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How corals can help us make predictions about our future under climate change (cross-posted from ReefBites)

This blog post originally appeared on ReefBites, the student blog of the International Society for Reef Studies.

Every two to seven years, the eastern equatorial Pacific climate oscillates between anomalously warm (El Niño) and cold (La Niña) conditions in a process known as the El Niño Southern Oscillation (ENSO). This process influences sea surface temperatures (SSTs), trade winds, and global teleconnection patterns, which together influence weather conditions all over the world (Collins 2010). Some scientists suggest that extreme El Niño events will happen more often with the warming climate (Federov and Philander 2000; Tudhope 2001; Cai 2014; Liu 2017), which would have profound impacts on communities around the world (for example, by altering patterns of global food production). Other scientists are undecided, pointing to the diversity of historical ENSO patterns, which confounds data that could suggest climate change is causing an impact (Collins 2010; Vecchi and Wittenburg 2010; Emile-Geay 2013, 2016). Fortunately, coral reefs hold a treasure trove of paleoclimate data that could be used to solve the mystery of past ENSO diversity, which would allow scientists to make more accurate predictions about how we can expect climate (and therefore weather) to change in the future.

It isn’t, however, an easy puzzle to solve. Scientists around the world have devoted huge amounts of resources to understanding how ENSO patterns will change as the climate continues to warm, but this has proven difficult because ENSO has historically exhibited differences in amplitude, temporal evolution, and spatial patterns (Capotondi 2015). Disagreements about what differences are caused by climate change and what is natural variation caused by radiative or orbital forcing have led to disagreement about future ENSO patterns. One thing that scientists do agree on, other than the absolute certainty that human-caused climate change is happening, is that in order to understand exactly what variations in ENSO are being influenced by a warming climate, scientists must first identify the background diversity of ENSO patterns, which requires going back potentially thousands of years (Collins 2010; Vecchi and Wittenburg 2010; Cobb 2013). This lack of information has limited the predicting power of climate models, leading to conflicting results.

So how can scientists get to the bottom of this? Instrumental records are limited in their usefulness because they tend to be short and sparse, particularly in remote regions of the Pacific where changes in SST are most pronounced (Emile-Geay 2013). Some proxy records, which are preserved physical characteristics of the environment that can stand in for direct measurements like ice cores and sediment records from lakes (NCDC NOAA, N.D.) may also be limited because they lack the temporal resolution needed to resolve ENSO patterns, which may vary seasonally (Cobb 2013). Luckily for us, coral reefs have been recording changes in the climate for hundreds of years at high resolutions. Similar to tree trunks, as they grow, corals record rings in their skeletons that reveal their age (Figure 1), and because corals are so sensitive to environmental fluctations, the chemistry in each ring can tell scientists about the temperature, rainfall, and water clarity from that year. By drilling into old corals and extracting a long sample (called a core), scientists can reconstruct monthly climate data over several hundred years. Corals therefore provide a hugely valuable source of data that could help us finally unravel the complicated history of ENSO, which in turn would help us accurately predict changes in our future climate.

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Figure 1: Each of the light/dark bands in this x-ray of a cross-section of a coral core formed during a year of growth (NASA Earth Observatory 2005).

Stable isotopes, which are elements with the same number of protons but different numbers of neutrons, are a power tool to understanding past climate. The environmental conditions at the time a coral grows its skeleton can influence the number of neutrons an element has. For example, a number of scientists have used stable oxygen isotopes (δ18O and δ16O) to reconstruct the history of sea surface salinity (Figure 2) (e.g. Nurhati 2009). Other scientists have used ratios elements, such as Stronium to Calcium (Sr/Ca) to reconstruct temperature (e.g. Thompson and van Woesik 2009). A clearer picture of climate variability has begun to emerge through the use of these climate proxies from coral cores. We know, for example, that there are two different types of El Niño events, one in which warm water is centered over the central Pacific (known as “CP El Niño”) and one where warm water is over the eastern equatorial Pacific (“EP El Niño”), and that CP El Niño, which is projected to increase with global warming, has happened more frequently in the 21st century than EP El Niño (Wang 2016). But data from across the Pacific are limited, and many of the studies identifying ENSO patterns use proxies from just a few coral cores, highlighting the need for more studies.

 

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Figure 2: Water vapor gradually loses 18O as it travels from the equator to the poles. Because water with heavy 18O isotopes in them condense more easily than normal water molecules, air becomes progressively depleted in 18O as it travels to high latitudes and becomes colder and drier. In turn, the snow that forms most glacial ice is also depleted in 18O. As glacial ice melts, it returns 16O-rich fresh water to the ocean. Therefore, oxygen isotopes preserved in ocean sediments [and coral cores] provide evidence for past ice ages and records of salinity (Riebeek 2005).

