Ocean Acidification Network

What can we learn from Earth's geological past about ocean acidification?

Can we determine what things will look like in the future? How will ocean acidification impact individual organisms and ecosystems?

What methods and approaches are being used?

Who is carrying out this research?

References used in this section

What can we learn from Earth's geological past about ocean acidification?

In the geological past, the Earth experienced rapid increases in CO2 or persistently high CO2 over long time periods. Studying these periods can give us insights into how the chemistry of the ocean and marine ecosystems will respond to our current rapid increase in CO2 and temperature. There are several periods in Earth's history that are particularly interesting for the ocean acidification problem (1):

(i) The seven glacial-interglacial transitions of the last 650,000 years, when atmospheric CO2 repeatedly increased by as much as 100 ppm (40%), accompanied by a surface ocean pH decrease of the order of 0.15 units.

(ii) The Eocene change in ocean pH levels 45-55 million years ago, which are suggested by the large change in the carbonate compensation depth, the depth at which carbonate minerals in the sediments are dissolved.

(iii) The Paleocene Eocene Thermal Maximum 55 million years ago, where abrupt processes (e.g. massive release and oxidation of methane hydrates) resulted in a sudden CO2 increase of the order of several hundred parts per million, presumably at a high rate that approximates the present situation. This event also provides the opportunity to observe recovery times of the Earth system to a CO2 perturbation.

(iv) The middle Cretaceous ~100-80 million years ago, as an example of an extreme and lasting greenhouse world, with estimated atmospheric CO2 concentrations three to ten times higher than present.

From such studies, we have learned that the ocean has experienced large perturbations in the past that may have included ocean acidification events, where many calcifying organisms became extinct and most corals died. After these events, it took several thousand to several hundred thousand years for the chemical structure of the ocean to recover, and millions of years for the ecosystem to recover its biodiversity. However, it is still not possible to definitively determine whether ocean acidification was directly responsible for these extinction events because so many other environmental factors likely changed at the same time, including light availability, ocean circulation patterns, and temperature increases. Further research is required to separate out the information about ocean acidification impacts in the past that may provide insights for the future.

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How will ocean acidification impact individual organisms and ecosystems? Can we determine what things will look like in the future?

The study of the impacts on organisms and ecosystem communities can be broken down into several categories (2):

(i) benthic communities, such as coral reefs, molluscs, echinoderms (such as crabs and urchins), and benthic calcifying species of phytoplankton and algae;

(ii) water column phytoplankton and zooplankton species, such as coccolithophores, foraminifera, pteropods, and other calcifying organisms;

(iii) linkages between the responses of single organism types and the effect on the whole ecosystem community structure; and,

(iv) controls and feedbacks of these changes on ocean chemistry and the global carbon cycle.

Key questions include:

How is calcification affected in organisms at different stages of their life cycles?
Why do some calcifying organisms seem to be less affected than others? How is adaptation and survival influenced by the different mechanisms of calcification or other physiological factors?
How do other environmental factors such as carbonate concentration, light levels, temperature and nutrient levels affect calcification processes?
How will communities with a mixture of calcifying and non-calcifying organisms respond to decreased calcification rates and what impact will this have on the marine food chain?

At present, our understanding is not sufficient to determine what future marine ecosystem may look like. The most optimistic view is that organisms with short generation times may adapt rapidly to changing conditions and simply replace the less tolerant species, with minimal impact on the marine food web. The most pessimistic view is that marine calcifiers will be unable to adapt and will be fully replaced by other non-calcifying organisms, which may lead to large perturbations in the food chain, including mass extinctions (3).

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What methods and approaches are being used?

Field Monitoring and Experiments

The international community has been developing a global observing system for ocean carbon over the last 10 years, using ships, buoys, and satellites to understand how the ocean absorbs atmospheric CO2 (4). This system, in conjunction with other field programs and monitoring stations, must be adapted to look at other variables in the carbon system as well as ecosystem parameters to provide the necessary data for studying ocean acidification. Other key environments requiring an adapted field monitoring strategy include coral reef sites and areas with naturally low pH, such as upwelling areas and hydrothermal vents.

