Marine Atlas: Maximizing Benefits for Fiji

CLIMATE CHANGE THREATS

HOTTER AND HIGHER: MEAN SEA SURFACE TEMPERATURE AND PROJECTED SEA LEVEL RISE Sea surface temperature (SST) is a limiting factor for much marine life. Climate change is leading to higher sea temperatures, as well as sea levels, thus compromising Fiji’s marine biodiversity.

Moreover, air masses in the Earth’s atmosphere are highly modified by SST. Warm SST is known to be a cause of tropical cyclones over the Earth’s oceans, with a threshold temperature of 26.5°C being a trigger mechanism (see also chapter “Stormy times”). At the same time, tropical cy- clones can also cause a cool wake, due to turbu- lent mixing of the upper 30 metres of the ocean. SST changes diurnally, like the air above it, but to a lesser degree due to its higher specific heat. There is less SST variation on windy days than on calm days. In addition, ocean currents can affect SST on multi-decadal timescales. Coastal SST can cause offshore winds to generate upwelling, which can significantly cool or warm nearby land masses, and additionally shallower waters over a continental shelf are often warmer. Onshore winds can cause a considerable warm-up even in areas where upwelling is fairly constant. The annual mean SST in Fiji’s waters ranges from just under 24°C in the south to nearly 30°C in the north, as the map shows. Across the year there is relatively little variation in the SST, with up to 2.5°C in the south and less than 1°C in the north. The northern parts of Fiji’s waters are strongly influ- enced by the South Equatorial Current (see also chapter “Go with the flow”), which brings warm water from the eastern tropical Pacific Ocean. This Blame it on the weatherman? In early 2016, water in the shallow reefs along Fiji’s Coral Coast rose to an uncom- fortable 36°C. Was it just a few hot sunny days or global warming that warmed the water way above its average temperature? To understand this, we need to look at two different things. On one hand cli- mate variability, which refers to shorter term (daily, seasonal, annual, inter-an- nual, several years) variations in climate, including the fluctuations associated with El Niño (dry) or La Niña (wet) events (see also chapter “Go with the flow”). On the other hand climate change, which refers to long-term (decades or longer) trends in climate averages such as the global warming that has been observed over the past century, and long-term changes in variability (e.g. in the frequency, severi- ty and duration of extreme events) (see also chapter “Stormy times”). There may always be particularly rainy weather in Suva, or a particularly hot week on the Coral Coast. Only by observing trends in the long term can we show how the climate is changing.

The following chapters explain how observed and predicted climate change will affect Fiji’s marine values, starting with SST, which is the water tem- perature close to the ocean’s surface. The very hot waters in 2016 at the Coral Coast and elsewhere in Fiji were not only uncomfortable for visitors, but for the ocean’s inhabitants too. Warm water holds less dissolved oxygen than cooler water and once the level of dissolved oxygen drops below a critical threshold, fish and invertebrates suffocate. This is especially bad in shallow-water habitats, which can rapidly heat up and lose dissolved oxygen, resulting in thousands of dead fish at the Coral Coast in 2016.

23°C and 29°C, hence they are confined to tropical regions of the globe. When the water temperature falls outside this range, they can become stressed and expel their symbiotic algae (see also chapter “Home, sweet home”) in a process known as bleaching. Coral bleaching is an increasing threat to coral reefs in tropi- cal regions and can have a negative impact on ecosystems, fisheries and tourism. An increase in SST of only 1°C for four weeks can trigger a bleaching event. When increased temperatures last for longer periods (eight weeks or more), corals begin to die. This shows how SST is an important factor in the distribution of ocean life, with many species confined to specific temperature ranges.

Corals also find hot water uncomfortable. Shallow-water corals grow optimally between

MEAN SEA SURFACE TEMPERATURE (°C)

30°C

22°C

Fiji Provisional EEZ Boundary Archipelagic Baseline

100 50

200 km

Sources : Becker et al, 2009; Claus et al, 2016; NASA Goddard Space Flight Center, 2014; Smith and Sandwell, 1997. Copyright © MACBIO Map produced by GRID-Arendal

MAXIMIZING BENEFITS FOR FIJI

CLIMATE CHANGE THREATS

60