The pH scale is used by scientists to describe strength of acids and alkalis.
Sea water normally had a pH around 8.3 but 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 -
Surface ocean pH has fallen by about 0.1 pH unit from preindustrial times to today — a 0.1-unit pH drop 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, and increase ocean hydrogen ion concentration (or acidity) by 100-150% above what it was in preindustrial 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.
Picture A shows healthy coral.
Picture B shows dead and dying coral.