Finding out what killed off the dinosaurs has been one of science’s greatest quests. In a geological blink of an eye, a species, which had dominated the world for 200-million years, stopped existing. Some 75% of all plant and animal species vanished with them, ushering in the era of mammals.
Two main culprits have been identified, thanks to their proximity to the extinction about 66-million years ago.
The one — constant volcanic activity — would have meant a slow extinction. Large-scale eruptions were frequent in the 800 000 years on either side of the die-off, releasing sulphur and carbon dioxide into the atmosphere. These gases deflected the sun’s rays and cooled the planet. With less sun, plants struggled to photosynthesise and died. The animals that relied on them for nutrition then followed suit and died off.
The second culprit would have been much more dramatic; a massive asteroid crashing into the Yucatan peninsula, in what is now Mexico. Smashing into the ground, its speed and 10km diameter created a crater 200km across and 20km deep. The amount of energy it hit the Earth with would have been the equivalent of a billion Hiroshima explosions.
An initial theory about this was that so much dust was thrown into the atmosphere, blocking sunlight and making photosynthesis impossible, that the global environment became too harsh for survival.
But new research, from the Potsdam Institute for Climate Impact Research, in Germany, zeroes in on the longer-lasting cooling effect that sulphate aerosols — released into the atmosphere by the collision — would have had on the world’s climate.
The sulphate aerosols would have been thrown into the atmosphere when the asteroid hit the ocean along the Yucatan peninsula, evaporating and melting everything in its path.
The study, Baby, It’s Cold Outside: Climate Model Simulations of the Effects of the Asteroid Impact at the End of the Cretaceous, uses various data sets to build up a more nuanced model of the global climate 66-million years ago.
It concludes that rapid cooling caused by the sulphates killed off dinosaurs. The team says: “The long-term cooling caused by the sulphate aerosols was much more important for the mass extinction [of dinosaurs] than the dust that stays in the atmosphere for only a relatively short time.”
Instead of a slow change in the global environment, this research indicates that the sulphates quickly changed the atmosphere. The average global temperature dropped by at least 26°C.
Temperatures in specific places, such as the north and south poles, dropped much quicker than anywhere else. In the tropics, the average land temperature dropped from 27°C to -22°C in a few years.
For cold-blooded dinosaurs, evolved over millions of years to live in warm climates, this was catastrophic. Whereas warm-blooded animals could adapt to changing temperatures because they regulate their own temperature, dinosaurs could not.
Short-term environmental disasters, such as wildfires, extreme heat, strong winds and tsunamis, exacerbated the situation. The latter radiated from the Yucatan impact crater, with tsunamis flooding up to half of North America.
In the world’s oceans, the sudden freeze cooled surface water. This dense water became heavier and sank to the bottom of the oceans, displacing warm water, which then rose to the surface. Packed with nutrients, the water fed the algae. That bloomed across vast tracts of ocean and choked off the nutrients to other plants and animals, doing for ocean life what sulphates were doing for life on land.
This lasted for some 30 years after the impact, before the world’s climate gradually returned to some sort of equilibrium.
The impact also had another, longer-term, effect — caused by all the carbon dioxide released. Rocks that were vaporised in that instant released the gas, which was trapped in the atmosphere and created long-term warming. As a result, a thousand years after the impact, the world was between 1°C and 2.6°C warmer.
These dramatic, short-term, changes, along with the longer-term impact of the asteroid, would have been too much for dinosaurs to survive. The Potsdam team conclude that the “dramatic reduction in temperature” that they model from that impact would have dovetailed with volcanic activity to create a world in which dinosaurs could not survive, ushering in the age of mammals — and Homo sapiens.