A Dangerous Intersection: Humans and Climate Destroy Reef Ecosystem
 
 

In late 1997, cool water from deep in the Indian Ocean was welling up to the surface along the coast of Indonesia. The cool water would have chilled the coral reefs near the Mentawai Islands off the west coast of Sumatra. If corals had a consciousness, however, they wouldn’t have been worried. Over the past 7,000 years, these cold spells had come and gone, and the reefs had barely acknowledged their presence.

  Page 2

Title photographs copyright Brian Nevins, Saraina Koat Mentawai

  Map of Indonesia and the Mentawai Islands

Meanwhile, to the east, a strong El Niño was drying out Indonesia’s tropical forests, especially in Borneo and Sulawesi. With rainfall 400-500 millimeters below the annual average, trees would have been slowing down photosynthesis and shedding leaves to prevent water loss. No stranger to El Niño, however, the forest, if it had a consciousness, probably wouldn’t have been too alarmed. It had withstood such droughts before.

 

The Mentawai Islands, fringed by coral reefs, lie 160 km (100 miles) off the coast of Sumatra, on the eastern edge of the Indian Ocean. (Map adapted by Robert Simmon)

 

What happened next, though, is a stunning and heartbreaking example of how human activity superimposed on Earth’s natural cycles of variation can disrupt the dynamic and delicate balance of life and climate. As 1997 became 1998, a sequence of climatic coincidences became a catastrophe when thousands of square kilometers of tropical forests damaged by human impacts went up in flames, indirectly killing almost the entire 400-kilometer Mentawai coral reef system off the coast of Sumatra.

Corals Tell a 7,000-year Story of the Indian Ocean

The cold water upwelling in the eastern Indian Ocean is part of a climate phenomenon called the Indian Ocean Dipole, during which the eastern half of the ocean becomes much cooler than the western half. Along with these changes in ocean temperature, strong winds blow from east to west at the equator, across Indonesia and the eastern Indian Ocean. The cool ocean temperatures begin to appear south of the island of Java in May and June along with moderate southeasterly winds. Over the next few months, both the winds and cool temperatures intensify and spread northeastward toward the equator. The southeastern Indian Ocean may become as many as 5 to 6 degrees Celsius cooler than the western part.

  Satellite image of Smoke over Sumatra, 1997

Thick smoke from an unprecedented outbreak of wildfires in Indonesia enveloped the archipelago in late 1997. The smoke was particularly heavy over Sumatra, the large island in the center of this image. The fires were partially to blame for the destruction of the Mentawai Islands’ (outlined in red) coral reefs. This image was acquired on October 29, 1997. (SeaWiFS Image Courtesy Norman Kuring)

  Map of Sea Surface Temperature Anomaly in the Indian Ocean

The cooling of the ocean in the coastal zone around the Mentawai Islands during Indian Ocean Dipole events influences the circulation of the atmosphere and rainfall, and it’s related to major droughts in Indonesia and Australia and floods in eastern Africa. In 2001, geologist and marine scientist Nerilie Abram was studying the climate history of the Indian Ocean as part of her Ph.D. studies in the School of Earth Sciences at the Australian National University, when she and her colleagues made a surprising discovery: nearly 100 percent of the Mentawai corals were dead!

 

Water temperatures around the Mentawai Islands dropped about 4° Celsius during the height of the Indian Ocean Dipole in November of 1997. During these events unusually strong winds from the east push warm surface water towards Africa, allowing cold water to upwell along the Sumatran coast. In this image blue areas are colder than normal, while red areas are warmer than normal. (Image based on data from the IRI/LDEO Climate Data Library)

 

“Our research group initially started working in the Mentawai Islands because this region is vital in controlling the climate of the Indian Ocean region,” explains Abram. They were planning to use the coral reefs to put together a 7,000-year record of the region’s climate. “As corals grow, the chemistry of their skeletons preserves a detailed record of the environmental conditions.” Because the kinds of chemicals that make it into the corals’ skeletons depend on ocean conditions like salinity and temperature, the chemical composition reflects the ancient climate. Equipped with scuba and snorkeling gear, Abram and her colleagues set out to sea for several months on an Indonesian dive boat. But when they arrived at the Mentawai reefs, she says, “we were surprised to find that the entire reef ecosystem had been killed. By talking to locals and other researchers working in the region we found that the coral and fish in the Mentawai reefs had all been killed when the ocean turned red in 1997, at around the peak of the 1997 Indian Ocean Dipole event.”

