Geoscience Reference
In-Depth Information
satellites flown by NASA and NOAA can reliably predict when and where mass
coral bleaching is likely to occur by tracking sea surface temperature anomalies
(Strong et al., 1997; Eakin et al., 2010; Strong et al., 2011). While the amount
of mortality following mass bleaching events on the Great Barrier Reef has been
relatively low (<10 per cent per event), observations from other Australian
reefs (e.g. Scott Reef, Hoegh-Guldberg, 1999) and internationally (Wilkinson
and Hodgson, 1999) reveal that coral reefs that experience elevated tempera-
tures for longer (e.g. 1-3°C for a month or more) tend to have mortalities that
may exceed 90 per cent of the corals on the reef. Heat stress events have also
been accompanied by changes in ocean pH and carbonate ion concentrations
(ocean acidification). While it is not possible to precisely identify the environ-
mental factor responsible, the observation of declining coral calcification since
1990 (unprecedented in 400 years of coral core record examined) suggests that
changing ocean conditions are beginning to impact on central processes such as
calcification (De'ath et al., 2009). Recently, observations of slowing coral growth
have been joined by rigorous long-term observations that show that the Great
Barrier Reef has lost approximately 50 per cent of its coral populations since the
early 1980s (De'ath et al., 2012). This is an extraordinary situation given that
the Great Barrier Reef is one of the most pristine and best-managed coral reefs
in the world.
Other factors, for example the changing characteristics of currents such as the
East Australian Current (EAC), are being identified with an increasing number
of changes within marine ecosystems such as the appearance and increased
abundance of warm-water zooplankton, fish and benthic invertebrate species
off eastern Tasmania (Pitt et al., 2010; Last et al., 2010; Johnson et al., 2011).
One of the most striking examples is the southward movement of the sea urchin
Centrostephanus rodgersii
into Tasmanian coastal waters over the past several
decades. Populations have expanded along the Tasmanian coast as a result of
the transportation of larvae across the Bass Strait due to the strengthening of
the EAC over the past several decades and rapid warming of coastal waters in
northern Tasmania (Johnson et al., 2005).
C. rodgersii
is a voracious herbivore
which has begun to change the structure of Tasmanian kelp communities, with
negative ramifications for coastal biodiversity, and industries such as abalone and
lobster fisheries (Ling, 2008). The scientific literature reveals that Australian
marine organisms and ecosystems are changing extensively, rapidly and funda-
mentally in response to climate change (see Poloczanska et al., 2007 and Johnson
et al., 2011, for more detailed information).
How can we know the future and what will the ocean be like in a
Four Degree World?
The observation of extensive, rapid and fundamental changes to Australian
marine ecosystems after a 0.7°C increase in the average global temperature raises
concern about what will happen if average global temperatures rise to 4°C and
beyond. Before answering this question, it is instructive to review how ecologists
