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Sentinels of the Sea: Ancient corals help predict severity of global warming

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For scientists, the debate over whether global warming is happening is over, but another debate remains: How serious will its effects be? Predictions of the consequences of global warming range from dire (major sea level rise, the extinction of polar bears, global economic depression, the spread of new infectious diseases) to rosy (opening up new routes for shipping in the Arctic and allowing income-producing crops to be grown in new places).

Terry QuinnFred Taylor
Dr. Terry Quinn (left) and Dr. Fred Taylor study fossil corals for clues to ancient climate and ocean conditions. This work helps improve climate models that forecast future climate.

With such a range of predictions from scientists, economists and environmentalists, it’s critical to know which ones to believe. One of the pieces of the puzzle may come from an unlikely source–corals.

Terry Quinn and Fred Taylor at The University of Texas at Austin’s Jackson School of Geosciences have pioneered the examination of ancient corals as a record of Earth's past climate. Their findings are proving critical to the global effort to model future climate change more accurately.

Back to the Future

Computer models that are used to predict future climate vary widely in their forecasts. For example, some predict Earth will warm by three degrees Fahrenheit by the end of the century, others put the number closer to 12 degrees. The models are based on fundamental physics of the ocean-atmosphere system, but also require simplifications and assumptions, which vary among models.

Even a three-degree rise in global temperatures could lead to dramatic changes. Smaller changes in the past century have already elevated sea levels, caused thousands of plant and animal species to shift their ranges toward the poles, changed flowering times of plants and sped up the melting of glaciers and ice sheets. Research published last year even linked climate change to more powerful hurricanes over the past several decades.

Climate scientists validate and improve their models by running them back in time to see how well they reproduce climate trends that actually occurred on Earth. That’s where Quinn’s coral research comes in.

Corals grow by adding new layers on the outside, like the trunk of a tree. And like trees, corals have annual growth bands that record information about what that year was like. As a result, ancient reefs provide one of the best records of climate in the tropical seas over the past 25,000 years.

“So we can tell the modelers, ‘OK, 20,000 years ago, the temperature and salinity of the oceans were like this, El Niño was this active, and so on,’ and that places constraints on the inputs to the climate models,” Quinn said.

According to Quinn, the key is a better understanding of uncertainties.

Porites coral colonies, such as this one in Ofu lagoon, American Samoa, hold a key to understanding Earth's past climate
“We look for big ugly ones,” said Quinn. Porites coral colonies, such as this one in Ofu lagoon, American Samoa, hold a key to understanding Earth’s past climate. Photo: Christina Kellogg, USGS.

“People say, ‘How do we know what’s going to happen?’” said Quinn. “We don't for sure, but we can think in terms of probable outcomes. It’s analogous with risk assessment and insurance. When I lived in coastal Florida, I had hurricane insurance. Now I live in Austin, I don’t have hurricane insurance. It’s still possible we’ll get hit by a hurricane, but not very likely. We want to use the past behavior of the Earth to lower the error bars.”

As the uncertainties diminish, Quinn said, U.S. policy makers might feel confident enough to start doing something about climate change.

Coral Hunting

Quinn joined the Jackson School this past summer, assuming a dual appointment as a professor in the school’s Department of Geological Sciences and senior research professor in the Institute for Geophysics.

Before coming to the Jackson School, Quinn directed the Global Change Research Center at the University of South Florida. He’s had a connection to the Institute for Geophysics for more than a decade through his collaboration with Taylor, a senior research scientist at the institute.

“Fred is the guy I always go to,” Quinn said, “when I want to know where to find corals that cover a time period and a climate problem I’m interested in.”

Quinn searches for just the right kinds of coral.

Researchers extract a core with a hydraulic drill on a Porites lobata coral colony at Clipperton Atoll in the North Pacific
Researchers extract a core with a hydraulic drill on a Porites lobata coral colony at Clipperton Atoll in the North Pacific. Photo: Maris Kazmers, NOAA Paleoclimatology Program/DOE.

“We look for big Volkswagen-sized corals,” he said. “Branching corals and big sea fans are pretty to look at, but they aren’t necessarily good for climate records. So we look for big ugly ones.”

To be precise, he looks for a type of stony coral called Porites, which go by the common names of lobe or mound coral. They look like big round bales of hay covered in purple, green or tan fuzz. To Quinn, these are also archives of Earth’s past climate. They tell of a time before scientists and naturalists and explorers had the instruments to accurately measure and record the world around them.