  Photograph of Diver Drilling Dead Underwater Coral

Nerilie Abram and her colleagues discovered the reef death in the Mentawai Islands while sampling corals in an effort to reconstruct past climate. Nearly all of the region’s coral had been dead since 1997, including this lifeless coral being sampled by a diver. (Photograph by Stewart Fallon)

 

Abram and her research colleagues spent several months collecting samples from the dead corals on the Mentawai reefs. They also collected core samples from fossil corals that were thousands of years old. These ancient corals are fossilized in paleo-reefs around the islands that have been uplifted and preserved by large earthquakes. According to their analysis, the cooling seen during the 1997 dipole event was about 4 degrees Celsius. That’s cool, but not the coldest ever according to Abram’s reef record. Indeed, about 4,400 years before present, the anomaly approached 6 degrees. Even more interesting, none of the numerous episodes of cooling associated with 7,000 years of the ocean’s warm-cool cycle appears to have ever caused such massive reef death. The 1997 event was unique.

  Photograph of Drilling Fossil Coral alon the Shoreline

The team drilled cores from fossil reefs along the shoreline, in addition to the recently dead underwater corals. The result was a record of coral growth and ocean temperatures that extended up to 7,000 years ago. (Photograph by Kriton Glenn)

Graph of Ocean Temperatures Dervied from Coral
 

Because previous Indian Ocean Dipole events had been colder than the 1997-98 one, the cold water probably wasn’t specifically to blame for the decimation of the Mentawai Reefs, whose productivity provided locals with food and livelihoods and whose beauty attracted tourists and surfers. But the cold water upwelling did play a role in the reefs’ fate. How could a phenomenon that had been coming and going for several thousand years without harm suddenly play a part in something so deadly?

The data used in this study are available in one or more of NASA's Earth Science Data Centers.

 

Temperature records of the Indian Ocean based on chemical analyses of fossil and modern corals (top) reveal that cold-water events (dips) more severe than the 1997 event have occurred in the past 7,000 years. These dips coincide with changes in coral growth and density that show up in X-ray images of core samples drilled out of the reefs (lined up below graph) as distinct white bands. Unlike the continuous layers of growth (bands) seen in other samples, X-rays of a core sample collected from a modern reef in June 2001 (far left) showed a dramatic break in these regular layers, indicating that reef growth stopped and began again several months later. (Graph by Nerilie Abram)

  A Dangerous Intersection: Humans and Climate Destroy Reef Ecosystem
 
 

In late 1997, cool water from deep in the Indian Ocean was welling up to the surface along the coast of Indonesia. The cool water would have chilled the coral reefs near the Mentawai Islands off the west coast of Sumatra. If corals had a consciousness, however, they wouldn’t have been worried. Over the past 7,000 years, these cold spells had come and gone, and the reefs had barely acknowledged their presence.

  Page 2

Title photographs copyright Brian Nevins, Saraina Koat Mentawai

  Map of Indonesia and the Mentawai Islands

Meanwhile, to the east, a strong El Niño was drying out Indonesia’s tropical forests, especially in Borneo and Sulawesi. With rainfall 400-500 millimeters below the annual average, trees would have been slowing down photosynthesis and shedding leaves to prevent water loss. No stranger to El Niño, however, the forest, if it had a consciousness, probably wouldn’t have been too alarmed. It had withstood such droughts before.

