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).

GBR SST anomalies

Figure 1 - Monthly average sea-surface temperatures for the Great Barrier Reef Marine Park

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

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.

Willis Island weather station

Figure 2 - Willis Island Observation station, with thermometer housing circled

Willis Island monthly mean temps

Figure 3 - Great Barrier Reef Marine Park sea surface temperatures (blue) and monthly mean temperatures at Willis Island (pink)

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.

GBR SST minus Willis Island temps

Figure 4 - GBR SST minus Willis Island monthly 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.

Willis Island 1940-2011

Figure 5 - Willis Island mean monthly temperatures Jan 1940 - Jan 2011

Willis Island temp. anomalies

Figure 6 - Willis Island monthly temperature anomalies (relative to 1961-90)

The temperature trend shown in Figure 5 is equivalent to an increase of 0.12 degrees/century, which negligible.

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.