Coral cores reveal truths about the earth’s climate history – and provide insights about its possible future.
I picked up the foot-long, cream-colored slab, a stony missive carved from a coral reef, far from its former home in the tropical Atlantic Ocean. It felt cool in my hands. About two inches across and just under a quarter of an inch thick, the slice, a vertical cross-section excised from a cylindrical coral core, resembled a piece of rough-sawn tile, bordered on one end with the lumpy edge of now-dead coral polyps – the tiny creatures’ most recent skeletal layer.
Lying on a table nearby was the printed image of the slab’s X-ray, an account of the corals’ internal layers, revealed in alternating bands of black and white. “The X-ray allows me to count the bands,” said paleoclimatologist Hali Kilbourne. “This is 2004, 2003, 2002, 2001,” she said, sliding her finger down the image and pointing out each layer with her finger. “I do the chemistry down-core and then I can [identify] exactly what year it was.” When the slab and the image were perfectly aligned, I could clearly pinpoint the years, decades, and centuries. Here in front of me was a timeline of the earth’s history, hard evidence of the past’s climate, etched in limestone.
I met Kilbourne during a recent visit to the University of Maryland Center for Environmental Science’s Chesapeake Biological Laboratory, a clutch of brick buildings that sits alongside the Patuxent River, in Solomon’s Island, Maryland. Kilbourne studies the earth’s climate from the past, before instruments for measuring and recording the weather were available. We talked about how she and her colleagues conduct their research – how they get to the truth about the earth’s past climate.
Kilbourne’s scientific background is in geology, but her interests lie in climate change, a subject she’s been studying for more than fifteen years. “I use natural archives of past environmental conditions to understand climate variability,” she said. Those natural archives can be found in ice caps, peat bogs, or tree rings – nature keeps very good records – but Kilbourne prefers the records found in coral reefs.
A coral reef is a collection of the external skeletons of millions of corals, tiny, soft-bodied creatures called polyps that claim distant kinship to jellyfish and anemones. Coral polyps prefer warm water that’s clear and shallow, so most reefs can be found in the tropical and subtropical oceans. The polyps draw on the stew of nutrients around them to excrete seasonal layers of calcium carbonate, the principal ingredient in their skeletons.
Despite their hard, sturdy appearance, corals are aquatic prima donnas. They possess a narrow range of tolerance to changes in their environment, making them excellent barometers for climate change.
Corals’ sensitivity manifests in their growth rate, which is faster or slower depending on variations in the surrounding ocean temperature, salinity, or clarity. These variable rates create thicker or thinner layers within the coral skeletons, producing an identifiable pattern of light and dark stripes – the black and white bands I observed on the X-ray – much like the concentric rings of a tree trunk or the stacked ribbons of lake sediments. The layers provide a record, a seasonal imprint of the passing of time that allows scientists to look back in history, from the very recent past to millions of years ago.
The use of corals in the search for answers about the earth’s climate is relatively new. “The recognition that corals could be used for reconstructing past temperatures occurred in the 1970s,” said Kilbourne, “but the analytical capability to measure some of the important chemistry inexpensively and with the precision needed didn’t catch up until the 1990s.”
Kilbourne analyzes the chemical properties of the coral skeletons’ layers to “read” their natural histories. “If I can understand some [chemical] process in the modern world, then I can understand how it occurred in the past,” Kilbourne said. That’s because chemistry doesn’t change. Specifically, she looks at the ratio of two minerals – strontium and calcium – to determine the climate conditions in which the corals grew.
Strontium shares many chemical and physical characteristics with calcium and can even serve as a stand-in for some of the calcium in a coral skeleton, depending on the surrounding ocean temperature. Corals incorporate less strontium into their skeletons when ocean temperatures are warm, and more when temperatures are cool.
“Let’s say we have a coral that grew in the 2000s. We had thermometers in the 2000s! So, I look at what the temperature was [when it was growing], and I look at what the strontium concentration was, and I can make a relationship between the two because they’re correlated,” Kilbourne said. “One drives the other.” The modern-day coral strontium-calcium ratios and temperature observations create a reference data set that allows Kilbourne to extend the correlation backward in history before thermometers were available. “And that’s how I can get a temperature record back in 1492 when Columbus sailed the ocean blue.”
Based on the differences in the strontium-calcium ratio, scientists can calculate ocean temperatures with great accuracy. “A single measurement by itself might have an uncertainty of plus or minus one degree Celsius,” said Kilbourne. But the coral-based climate records she and her colleagues rely on are made up of hundreds to thousands of measurements. With such large datasets, the results become much more certain – to within one-tenth of one degree Celsius.
Measuring other elements in the skeletons, such as different varieties of oxygen (“heavy” or “light” oxygen, called isotopes), Kilbourne back-calculates levels of the ocean’s salinity. A higher concentration of light oxygen is a sign of heavy rainfall, which results in lower salinity. “So, with the strontium-calcium [ratio] and that salinity information, I can see floods, I can see droughts.” Kilbourne can even identify years in which El Niño events, intense hurricanes, or large volcanic eruptions occurred.
