Climate Change

In this second section of this topic, we look at the issue of rising sea level.

2. Sea level rise

Quoting CSIRO (2009):

"Sea level is rising as a result of increasing concentration of greenhouse gases in the atmosphere. Sea level rise contributes to coastal erosion and inundation of low-lying coastal regions, particularly during extreme sea level events. It also leads to saltwater intrusion into aquifers, deltas and estuaries. These changes impact on coastal ecosystems, water resources, and human settlements and activities. Regions at most risk include heavily populated deltaic regions, small islands (especially coral atolls), and sandy coasts backed by major coastal developments."

View projections for sea level rise on the CSIRO site. There is no doubt that sea level has been rising steadily but there is still debate about whether the rate of rise has increased recently and about whether it may be expected to increase further. Some scientists are also concerned about the possibility of rapid and catastrophic increases in sea level: we look at this a little in the activity. For the purposes here, it is sufficient to acknowledge that sea level is rising and the rate may be increasing.

Mangrove forests

Mangrove forests grow on sheltered shorelines in temperate and tropical regions around the world. They are most diverse on coastlines around south-east Asia.

  • Read some general information about mangroves in Wikipedia.
    Note: As the article itself says, it needs some work!

Sea level rise and mangrove forests

Mangrove forests, like all intertidal habitats, may be threatened by rising sea levels. Mangroves inhabit particular parts of the intertidal region and, if that region shifts, they must move or perish.

  • First, read the abstract of  Gilman, E., J. Ellison, et al. (2007). "Assessment of Mangrove Response to Projected Relative Sea-Level Rise And Recent Historical Reconstruction of Shoreline Position." Environmental Monitoring and Assessment 124: 105-130. They present four "scenarios" representing possible responses of mangrove forests to sea level rise, depending upon the conditions prevailing at the site.Read the abstract: We predict the decadal change in position of three American Samoa mangroves from analysis of a time series of remotely sensed imagery, a geographic information system, tide gauge data, and projections for change in sea-level relative to the mangrove surface. Accurate predictions of changes to coastal ecosystem boundaries, including in response to projected relative sea-level rise, enable advanced planning to minimize and offset anticipated losses and minimize social disruption and cost of reducing threats to coastal development and human safety. The observed mean landward migration of three mangroves' seaward margins over four decades was 25, 64, and 72mma−1, 12 to 37 times the observed relative sea-level rise rate. Two of the sites had clear trends in reductions in mangrove area, where there was a highly significant correlation between the change in position of the seaward mangrove margin and change in relative sea-level. Here it can be inferred that the force of sea-level rise relative to the mangrove surface is causing landward migration. Shoreline movement was variable at a third site and not significantly correlated with changing sea-level, where it is likely that forces other than change in relative sea-level are predominant. Currently, 16.5%, 23.4%, and 68.0% of the three mangroves' landward margins are obstructed by coastal development from natural landward migration. The three mangroves could experience as high as a 50.0% reduction in area by the year 2100. A 12% reduction in mangrove area by the year 2100 is possible in the Pacific islands region.
  • The paper by Woodroffe, C. D. (1995). Response of tide‐dominated mangrove shorelines in Northern Australia to anticipated sea‐level rise. Earth Surface Processes and Landforms, 20(1), 65-85 was one of the first attempts to look in detail at the potential responses of a coastal ecosystem – in this case, mangrove forests – to rising sea levels. Read the abstract: The exact response of mangrove shorelines to anticipated sea-level rise will depend upon the balance between sedimentation and sea-level change. Within the Top End of the Northern Territory of Australia there are extensive, relatively unmodified, tide-dominated mangrove forests, where tidal processes redistribute sediment. Harbours, such as Darwin Harbour, and tidal rivers, such as the South Alligator River with its associated coastal and estuarine plains, represent opposite extremes in terms of Holocene sedimentary infill, and will respond differently to sea-level rise. In Darwin Harbour, mangrove assemblages can be recognized in geomorphologically defined habitats. Similar topography within and between creeks implies morphodynamic equilibrium with tidal processes. Tidal reworking of sediment may maintain an equilibrial profile under gradually rising sea level, with resuspension of lower intertidal and subtidal muds and their redeposition within upper intertidal mangrove habitats. In contrast, the plains along the coast and tidal rivers draining into van Diemen Gulf developed during the post-glacial marine transgression, and since sea level stabilized, around 6000 years ago, coastal plains have prograded. These broad plains are presently not extensively influenced by salt water, but are often at elevations close to, or even below, modern high-tide levels. They may, therefore, revert to saline conditions particularly rapidly if the sea rises. The pattern of change may not be directly analogous to marine incursion experienced in the early Holocene, because broad plains have been able to prograde during the last 6000 years of relatively stable sea level
  • His conclusion is that for Darwin Harbour, the mangroves may move further up-shore but otherwise, not change greatly. This may very well not be the case elsewhere: he refers to the fact that, in some places, the sediment has become compacted, so that the surface of the land is actually below the level of high tide.
  • It is important to remember that sea level has been rising for about the last 6,000 years; rapidly at first but, over the last one hundred years, at a rate of about 1.8 mm year (see Wikipedia and references in the papers listed in the references section below). As a consequence, even without any potential additional effects of climate change, there were likely to be impacts. One of the major problems for us – meaning humans – is that many of us like to live near the coast but, all too often, have not considered what might happen if the coast moves, especially if it moves inland. For mangroves, and similar coastal ecosystems, one of the consequences of this is illustrated in Scenario D in Gilman et al. (2007): human developments, of one sort or another, obstruct, or hinder, the landward movement of coastal ecosystems, resulting in their loss.