Another challenge is deciphering the cores themselves. Recent studies have called into question temperature data derived from coral cores using the common Sr/Ca proxy, because biological processes known as “vital effects” can influence and even override Sr/Ca relationships to temperature in corals during the biomineralization process (Alpert 2016, DeCarlo 2016). As a result, DeCarlo (2016) suggested a new proxy record that can be used to record past SST by combining Sr/Ca and the ratio of Uranium to Calcium (U/Ca) to create a new proxy, which they dubbed “the Sr-U thermometer.”

The need to address climate change only gets more urgent as time passes, which emphasizes how important this research is. Scientists cannot accurately predict the ways that climate change will influence humanity without understanding ENSO diversity. Coral have recorded climate variability in their skeletons for hundreds of years and are therefore a source of high-resolution, long-term data that could prove invaluable if we can only figure out the best way to decipher it. If scientists can understand ENSO’s patterns in the past, we can account for those patterns in climate models, and therefore predict how future ENSO will be influenced by climate change. This would allow us to make clear, accurate predictions about climate change in general, such as how rainfall patterns would impact food production, which could prove critical to the future of humanity.

References:

 Alpert AE, Cohen AL, Oppo DW, DeCarlo TM, Gove JM, Young CW (2016) Comparison of equatorial Pacific sea surface temperature variability and trends with Sr/Ca records from multiple corals. Paleoceanography 31:252-265 (doi: 10.1002/2015PA002897)

Cai W, Borlace S, Lengaigne M, van Rensch P, Collins M, Vecchi G, Timmermann A, Santosa A, McPhaden MJ, Wu L, England MH, Wang G, Guilyardi E, Jin FF (2014) Increasing frequency of extreme El Niño events due to greenhouse warming. Nature Climate Change 4:111-116 (doi: 10.1038/nclimate2100)

Capotondi A, Wittenberg AT, Newman M, Di Lorenzo E, Yu JY, Bracconot P, Cole J, Dewitte B, Giese B, Guilyardi E, Jin FF, Karnauskas K, Kirtman B, Lee T, Schneider N, Xue Y, Yeh SW (2015) Understanding ENSO Diversity. American Meteorological Society 921-938 (doi: 10.1175/BAMS-D-13-00117.1)

Cobb KM, Westphal N, Sayani HR, Watson JT, Di Lorenzo E, CHeng H, Edwards RL, Charles CD (2013) Highly Variable El Niño-Southern Oscillation Throughout the Holocene. Science 339:67-70. (doi: 10.1126/science.1228246)

Collins M, An SI, Cai W, Ganachaud A, Guilyardi E, Jin FF, Jochum M, Lengaigne M, Power S, Timmermann A, Vecchi G, Wittenberg A (2010) The impact of global warming on the tropical Pacific Ocean and El Niño. Nature Geoscience 3:391-397. (doi: 10.1038/ngeo868)

DeCarlo TM, Gaetani GA, Cohen AL, Foster GL, Alpert AE, Stewart JA (2016) Coral Sr-U thermometry. Paleoceanography 3:626-638. (doi: 10.1002/2015PA002908)

Emile-Geay J, Cobb KM, Mann ME, Wittenberg AT (2013) Estimating Central Equatorial Pacific SST Variability over the Past Millennium. Part II: Reconstructions and Implications. Journal of Climate 26:2329-2352. (doi: 10.1175/JCLI-D-11-00511.1)

Emile-Geay J, Cobb KM, Carre M, Braconnot P, Leloup J, Zhou Y, Harrison SP, Correge T, McGregor HV, Collins M, Driscoll R, Elliot M, Schneider B, Tudhope A (2016) Links between tropical Pacific seasonal, interannual and orbital variability during the Holocene. Nature Geoscience 9:168-175. (doi: 10.1038/NGEO2608)

Federov AV, Philander SG (2001) A Stability Analysis of Tropical Ocean-Atmosphere Interactions: Bridging Measurements and Theory for El Niño. Journal of Climate 14:3086-3101. (doi: 10.1175/1520-0442(2001)014<3086:ASAOTO>2.0.CO;2)

Liu Y, Cobb KM, Song H, Li Q, Li CY, Nakatuska T, Zhisheng A, Zhou W, Cai Q, Li J, Leavitt SW, Sun C, Mei R, Shen CC, Chan MH, Sun J, Yan L, Lei Y, Ma Y, Li X, Chen D, Linderholm HW (2017) Recent enhancement of central Pacific El Niño variability relative to last eight centuries. Nature Communications:15386. (doi: 10.1038/ncomms15386)

NASA Earth Observatory (2005) Climate Close-up: Coral Reefs. From https://earthobservatory.nasa.gov/Features/Paleoclimatology_CloseUp/paleoclimatology_closeup_2.php. Accessed 18 October 2018.

