More than half of the world’s oceans are already affected by climate change

More than 50% of the world’s oceans could already be affected by climate change, with this figure rising as high as 80% in the coming decades, a new study has shown.

Scientists used climate models and observations in deeper areas of the ocean worldwide to calculate for the first time the point at which changes in temperatures and salt levels – good indicators of human effects on climate change – transmit natural variations.

The study, published in Nature Climate change, estimates that 20-55% of the Atlantic, Pacific and Indian Oceans now have markedly different temperatures and salt levels, while this will rise to 40-60% by the middle of the century, and to 20-80% by 2080 .

It also found that the oceans of the Southern Hemisphere were more rapidly affected by climate change than the Northern Hemisphere, with changes observed there since the 1980s.

Professor Eric Guilyardi, co-author of the University of Reading and LOCEAN-IPSL, Laboratory for Oceanography and Climate in Paris, said: “We have detected ocean surface temperatures due to climate change for several decades now, but changes in large areas of the ocean, especially deeper parts, are much more challenging to detect. “

Yona Silvy, a doctoral student at LOCEAN-IPSL / Sorbonne University, and lead author of the study, said: “We were interested in whether the levels of temperatures and salt were great enough to overcome natural variability in these deeper areas, that is, if they were increased or dropped higher than they would ever do in normal peaks and troughs, this would affect global ocean defenses, rising sea levels and a threat to human societies and ecosystems.

Previous studies have measured the impact of climate change on the ocean by looking at surface temperature, precipitation and sea level rise, but few have looked at the regional effects deeper down in the ocean to get a more complete picture.

The effects of climate change are harder to detect in deeper, more isolated parts of the ocean, where heat and salt are spreading at a slower rate due to weaker mixing processes. It is also troublesome in areas that are poorly perceived or when natural variability is high.

Yona Silvy and her co-authors used model simulations with and without the influence of human activity and an analysis that combines both temperature and ocean salt to detect significant changes and their date of probable detection, also called “time of emergence”. Yet these are regions that have kept the memory of these changes for decades to centuries.

Changes detectable above natural variability were calculated to be seen in the oceans of the Northern Hemisphere between 2010-2030, meaning that temperatures increase or decrease and salt levels are likely to have already occurred.

The rapid and earlier changes seen in the Southern Hemisphere emphasize the importance of the Southern Ocean for global warming and carbon storage, as surface water there makes its way to the deeper ocean more easily. However, this part of the world is also particularly poorly perceived and sampled, meaning that changes are likely to be no longer detected.

The scientists argue that improved ocean observation and greater investment in ocean modeling are needed to control the extent of the impact of climate change on the world’s oceans, and more cautiously predict the broader effect this may have on the planet.