When he first began studying corals for clues about past climate about 15 years ago, Quinn went on large research ships with an array of other researchers working in different fields, expensive equipment and technical support. In the early days, he made several field trips with the Ocean Drilling Program. But over the years, he’s gotten more nimble. Now, when he goes out in the field, it’s often with just two or three other researchers on a small boat. He usually just rents the boat and its captain locally for a few weeks.

The researchers spend entire days in wet suits, drilling into underwater corals, often eight or ten meters down.

“We go to remote places,” usually on or near tropical islands, he said. “So we need simple and robust tools. There are no Home Depots in places like the Solomon Islands or New Guinea.”

Quinn uses a hydraulic coring device designed by Taylor. It runs on power from the boat delivered through an umbilical cord.

“Most of the people who drill modern corals use the UT drill,” Quinn said. “It’s evolved and improved over the years of course.”

Quinn is sometimes asked about the damage that drilling can do to corals, which are in decline in much of the world.

“We fill the holes in with marine cement,” he said. “The living animals around it will grow over them. Like a scab, it heals itself. You go back three years later and you can’t tell where you drilled.”

Graduate student Hali Kilbourne lays next to coral cores for length comparison
Graduate student Hali Kilbourne lays next to coral cores for length comparison.

Quinn and his team bring back solid cylinders of limestone three and a half inches, or nine centimeters, wide. Some cores shipped back to the lab might be as long as 10 feet, or three meters. Following each field trip, it takes years of lab work and analysis to interpret what details the cores have preserved about past climate.

Increasing Confidence

Few things are harder to predict than the weather. The television weather forecaster, if he or she is bold, might venture to guess what the weather will be like a week from now.  Maybe.  So it’s not surprising that critics are dubious of climate modelers’ ability to make a forecast for the whole planet, decades in advance.

But climate modelers have made great strides in recent years. Quinn and others have shown that the models can be trusted to at least get the general trends right. Take, for example, El Niño—a warming in the eastern equatorial Pacific every few years that changes global weather patterns. The coral record shows that these events were weaker and less frequent 6,000 to 10,000 years ago than they are today. When modelers ran their climate models back thousands of years, they found the same reduction in El Niño activity.

“If climate simulations of past climates are in agreement with the geologic record of climate, then model-based predictions of future climate can be viewed with more confidence,” Quinn said.

And the models keep improving. According to Quinn, climate modelers once assumed that tropical sea surface temperatures were about the same 20,000 years ago, during a time called the Last Glacial Maximum, as they are today. Now, thanks to the fossil coral record, scientists think it was two to three degrees Celsius colder back then. That information has led to further refinements in the climate models.

How do corals record
climate and ocean conditions?

Drawing of a coral polypA colony of stony coral is made up of thousands of colorful little animals called polyps. Each polyp, which is a clone of all the others, builds a hard cup-shaped scaffold of calcium carbonate. It absorbs the calcium it needs from the surrounding seawater.

“It starts as a one-story house," said Terry Quinn, a research scientist at the University of Texas at Austin's Jackson School of Geosciences. "The polyp lives in it for a year, vacates it and then starts building a second story on top.” Slowly, layer by layer, a hard “coral head” forms beneath. All the living coral is on the outer surface.

Continue reading about how corals record climate and ocean conditions.

Quinn is especially proud of research he, Taylor and their colleagues published in the journal Paleoceanography in August 1998. They reconstructed a history of ocean conditions at New Caledonia—a French island east of Australia—back to the year 1657. French scientists have collected daily measurements of sea surface temperatures and salinities at that same spot continuously since 1967. That allowed Quinn and his team to precisely match part of their record to actual observations to verify and calibrate his results.

The record showed seasonal cycles and El Niño/southern oscillation cycles that were familiar. But it also revealed longer, previously unknown cycles that unfolded over 16, 32 and 60 years.

“We still don’t know the causes of these decadal changes,” Quinn said.

He and a student are looking at other corals at nearby sites to paint an even more detailed picture of what happened. This work might reveal new mechanisms that need to be incorporated into climate models.

Now, just as humanity faces some of the most troubling and difficult questions imaginable, it is an amazing fact that for thousands of years corals, stoic sentinels of the sea, have been diligently monitoring the planet’s health. Fortunately, someone is asking them to share their secrets.

BY Marc Airhart

Photo of Dr. Taylor: Marsha Miller

Banner photo: NOAA Photo Library-Coral Kingdom

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  Updated 20 November 2006
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