 

The Mentawai Islands, fringed by coral reefs, lie 160 km (100 miles) off the coast of Sumatra, on the eastern edge of the Indian Ocean. (Map adapted by Robert Simmon)

 

What happened next, though, is a stunning and heartbreaking example of how human activity superimposed on Earth’s natural cycles of variation can disrupt the dynamic and delicate balance of life and climate. As 1997 became 1998, a sequence of climatic coincidences became a catastrophe when thousands of square kilometers of tropical forests damaged by human impacts went up in flames, indirectly killing almost the entire 400-kilometer Mentawai coral reef system off the coast of Sumatra.

Corals Tell a 7,000-year Story of the Indian Ocean

The cold water upwelling in the eastern Indian Ocean is part of a climate phenomenon called the Indian Ocean Dipole, during which the eastern half of the ocean becomes much cooler than the western half. Along with these changes in ocean temperature, strong winds blow from east to west at the equator, across Indonesia and the eastern Indian Ocean. The cool ocean temperatures begin to appear south of the island of Java in May and June along with moderate southeasterly winds. Over the next few months, both the winds and cool temperatures intensify and spread northeastward toward the equator. The southeastern Indian Ocean may become as many as 5 to 6 degrees Celsius cooler than the western part.

  Satellite image of Smoke over Sumatra, 1997

Thick smoke from an unprecedented outbreak of wildfires in Indonesia enveloped the archipelago in late 1997. The smoke was particularly heavy over Sumatra, the large island in the center of this image. The fires were partially to blame for the destruction of the Mentawai Islands’ (outlined in red) coral reefs. This image was acquired on October 29, 1997. (SeaWiFS Image Courtesy Norman Kuring)

  Map of Sea Surface Temperature Anomaly in the Indian Ocean

The cooling of the ocean in the coastal zone around the Mentawai Islands during Indian Ocean Dipole events influences the circulation of the atmosphere and rainfall, and it’s related to major droughts in Indonesia and Australia and floods in eastern Africa. In 2001, geologist and marine scientist Nerilie Abram was studying the climate history of the Indian Ocean as part of her Ph.D. studies in the School of Earth Sciences at the Australian National University, when she and her colleagues made a surprising discovery: nearly 100 percent of the Mentawai corals were dead!

 

Water temperatures around the Mentawai Islands dropped about 4° Celsius during the height of the Indian Ocean Dipole in November of 1997. During these events unusually strong winds from the east push warm surface water towards Africa, allowing cold water to upwell along the Sumatran coast. In this image blue areas are colder than normal, while red areas are warmer than normal. (Image based on data from the IRI/LDEO Climate Data Library)

 

“Our research group initially started working in the Mentawai Islands because this region is vital in controlling the climate of the Indian Ocean region,” explains Abram. They were planning to use the coral reefs to put together a 7,000-year record of the region’s climate. “As corals grow, the chemistry of their skeletons preserves a detailed record of the environmental conditions.” Because the kinds of chemicals that make it into the corals’ skeletons depend on ocean conditions like salinity and temperature, the chemical composition reflects the ancient climate. Equipped with scuba and snorkeling gear, Abram and her colleagues set out to sea for several months on an Indonesian dive boat. But when they arrived at the Mentawai reefs, she says, “we were surprised to find that the entire reef ecosystem had been killed. By talking to locals and other researchers working in the region we found that the coral and fish in the Mentawai reefs had all been killed when the ocean turned red in 1997, at around the peak of the 1997 Indian Ocean Dipole event.”

