Climate change and the tropical marine environment

Tropical marine environments such as coral reefs and mangrove forests around the world are under unprecedented pressure due to climate change, changes in water quality from terrestrial runoff and overexploitation. Coral reefs are iconic tropical ecosystems represented by Australia’s irreplaceable Great Barrier Reef (GBR) and the less explored reefs off Western Australia. Corals thrive in locations which also happen to be near their physiological limits, making them sensitive to stresses caused by rising sea surface temperature and an increase in ocean acidity linked to rising carbon dioxide (CO2) in the atmosphere. Coral reefs and mangrove forests contribute greatly to tropical coastal productivity and provide habitat for myriad fish and other species.

AIMS contributes to understanding the implications of a changing climate by monitoring and modelling ocean climate changes, assessing impacts of climate change on coral reef and other organisms, identifying potential adaptation mechanisms and identifying characteristics and locations that may provide refuge for marine species in a rapidly changing world.

What we know

  • Atmospheric levels of CO2 measured from many locations around the world have been rising steadily for decades. This includes measurements taken from the AIMS headquarters site at Cape Ferguson in North Queensland where the level of atmospheric CO2 has risen from 355 parts per million (ppm) in 1992 to 379 ppm in 2006, an average increase of approximately 1.7 ppm per year (
  • The long-term average temperature for the waters of the Great Barrier Reef has increased by about 0.4oC since the 19th century and the Reef system has experienced two mass coral bleaching events (1998 and 2002) caused by long periods of coral exposure to unusually warm seawater.

The consequences of increasing atmospheric carbon dioxide

The amount of CO2 in the atmosphere has risen to the current level of 383 parts per million (ppm) from about 200 ppm in the days before the Industrial Revolution more than 200 years ago. Measurements of atmospheric CO2 taken from AIMS headquarters outside Townsville show broad agreement with this global figure (see page 1 of this document).

Under current IPCC projections and assuming no measures are adopted to reduce CO2 emissions, atmospheric CO2 concentrations are likely to reach 500 ppm in the second half of this century. If that is the case, global temperature averages may increase a further 2oC and possibly more.

Coral reefs provide ecosystem services essential to our national identity and wealth. The GBR contributes more than $5 billion annually to the Australian economy. Ningaloo Reef in WA is of growing economic and environmental importance, as are the Kimberley Coast and Oceanic Shoals off WA’s northwest. While Australia’s coral reefs are well managed, they are not isolated from global atmospheric and ocean changes.

Ocean acidification

Ocean acidification is a predicted consequence of increasing atmospheric CO2, in which large quantities of carbon dioxide from the atmosphere dissolve in the oceans, causing their alkaline/acid balance (their “pH”) to shift towards acidic.

The coral animal is a calcifying organism – it takes dissolved material from the surrounding sea water and turns it into a calcium carbonate skeleton. Corals are not alone in doing this – many marine plants and animals convert nutrients into calcareous and other inorganic skeletons or shells. There is a simple chemical equation governing the ability of all marine calcifying organisms to undertake this process, linked to the pH of seawater. Some marine scientists have hypothesised that this simple chemistry is being thrown out of kilter by the rise in atmospheric CO2 and its subsequent absorption by the oceans.

The oceans have been efficiently absorbing a large proportion of that extra carbon – in fact, if they hadn’t done so we may have experienced more global warming than we have already. Oceans are naturally slightly alkaline and this is conducive to the calcifying process. As seawater heads down the pH scale to become slightly more acidic, a problem with coral growth (calcification) is predicted to arise.

If projections are correct that pH could decrease by up to 0.4 pH units by the end of this century, this would be well outside the realms of anything organisms have experienced over hundreds of thousands of years. While 0.4 pH units might not seem like a lot, because pH is measured on a negative log scale this equates to a 2.5-fold increase in the concentration of hydrogen ions. Scientists are still uncertain about what this will mean for coral reefs and how resilient they may be to these changes if they come about. Nor is there any certainty about the effects on the multitude of other organisms that live in and on reefs and other tropical marine environments.

Many processes depend upon the pH of the surrounding environment being within a certain range for them to function properly. These may include deposition of marine cements to enable benthic organisms to attach to surfaces, or fertilisation of eggs by sperm which may affect the number of larvae produced by organisms that release their gametes into the ocean.

Absorption of CO2 by seawater may not only result in changes to seawater pH, it also changes the composition of the dissolved gases available for animals to breathe through their gills. Absorbing more CO2 into their bodies may have an effect on their health and behaviour and may unnaturally alter the pH of their body fluids.

AIMS, 19 December 2009. Full article.

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