February 3, 2011

Good News for Sea Turtles from the Great Barrier Reef

If you haven’t heard the news, global warming is causing sea level to rise and causing storms to become more severe, and the net result is shoreline erosion throughout the world. This pillar of the apocalypse is particularly easy to sell—gather up some pictures of shoreline erosion, throw in some images of turtle nest destruction, and you are on your way to winning a Nobel Prize for putting all the pieces together.

A recent issue of Global and Planetary Change contains an article on this subject written by two scientists with the School of Earth and Environmental Sciences at James Cook University in Townsville, Queensland; funding was provided by the Environmental Protection Agency-Queensland. Dawson and Smithers focused on Raine Island located on the northern portion of the Great Barrier Reef, and if you don’t know, Raine Island is “a globally significant turtle rookery.” So it’s all here—an island on the Great Barrier Reef, turtles, sea level rise, relatively frequent tropical cyclones, sand beaches easily eroded—we are sure the global warming alarmists cannot wait to see how bad things have become at this sacred location.

But, alas, the results from Raine Island are about to rain on their parade of pity.


Turtles love Raine Island (image from seaturle.org)

From the beginning of the article, we thought the authors were potentially off the reservation. They lead off by stating “It is projected that global sea level will rise ∼0.03 m in the next decade and between 0.18 and 0.79 m by 2100, and that tropical cyclones will become more intense.” OK—that sounds like they are onboard the alarmists’ train. But next they write “Low-lying reef islands are widely perceived to be particularly sensitive to these changes, although a number of geomorphologists have argued that rising sea levels do not always cause reef islands to erode.” What? There might be more to this story?? Still in their first paragraph that explain “For example, a rise in sea level may promote reef island growth by: i) increasing accommodation space for new sediment; ii) reinvigorating carbonate production on reef flats where further reef growth has been inhibited by a stable sea level; and iii) increasing the efficiency of waves to transport new and stored sediment to an island depocentre”. Imagine that—sea level rise might just cause these low-lying reef islands to increase growth? We liked where this was heading from the outset.

Dawson and Smithers state clearly that “Raine Island is a vegetated coral cay located on the far northern outer Great Barrier Reef (GBR), recognised as a globally significant turtle rookery. Cay geomorphology, specifically the morphology of the beach and swale, dictate the availability of nesting sites and influence nesting success. Understanding short and long-term shoreline change is critical for managers charged with protecting the nesting habitat, particularly as climate change progresses.” We agree totally that this is an important location that merits our attention.

The authors used historical topographic surveys, a simple numerical model, and geographic information system (GIS) techniques to reconstruct a 40-year (1967–2007) shoreline history of Raine Island. The resultant map (Figure 1) shows a lot of green (volume increase) and little red (volume decrease) around the island. They report “Results show that significant shoreline change has occurred on 78% of the island’s shoreline between 1967 and 2007; 34% experienced net retreat and 44% net progradation during the study interval”.

Don’t look now, but the island is not eroding overall—it is adding to its mass!


Figure 1. Cut and fill analysis of the subaerial beach of Raine Island (1998–2007) using georeferenced digital elevation models (DEM’s). The subaerial beach is defined as the portion of beach between the berm crest (dashed line) and the cliff. The position of the beach toe (dotted line) is also given. Calculated cut (erosion), fill (accretion), and total (net) island volume change are in m3. Planform areas (m2) and average change in beach elevation are also given for areas of cut and fill. It is assumed that the island interior is stable and has not changed over the 9-year period, therefore the Island interior has been omitted from the analysis to remove the potential for volumetric change arising from inconsistencies in topography (from Dawson and Smithers, 2010).

Dawson and Smithers provide more details, noting:

The volume of sediment deposited on Raine Island between 1967 and 2007 increased by ∼68,000 m3 net, but accretion rates varied significantly seasonally and from year to year. The largest volumetric changes have typically occurred over the last 23 years (1984–2007). Despite the recent concern that Raine Island is rapidly eroding, our data demonstrate net island growth (6% area, 4% volume) between 1967 and 2007. Perceptions of erosion probably reflect large morphological changes arising from seasonal, inter-annual and inter-decadal patterns of sediment redistribution rather than net loss from the island’s sediment budget.

The authors have this advice for conservation managers:

It is clear that future management strategies of Raine Island and other islands of the Great Barrier Reef should recognise that perceptions of reef island erosion can arise from large short-term seasonal and storm derived sediment redistribution from one part of the island to another or to a temporary storage on the adjacent reef flat, and not necessarily a net permanent loss from the island sediment budget.

That is probably good advice for us all—our perceptions of links between various short-term (e.g. weather) events and large term climate changes are not always a good indication about how thuings actually are.

And as a final thought, one that we like to beat into the ground here at World Climate Report, had Dawson and Smithers found erosion of the globally important turtle rookery, they would have received coverage from shore to shore all over the world. But when they found no net erosion and a gain of mass, they were reduced to coverage at World Climate Report.

Reference:

Dawson, J.L. and S.G. Smithers. 2010. Shoreline and beach volume change between 1967 and 2007 at Raine Island, Great Barrier Reef, Australia. Global and Planetary Change, 72, 141–154.




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