  Photograph of Diver Drilling Dead Underwater Coral

Nerilie Abram and her colleagues discovered the reef death in the Mentawai Islands while sampling corals in an effort to reconstruct past climate. Nearly all of the region’s coral had been dead since 1997, including this lifeless coral being sampled by a diver. (Photograph by Stewart Fallon)

 

Abram and her research colleagues spent several months collecting samples from the dead corals on the Mentawai reefs. They also collected core samples from fossil corals that were thousands of years old. These ancient corals are fossilized in paleo-reefs around the islands that have been uplifted and preserved by large earthquakes. According to their analysis, the cooling seen during the 1997 dipole event was about 4 degrees Celsius. That’s cool, but not the coldest ever according to Abram’s reef record. Indeed, about 4,400 years before present, the anomaly approached 6 degrees. Even more interesting, none of the numerous episodes of cooling associated with 7,000 years of the ocean’s warm-cool cycle appears to have ever caused such massive reef death. The 1997 event was unique.

  Photograph of Drilling Fossil Coral alon the Shoreline

The team drilled cores from fossil reefs along the shoreline, in addition to the recently dead underwater corals. The result was a record of coral growth and ocean temperatures that extended up to 7,000 years ago. (Photograph by Kriton Glenn)

Graph of Ocean Temperatures Dervied from Coral
 

Because previous Indian Ocean Dipole events had been colder than the 1997-98 one, the cold water probably wasn’t specifically to blame for the decimation of the Mentawai Reefs, whose productivity provided locals with food and livelihoods and whose beauty attracted tourists and surfers. But the cold water upwelling did play a role in the reefs’ fate. How could a phenomenon that had been coming and going for several thousand years without harm suddenly play a part in something so deadly?

 

Temperature records of the Indian Ocean based on chemical analyses of fossil and modern corals (top) reveal that cold-water events (dips) more severe than the 1997 event have occurred in the past 7,000 years. These dips coincide with changes in coral growth and density that show up in X-ray images of core samples drilled out of the reefs (lined up below graph) as distinct white bands. Unlike the continuous layers of growth (bands) seen in other samples, X-rays of a core sample collected from a modern reef in June 2001 (far left) showed a dramatic break in these regular layers, indicating that reef growth stopped and began again several months later. (Graph by Nerilie Abram)

 

Restocking the Nutrient Shelves at the Surface of the Ocean

 

Most ocean life goes about its business in the surface waters of the world’s oceans, where sunlight feeds plants, and plants feed animals. Over eons, the remains of millions of creatures that lived and died at the ocean surface sink to the ocean floor and decay, filling the bottom waters with nutrients. When this deep water bubbles up to the surface, it’s like a shot of vitamins for surface-dwelling ocean life. Spiked with nutrients, the water explodes with a great bloom of marine plants called phytoplankton.

Phytoplankton blooms, though, aren’t really a rare occurrence in the region. In many cases, such blooms are a benefit and even a necessity to maintaining a healthy ecosystem; these tiny plants are the foundation of the food chain. Although most nutrients that the ocean plants need to grow are widely available in coastal upwelling zones such as the waters around the Mentawai Islands, there is another necessary nutrient that is more difficult for phytoplankton to come by: iron. The major source of iron in the ocean is dust that blows off the land surface and settles on the ocean. In general, this transport of iron doesn’t keep pace with the availability of the other nutrients driven to the surface by the upwelling, and so plant growth is limited by the availability of iron.

  page 1Page 3
  Comparison of SeaWiFS Chlorophyll Data from December 1997 and 2000

Phytoplankton populations can skyrocket when the ocean receives a sudden influx of iron-rich dust; Saharan dust storms blowing across the Atlantic Ocean have long been known to cause devastating red tides in the Gulf of Mexico. The massive red tide that destroyed the reefs off Sumatra seemed suspiciously similar, but unfortunately, there were no scientific studies of the event that Abram could turn to. Instead Abram used satellite data from the Sea-viewing Wide Field-of-view Sensor (SeaWIFS) to verify local reports of an atypical red tide. “We used SeaWiFS data to examine the productivity in the ocean around the Mentawai Islands at the time of the reef death. The chlorophyll-a concentrations detected by SeaWiFS in December 1997 show a region of elevated productivity around the Mentawai Island chain that is consistent with local observations of the red tide,” says Abram. A high concentration of chlorophyll in the water means a lot of plant life is present.

