Seeking to explain the major changes of the Earth’s climate that have occurred in the past half a billion years, scientists have looked at various geological, astronomical, biological, geomagnetic, and cosmic factors. They have even considered the possibility of visitors from other planets, who might have used some sort of climate weapons. But scientists found no trace of action by aliens. What they found was that the temperature on our planet in the last few hundred million years was determined by the location of its continents.
Moving continents
The Earth’s crust is only the thin top layer of our planet (Fig. 1.3.3). Beneath it begins the mantle, which is the main part of the planet and which becomes a very hot and sticky liquid deeper down. The crust and top layers of the mantle consist of relatively hard (‘lithospheric’) plates, which can crack, move apart or come together, shifting just a few centimetres each year, butcoveringthousandsofkilometersovermillionsofyears! This is called ‘continental drift’. The single, ancient continent of Pangaea gradually divided into separate continents, which moved apart and collided with one another (Fig. 1.3.4). If you look at the western side of Africa and the eastern side of South America, you can see that they fit together like pieces of a jigsaw puzzle, and the reason for this is that they were once part of one single continent that split apart.
Figure 1.3.3 Layers of the Earth
Continents that are close to the equator do not accumulate ice, but if they are close to the poles, then they are soon buried under the glaciers (ice masses) that we now see in Antarctica and Greenland. The white surface of ice and snow reflects solar radiation back into space, ensuring that the ice and snow remain cold, while the dark surfaces of earth or water almost completely absorb solar radiation and therefore heat up.
Figure 1.3.4 Continental drift over the past 500 million years.
А – the formation of Pangaea
B – the division of Pangaea, formation of Laurasia and Gondwana
C – the splitting of Gondwana, formation of Hindustan, Australia and Antarctica
D – the formation of South America, beginning of the division of Laurasia
When this occurs over a large area, it becomes the main factor influencing the climate of the entire planet. For most of the time in the last half a billion years the continents had less ice cover than they have now, so the earth’s climate was warmer.
The white surface of ice and snow reflects solar radiation back into space, ensuring that the ice and snow remain cold, while the dark surfaces of earth or water almost completely absorb solar radiation and therefore heat up.
When there was a major change of climate, particularly when there was a cooling, so-called ‘bio-catastrophes’ occurred: whole species of living organisms died out and only those survived which were best suited to the new conditions.
One of these cold spells about 60 million years ago led to the disappearance of the last dinosaurs. This must have been a gradual process lasting more than a thousand years. The exact cause of the extinction of the dinosaurs is unknown, and there may have been several and not just one cause.
Why did the dinosaurs become extinct?
Dinosaurs finally died out on Earth around 60 million years ago. Scientists are still unsure exactly why.
One theory is that the dinosaurs were unable to compete with more ‘sophisticated’ living organ- isms. For example, with warm-blooded mammals, that were no larger than a squirrel, but which could eat the dinosaurs’ eggs or attack them by night, when the cold-blooded dinosaurs were unable to move.
According to another theory, a huge meteorite struck Earth around the present Caribbean Sea, causing gigantic amounts of dust to spread through the atmosphere, blotting out the rays of the sun for a considerable period. Birds, mammals, and many other organisms adapted to the new temperatures, but dinosaurs did not.
There is one other version. It is known that for some reptiles (crocodiles, turtles) the ground temperature determines whether males or females will hatch from eggs laid in the sand along riverbanks and coasts. Biologists suggest that this dependence might also have applied to dinosaurs, which were also reptiles, only very large ones. If the temperature was such that only females (or males) hatched from dinosaur eggs, the species would have quickly disappeared without any need for disasters or falling meteorites.
The change from an invariable, moist climate to one with seasonal changes (even small changes) could produce short periods of cold nights when the huge reptile bodies of the dinosaurs could not retain sufficient warmth. Many of the animals would weaken and finally die.
But the most important climate event happened 50 million years ago, when the continents moved away from the poles. Snow and ice cover shrank, and temperatures rose to a level about 12°C higher than nowadays. Then, ‘suddenly’, India, which had previously been a small, separate lithospheric plate, crashed into Eurasia. The Himalaya mountains emerged at the place of the collision. The other plates moved around so that Antarctica took its place at the South Pole and was covered with a layer of ice (30–40 million years ago). The temperature on Earth began to fall sharply as the white ice of Antarctica began to reflect solar radiation back into space.
About 10 million years ago Greenland reached its present location and was covered by a layer of ice that lowered the temperature still further, to levels close to those we have today.
It was much warmer on Earth 100 million years ago than it is today. Antarctica became covered with ice 30–40 million years ago and Greenland 10 million years ago, causing temperatures to drop to their current levels.