National Climatic Data Center, National Ocean and Atmospheric Administration (N.D.) What Are “Proxy” Data? From http://www.ncdc.noaa.gov/news/what-are-proxy-data, accessed 14 October 2018.

Nurhati IS, Cobb KM, Charles CD, Dunbar RD (2009) Late 20th century warming and freshening in the central tropical Pacific. Geophysical Research Letters 36:L21606. (doi: 10.1029/2009GL040270)

Reibeek H (2005) Paleoclimatology: the Oxygen Balance. NASA Earth Observatory: https://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance, accessed 18 October 2018.

Thompson DM, van Woesik R (2009) Corals escape bleaching in regions that recently and historically experienced frequent thermal stress. Proceedings of the Royal Society B 276:2893-2901 (doi: 10.1098/rspb.2009.0591)

Tudhope AW, Chilcott CP, McCulloch MT, Cook ER, Chappell J, Ellam RM, Lea DW, Lough JM, Shimmield GB (2001) Variability in the El Niño-Southern Oscillation Through a Glacial-Interglacial Cycle. Science Magazine 291:1511-1516. (doi: 10.1126/science.1057969)

Vecchi GA, Wittenberg AT (2010) El Niño and our future climate: where do we stand? WIREs Climate Change 1:260-270. (doi: 10.1002/wcc.33)

Wang C, Deser C, Yu JY, DiNezio P, Clement A (2016) El Niño-Southern Oscillation (ENSO): A review. In: Reefs of the Eastern Pacific, Glymn P, Manzello D, and Enochs I, Eds., Springer Science Publisher:85-106. (doi: 10.1007/978-94-017-7499-4_4)

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Getting to know Tarawa

It was exciting to finally step foot in Tarawa, the capital of the Republic of Kiribati, after hearing about it for so long — my advisor has worked here for a decade or so, and I’ve spent my last three years as his student hearing about his work and its accompanying adventures. I’ve also spoken with other folks who have worked and visited Tarawa, and I’m sorry to say that I’ve read Maartin J. Troost’s, The Sex Lives of Cannibals (I could write a whole blog post about why this is not a good book, but its exploitative and he is neither fair nor charitable with his descriptions of life here). All that goes to say that I had very clear images of what Kiribati would be like, both for better and for worse, before stepping off the plane on Thursday.

To be fair, you won’t be greeted at the Kiribati airport with a lei like in some of the more touristy destinations of the Pacific. The beaches have white sand and turquoise water, but are often littered with both trash and the relics left behind by the bloody battles that took place here during World War II. It’s stunningly beautiful, but it’s not exactly paradise the way it would be defined in romanticized Western narratives (although I don’t think such a place really exists).

Having context is paramount to understanding what the Pacific Islands are like. The same countries that tend to judge Kiribati harshly have caused the problems facing the people here (such as climate change and the poverty that was left in the wake of colonization). It is unfair for westerners to criticize Kiribati for the ways they’re using their limited resources to surviving challenges that were forced upon them through no fault of their own.

The people in Kiribati are nothing if not resilient. We had a lot of logistics to organize when we first arrived, which meant that we spent most of our first day driving up and down the one paved road connecting the islands in South Tarawa. It’s immediately evident, even from the plane, that Tarawa is battling against climate change; land is limited, sand bags and sea walls line the coastlines, and road construction projects to fortify the main road have been happening for years. We also saw the products of a mangrove planting project, which will help to protect the coastline from erosion caused by rising sea levels and waves. There is even a recycling program in Tarawa, no small feat for a small island that has to ship their trash elsewhere, and Erietera, who works for Kiribati’s fisheries program and is joining us on our dives, told us about an initiative in his village to ban all plastics. I’ve not heard of any programs like this elsewhere in the Pacific, and the Kiribati people are rightfully proud of their hard work.

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Young mangroves growing near the airport in Tarawa. These will help to protect the shoreline from erosion.

My advisor is fond of talking about how much people in Kiribati love to laugh, and I’ve found that to be true in the most delightful ways. As a woman, it can be challenging to do fieldwork in the Pacific Islands, some of which are very conservative (here, we need to keep our legs covered above the knees and have to wear shirts that cover our shoulders, even while on the boat and in the water). While it doesn’t erase or minimize the difficulties, being able to joke with the people we are working with (all of whom are men, although there have been women doing diving work for the fisheries department in the past) helps to break the ice.

