Is an ice-free winter at the poles possible?

maybe ? Yes, it is, at least in principle. But is this how far we will go? That's another story, but it may happen in the future if nothing is done to significantly reduce greenhouse gas emissions – I'll come back to that in a moment.

As for the “feasibility” of ice-free polar winters, let's assume that they actually happened in the (distant) past, indicating that there is no hard impossibility in that. According to climatic reconstruction of the past 500 million years Published in science In 2019The Earth was once much warmer than it is now, with the average ocean temperature generally fluctuating between 27 and 32 degrees Celsius (!), compared to just 14 degrees Celsius today.

When land plants appeared and began consuming carbon dioxide from the atmosphere, about 400 million years ago, the greenhouse effect diminished dramatically and temperatures dropped, but the planet still experienced other extremely hot periods, when volcanic activity became strong enough to charge the atmosphere. Atmospheric carbon. dioxide.

An example of this is the end of the Cretaceous period – about 90 to 95 million years ago the “golden age” of dinosaurs. Many studies have attempted to estimate ocean temperatures from this era at different latitudes, often by analyzing certain “isotopes” in the shells of fossil mollusks.

Roughly speaking, isotopes are different “versions” of the same element, each atom of which can have a little more or fewer particles than the average in its nucleus, and thus be a little heavier or a little lighter. But we know that when the carbonates that make up mollusc shells form, the colder the waters surrounding the molluscs are, and The more heavy isotopes we will find Oxygen and carbon in its crust. Which obviously gives an indication of past temperatures.

In works of this kind Published in 2018 in Global and planetary changeAn American team estimated that at the end of the Cretaceous period, below the Antarctic Circle, the water temperature at the ocean floor should have been about 21 degrees Celsius (!!) and 27 degrees Celsius at the surface. There is nothing to be done, we think, with the current temperatures, which are around 0°C.

Other business He even concluded that ocean temperatures were over 30 degrees Celsius in Antarctica at that time.

It was the same near the North Pole. In fact, the researchers found fossils of an apparently densely packed population (there were many juvenile skeletons) of “champosaurs,” a type of freshwater crocodile that lived on what is now Axel Island. Highberg, in the Canadian Arctic.

Because of continental drift, the site was not far north at the end of the Cretaceous, but paleomagnetic data indicate that it was still at about 71 degrees latitude – which corresponds to the present-day location of Baffin Island, Nunavut! – We read in A Study published in 2007 in Paleogeography, paleoclimatology, paleoecology.

So, if “crocodiles” live there, it shouldn't freeze so much in the winter, let's say…

And now?

It is therefore possible for the poles to be free of ice all year round, because this has already happened. But that was a very long time ago. Is this “on the horizon” for us in light of current global warming?

It must be said here that at the end of the Cretaceous period, the atmosphere contained… a lot More carbon dioxide than now: About 1000 ppm (ppm), compared to about 280 ppm in the 19th century, and 415 ppm today. Therefore, the phenomenon of global warming was much greater than that prevailing today.

So tomorrow we won't see frost-free poles year-round – if we get that far. But…but it could happen, and perhaps faster than we think.

First, it's already there Last work Which predicts that in September, the time of year when sea ice is at a minimum, the Arctic Ocean could be “practically ice-free” (the expression defined in the study as “less than 1 million square kilometres”, compared to 3 to 10 million square kilometers between 1979 and 1998) by 2050.

“With the RCP8.5 scenario,” adds Bruno Tremblay, a climate science researcher at McGill University [c’est-à-dire les modélisations business as usual du GIEC où aucune mesure n’est prise pour réduire nos émissions de gaz à effet de serre, ni maintenant ni dans les prochaines décennies]“We will reach 1,200 ppm by 2100,” which is roughly equivalent to, if not more than, what it was in the Cretaceous period.

This would be enough to melt the ice, according to the simulations Mr Tremblay took part in but the results of which have not yet been published in the scientific literature – so they must be viewed with a certain caution as the study has not yet been peer-reviewed.

“If greenhouse gas emissions continue [le scénario business as usual]We will see an ice-free Arctic Ocean even in winter in the first half of the next century, Mr. Tremblay told me in an email exchange. Although it will always be cold in winter at the pole [il n’y a pas de soleil à ce moment-là de l’année]Solar energy stored in the water during the summer would take almost the entire winter before being released into the atmosphere, keeping the surface free of ice.

It is clear that this “worst-case scenario” is not guaranteed to come true, as we will not be able to reduce the amounts of greenhouse gases emitted into the atmosphere in the coming years or decades (ideally). But he shows that the idea of ​​an ice-free Pole year-round is not impossible, and if nothing is done, it is not far away either.

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