Sea-surface Temperatures along the Great Barrier Reef
20 Jul 2011 Category: AUSTRALIA
We're often told that the sea temperature along the Great Barrier Reef (GBR) is increasing and that soon
the coral will be bleached and the reef be destroyed. But what's the real story according to the data?
The USA's National Oceanographic and Atmospheric Administration (NOAA) has a web page with recent data and maps, along with links to archived data of sea surface temperatures since 1982. The data is matched to grid cells of 1 degree Latitude and 1 degree longitude and from it I extracted the data applying to the GBR Marine Park and calculated the average across the park for each month.
Figure 1 (below) shows those monthly averages along with the 12-month running average (current month and 11 previous).
The sea surface temperature (SST) clearly fluctuates throughout the year
by about 5 degrees, typically with highest temperatures in January and lowest temperatures in August. It is
also clear that The SST rises with the onset of El Nino events and falls with the onset of La Nina events.
We can also compare the monthly average SST for the GBR Marine Park with the monthly mean temperatures for the Australian Bureau of Meteorology's observation station on Willis Island, which is located at Lat. 16.3S Long. 150.0E on the Great Barrier Reef, 455km almost due East from the city of Cairns. The data shown here for Willis Island comes from NASA's GISS (see http://data.giss.nasa.gov/gistemp/station_data/).
An image of the Willis Island observation station (from the Bureau of Meteorology website) is shown in Figure 2,
with the Stevenson screen housing for thermometers circled in red. The ground surface in the immediate vicinity
of the Stevenson screen suggests that reflected heat might be a seasonal problem.
The comparison of the two sets of data, GBR marine park and Willis Island, is shown in Figure 3.
Figure 3 shows that the average SST is usually very similar to the mean temperature
derived from monitoring at the Willis Island observation station, except that in winter the SSTs were
usually cooler than the Willis Island temperatures up to about 1993. This might be due to cooler ocean currents at that
time of the year or perhaps the surface of Willis Island has advantageous thermal properties, especially
heat being retained near the ground surface and easily transferring heat to the air. Some data is missing for Willis
Island and we can only assume that it is likely to follow similar patterns to those which is shown.
The temperature trends shown on this graph of monthly data indicate warming equivalent to 1.2 degrees/century for the SST and 0.6 degrees/century for Willis Island. Some of the difference in these two trends may be due to that winter variation in the two temperatures as mentioned above.
Figure 4 shows the difference between GBR SST and Willis Island (former minus latter). Winter SSTs are almost always lower than Willis Island temperatures.
The correlation coefficient between the two sets of data in Figure 3 is 0.951, meaning that it is very strong. On the basis of this good correlation, earlier (i.e. pre-1982) data from Willis Island can provide a good indication of the monthly average SST across the GBR Marine Park. Willis Island monthly temperature data from 1940 to 2010 is shown in Figure 5, with the corresponding temperature anomalies (relative to 1961-90 averages) shown in Figure 6.
The temperature trend shown in Figure 5 is equivalent to an increase of 0.12 degrees/century, which
This proxy data from temperature monitoring on Willis Island suggests that sea surface temperatures on the Great Barrier Reef have varied very little over the last seventy-one years and show no sustained periods of significant warming.