The pH scale is used by scientists to describe strength of acids and alkalis.
Sea water normally had a pH around 8.2
It has now reduced to 8.1, and will continue to reduce, as more CO2 is added to the air by human activities.
Some of the extra CO2 in the air dissolves in the sea, and this affects sealife.
Here is what one expert scientist has said about this -
"A drop of 0.1-unit pH is equivalent to about a 26% increase in the ocean hydrogen ion concentration."pH is likely to drop by 0.3-0.4 units by the end of the 21st century.
"This will increase ocean hydrogen ion concentration (or acidity) by 100-150% above what it was in pre-industrial times."
Humanity's greenhouse gas emissions may be acidifying the oceans at a faster rate than at any time in the last 300 million years.
The stony skeletons that support corals and reefs will dissolve.
The left-hand picture shows healthy coral. The right-hand picture shows dead and dying coral.