'Nature's historians': What clues from coral reefs' past tell us about the future

A coral reef landscape with the ocean surface above

As corals build reefs, they capture snapshots of the ocean water's chemical signature and preserve them, not unlike a natural "history book." By collecting and analyzing reef samples, scientists can gather clues about past climate conditions.

It's a multibillion-dollar question: What will happen to water availability as temperatures continue to rise? There will be winners and losers with any change that redistributes where, when and how much water is available for humans to drink and use.

To find answers and make informed predictions, scientists look to the past.

Reconstructions of past climate change using geologic data have helped to show the far-reaching influence of human activity on temperatures since the industrial age. But assembling hydroclimate records of the water cycle for the same timeframe has proved to be much harder.

Diane Thompson, associate professor in the University of Arizona Department of Geosciences, in the College of Science, is part of PAGES, or Past Global Changes, an international project aimed at improving researchers' understanding of past climate and environmental changes. A new study by PAGES researchers, including Thompson, takes an important step toward reconstructing a global history of water over the past 2,000 years.

Using geologic and biologic evidence preserved in natural archives – including 759 different paleoclimate records from globally distributed corals, trees, ice, cave formations and sediments – the researchers showed that the global water cycle has changed during periods of higher and lower temperatures in the recent past.

The study was published this week in the journal Nature Geosciences.

In this Q&A, Thompson discusses the significance of studying how water moves around on Earth, how to go about collecting a core sample of coral and where her research is headed next.

Q: What are the main takeaways from your research, and what makes it important?

A: Water is a hugely important resource, and we are very concerned about the impacts of warming temperatures on the hydrological cycle – how water moves around on Earth – and how it translates to water availability. For years, researchers have compiled temperature-sensitive records covering the past 2,000 years, and we now understand the temperature change over this period very well. But the impacts of warming on the water cycle are still not well understood. We found a very strong relationship between temperatures and the distribution of water isotopes, which essentially are variations in the atomic weight of oxygen and hydrogen comprising the molecule.

One of the things that was powerful about this particular study is that we were able to utilize different types of natural archives, different types of nature’s "history books," if you will, to tell us about different aspects of the water cycle. For example, some records tell us about precipitation. If you have extreme precipitation, you can get flooding, and if you lack precipitation, you can get droughts. Both obviously have major impact on humans. Perhaps more important, though, is the overall balance of the water cycle: how much precipitation you're getting relative to how much evaporation is happening. That's what we call precipitation-evaporation balance, and it is really important for things like recharging groundwater reservoirs and overall availability of water in an area. When you go to the tap to get a glass of water, that is going to be impacted a lot by the groundwater reservoir in most areas.

Diane Thompson uses a drill to collect a core sample from a subfossil coral reef

Diane Thompson uses a drill to collect a core sample from fossil corals in the Marshall Islands, a remote and isolated archipelago in the Pacific. By piecing together such samples, her research group can piece together histories of ocean change and gain insights into about the longer-term climate trends in the past.

Hussein Sayani

Q: Tell us more about water isotopes. What are those, and why are they important?

A: Water molecules can have different signatures, depending on the composition of the two hydrogen atoms and the oxygen atom that make up each water molecule. These isotopes can combine in different ways, and what's nice about them is that they are very sensitive to changes in the water cycle. They are recorded in what we call natural archives, which are like nature's historians in a way. Whether that's the skeleton of a coral in the case of the work I do, or tree rings, they record information about past climates. We then compile all those types of historical information from nature and these very sensitive changes in in isotopic variability contained within them to infer global scale changes in the water cycle associated with temperature. We know that regionally, for example, temperature impacts the behavior of water isotopes, but this is the first time that we have shown that this relationship is operating globally over short timescales (decades to millennia), in addition to the well-understood global variations over tens or hundreds of thousands of years associated with ice ages.

Q: How did you go about this study?

A: Sorting and interpretating different climate records is a bit like trying to compare apples to oranges. We needed to assemble a team of international experts to sort it in a meaningful way that would allow us to make the interpretations in this paper. A previous outcome of that effort was a database that we published in 2020, and which has already had quite an impact. In this paper, our team of experts asked, "What are first order conclusions we can make from this really new, nuanced dataset?"

The water cycle is extremely complex and consists of many moving parts. Water might be evaporating off the surface in the tropical Pacific Ocean and raining out over western North America. We have to understand the global picture of the water cycle to really predict how warming is going to impact what we care about, which is, for example, whether we have enough water in the desert Southwest. We did a lot of analysis, interpretation and working with climate models to figure out what it all means. For example, we used certain records to learn about effective moisture, or the balance between evaporation and precipitation in an area — an important factor for overall water availability.

Thompson and her research team collecting core samples from a coral reef in Galapagos.

Thompson and her research team are seen here collecting core samples from a coral reef in the Galapagos.

Courtesy of Diane Thompson

Q: How can a piece of coral tell us about past climates?

A: Corals (the types that build reefs) are tiny polyps, animals that build a skeleton very similar to the shells that you find on a beach. It's composed of the same mineral, calcium carbonate, which stores the information about water isotopes we talked about earlier. In other words, that's the "history book." As the coral grows, the isotopic composition of the oxygen it builds into its skeleton is dependent on mainly temperature and the isotopic composition of the surrounding seawater. As paleoclimate scientists, we measure the chemical and isotopic composition of its skeleton. In the past, people used to think corals could mainly inform us about past temperatures, but we were able to tease out records that provided information about the hydrological cycle as well. For example, in our study, corals in the western Pacific provided key insights into changes in the water cycle over this region.

Q: What does your field research look like?

A: When we take samples from living coral reefs, we use scuba gear and underwater drills to extract a core. In the lab, we split these core samples open and then very painstakingly subsample the skeleton millimeter by millimeter to measure their isotopic composition. Sometimes we're lucky and come across sites where we find what we call subfossil corals. They were tossed onto the beach by strong wave action, or a tsunami. Those provide us with a longer history of climate change because they grew sometime in the past, unlike living corals, which provide us with a snapshot of current conditions. If we get enough of those subfossil corals, we can piece together an even longer history of ocean change, and we can then start to say something about the longer-term climate trends.

Corals give us information like tree rings, with year-to-year changes, which allows us to resolve small timescales. And this allows us to say things not just about overall changes of hydrological cycle, but changes from year to year, particularly since the pre-industrial era when coral records are abundant. That was primarily my role in this project.

Q: Where are you planning on taking this research from here?

A: I think a difficult but important target is to really understand and improve regional hydroclimate prediction. To do that, we'll have to continue to expand and probe databases like the one that we collated. One of the big questions is, "What are the spatial patterns of changes in the water cycle, or the 'winners' and 'losers' (so to speak)," which is obviously very important for societies. Are we going to get water in Tucson, or is the Pacific Northwest going to get it? And while we can't necessarily answer that question in full yet, one of the really interesting results that we found is an opposing pattern in the isotopic signatures between both ends of the Pacific – Southeast Asia and the Pacific Northwest – with regard to the balance between rainfall and evaporation. This suggests that variability in the state of the Pacific Ocean over decadal to millennial timescales likely matters quite a bit for the water cycle in the Western U.S.

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