Where sea-level rise isn’t what it seems

Whilst working for the Maldives government I was always aware of the need to resist the temptation of making sweeping statements about the impacts of climate change and sea-level rise in the service of wider political ends. I saw part of my role as advisor to push back against the simplistic view that given that we know that the planet is warming, and the seas are rising, surely the impacts  – in terms of erosion, flooding events and disasters – should increasingly be visible now, right?

A new paper published in the AGU’s house journal Eos Transactions shows why caution is often justified. Here (via a screengrab, as the entire thing is behind a password) is the 1993-2011 sea level trend data from Tarawa atoll, part of Kiribati in the central Pacific:

Whoa! No sea-level rise there, then. And yet of course climate campaigners – and even the Kiribati government – understandably anxious to highlight the future existential threat to the islands, have used storm surges, flooding events and suchlike as evidence of current sea-level rise impacts. Which they are almost certainly not, at least not in Tarawa atoll anyway.

To me the graph is interesting for two reasons. The first is the absence of any trend over the last 20 years towards increased sea levels in that part of the Pacific. This should be expected, because sea level rise as a computed average means that the oceans are rising in more places than they are falling, but they are falling in some places nonetheless. (Just as a few areas of the globe have got colder over recent years.) The second is the sheer up-and-down massive variability in actual sea levels, which is linked to the El Niño cycle. The author (Simon Donner, a geographer from the University of British Columbia, Canada) points out in the Eos paper that the monthly mean sea level dropped by nearly half a metre (45cm) between March 1997 and February 1998 because of switch from El Niño to La Niña conditions, and peaks of 15cm were seen in each of the recent El Niño events – which as the author points out is “equivalent to 50 years of global sea level rise at the rate observed since 2000 of 3 mm per year”.

So the problem with attributing sea-level rise impacts is the same as with attributing heat-waves, droughts, floods or other extreme events to climate change – you have to try to figure out what would have happened absent the global warming trend (in order to distinguish genuine impacts from noise), and also distinguish background changes from more direct anthropogenic interference which might confuse the picture. In a heatwave, for instance, were the extreme temperatures caused by the urban heat island effect in a more built-up area?

In Tarawa atoll, direct human interference probably explains the majority of what is often pointed to as evidence of sea-level rise impact, according to Donner. Because few readers will be able to access the paper online, I will take a leaf out of Judith Curry’s book and quote extensively from it here:

The combination of natural weather – and climate – driven variability in sea level and the astronomical tidal cycle can lead to flooding and erosion events, particularly in sand-dominated systems like atolls and barrier islands. For example, the 2004–2005 ENSO event contributed to two major flooding events in Tarawa. During a ‘king’ tide on 10 February 2005, water flooded several causeways between the islets in South Tarawa and damaged the hospital in the town of Betio. A second flooding event occurred 2 weeks later, despite the lower daily tidal range, because of record high winds (47 knots at Betio) and record-low surface level pressure (999.2 hectopascals). Even though the maximum gauge height was 25 centimeters below that reached 2 weeks earlier, the northwest winds generated lagoon waves that again breached sea walls, flooded causeways, and damaged homes and public infrastructure.

These flooding events, though statistically more likely to happen as global average sea level rises, are themselves no more evidence of rising sea level than an individual heat wave is evidence of rising global temperatures. Despite a continued global average sea level rise, the gauge height reached on 10 February 2005 in Tarawa has not been surpassed since.

The paper then goes on to discuss some direct human impacts which can impact shoreline dynamics with or without a sea-level trend:

Three types of shoreline modification that are typical in low-lying island nations have altered sediment supply and island shape in South Tarawa [Webb, 2005]. First, land reclamation, accomplished by infilling behind a constructed sea wall, has increased land area in some locations but exacerbated erosion and inundation in others. The shoreline of islets like Bairiki has been extended lagoonward through the construction of government facilities, landfills, maneabas (community meeting houses), and individual homes [Webb and Kench, 2010]. At the same time, poor engineering of sea walls has led to erosion at the airport and the hospital [Webb, 2005] on the islet of Bikenibeu and also led to inundation of reclaimed lands along the lagoon shoreline in Abarao and other islets.

Second, the practice of mining of beaches and barrier reefs for construction materials, common in Kiribati, Tuvalu, and other atoll nations, can make the shoreline more vulnerable to tidal extremes and storms [Webb, 2005]. Almost three quarters of the households in South Tarawa mine sand, gravel, and reef rock from the lagoon or the ocean reef, with one third doing so more than once a week [Greer Consulting Services, 2007]. Although the effect of beach mining on the shoreline is difficult to distinguish from that of other coastal processes, concern is sufficient to warrant European Union investment in a midlagoon dredging project to provide an alternative source of fill.

