GREAT BARRIER REEF
// Outlook Report 2014
In addition, cyclonic winds can cause storm surges — onshore rises of water above the predicted tide.64 Over the past 100,000 years sea levels have risen and fallen many times, shifting the position of reef growth on the continental shelf.65 The role of sea level in the geomorphological evolution of the Great Barrier Reef is recognised in its world heritage listing.3 Sea level is rising in Australian waters, with the fastest rises being recorded in northern areas.64,66 In the Region, sea level is rising by an average of about 3.1 millimetres per year.64,67,68 Sea level data presented in the Outlook Report 2009 showed the Townsville area had experienced an average increase of 1.2 millimetres per year between 1959 and 2007 and the rate may be increasing. Since then, the rate of increase has accelerated, peaking in 2010 at 125 millimetres above the long-term (1959–2012) average (Figure 3.5).69 Sea level at Townsville has now risen an average of 2.6 millimetres per year from 1959 to 2012 and an average of 11.8 millimetres per year between 2007 and 2012.69 Most reefs in the Region will probably be able to accommodate the current rate of sea level increase as the maximum rate of reef growth is about twice this.70 However, sea level rise is predicted to increase at a higher rate (see Section 6.3.1) and coral reef growth may not be able to keep pace.71 The shape and existence of some coastlines, cays and islands may also be affected.22,72 Even modest rises in sea level may have substantial consequences for other aspects of the Region, especially when combined with natural variability arising from the El Niño–Southern Oscillation. For example, the ability of marine turtles to nest and the survival of their eggs may be reduced if islands are inundated.72
The fastest rates of sea level rise in Australian waters are in northern areas.
Figure 3.5 annual average sea level, townsville, 1959–2012
From 1959 to 2012 sea level in Townsville has varied 235 millimetres around the average for that period. Since the 1980s, the deviation from the average sea level has tended to be above the average.
Source: Permanent Service for Mean Sea Level 2013 69
3.2.6 Sea temperature
Sea temperature is a key environmental factor controlling the distribution and diversity of marine life.73 It is critical to reef building and is one of the key variables that determine coral reef diversity and the north-south limits of coral reefs.74 The average sea surface temperature in the Coral Sea has risen substantially over the past century. Since instrumental records began, 15 of the 20 warmest years have been in the past 20 years75 (Figure 3.6 and see Section 6.3.1). When temperature limits are exceeded, physiological processes may break down.75,76 For reef habitats, the most critical mechanism affected is the symbiotic association between animals (such as corals and clams) and the microscopic algae which live within their tissues and provide much of their nutrition through photosynthesis. If sea temperatures exceed a certain threshold these algae are expelled — an effect known as bleaching.77
Figure 3.6 Sea surface temperature anomalies for the coral Sea, 1900–2013
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The hottest five-year running averages of sea surface temperature have all been in the last 15 years. This graph uses the 1961 to 1990 average as a baseline for depicting change. Source: Bureau of