Choosing the dive site where Kilbourne will collect the coral samples is an important part of the research. “We want to [collect from] several sites in order to get the bigger picture more confidently,” said Kilbourne. “If you have a really enclosed lagoon, you might have processes that alter the seawater chemistry. It’s not reflective of the open ocean conditions, which is what we’re trying to look at in the [global] perspective.” Although she and her colleagues have been as far south as islands off the coast of Brazil, Kilbourne spends most of her time in the Caribbean and the tropical Atlantic, in places like Grenada and the Lesser Antilles. “It’s easier to count the islands I haven’t been to,” she said.
Even though Kilbourne studies coral reefs in balmy waters far from where I live, in northern Virginia, she makes an excellent case for why tropical ocean temperatures should matter to me. “The Caribbean and tropical Atlantic are like a pot of warm water letting off steam. That steam is the moisture that provides rain for North America, South America, Africa, and Europe. If you live in one of those four continents, and a lot of people do, including us, it’s pretty important,” she said. That’s because we really have only one ocean, and it’s a global one, a massive, interconnected reservoir of heat and the principle regulator of the globe’s weather.
The type of coral Kilbourne samples is important, too. The delicate arms and fanned sprays of branching corals that most people are familiar with don’t provide the long, continuous records that Kilbourne needs for her research, so she looks for coral species that tend to form solid boulder-like formations. “In the Pacific, we go after the Porites genus, but in the Atlantic, we are more often looking at the Orbicella, which is the boulder star coral.” Orbicellas grow in massive stony mounds with dome-like tops or ruffled, skirted edges, in shades of green, orange, brownish yellow, and gray. The mounds cluster in colonies that may reach ten feet in diameter.
Outfitted in scuba gear and using a hand-held hydraulic drill with a long, hollow, cylindrical coring bit attached, Kilbourne removes two-and-a-half feet-long segments of the coral skeleton, roughly the diameter of a soup can, at a time. Kilbourne is petite, and working underwater for long periods of time is grueling, but she has grown accustomed to the physical demands of her profession. She breaks off each segment, removes it from the bit, and then continues drilling. The largest coral core Kilbourne drilled was more than eight feet long. When pieced back together, she said, it served as a stony, cylindrical “biopsy” of the reef.
Keeping accurate records of where the cores come from allows Kilbourne to collaborate with other scientists who drill in nearby locations and to analyze the data within the broader context of weather around the globe. “We take GPS coordinates at the site,” said Kilbourne. “Then we bring [the cores] onboard and use a waterproof marker to label them with the GPS coordinates and our depth. We take extensive field notes as to where we are and take pictures.” When Kilbourne returns to her lab, she uses a tile saw to cut the cores into slabs and X-rays them to observe the growth patterns, she said.
Corals are natural products, however. They aren’t perfect proxies for past climate change, but they’re the gold standard in tropical regions for now. “A very rigorous quality control analysis must be applied to each record to ensure that it represents climate and not other factors,” said Kilbourne.
In cooler parts of the world, where corals are unable to grow, paleoclimatologists rely on temperature and rainfall records preserved in land archives such as ice (from high-latitude polar regions) or trees (from mid-latitude regions) to complement the data from tropical region coral cores. Piecing together the data from these complementary sources, Kilbourne and her fellow paleoclimatologists, a consortium of climate scientists from around the world, have observed a significant cooling trend in the Indian, western Pacific, and Atlantic oceans over the past 2000 years, with the steepest drop occurring between 1400 and 1800 of the Current Era. Their findings, published in the August 2016 issue of the journal Nature, reveal that the cooling trend came to an abrupt halt around 1830 – the infancy of the Industrial Age and the widespread production of greenhouse gases from fossil fuels, especially coal – some 20 years earlier than most climate change models suggest, an indication that even small amounts of the gases can change the earth’s climate.
The data gleaned from Kilbourne’s lab support and expand on what scientists already know about climate change: The trend toward increasing global temperatures has already altered local and regional weather patterns, incurring threats to food production, freshwater supplies, and the safety and welfare of people living in coastal areas. “It’s not a trend that’s going to change unless humans change the carbon dioxide content of the atmosphere in a different way other way than up,” said Kilbourne. But the findings also suggest that the earth’s climate can respond to even small changes in greenhouse gas emissions, perhaps offering humans a means of slowing down the warming process.
Despite the bad news about changes in the world’s climate, Kilbourne is optimistic. “To me, the important [thing] is that it’s not too late and that we need to change our ways now. Smart people have thought about solutions to this; I’m trying to understand the problem, but we already understand it enough to take action.”
Abram, N. J., McGregor, H. V., Tierney, J. E., Evans, M. N., McKay, N. P., Kaufman, D. S., & PAGES 2k Consortium. (2016). Early onset of industrial-era warming across the oceans and continents. Nature, 536(7617), 411-418.
Tierney, J. E., Abram, N. J., Anchukaitis, K. J., Evans, M. N., Giry, C., Kilbourne, K. H., & Zinke, J. (2015). Tropical sea surface temperatures for the past four centuries reconstructed from coral archives. Paleoceanography, 30(3), 226-252.