The local reports and the SeaWiFS imagery confirmed that there was indeed a unusual red tide around the Mentawai Islands in late 1997, which left only one question: what caused it? If the dipole had been occurring for 7,000 years without killing off the reefs, then clearly some other event must have coincided with the upwelling that had never occurred in at least 7,000 years. Since the Mentawai Island region of the Indian Ocean is an iron-limited region, it wasn’t too much of a leap to assume that some sort of massive iron fertilization had occurred. Abram didn’t have to hunt very long to identify a potential culprit: a staggering and probably unprecedented number of forest fires burning just to the east in the jungles of Sumatra and East Kalimantan, Borneo.

 

According to locals, the Mentawai reefs died in 1997 “when the sea turned red,” the result of a bloom of microscopic plants including red algae. In December of that year, the Sea-viewing Wide Field-of-view Sensor recorded chlorophyll concentrations (which are proportional to the density of plant populations) that ranged from 1 milligram per cubic meter of water (green) to nearly 60 milligrams (red). In 2000, a more typical year, the chlorophyll concentration around the reefs didn’t exceed 1 milligram. The amount of iron needed to support such a large red tide would have been between 10 and 50 times what local sources could have provided. (Images courtesy SeaWiFS Project.)

  Photograph of Indonesian Forest Fire

Forest Fires Add the Missing Ingredient

Different parts of the world feel the effects of the El Niño-Southern Oscillation ocean cycle more or less strongly. Indonesia is one of the places strongly influenced by it; El Niño produces drought across most of the region. In late 1997, the drought was hammering the whole of Indonesia, with rainfall deficits of 400-500 millimeters in many places. In the midst of this exceptional drought, the growing use of fire as a forest and agricultural management tool in the region became disastrous. In the absence of sufficient caution, forest fires exploded across the dry landscape.

 

Uncontrolled forest fires sprang up all over Indonesia in late 1997 and early 1998, especially on the islands of Sumatra and Borneo. This smoky fire occured in Borneo’s East Kalimantan province. (Photograph copyright Global Fire Monitoring Center)

  Image of Fires and Plantation in Sumatra

A study by German scientists revealed that recently logged forests were most severely affected. In the near-14-million-hectare study area (almost all of East Taklimakan), 73 percent of peat swamp forests, 75.5 percent of secondary forest, plantation, or farmland, and 81 percent of wetlands burned during the 1997-98 El Niño season fires. Those that didn’t burn completely left behind fuel for the next round of devastating fires. The scale of the disaster was unprecedented. And the impact of fires isn’t just on the land; it’s also in the air.

“When fires burn,” explains Abram, “nutrients from the plants and soil go into the atmosphere in smoke, and as this smoke settles, the nutrients fertilize surrounding environments.” Ash fallout from the staggering number of fires burning in the tropical forests of Sumatra could have added the final fatal ingredient needed to push algae populations—already primed with the nutrients from the Indian Ocean Dipole upwelling—to deadly levels. Abram tracked the spread of the smoke with satellite observations from NASA’s Total Ozone Mapping Spectrometer—TOMS for short. TOMS measures more than just ozone, including smoke and other air pollution.

 

This image shows a plantation near Jambi, Sumatra, being cleared out of the forest using fire. The SPOT 2 satellite captured the image on August 20, 1997. It is made from a combination of visible and infrared light. Vegetation appears bright red, while the burned area appears charcoal gray. Many of the wildfires that plagued Indonesia in late 1997 were ignited intentionally to clear land for agriculture, but got out of control due to the unusually dry conditions. (Image courtesy Centre for Remote Imaging, Sensing and Processing copyright CNES)

  TOMS Aerosol Index October 23, 1997

TOMS observations revealed that with the Indian Ocean Dipole event driving winds from the east, as much as 46 percent of the smoke from the Sumatran wildfires may have spread across the Mentawai reef area. The cool ocean waters caused by the Indian Ocean Dipole upwelling had chilled the air, and cold air tends not to rise, keeping the smoke over the reefs. Abram thinks the smoke is the most likely culprit for fertilizing the algae bloom that suffocated or poisoned nearly every living coral for hundreds of square kilometers.