In this module, we have focussed on two potential impacts arising from climate change: increasing disturbance and rising sea level. These are, of course, not the only things which may vary with climate change. There is, obviously, an increase in CO2 in the atmosphere – and this can affect plant growth – and their are potential changes in rates and patterns of precipitation. The last reading for this module, and the unit, Gilman et al. (2008) is a recent attempt to examine the consequences of a range of changes on mangrove ecosystems.
Read Gilman et al. (2008) Threats to mangroves from climate change and adaptation options: A review and review the information in Tables 1 and 2.
Note: In the conclusions, they state: "To date, relative sea-level rise has likely been a smaller threat to mangroves than non-climate related anthropogenic stressors, which have likely accounted for most of the global average annual rate of mangrove loss, estimated to be 1–2%, with losses during the last quarter century ranging between 35 and 86%." In other words, the average annual rate of mangrove loss, due to human activity, is about 1–2%! A little later, they write: "Based on this limited information, relative sea-level rise could be a substantial cause of future reductions in regional mangrove area, contributing about 10–20% of total estimated losses." So sea level rise could reduce the area occupied by mangrove forests but 80–90% will still be due to other things. (And they note that other climate change factors may not have much impact on mangroves.) Does this alter your view of the significance of sea level rise for mangroves?

It is, perhaps, appropriate to conclude the main part of this unit by looking at mangroves. They are one of the most important ecosystems for people living in tropical regions and they are also of importance in many temperate locations. And this significance is, perhaps, recognised by the fact that the background photograph for this site – the one that frames every page – is of a mangrove forest!

The mangrove forests of the Darwin Harbour region: features and response to change is presented in the following video lecture by Dr Keith McGuinness: https://www.youtube.com/watch?v=ebmC0YyfTtY&feature=emb_logo

Mangrove Walk 360°
This view is from a mangrove walk at the Casuarina Campus of CDU done 25 May, 2017. Update: The link goes to a special site where you can pan, tilt and zoom the scene to look around. Almost like being there (with no mud)!
https://theta360.com/s/bNWKBzz4uije9t6d4Zset7qBI

Activity 10

Activity 10 looks further at the issue of sea level rise and public policy.

References

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