Marine Atlas: Maximizing Benefits for Tonga

and information on sea level rise are necessary in order to plan effectively for these changes.

Sea level rise, as a consequence of global warm- ing, threatens many low-lying regions of the world. The Fifth International Panel on Climate Change assessment projects a global rise in mean sea lev- el for 2081–2100 relative to 1986–2005 of between 0.2 and 0.98 metres, depending on different emis- sions scenarios. Furthermore, the western tropi- cal Pacific Island region is considered one of the most vulnerable regions under future sea level rise (Nicholls and Cazenave, 2010). Sea level rise is not uniform across the western Pacific and is affected by ENSO events. These have a strong modulating effect on inter-annual sea level variability, with low- er than average sea level during El Niño and higher than average during La Niña events (of ±20–30 cm). In addition, there is also an observed low-fre- quency (multi-decadal) variability, which in some areas adds to the current global mean sea level rise due to ocean warming and ice melting (Becker et al., 2012). Tonga is a mix of high volcanic islands and low- lying coral atolls. Vulnerability to sea level rise is influenced by coastal geography and prevailing ocean currents. Islands exposed to higher wave energy in addition to sea level rise can experience higher rates of erosion than their more sheltered counterparts. However, the coral atolls of Tonga may be able to adjust their size, shape and po- sition in response to sea level rise, as has been suggested for other reef islands such as Funafuti Atoll in Tuvalu (Kench et al., 2015). Vertical reef accretion that occurs in response to sea level rise may be able to prevent the significant increases in shoreline wave energy and wave-driven flooding that are predicted in the absence of reef growth (Beetham et al., 2017).

Satellite data indicate the sea level has risen near Tonga by about 6 millimetres per year since 1993, compared with the global average of 2.8–3.6 milli- metres per year (Australian Bureau of Meteorology and CSIRO, 2011). The impacts of rising sea level can be compounded by other factors, such as natural disasters. Tonga was identified as the sec- ond most-at-risk country for natural disaster, as measured by the World Risk Index (Garschagen et al., 2016). Sea level rise coupled with storm surge during a cyclone can lead to significant inundation of lowlands, damaging coastal infrastructure and property and affecting livelihoods. There is there- fore a planning need for coastal infrastructure to take this risk into consideration. The map indicates that by 2030, Tonga will experi- ence a sea level rise of between 0.14 and 0.16 me- tres. The southernmost parts of the EEZ will see the largest increase, with predicted sea level rise of 0.15 metres around the main island of Tongatapu.

This is likely to be accompanied by increases in episodes of flooding and wave inundation in some coastal areas, especially in the southernmost is- lands. Pacific Island nations are therefore focused on developing adaptation strategies to address the predicted continued rise in sea level. In the past, atolls and islands, which often rise a mere metre above the waves, were only flooded by the ocean every couple of decades. That trend has since changed, with an increased frequency in these flooding events. When these events become too frequent, it makes it difficult for islands to recover. The land becomes too salty, the freshwa- ter reserves in the lagoons become undrinkable and the islands themselves can no longer support human habitation. It is becoming clear that in a warming world, Tonga’s sea will become hotter and higher, with drastic con- sequences for coastal habitats and their inhabitants.

PROJECTED SEA LEVEL RISE RCP 4.5 2030 0.16 meters

NORTH PAC I F I C OCEAN

0.11 meters

Exclusive Economic Zones (EEZ)

Copyright © MACBIO Map produced by GRID-Arendal Sources : Becker et al, 2009; Claus et al, 2016; CSIRO Australia, 2014; Smith and Sandwell, 1997.

SOUTH PAC I F I C OCEAN

MAXIMIZING BENEFITS FOR TONGA

CLIMATE CHANGE THREATS

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