Ocean Acidification


"The evil twin of the greenhouse effect". 

Absorption of C02 by the ocean

Since 1950's: Mauna Loa observatory 

Since 1750: Using Siple Ice Core

Can see that rates of C02 emissions are higher than the concentration in the atmosphere...

Where has the C02 gone?

Into the oceans- carbon sink:

1/3 of the CO2 released by burning of fossil fuels over the last 200 years

C02 from anthropogenic activites has a unique signiture- can track it.

Highest concentrations are at the surface- but down to beneath 1000m in depth

Lowest concentrations at equator- warmer water- CO2 is released- gases come out of solution.

But at higher latitudes near poles- greater drop in pH due to higher adsorption of CO2

What happens?

CO2 dissolves into the water C02+H20= H2C03 (Carbonic acid)

Carbonic acid- breaks down= Bicarbonate ions HC03-1 and Hydrogen ions H+

Bicarbonate ions break down into Carbonate ions C03-2 and Hydrogen ions H+

The pH of the water becomes more acidic as the H+ ion concentration rises

This limits organism access to carbonate ions- needed to form shells etc.= Deformed shells.

(IPCC, 2013): As C02 levels in the surface ocean have increased, pH has decreased. 

Average drop in pH= 8.25 - 8.14 = 30% increase in H+ ions (logarithmic scale)


Hopefully we will stop using fossil fuels? But in the meantime- more CO2 into atmosphere. We've so far lost about -0.1 in pH, By 2050- due to loose another -0.1 pH (Caldera & Wickett, 2003). Process is irreversible for thousands of years.

Period of change is 100x greater than any time over 10's of millions of years. 

Magnitude of ocean acidification can be predicted with high levels of confidence.

Impacts on marine organisms and their ecoystems are less certain.


  • Ocean acidification has been implicated in several mass extinction events
  • At the end of the Permian - massive drop in aquatic species diversity- (Permian Triassic Extinction Event- PTEE) 250 million yrs ago. 96% species wiped out. Calcifying organisms were particularly hard hit. Cause? Volcanism igniting coal beds- closer to the surface than today. Massive natural fires. 
  • PETM- Paleocene-Eocene Thermal Maximum (56yrs ago)- global average temperature increased 6oC in 20,000 years- geologically speaking very rapid…


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