In September 2019, the IPCC published a special report entirely dedicated to the ocean and the cryosphere. It states that since the 1950s, marine species have been moving toward the poles by about 52 kilometers per decade near the surface.
This figure is not noticeable in a single dive. It is read over decades.
Our article on ocean warming and what divers observe first already covers bleaching and gorgonians. Here, we look at the rest of the picture: species that are moving, acidifying water, decreasing oxygen, and what all this changes for the wildlife you encounter while diving.
Why are marine species changing their zones?
Many species follow the temperature that suits them. When the water warms, their distribution area shifts toward the poles or deeper waters. The IPCC quantifies this shift since the 1950s at 52 kilometers per decade, plus or minus 33, for organisms in the top 200 meters, and 29 kilometers for those living on the seabed.
The previous version of this page mentioned migrations of “70 to 1000 km per decade.” We found no serious source for this figure, and it far exceeds the ranges published by the IPCC, so we removed it without regret to keep the values from the 2019 report.
These averages hide large differences.
However, a diver might quickly want to link an unusual encounter to climate, for example, a southern species seen on a Breton site in the middle of summer. The report talks about trends measured over thousands of observations and decades. A single encounter could come from a temporary current, a mild winter, or transport by a ship. Or pure chance. Impossible to decide from here, hence the importance of dated and localized reports.
What does acidification change for corals and shellfish?
The ocean absorbs some of the CO2 we emit, and this gas makes the water more acidic. According to NOAA, surface pH has dropped by about 0.1 units since the industrial revolution. Since the scale is logarithmic, this represents an increase in acidity of about 30%.
The problem primarily affects animals that build calcium carbonate. More acidic water contains fewer available carbonate ions, and corals, oysters, or pteropods (tiny swimming snails, the base of many food chains) have more difficulty building and maintaining their shells or skeletons. NOAA describes a telling experiment. Pteropod shells placed in water under conditions projected for 2100 slowly dissolved over 45 days. The IPCC estimates that the ocean has absorbed 20 to 30% of human CO2 emissions since the 1980s.
Thus, acidification remains invisible to the naked eye.
You won’t notice it while diving, unlike bleached coral. It acts silently on the growth of reefs and shellfish, and that is precisely what makes it hard to understand. We talk about a stress that adds to heat and local pressures.
Is the ocean really losing oxygen?
Yes, and it is measured. According to the IPCC, data series from 1970 to 2010 show that the open ocean lost between 0.5 and 3.3% of its oxygen in the top 1000 meters. Warmer water dissolves less gas and mixes less well with deeper layers.
Looking ahead, the report projects a 3 to 4% decrease in ocean oxygen content by 2081-2100 under the high emissions scenario (RCP8.5), compared to 2006-2015. The confidence level assigned is medium.
It seems small.
But for a fish, dissolved oxygen sets the limit of what it can do. Active species and large individuals require more, and even a modest decrease can reduce the space where they live comfortably. The previous version of this article mentioned changes in reproductive cycles and population reductions without citing studies. We kept the general idea from the IPCC (changes in species composition, abundance, and production) and removed detailed claims that we could not verify. In short, the trend is solid, but species-by-species details require studies that cannot be summarized in one line.
What does the IPCC predict for coral reefs?
The diagnosis is severe. The IPCC projects that almost all warm-water coral reefs will suffer significant surface losses and local extinctions, even if warming is limited to 1.5 °C. The report adds, with very high confidence, that the remaining reefs will not have the same composition as those of today.
Concretely, a reef can survive by changing its appearance, with fewer coral species and different surrounding fauna. The causes accumulate. Heat causes coral bleaching, acidification slows skeleton building, and local pressures like pollution or overfishing further weaken the colonies. The old page also claimed that reefs harbor “30% of marine biodiversity” and cited a bleaching figure on the Great Barrier Reef in 2016 without a source. These were removed, as they could not be linked to a specific publication.
In short, another reef.
And the issue of blooms often comes up in clubs. Jellyfish are the most cited example, and our file on jellyfish invasions details their possible causes, which are not limited to climate.
What will happen to fauna and fishing by 2100?
The IPCC projects, by the end of the century and in the high emissions scenario, a 15% decrease in the global biomass of marine animals, plus or minus 6 percentage points, and a decline of 20.5 to 24.1% in the maximum catch potential of fisheries compared to 1986-2005.
These figures strongly depend on emissions. The same paragraph specifies that these changes would be three to four times greater in the high scenario than in the low scenario (RCP2.6). In other words, the trajectory is not set. It remains linked to decisions made outside the water. The old text spoke of “new exploited species” and shifted fishing zones without dated examples; we preferred to stick to the report’s quantified projections.
However, diving is not a neutral observation post.
You can note what you see with the date, site, depth, and temperature read, then transmit it to a monitoring program. The role of citizen divers in ocean monitoring begins with this well-kept logbook. On an already fragile site, the rules for observing without disturbing matter even more. The cost remains modest: five minutes of notes after each dive and a bit more distance from the reef.
FAQ
How fast are marine species moving toward the poles?
According to the IPCC special report of 2019, the average movement since the 1950s is about 52 kilometers per decade for species in the top 200 meters. It is about 29 kilometers per decade for deep-sea species. These are averages with wide margins of uncertainty.
Can ocean acidification be seen while diving?
No. It affects the chemistry of the water and the ability of corals, oysters, or pteropods to build their calcium carbonate shells. According to NOAA, surface pH has dropped by about 0.1 unit since the industrial revolution, which is about 30% more acidity.
How much oxygen has the ocean lost?
The IPCC indicates a loss of 0.5 to 3.3% in the top 1000 meters of the open ocean, based on data covering 1970 to 2010. It projects a decrease of 3 to 4% by 2081-2100 in the high emissions scenario.
Does an unusual species on my site prove climate disruption?
Not by itself. A single observation may come from a current or a particular season. Trends emerge from thousands of dated records, hence the importance of noting and transmitting your observations.
Sources
- Summary for Policymakers, Special Report on the Ocean and Cryosphere in a Changing Climate (IPCC, 2019)
- Ocean acidification (NOAA, updated 2026)
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