Climate change has many complex layers, including the multiple factors that can affect weather patterns and the subsequent ecological impacts. Through this project, we work on making this information more accessible and easy to understand for the general audience. With information drawn from the 2020 Pacific Islands Regional Climate Assessment report, the video presented provides simple and basic ideas at the elementary level behind climate change and its impact on Palau. This web page complements the video by providing a more in depth look at patterns of key parameters of climate change: sea temperature and sea level in Palau. Here, we focus on how climate change specifically affects Palau using a 20-year data set to look at short term variation of sea temperature and sea level in relation to ENSO events and overall long term trends.
El Niño – Low sea water temperatures (due to shallow thermoclines) and low mean sea level (MSL). This is generally good for corals and for Jellyfish Lake. However, extreme low mean sea levels can fatally expose shallow water corals to air. Strong El Niño events produce extreme droughts in Palau.
La Niña – High sea water temperatures and high mean sea level (MSL). This can cause coral bleaching and mortality throughout the water column in an extreme event, and was the cause of the massive coral bleaching in Palau in 1998. Mean sea level is also high, but this does not adversely affect reefs through aerial exposure. Strong La Niña events can produce extreme amounts of rain, causing environmental damage.
Images reproduced by permission of Bureau of Meteorology, © 2017 Commonwealth of Australia
In Palau, El Niño and La Niña variation occurs regularly and has effects on most physical parameters, including sea water temperature, mean sea level (MSL), amount of rain etc. This variation is integral to the marine environment and underlies much of the diversity of conditions which support the marine habitats of Palau. Let’s take a look at some of these specific parameters.
The changes in mean sea level over months to years are often confusing as multiple things can occur at once, some working together, some working in opposition to each other. As the figure above shows, the day to day MSL varies up and down by about 300 mm over periods of weeks to months. An observer may see a dramatic rise or decrease in sea level (other than the daily tides) and think they are seeing the results of global sea level rise, when in fact it is simply variation caused by ENSO, or El Niño/La Niña variation. Scientists can be fooled also if they limit their perspective to a small time period without looking at the big picture.
An extreme example of this variation is the time period at the end of 2015 and early 2016, indicated above by the red arrow. This period represents the shift between a strong El Niño and La Niña which was documented by Schönau et al. 2019.
This is also seen in the graphs below, mean sea level (upper) and sea water temperature (lower). MSL dropped slowly and gradually by 600 mm (60 cm) between mid-2013 and the end of February 2016 as El Niño developed, a decrease of about 0.66 mm per day over 960 days. That’s a very small number, less than 1 mm per day. What happened next was almost oceanographically unbelievable! The ENSO state rapidly changed from a strong El Niño to a La Niña. In early March 2016, MSL started rising in Palau, rocketing up the 600 mm in only ~ 70 days, reaching a rate of nearly 8 mm PER DAY for much of the time (upper graph). With mean sea level being interrelated with temperature, the water temperature climbed from some of the lowest shallow water temperatures ever seen in Palau to ~ 30ºC (lower graph) over this very short time period. This 30ºC temperature on Palau’s reefs is associated with coral bleaching. Fortunately the La Niña was short lived, and the ENSO state returned to neutral quickly, resulting in more typical temperatures and mean sea level conditions.
Long Term Sea Level
Globally sea levels are rising at about 3 mm per year, the result of melting of ice on land and thermal expansion of the water column with rising temperatures. If the last 60 years are considered, based on Palau’s tide gauge data from Malakal, Palau fits right into this upward trend of 3 mm per year (see long term sea level graph above, red line).
Another way to look at mean sea level rise in Palau is to look at the trend of the highest high tides, and the lowest low tides- BOTH show an upward trend. The gold line shows an upward trend for the lowest sea level recorded in one year, and the green line shows an upward trend for the highest sea level recorded in one year, since 1971.
Prolonged low sea levels can affect reefs. In Palau from mid-2010 to mid-2013 corals grew up to their limits during the high MSL, and in early 2016 extremely low MSL caused prolonged exposure to air on low tides of these same corals. Many of these corals died, as documented by Colin and Schramek 2020 below.
Higher than usual sea levels can lead to coastal erosion and flooding of low lying areas, taro patches and buildings along the coast. Salt water inundation into taro patches and homes along the coast usually occurs during La Niña events. During neutral or El Niño events, with normal to lower than usual sea levels, these areas are not flooded. An understanding of the variation of mean sea level during ENSO events is particularly important for building homes and structures near the coast, especially if building during an El Niño phase. Imagine land-filling and building according to the low mean sea level during an El Niño phase, only to be inundated during a neutral or La Niña event when mean sea levels are higher.
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