Geoscience Reference
In-Depth Information
to shift its ecosystems by 2,000 km over 100 years. This would require migration
of this complex marine ecosystem at the rate of 2,000km per 100 years or 20km
per year. A similar calculation for a 4°C rise in sea temperature would require a
speed of movement of the Great Barrier Reef (GBR) ecosystem of around 40 km
per year. Given that the ability for coral species to disperse significantly over one
generation is probably <10 km (Hoegh-Guldberg, 2009), and that many of them
organisms and ecosystem processes would have to move and become established
at sites 20-40 km away each generation, it is unlikely that the GBR ecosystem
will migrate at the necessary speeds to keep up with climate change. The other
issue associated with the migration of marine ecosystems such as the GBR is
the lack of suitable conditions at higher latitudes. Coral reefs require high light
levels and carbonate ion levels of >200 µmol kg
-1
(water), both of which decrease
significantly as one moves to higher latitudes (i.e. toward the poles). It is likely
to be the combination of the three factors (i.e. light, chemistry and temperature)
that controls the distribution of coral reefs worldwide (Kleypas et al., 1999).
Consequently, the arrival of a few isolated coral species at higher latitudes is not
proof that highly productive carbonate coral reef ecosystems will also migrate at
rates that will match the speed required by rapid anthropogenic climate change.
Given these fundamental obstacles, the prospect for many marine ecosystems
is not particularly good if average global temperature exceeds 2°C or progresses to
4°C and above over the next 100 years. Again, these issues have been explored
extensively for coral reef ecosystems, which serve as an illustration of the likely
changes to marine ecosystems at average global temperatures of 2°C or 4°C
above those seen in the preindustrial period.
Figure 5.4
provides an illustration of how the Great Barrier Reef is likely
to change over the coming decades and century. A fuller explanation of these
scenarios and the supporting assumptions and evidence is presented and discussed
by Hoegh-Guldberg et al. (2007). A situation similar to today in which regular
bleaching events have an impact on reefs, but reefs mostly recover is represented
in Panel A. Panel B illustrates the impact of high temperatures (
1
2°C) and
reduced carbonate ion concentrations (~200 mmol kg
-1
water). At this point,
coral communities have lost the more sensitive species and are composed of
tougher, less 'charismatic' massive corals that are in low abundance relative to
a range of other more competitive organisms such as seaweeds, sponges and soft
corals. Pushing sea temperatures beyond 3°C for more, and driving carbonate ion
concentrations well below 200 µmol kg
-1
(water), results in the almost total loss
of reef building corals. Under these conditions, community calcification almost
certainly rates fail to keep up with physical and biological erosion, resulting in a
decrease in habitat complexity as the carbonate structures of coral reefs begin to
break down. While the latter may take significant amounts of time to occur, the
eventual loss of reef structure removes habitat for thousands of species, many of
which are important to fisheries and tourism. The reduced carbon framework of
coral reefs also eventually decreases their ability to protect coastlines from wave
action, triggering a range of other impacts on associated marine habitats such as
mangroves, beaches and seagrass beds.
