The state of ocean acidification

By Fiona Brown, Chris GerbingNovember 25th, 2020

Ocean acidification is often thought of as a future impact of our changing climate. But exactly what is it, what are its impacts and is it really a problem of the future?

As reported in the CSIRO and Bureau of Meteorology’s latest State of the Climate report, oceans around Australia are acidifying 10 times faster than at any point in the last 300 million years. When coupled with ocean warming and deoxygenation, this is putting considerable pressure on our marine environments. Recent research from CSIRO and AIMS has highlighted the changing conditions on the Great Barrier Reef. Drawing on over a decade of observations collected as part of Australia’s Integrated Marine Observing System (IMOS) the team found that the Reef’s rich carbonate seafloor is not buffering against ocean acidification, a process that might offset ocean acidification.

Ocean acidification 101

Although many people have heard of ocean acidification and the role the oceans play in sequestering carbon dioxide from our atmosphere, exactly how these processes occur, and the degree to which they will impact our oceans are somewhat more complex. So, let’s take a step back – all the way back to your high school chemistry class!

Ocean acidification is the term used for the chemical reactions that occur when carbon dioxide is absorbed by sea water. These reactions reduce seawater pH, carbonate ion concentration, and the saturation states of the biologically-important calcium carbonate minerals – but more on that later.

The State of the Climate report showed that the average pH of surface waters around Australia has decreased by about 0.12 between the periods 1880–1889 and 2010–2019. Being a logarithmic scale, a decrease in the pH of one unit translates to a 10-fold increase in acidity. Therefore, acidity of surface waters around Australia has increased more than 30 per cent over the past 140 years.

The acidity of waters around Australia is increasing (pH is decreasing)

The pH of surface waters around Australia. Change between 1880–1889 and 2010–2019. Calculations are based on present-day data on the carbonate chemistry of surface seawater around Australia from IMOS and other programs, and extrapolation of atmospheric carbon dioxide concentration changes since the 1880s. ©CSIRO

The cause of this drop in pH is rising carbon dioxide levels in the atmosphere. When carbon dioxide levels in the atmosphere are higher than those of ocean surface waters, the water absorbs carbon dioxide. At high latitudes, these carbon dioxide-laden surface waters become cold and salty enough to sink to the deep sea. Deeper water is then pushed up to replace the sinking surface water. This new surface water absorbs more carbon dioxide and so the cycle starts again. It is this connection between surface and deep waters in seas at higher latitudes that is the main pathway enabling uptake of carbon dioxide emissions by our oceans.

And why does absorbing carbon dioxide impact pH? We’re glad you asked! When carbon dioxide is absorbed by seawater it forms carbonic acid, which in turn separates to form hydrogen ions and bicarbonate ions. The acidity of a solution is based on its concentration of hydrogen ions; the higher the levels of hydrogen ions the lower pH of the water and the more acidic. Therefore, the more carbon dioxide absorbed, the higher the water’s acidity.

Another impact of elevated hydrogen ions in sea water is a reduction in carbonate ions, due to the excess hydrogen ions reacting with the carbonate ions to create bicarbonate. This matters because carbonate ions are used by marine organisms to build shells and skeletal material. As carbonate ion concentrations decrease, this is likely to affect the ability of marine life to produce and maintain their shells.

Monitoring ocean acidification

Given the harmful effects of ocean acidification around the world’s oceans many institutions are developing and deploying technologies to monitor these ocean changes. Many of these stations around the world show similar acidifying trends to those reported in State of the Climate.

In Australia, the Integrated Marine Observing System, or IMOS, operates a network of coastal and ocean observing systems that enable us to track many features of the ocean. CSIRO is responsible for running the network of IMOS ocean acidification moorings and observations that provide data back to scientists who can interrogate changes in ocean carbon. The moorings at Maria Island (Tasmania), Kangaroo Island (South Australia) and Heron Island (Queensland, Great Barrier Reef) provide data at high frequency contributing to national and international studies.

Two technicians attend to a floating yellow mooring

Researchers can follow the changes of our coasts and oceans thanks to a unique partnership between IMOS, CSIRO and other institutions. The Maria Island, Tasmania, mooring is pictured here. © CSIRO Carlie Devine

The future is now for the Great Barrier Reef

New research published by the Australian Institute of Marine Science (AIMS) and CSIRO has shown that ocean acidification is no longer a thing of climate projections, but a present-day reality.

Dr Katharina Fabricius, a Senior Principal Research Scientist at AIMS said people talk about ocean acidification in terms of 50 years’ time, but for the first time our study shows how fast ocean acidification is already happening on the Great Barrier Reef.

Dr Bronte Tilbrook is a Senior Principal Research Scientist at CSIRO who leads IMOS’ observational projects for CO2 and ocean acidification. He is a co-author of the new research and said that it shows that acidification is rapidly changing the conditions that support the growth of corals on the Reef.