 

NASA’s satellite-based Total Ozone Mapping Spectrometer tracked the smoke from Indonesian wildfires as it spread westward and blanketed the Mentawai Islands. Aerosol index indicates smoke thickness. The minimum value of 1.0 represents smoke that is roughly as thick as a smoggy day in Los Angeles. Gray, yellow, and red areas are covered by progressively thicker smoke. As iron-rich ash settled on the ocean, it unleashed a massive red tide that killed off the Mentawai coral reefs.

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(Image courtesy TOMS Science Team)

 

A Future of Freak Events

 

It seems almost too coincidental to be real—a freak occurrence of negative synergy in which an unlikely sequence of seemingly unrelated events collided in a dangerous intersection to produce something not better than the sum of the parts, but far, far worse. The Indian Ocean Dipole brought lots of cool, nutrient-rich water up from the ocean floor, while El Niño caused a drought over Sumatra. Because humans had degraded the forest through logging, clearing, and accidental burning, forest fires raged out of control across Indonesia. With the Dipole came strong easterly winds that blew the smoke out over the ocean. The Dipole’s cool ocean temperatures chilled the air, and so the smoke didn’t rise away from the reefs. The iron-rich fallout from the smoke added the final nutrient to the already rich broth of deep-sea water, spawning a massive red tide that destroyed nearly 100 percent of a 400-kilometer long reef system.

It would be nice to hope that such a devastating event was a fluke so coincidental that it would likely never happen again. Unfortunately, that hope may be misplaced. “The combination of natural and human influences in 1997 created a situation that appears to have been unprecedented over at least the past 7,000 years,” says Abram. “However, wildfires are becoming stronger and more frequent with increasing human pressures on tropical forests, and pollution and over-fishing are reducing the ability of reef ecosystems to naturally limit the extent of algae blooms, so it is likely that the threat [to reefs] from wildfires will increase in the future. The increasing threat from wildfires and subsequent large algae blooms applies not only to coral reefs, but also to any coastal marine ecosystem.”

  page 2 Photograph of Coral Regrowth

Years after the 1997 catastrophe, the Mentawai Island reefs are slowly recovering. This juveneil coral, photographed in mid-2001, is growing on the skeleton of a coral killed in 1997. (Photograph courtesy Kriton Glenn)

  Landsat Image of Mentawai Reefs

Abram says that we can shield reefs from such catastrophe by reducing over fishing, water pollution, and other pressures that weaken the natural defense mechanisms that reefs use to fight off algae. But Abram’s study reveals that we must turn our gaze toward the land as well, becoming better caretakers and managers of tropical forest ecosystems. The closer we look at the way the world works, the more we realize how complex and interconnected it is. If we are going to prevent catastrophes like the Mentawai Reef death, strategies for exploiting and protecting natural resources are going to have to match the natural world’s intricacy and sophistication.

  • References:
  • Coral Reef Death During the 1997 Indian Ocean Dipole Linked to Indonesian Wildfires. Abram, N.J. , Gagan, M.K., McCulloch, M.T., Chappell, J., Hantoro, W.S. (2003) Science 301, 952-955
  • Indonesia Country Case Study: Impacts and Responses to the 1997-98 El Niño Event. Kishore, K., Setiana, A., Subbiah, A., Sribimawati, T., Diharto, I.R., Alimoeso, S., and Rogers, P. Full report: Reducing the Impact of Environmental Emergencies Through Early Warning and Preparedness—The Case of El Niño-Southern Oscillation (ENSO). Published in Once Burned, Twice Shy: Lessons Learned from the 1997-98 El Niño. (2001) Glantz, M.H. (Ed.), Tokyo, Japan: UN University Press. Accessed online December 2003
  • Increased damage from fires in logged forests during droughts caused by El Niño. Siegert, F., Ruecker, G., Hinrichs, A., and Hoffmann, A. A. (2001) Nature 414, 437-440.
  • A dipole mode in the tropical Indian Ocean. Saji, N.H., Goswami, B.N., Vinayachandran, P.N., Yamagata, T. (1999) Nature 401, 360-363.
 

