July 2017

From 1991 to 2010, the global climate risk index ranked Tonga 19th of 179 countries in terms of observed average annual losses as a percentage of gross domestic product due to climate-related disasters and in terms of average of climate-related deaths per 100,000 people.

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In recent years Tonga experienced higher variability of rainfall causing localised flooding and droughts related to El Niño events. Increased ocean temperatures have caused coral bleaching and destruction of habitats for reef species. Sea level rise is contributing to coastal erosion and subsequent damage to infrastructure and property. Coral bleaching destroys natural coastal barriers and together with the increase in sea level rise puts coastal community livelihoods and infrastructure at risk, for example for tropical cyclones and storm surges. 

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As part of the substantive investment required to help adapt to and manage these effects, the ADB Climate Resilience Sector Project is a five-year activity linked to enhancing climate resilience in Tonga. It is also part of the Pilot Program for Climate Resilience funded by the Climate Investment Fund. 

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The EO support project delivered regional statistics on marine parameters and specific coral habitat stressor maps, in an effort to improve the acknowledgement of Tongan climate in general, and to demonstrate the capabilities of satellite-based observations to complement existing data. 

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Coastal habitat (coral reef) oceanographic stressor mapping 

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This service provided a number of parameters relating to water and weather in the area surrounding Tongatapu and ʻEua islands, starting from 1990 to today. For each parameter, maps and time series were presented, including statistics based on monthly, seasonal and annual aggregate measurements (extreme values, mean values, P90 values, etc.) and temporal trends using linear regression. Additionally, anomalies and exceptional events were analysed. Where available, seasonal aggregation was calculated with two seasons: from April to November, and from December to March. The studied parameters were:

• Sea surface height: episodic but progressive changes will reduce or eliminate endemic terrestrial species. Coral reef ecosystems are likely to be affected by the mid-century as the upward growth rates of corals are expected to slow in response to rising sea levels.

• Wave height and direction: reefs dissipate wave energy, reducing routine erosion and lessening inundation and wave damage during storms.

• Currents: areas with strong currents and high mixing rates indicate a potential of cooling to counter increased sea surface temperature.

• Sea surface temperature: fluctuations of this important parameter are closely linked to, among others, coral proneness to stress during a heat wave.

• Sediments concentration (turbidity): high sediment concentrations could signify poor water quality and pollution which reduces coral resistance.

• Sea surface salinity: fluctuation or especially a steep drop in salinity has already caused significant coral extinction.

• Chlorophyll-a: just like in the case of sediments, high concentrations could signify poor water quality and pollution which reduces coral resistance.  

• Photosynthetically Active Radiation (PAR): high solar irradiance at the water surface indicates a potential to heating and photochemical damage.  

• Wind: high wind velocity enhances the water mixing and affects the air-water interface and thus solar radiation reach. 

• UV radiation (ozone): high solar UV irradiance indicates a potential to heating and photochemical damage. decreased photosynthetic performance and altered community structure. 

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Ocean climate parameters listed in the previous section form the environment in which coral reefs have existed for millions of years, within specific limits of temperature, light, wave energy, etc. These key variables are also impacted by other atmospheric and ocean processes – for example wind velocity enhances the mixing of ocean water, with an overall effect of cooling. Similarly, strong currents could also have a cooling effect and at the same time affect the mechanical structure of the coral colonies. Chlorophyll and total suspended matter can be observed from satellites via changes in the optical properties of water. These variables can also be used as proxies of pollution from sediments and nutrients, which in turn can also be beneficial to corals as they create a ‘shading effect’, protecting corals from direct sunlight. Given that corals depend on light for photosynthesis, too much shading can prevent primary production. 

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Considering the above, the marine environment parameters were combined to estimate ‘good’ and ‘bad’ conditions as a gradient from 0 to 1, where 1 indicates high stress, and zero indicates relatively low or no stress to corals. Processing involved aggregating each of the parameters in time (2003–2011), establishing the effect of individual parameters on coral reef biological response, and finally, scaling the parameters to the established response behaviour. 

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EO-derived information provides a unique capability to cover the extended time periods and spatial extent required to ensure valid and comprehensive characterisation of climate change-driven trends and pressures on the Tongan coasts. Based on EO measurements, a wide range of climate-driven processes and their evolution over a 10-year were be generated in a consistent and comparable manner, enabling their combination to ensure a complete characterisation of climate-driven stress. These include the evolution of basic meteorological and oceanographic parameters (e.g. wind speed, ultraviolet radiation levels, sea surface temperature, ocean transparency/sediment load and surface wave height) as well as more complex information such as stress characterisation on the coral reefs surrounding Tonga. With the availability of data from satellites such as Sentinel-1, 2 and 3, continuous monitoring is ensured and small island states such as Tonga can access reliable and complete information on the changing environmental conditions in the surrounding area as well as the resulting changes in critical coastal habitats such as mangroves. As Tonga is typical of many Pacific Island states, we expect the capabilities demonstrated here to also be applicable to the other islands in the region.