Last, the construction of causeways between islets has altered islet evolution. Unlike a bridge, a solid, hard-topped causeway limits or blocks the natural flow of sediment between the ocean and the lagoon. Causeway construction allowed nearshore currents to deposit sediment along the lagoon beaches of South Tarawa islets like Bairiki and Nanikai [Solomon and Forbes, 1999]. These densely populated islets have actually grown in area over the past few decades [Webb and Kench, 2010] because of the unintentional impacts of local development on sediment supply, land reclamation, and natural processes. This accretion, however, came at the cost of other islets. The lagoon islet of Bikeman, which was dotted with coconut trees during the Battle of Tarawa, is now a sandbar that disappears from view at high tide. Despite some claims to the contrary by climate activists, the loss of this once popular resting spot for fishermen is primarily due to the construction of the Betio-Bairiki causeway, which redirects sediment flow.

This leads to important communications issues, of course.  So whilst I have no truck with sceptics like Nils-Axel Mörner, who use highly-questionable anecdotes in one small area to ‘prove’ that the entire global rise in sea levels isn’t happening, I do feel that any degree of exaggeration simply leaves an open goal for sceptics to belittle the real challenges these island countries face. It is therefore counter-productive as well as dishonest (intentionally or unintentionally). Ergo:

The failure to consider the contribution of natural variability and direct human modifications can lead to misattribution of flooding events or shoreline changes to sea level rise. Tarawa, the most easily accessible atoll in Kiribati, is a popular destination for journalists and activists interested in observing and communicating the impacts of sea level rise on a low-lying nation. For example, a Greenpeace slide show within an explanation of what sea level rise means that depicts the 2005 flooding remains among the top responses to an Internet query of “Kiribati” and “sea level rise.” These common images of flooded homes and waves crashing across the causeways—collected during an anomalous event on islets susceptible to flooding due in part to local modifications to the environment—can provide the false impression that Tarawa is subject to constant flooding because of sea level rise.

The attribution problem is further magnified by the political situation. The Kiribati government faces the difficult challenge of raising international awareness about the local impacts of climate change to support adaptation and mitigation efforts. Interpreting the causes of shoreline changes or flood events, as well as predicting the local impacts of sea level rise, is challenging for a developing country with limited resources for scientific investigations. Many individual observations of erosion, flooding, or groundwater salinization, recorded in community consultations for internationally funded climate change adaptation programs, are thus attributed to climate change without scientific analysis [e.g., Mackenzie, 2004]. These events are presented as examples of climate change impacts in promotional materials and at international events (e.g., “Our Road to Copenhagen,” a Kiribati side event at COP15 in Copenhagen), without any mention of ENSO-driven natural variability or local shoreline modification.

Such unverified attribution can inflame or invite skepticism of the scientific evidence for a human-caused increase in the global sea level. After Webb and Kench [2010] reported that the area of 23 atoll islets in Kiribati and neighboring countries had remained stable or increased over the past 20–60 years, some of the international news media reported that the effects of sea level rise on atoll nations were exaggerated and that Kiribati is not threatened by future sea level rise (e.g., R. Callick, Coral islands left high and dry, The Australian, 2010). Though the study did show evidence that atoll islets were dynamic and do not necessarily decrease in area in response to sea level rise, the islets in question remain vulnerable to inundation from global mean sea level rise in the future, as the authors stressed in a subsequent briefing note.

The challenge of differentiating between observed changes in the coastal environmental and the projected impact of sea level rise is not unique to Kiribati. For example, the Carteret Islanders of Papua New Guinea have been migrating from their home atoll for decades because overpopulation, human development, and natural disasters, in addition to sea level rise, have caused coastal erosion and reduced water availability [Connell, 1990]. Nevertheless, the Carteret Islanders are commonly called the world’s first climate change “refugees” in outreach and documentary films (e.g., The Rising Tide).

The upshot?

Instead of incorrectly attributing individual flood events or shoreline changes to global sea level rise, scientists and climate communicators can use such occurrences to educate the public about the various natural and human processes that affect sea level, the shoreline, and the shape of islands. This would better prepare the public and policy makers for the changes that societies are likely to experience as global sea level rises in the coming decades.

I couldn’t agree more. If island nations are making themselves more vulnerable to the slow process of sea-level rise by mining sand, destroying reefs and so on, they need to know about it – and not be encouraged to blame it all on outsiders causing climate change. In the case of the impacts currently affecting Kiribati, ‘mitigation’ means changing local practices as much as changing global ones. And therein lies a lesson for us all.

© Mark Lynas
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