The world’s underwater ecosystems, including these reefs north of the Mentawai islands, but still in the zone of reef death, continue to be at risk from human activities. Direct threats like pollution and over-fishing can weaken reefs and make them unable to cope with shifts in climate or sudden environmental shocks, such as the massive red tide of 1997. This true-color image was acquired on July 9, 2001. (Landsat 7 image courtesy Remote Sensing of Coral Reefs)

  A Dangerous Intersection: Humans and Climate Destroy Reef Ecosystem
 
 

In late 1997, cool water from deep in the Indian Ocean was welling up to the surface along the coast of Indonesia. The cool water would have chilled the coral reefs near the Mentawai Islands off the west coast of Sumatra. If corals had a consciousness, however, they wouldn’t have been worried. Over the past 7,000 years, these cold spells had come and gone, and the reefs had barely acknowledged their presence.

  Page 2

Title photographs copyright Brian Nevins, Saraina Koat Mentawai

  Map of Indonesia and the Mentawai Islands

Meanwhile, to the east, a strong El Niño was drying out Indonesia’s tropical forests, especially in Borneo and Sulawesi. With rainfall 400-500 millimeters below the annual average, trees would have been slowing down photosynthesis and shedding leaves to prevent water loss. No stranger to El Niño, however, the forest, if it had a consciousness, probably wouldn’t have been too alarmed. It had withstood such droughts before.

 

The Mentawai Islands, fringed by coral reefs, lie 160 km (100 miles) off the coast of Sumatra, on the eastern edge of the Indian Ocean. (Map adapted by Robert Simmon)

 

What happened next, though, is a stunning and heartbreaking example of how human activity superimposed on Earth’s natural cycles of variation can disrupt the dynamic and delicate balance of life and climate. As 1997 became 1998, a sequence of climatic coincidences became a catastrophe when thousands of square kilometers of tropical forests damaged by human impacts went up in flames, indirectly killing almost the entire 400-kilometer Mentawai coral reef system off the coast of Sumatra.

Corals Tell a 7,000-year Story of the Indian Ocean

The cold water upwelling in the eastern Indian Ocean is part of a climate phenomenon called the Indian Ocean Dipole, during which the eastern half of the ocean becomes much cooler than the western half. Along with these changes in ocean temperature, strong winds blow from east to west at the equator, across Indonesia and the eastern Indian Ocean. The cool ocean temperatures begin to appear south of the island of Java in May and June along with moderate southeasterly winds. Over the next few months, both the winds and cool temperatures intensify and spread northeastward toward the equator. The southeastern Indian Ocean may become as many as 5 to 6 degrees Celsius cooler than the western part.

  Satellite image of Smoke over Sumatra, 1997

Thick smoke from an unprecedented outbreak of wildfires in Indonesia enveloped the archipelago in late 1997. The smoke was particularly heavy over Sumatra, the large island in the center of this image. The fires were partially to blame for the destruction of the Mentawai Islands’ (outlined in red) coral reefs. This image was acquired on October 29, 1997. (SeaWiFS Image Courtesy Norman Kuring)

  Map of Sea Surface Temperature Anomaly in the Indian Ocean

The cooling of the ocean in the coastal zone around the Mentawai Islands during Indian Ocean Dipole events influences the circulation of the atmosphere and rainfall, and it’s related to major droughts in Indonesia and Australia and floods in eastern Africa. In 2001, geologist and marine scientist Nerilie Abram was studying the climate history of the Indian Ocean as part of her Ph.D. studies in the School of Earth Sciences at the Australian National University, when she and her colleagues made a surprising discovery: nearly 100 percent of the Mentawai corals were dead!