Bleached corals with small fish in background

The oceans are slowing climate change by absorbing excess heat, but changes in carbon chemistry of sea water is contributing to coral bleaching. © CSIRO, Christopher Doropoulos.

The study has filled this important knowledge gap by analysing 10 years of CO2, pH and aragonite saturation state data (2009–2019). These data were collected as part of Australia’s IMOS network at two long-term monitoring stations, located 650 kilometres apart at contrasting locations on the central and south Great Barrier Reef.

The researchers found the range of CO2 concentrations measured today were already greater than the range expected 60 years ago, even after accounting for the effects of temperature, nutrients, salinity, and daily and seasonal changes.

“We know now that oceans are taking up about 23% of the excess CO2 from the air. They actually provide a service to humanity by slowing climate change. But the price to pay is that the seawater’s carbon chemistry is changing, and we didn’t know it was happening in dynamic coastal waters at such fast rates,” Dr Fabricius said.

“Ocean acidification is not just a climate change issue, but can be addressed and managed in its own right,” added Dr Fabricius.

Back to the future – what are the options?

Our changing climate is a slow process, and some further changes in the climate system are locked in based on greenhouse gas emissions that are already in the earth system. This means that adaptation measures for ecosystems affected by ocean acidification will be needed over the near term.

Earlier this year a team of researchers from CSIRO, AIMS and the University of Melbourne used synthetic biology approaches to create coral that is more tolerant to temperature-induced bleaching. They did this by bolstering the heat tolerance of the corals microalgal symbionts – tiny cells of algae that live inside the coral tissue.

This is a promising breakthrough, and demonstration of the research required to assist marine and coral ecosystems to adapt to climate change.

The State of the Climate report is clear on what the future holds for Australia’s climate. The amount of climate change expected in the next decade is similar under all plausible global emissions scenarios. However, by the mid-21st century, higher ongoing emissions of greenhouse gases will lead to greater ocean acidification and warming – with all their associated impacts. Reducing emissions will lead to less ocean acidification and warming, and fewer impacts.

Australia’s Integrated Marine Observing System (IMOS) is enabled by the National Collaborative Research Infrastructure Strategy (NCRIS). It is operated by a consortium of institutions as an unincorporated joint venture, with the University of Tasmania as Lead Agent.

State of the Climate draws on the latest monitoring, science and projection information to describe long-term changes in Australia’s climate.

In this upcoming webinar, discover the science behind the report and have your questions answered by the authors of the report from the Bureau of Meteorology and CSIRO.

Register now to find out more about what’s changed, including temperature, fire weather, rainfall, oceans and atmosphere and our future climate.


  1. Can you please explain the equation and how you arrived at the figure of 30% acidification.

    1. As carbon dioxide (CO2) enters the ocean, it reacts with water to form carbonic acid, which subsequently dissociates, leading to an increase in Hydrogen ion concentration [H+] causing an increase in acidity and a corresponding decrease in pH (the negative logarithm of [H+]).
      CO2atm + H2O <-> CO2aq + HCO3– + CO32-
      This in turn leads to a shift in the forms in which dissolved inorganic carbon is stored in the ocean. As atmospheric carbon dioxide levels increase, carbonate ions and their corresponding saturation decrease whereas increases occur in bicarbonate and aqueous CO2. These changes in carbon are collectively known as ocean acidification
      There when we say “greater than 30% in increasing ocean acidification” we are referring to increase in hydrogen ion concentration [H+]. However, as we often use the pH scale to talk about acid and bases, we convert the change in [H+] into a change in pH (0.12). So they are exactly the same thing, just on different scales.

  2. Great article!! Appreciated the refresher in real world chemistry too. Well worth the read in getting an understanding of the fundamentals of ocean acidification and effect. Thanks for a well written report. Now, is anyone doing CO2 and how it causes climate change? Haven’t seen that.

    1. Thanks Eric. The greenhouse gases section of State of the Climate presents our monitoring and research of CO2 in Australia. You can read that here >

  3. Thank you for the article presenting the implications of the observed downward trend in marine pH. It raises many questions and as environmental chemist I would be very interested in any information or links you may be able to share.
    1. What is the statistical correlation between atmospheric CO2 with oceanic pH over the study period?
    2. pH trend reminds me of titration experiments, what is the projected pH when CO2 reaches saturation in marine waters?
    3. with lower pH some metals will become more mobile (others less). I am thinking particularly of cadmium and zinc. What is being observed with metal mobilisation?
    4. speaking of cadmium mobilisation, how are the chlorine levels being effected?

  4. Thank you Chris for your reply.
    I had already worked out all the chemistry but I wasn’t sure about the maths and converting from one scale to the other.

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