Has the Earth been so hot already?

Would you like to go on vacation to the North Pole? Unless you like negative temperatures and Nordic hikes, probably not. But if you lived 56 million years ago, you could respond differently. At the time, you would have benefited from mild temperatures and a lush landscape (though you should have paid attention to crocodiles). Indeed, the world was going through an extreme period of global warming called the Paleo-Eocene Thermal Maximum, when the Earth was so hot that even the poles reached almost tropical temperatures.

But has the planet ever been as hot as today, when every month the world seems to be breaking records for high temperatures?

It turns out that the Earth has gone through periods of extreme warming more than once. The poles have frozen and thawed and are frozen again. Now the Earth is warming up again. Despite this, the current climate change is a different beast, and this is clearly not part of a larger natural cycle, Stuart Sutherland, a paleontologist at the University of British Columbia, told Live Science. [How Often Do Ice Ages Happen?]

The Earth's climate oscillates naturally – over tens of thousands of years, its rotations around the sun are changing slowly, causing variations in all kinds of conditions, from seasons to sunlight. Partly because of these oscillations, the Earth traverses glacial periods (better known as glacial ages) and warmer interglacial periods.

However, to create a massive warming event, such as the maximum paleo-Eocene heat, it takes more than a change in tilt of the Earth's axis or the shape of its path around it of the sun. Extreme warming events always involve the same invisible culprit, a problem we know too well today: a massive dose of carbon dioxide, or CO2.

This greenhouse gas was almost certainly responsible for the paleo-Eocene thermal maximum. But how did CO2 concentrations become so high without human presence? Scientists are not absolutely sure, said Sebastian Castelltort, a geologist at the University of Geneva. Their best guess is that volcanoes have released carbon dioxide into the atmosphere, trapping heat and possibly melting pockets of methane jellies, a more potent greenhouse gas than long-sequestered CO2 under the # 39; ocean. The mere fact that greenhouse gases have caused extreme warming does not mean that they are safe. Take the example of the Permian-Triassic extinction, which took place a few million years before the dinosaurs appeared on the planet. If the word "extinction" is not a clue, here is a revelatory: it was an absolute disaster for the Earth and all that is found there.

This warming event, which occurred 252 million years ago, was so extreme that Sutherland called it "a child of the poster for the year. 39, greenhouse effect galloping ". This warming event, also caused by volcanic activity (in this case, the eruption of a volcanic region called the Siberian traps), triggered climatic chaos and widespread death.

"Imagine an extreme drought, dying plants, the Saharah spreading all over the continent," Sutherland told Live Science.

Temperatures increased by 10 degrees Celsius. (This is compared to the temperature rise of 2.1 F (1.2 C) that we have seen since humans started burning fossil fuels). About 95% of marine life and 70% of terrestrial life has disappeared.

"It was too hot and too unpleasant for creatures to live," said Sutherland.

Were concentrations of greenhouse gases high during Permian-Triassic extinction? They were probably much higher than today. Some models suggest an increase of up to 3,500 parts per million (ppm). (As an indication, current carbon dioxide concentrations are just over 400 ppm – but this is still considered high).

But it is the rate of change in CO2 concentrations that makes the current situation unprecedented. Upon extinction in the Permian Triassic, it took thousands of years for temperatures to rise as high as they did – according to some studies, up to 150,000 years ago . During the Paleo-Eocene thermal maximum, considered as a case of extremely rapid warming, it took 10 000 to 20 000 years for the temperatures to reach their height.

The warming of today has only taken 150 years.

This is the biggest difference between today 's climate change and past climatic peaks. This is also what makes the consequences of the current climate change so difficult to predict, Castelltort said. The worry is not just "but the planet is warming up". The problem is that we do not know how fast is too fast for life to adjust, he said. On the basis of past warm-up events, no expert could say that the current rate of warming will not have dramatic consequences, he said. "We do not know how dramatic it is," he added.

Originally published on Science live.