 

Water temperatures around the Mentawai Islands dropped about 4° Celsius during the height of the Indian Ocean Dipole in November of 1997. During these events unusually strong winds from the east push warm surface water towards Africa, allowing cold water to upwell along the Sumatran coast. In this image blue areas are colder than normal, while red areas are warmer than normal. (Image based on data from the IRI/LDEO Climate Data Library)

 

“Our research group initially started working in the Mentawai Islands because this region is vital in controlling the climate of the Indian Ocean region,” explains Abram. They were planning to use the coral reefs to put together a 7,000-year record of the region’s climate. “As corals grow, the chemistry of their skeletons preserves a detailed record of the environmental conditions.” Because the kinds of chemicals that make it into the corals’ skeletons depend on ocean conditions like salinity and temperature, the chemical composition reflects the ancient climate. Equipped with scuba and snorkeling gear, Abram and her colleagues set out to sea for several months on an Indonesian dive boat. But when they arrived at the Mentawai reefs, she says, “we were surprised to find that the entire reef ecosystem had been killed. By talking to locals and other researchers working in the region we found that the coral and fish in the Mentawai reefs had all been killed when the ocean turned red in 1997, at around the peak of the 1997 Indian Ocean Dipole event.”

  Photograph of Diver Drilling Dead Underwater Coral

Nerilie Abram and her colleagues discovered the reef death in the Mentawai Islands while sampling corals in an effort to reconstruct past climate. Nearly all of the region’s coral had been dead since 1997, including this lifeless coral being sampled by a diver. (Photograph by Stewart Fallon)

 

Abram and her research colleagues spent several months collecting samples from the dead corals on the Mentawai reefs. They also collected core samples from fossil corals that were thousands of years old. These ancient corals are fossilized in paleo-reefs around the islands that have been uplifted and preserved by large earthquakes. According to their analysis, the cooling seen during the 1997 dipole event was about 4 degrees Celsius. That’s cool, but not the coldest ever according to Abram’s reef record. Indeed, about 4,400 years before present, the anomaly approached 6 degrees. Even more interesting, none of the numerous episodes of cooling associated with 7,000 years of the ocean’s warm-cool cycle appears to have ever caused such massive reef death. The 1997 event was unique.

  Photograph of Drilling Fossil Coral alon the Shoreline

The team drilled cores from fossil reefs along the shoreline, in addition to the recently dead underwater corals. The result was a record of coral growth and ocean temperatures that extended up to 7,000 years ago. (Photograph by Kriton Glenn)

Graph of Ocean Temperatures Dervied from Coral
 

Because previous Indian Ocean Dipole events had been colder than the 1997-98 one, the cold water probably wasn’t specifically to blame for the decimation of the Mentawai Reefs, whose productivity provided locals with food and livelihoods and whose beauty attracted tourists and surfers. But the cold water upwelling did play a role in the reefs’ fate. How could a phenomenon that had been coming and going for several thousand years without harm suddenly play a part in something so deadly?

 

Temperature records of the Indian Ocean based on chemical analyses of fossil and modern corals (top) reveal that cold-water events (dips) more severe than the 1997 event have occurred in the past 7,000 years. These dips coincide with changes in coral growth and density that show up in X-ray images of core samples drilled out of the reefs (lined up below graph) as distinct white bands. Unlike the continuous layers of growth (bands) seen in other samples, X-rays of a core sample collected from a modern reef in June 2001 (far left) showed a dramatic break in these regular layers, indicating that reef growth stopped and began again several months later. (Graph by Nerilie Abram)