the hard fragments contain oxygen isotopes that in turn can answer questions about the atmospheric temperatures at the time the ancient fish swam in ancient oceans.
And in this series of sediments is a thin red layer rich in the kind of evidence to be expected from a colossal impact with an interplanetary fireball.
No abrupt cooling
What the scientists did not find was evidence of a sudden, brief dramatic cooling, but they didn’t expect to. But they did find, they say, evidence that “matches expectations for impact-initiated greenhouse warming.”
The impact probably extinguished three fourths of all life on Earth. As so often happens in research, a second, almost simultaneous study in a different publication of a different series of geological sediments – in North Dakota in the US – yielded more details about the Cretaceous calamity.
Plant fossils, pollen and spores, according to a report in the journal Current Biology, confirm indirectly that not only were the world’s forests incinerated during and after the impact, but perhaps all tree-dwelling birds of the time.
Today’s finches, falcons and guinea fowl all seem on separate evidence to have evolved from the ancestors of the kiwi, the ostrich, the cassowary and other ground-dwellers.
Because Earth is a once-only experiment, the only lessons for how climate change happens without human help are to be found in the deep past. But the past is a mysterious and sometimes enigmatic landscape.
Climate change happens because of tectonic plate movements, or shifts in planetary orbit, or dramatic losses of oxygen in the oceans, but these changes often happen imperceptibly, over very long periods.
But the change associated with the human expansion and the profligate combustion of fossil fuels – sometimes called the Great Acceleration – in the last 200 years is far, far faster.
Thanks to evidence from the last days of the Cretaceous, though, climate scientists have found an accelerated change even faster than anything humans have yet managed.
So the latest study provides, the scientists say, “a perspective on the response of Earth systems to extremely rapid global perturbations.” So far, that is all it provides: a perspective. There are many more questions to be settled before the dying convulsions of the dinosaurs become a model for what might happen to humanity in the coming century.
Source: By Tim Radford– Climate News Network