Laboratory Experiments

There is still very little known about how different organisms may respond to expected changes in pH and carbonate ion concentration, and how these impacts may be enhanced or decreased because of response to changes in other environmental conditions such as temperature, circulation or light. Laboratory experiments will be required to examine responses of a wide range of organisms to environmental perturbations over a range of timescales and conditions that can only be controlled in a laboratory setting.

Mesocosm Experiments

While laboratory experiments are crucial for improving our understanding of how different organisms will respond to changes in conditions, these responses must also be understood in terms of their effects not just on a single organism, but on the full ecosystem in which it resides. Mesocosms, bags or corral enclosures set in natural seawater environments, allow environmental conditions to be manipulated while examining the effects on natural populations of organisms. Recent mesocosm experiments have been successful in measuring primary production and calcification associated with phytoplankton blooms, and also allow for quantification of vertical fluxes of organic and inorganic carbon in the water column.

Numerical Modeling

Projections of future conditions and likely impacts must rely heavily on models that are designed to address ocean acidification research issues. Models must be used to understand the future chemical environment of pH and carbonate saturation states as a function of changes in factors such as atmospheric CO2 concentrations, temperature and its effects on ocean circulation and mixing, and chemical interactions with sediments. On a different scale, modeling must also be used to examine the complex interactions between organism responses to environmental changes and how these responses propagate up the marine food web. Because different ecosystems and regions are expected to be affected differently by ocean acidification, models will need to be downscaled and targeted to specific regions, taking into account each region’s physical, chemical, and ecosystem features in order to make meaningful projections of potential impacts.

Photo credits (from top to bottom): Chris Sabine; Reefbase / Christian Perthen; IAEA Marine Environmental Laboratory; Ulf Riebesell and Jean-Pierre Gattuso; Scott Doney.

The Free Ocean CO2 Enrichment (FOCE) System

One special type of field experiment being investigated is the Free Ocean CO2 Enrichment (FOCE) System, based on a similar system already in use to study the response of trees and plants to increased CO2 levels. In this experiment, a ring of towers would release carefully controlled amounts of CO2 into the water to study the effects of organisms and ecosystems to pH levels we expect to see at the end of this century. A conceptual drawing of this system is shown below.

The Free Ocean Carbon Dioxide Enrichment (FOCE) system. Figure reprinted from Kirkwood et al. (5), courtesy of Bill Kirkwood and Peter Brewer, Monterey Bay Aquarium Research Institute.

This system could be set up on a variety of scales to study the impacts on coral reefs and other benthic ecosystems as well as natural populations of phytoplanton and zooplankton in the water column. The design and implementation of such a system is a considerable engineering challenge, mainly owing to the difficulty of precisely controlling the pH levels in the experimental patch in the presence of ocean currents. Initial tests and sea trials of a small prototype system have demonstrated the feasibilty of this system, and research is continuing to adapt the prototype system to cope with more dynamic, shallow water environments (5).

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Who is carrying out this research?

Understanding ocean acidification and its impacts on the marine ecosystem is one of the greatest challenges facing ocean scientists this century. It will require a combination of individual scientists working on targeted research issues and international collaborations to bring together all the pieces of the puzzle. Listed are the major international, regional, and national programs coordinating or carrying out ocean acidification research.

See the list of international, regional, and national research programs in the Resources section of this site.

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References used in this section

(1) Adapted from IGBP-SCOR Fast Track Initiative on "Ocean Acidification"

(2) Adapted from Kleypas, J.A et al. (2006) Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Future Research, report of a workshop held 18-20 April 2005, St. Petersburg, FL, sponsored by NSF, NOAA, and the U.S. Geological Survey, 88 pp.

(3) IGBP-SCOR-PAGES workshop report (2006) Ocean Acidification - Modern Observations and Past Experiences.

(4) See the International Ocean Carbon Coordination Project for details.

(5) Kirkwood, W.J. and P.G. Brewer (2006) Free Ocean CO2 Enrichment (FOCE) System: Technology for Chemical and Biological Studies of a High CO2 Ocean, presented at the workshop on Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers (see reference 2).