Marine Atlas: Maximizing Benefits for Kiribati

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 of Kiribati’s marine life. Climate change is leading to higher sea temperatures, as well as sea levels, compromis- ing Kiribati’s marine biodiversity.

The following chapters explain how ob- served and predicted climate change will affect Kiribati’s marine values, starting with SST, which is the water temperature close to the ocean’s surface. Warm water holds less dissolved oxygen than cooler water and once the level of dissolved oxygen drops below a critical threshold, fish and inverte- brates suffocate. This is especially bad in

shallow-water habitats, which can rapidly heat up and lose dissolved oxygen, resulting in thousands of dead fish. Corals also find hot water uncomfortable. Shallow-water corals grow optimally between 23°C and 29°C, hence they are confined to tropical regions of the globe. When the water temperature falls outside this range,

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 tempera- ture ranges. Moreover, air masses in the Earth’s atmos- phere 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 cyclones can also cause a cool wake, due to turbulent 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 breezy 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 Kiribati’s waters ranges from 25°C to 29°C. The warmest water temperatures occur in the Gilbert group and the southern parts of the Phoe- nix group, and to a lesser extent, the Line group. The central part of the Line group has the coolest temperatures. Across the year there is variation in the SST, with the largest seasonal variation of ±3.5°C in the central part of the Line group and the smallest seasonal variation of ±1.5°C in the Gilbert and Phoenix groups. The south- ern parts of Kiribati’s waters are strongly

they can become stressed and expel their symbiotic algae (see also chapter “Home, sweet home”) in a process known as bleach- ing. Coral bleaching is an increasing threat to coral reefs in tropical 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

Blame it on the weatherman?

Water in Kiribati can get poisonously hot. Tiny dinoflagellates, such as Gambierdiscus toxicus, like it warm and produce a certain poison that adheres to algae, which is eaten by reef fish. These fish are, in turn, eat- en by humans. In warm periods, this so-called Ciguatera poisoning can become a serious problem for I-Kir- ibati, who rely on reef fish as their main source of food and nutrition. But is it just a few hot, sunny days or global warming that is warming the water to a poisonous tempera- ture? To understand this, we need to look at two different things. On one hand climate variability, which re- fers to shorter term (daily, seasonal, annual, inter-annual, 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 cli- mate 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, severity and dura- tion of extreme events) (see also chapter “Stormy times”).

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

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CLIMATE CHANGE THREATS MAXIMIZING BENEFITS FOR KIRIBATI

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