Understand | The frightening warming of the oceans

For two years, surface temperature anomalies in the Atlantic Ocean, which reach levels never before observed, have troubled the scientific community. “It’s really frightening, we are entering an unknown zone,” says paleoclimatologist Anne de Vernal, professor in the department of Earth and atmospheric sciences at UQAM. Here are sources to better understand the phenomenon and its effects.

A graph (and others as a bonus)

If average temperatures on the ground have been increasing since the beginning of the pre-industrial era, this is also the case in the oceans, but at a slower rate, points out Anne de Vernal. It should be remembered here that the oceans occupy 70% of the Earth’s surface. Without them, global warming would be much greater since the oceans have so far absorbed 90% of the excess heat caused by climate change. They also absorb nearly 30% of CO₂ issued each year.

“The ocean is thus a reservoir of heat which accumulates energy,” explains M.^me of Vernal. However, the reservoir homogenization times are very long, of the order of a thousand years. » Concretely, this means that the current warming of the oceans is irreversible. In the event that humanity stops emitting greenhouse gases (GHG) today, the time it will take for the oceans to cool down will take thousands of years. Conversely, not reducing our emissions contributes to accelerating the warming of the oceans, which we now observe at depths of up to 2000 meters.

See how deep the oceans are warming

Discover the evolution of the daily sea surface temperature (in English)

View global warming charts on Berkeley Earth

The ocean is out of breath

This is the title of an analysis published by researchers from the Ocean & Climate platform, supported by UNESCO, one of Anne de Vernal’s reading suggestions. We are referring here to the fact that warming of the oceans leads to a drop in oxygen levels. “When water is warmer, it is less dense. Ocean surface waters, by warming, therefore lead to a stratification of the surface layer limiting exchanges with deep waters (therefore less penetration of oxygen into the seabed waters),” explains the climatologist.

We can observe this phenomenon almost everywhere on the planet. In a recent report, scientists from the Maurice-Lamontagne Institute, located in Mont-Joli, in Bas-Saint-Laurent, reported that oxygen levels of around 10% are currently observed in deep waters. located opposite Rimouski. Water considered low in oxygen has a saturation rate of 30%. At 20%, the level can be fatal for several aquatic species. More tolerant species, such as halibut and northern shrimp, cannot survive at oxygen levels below the 10 to 15 percent range. In short, the deoxygenation of the oceans associated with the acidification of the waters concerns the scientific community.

View the report The ocean is out of breath

Rising oceans

One of the consequences of climate change is the rise in sea levels, which is also irreversible. Two phenomena are at the origin of this increase, specifies Anne de Vernal. First there is the thermal expansion of the ocean. “The volume of water increases with temperature. A direct effect of warming is therefore an increase in the volume of ocean water. » Sea level has been rising by 1.3 mm per year since 2005 due to thermal expansion. The other, more well-known explanation is the melting of the ice caps, “notably that of Greenland which loses around 268 gigatons of ice each year, according to satellite measurements since 2002”, adds M^me of Vernal. The melting of ice caps has led to a rise in ocean levels of 2.2 mm per year since 2002. Over the past 30 years, ocean levels have risen by 103.3 mm or 10 cm on average, according to official data. An essential reference on this subject remains the Sea Level Change site, produced by NASA, which contains a multitude of data on sea level rise.

Visit the Sea Level Change page

Decline of marine biodiversity

Ocean warming and deoxygenation, like acidification, are not without consequences for marine biodiversity, recalls Anne de Vernal. “The latest estimates from the United Nations Educational, Scientific and Cultural Organization warn that more than half of marine species could be on the brink of extinction by 2100. With a current temperature rise of 1.1°^⁠C, it is estimated that 60% of the planet’s marine ecosystems have already been degraded or are being used in an unsustainable manner,” indicates the UN in a recent publication recommended by the researcher. We know, for example, that 70 to 90% of coral reefs will disappear with a warming of at least 1.5°C by the end of the century, a proportion which will increase to 99% with a warming of at least 2°C. vs. To better understand coral bleaching, she suggests reading about it on the Woods Hole Oceanographic Institution website.

View a report on coral bleaching

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