Geoscience Reference
In-Depth Information
The impact of warming and acidifying ocean waters
Rising atmospheric CO
2
levels have resulted in a steady increase in the heat
content of the upper 700m of the ocean, with a resulting 0.5°C increase in the
average surface temperature of the ocean during the 20th century (IPCC, 2007;
Levitus et al., 2012). Uptake of anthropogenic CO
2
has also resulted in the ocean
becoming more acidic (as measured by decreasing pH), which has reduced the
concentration of carbonate ions. Carbonate ions are part of the raw material
that is used to make the calcium carbonate skeletons of many marine organisms
including corals, molluscs and a range of marine plants. There is growing
evidence that relatively minor perturbations in ocean carbonate chemistry could
lead to profound changes in the carbon cycle, marine food webs and nutrient
cycles with potential impacts on economically important fisheries (Doney et al.,
2009).
Since the Industrial Revolution, the pH of the world's ocean waters has
decreased by 0.1 pH units, which has caused a 30 per cent decrease in the
concentration of carbonate ions (Caldeira and Wickett, 2003). These changes
are rapidly approaching conditions that are unprecedented in the last million
years at least (Raven et al., 2005, Pelejero et al., 2010). Changes to ocean heat
content are also increasing sea levels, intensifying storm systems, changing
ocean mixing and ventilation, wind patterns and currents, melting sea ice and
the distribution of clouds and solar radiation (IPCC, 2007; Hoegh-Guldberg and
Bruno, 2010).
Conditions are also changing rapidly within Australian waters. Sea surface
temperatures (from the upper layers of the ocean) have increased by approxi-
mately 1°C since 1900, with some of the warmest temperatures being seen
along half of Australia's coastline in 2010 (Braganza et al, 2013,
Chapter 2
,
this volume). Monitoring of ocean alkalinity within Australia's tropical waters
has revealed a downward trend in pH and carbonate ion concentrations similar
to that seen globally (Tilbrook et al., 2011). Atmospheric warming has led to
changes to ocean currents, such as the strengthening of the East Australian
Current, which corresponds to a southward shift in sea surface climatology in
south-eastern waters of around 3 degrees latitude or 350km over 1944-2002
(Ridgway et al., 2008). Sea levels around Australia have risen as the volume
of the ocean has expanded as a result of heating and the increased addition of
glacial water. As result, the risk of coastal inundation has increased, particularly
in combination with extreme storm events (Church and White, 2006). The
range of conditions that will influence marine environments around Australia
are summarized in
Figure 5.1
.
Biological responses to recent climate change
Changes in the physical and chemical nature of the world's oceans are driving
fundamental changes to marine ecosystems (Hoegh-Guldberg and Bruno,
2010). While oceans and marine ecosystems have not received the attention