 

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A sleeping hut on the ocean side of South Tarawa. The ocean breeze keeps it cool and also discourages mosquitos.

 

I’ve only been here for a few days and I’m still getting my bearings, but it’s been a lot of fun so far. We did our first day of diving yesterday, and it went relatively smoothly, despite a few minor mishaps (but it always takes a day or two to get the swing of things). I’ll post more about the science we’re doing soon, if you’re curious — there’s a lot more going on this trip than what we did in the Marshall Islands, and I’m pretty excited about what we’ll learn. In the meantime, I’m enjoying the opportunity to explore and get to know Tarawa and all the wonderful people we’re having the chance to meet!

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Counting down to fieldwork in the Gilbert Islands

In just over a month, I’ll be boarding a plane and heading to Tarawa, an atoll in the Gilbert Islands of Kiribati. I’ll be staying in Tarawa and the nearby Abaiang Atoll for about a month to conduct the first stage of my Ph.D. fieldwork. It’s been a long, dark, rainy winter in Vancouver and I’d be lying if I said I wasn’t looking forward to getting to spend some time in the tropics, although I expect that my lack of exposure to light means I will be especially susceptible to sunburn.

Tarawa is far away — about 8,000 kilometers — and it takes quite a bit of travel to get there. My advisor, a fellow graduate student, Heather, and I will all fly from Vancouver to Los Angeles to Nadi, Fiji and from there, finally, to Tarawa. While I’ve spent some time in other places in the Pacific (specifically Yap, FSM and the Marshall Islands), this will be my first time in the southern hemisphere. I guess I will finally have the opportunity to test whether the toilet flushes in the opposite direction on the other side of the equator!

The Gilbert Islands are just south of the Marshall Islands and north of Tuvalu. Map via http://www.geographicguide.com/oceania-maps.

I’ll be doing similar work to what I was doing in the Marshall Islands for my MSc (which I finally finished in August 2017!). Tarawa, like Majuro, is more developed and has a large population, while Abaiang is more similar to Arno and is less populated. We will use the data we collect to test the hypothesis that past sea surface temperatures may influence how likely corals are to bleach when temperatures are high. In the Gilbert Islands, the reefs experience a lot of temperature variability because of El Niño and La Niña events, so corals there may have had a chance to adapt to temperature fluctuations. Corals in other places with more stable sea surface temperatures, like the Marshalls, could be more likely to bleach when temperatures are warm because they haven’t been exposed before. I’m also going to be using a fancy contraption called a diving PAM (a pulse-amplitude modulated fluorometer that can be used underwater) to measure photosynthetic rates of corals, which is a way to get an idea of how healthy the corals are at different sites. (This is not the same one that we have, but it will give you an idea of what it does and what it looks like.) Meanwhile, Heather, a MSc student in my lab, will be doing a really cool project to map the complexity of the reefs.

Preparing for this trip looks very much like my prep two years ago for my trip to the Marshalls, except I am leaving immediately after the semester ends so time is a bit more limited. I am in the process of renewing my Dive Accident Network first-aid and oxygen administration training, as well as doing check-out dives with UBC’s Dive Safety Officer (we have to do all of this, plus the full dive physical, every two years). I also need to get my dive gear serviced and do a bit of shopping to get appropriate clothing — in Kiribati, women generally keep their shoulders and their legs above the knees covered. Because there isn’t a lot of soil on low-lying atolls like Tarawa, fresh fruit and vegetables are limited, so we also need to bring things like dried fruits and electrolyte powders to supplement our meals. And, my advisor got dengue fever on a previous trip to Kiribati, so we will need to be extra careful to avoid mosquito bites as much as possible (I, unfortunately, am usually a mosquito magnet), which means buying a mosquito net and lots and lots of bug spray.

I’m also doing coursework at the moment and am working as a teaching assistant for two undergrad geography classes. In addition to finishing my assignments and the piles of marking I have to do before I go, probably the largest thing on my to-do list is to get comfortable with the diving PAM and to make sure I know exactly what we need to bring with us to use it. We think we will likely take small coral samples from each site and will then conduct measurements on shore, so we’ll need to bring whatever we need to collect samples and to keep them alive in a hot boat that’s sitting in the equatorial sun all day (although fortunately, the boat is partially covered so we will have some shade). This means a fair amount of research on my end as all of this is new to me — I’ve never collected samples of live corals or used a diving PAM before. I’ll be doing some practice with the PAM here in Vancouver to make sure I’m comfortable taking measurements with it, but of course I won’t be able to test this with corals until we’re in the field, where we likely won’t have access to a lot of materials.

I will be keeping the blog up-to-date during our fieldwork, so stay tuned for updates once we make it to Kiribati in April!

 

 

 

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