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Book Review

The Indian Ocean Dipole: How Climate Change affects South East Asia and Beyond

Reviewing: Abram, N.J., Wright, N.M., Ellis, B. et al. Coupling of Indo-Pacific climate variability over the last millennium. Nature 579, 385–392 (2020).

Often Overlooked: The Indo-Pacific

When we read about climate change impacting our society, the media typically covers droughts in California, hurricanes arriving in Florida, or maybe even heat waves in Europe. While these events are important to the many millions living there, our Eurocentric focus on global warming often neglects other regions of the world that potentially deserve more attention. For example, in the map below, you might be surprised to learn that more people in the world live inside the circle surrounding South East Asia—a region termed the Indo-Pacific—than outside of it! In a recent study, Dr. Abram and others investigated how climate change is affecting the Indo-Pacific, with important implications for the billions of people who call this part of the world home.

A global map with a circle marking the Indo-Pacific region. More people live inside than outside of the circle. (Credit: Stephen Haptonstahl)

The Indian Ocean Dipole (IOD)

One of the most important climate phenomena that affects the Indo-Pacific is the El-Nino Southern Oscillation, (click here for a quick overview).  Equally as important is the Indian Ocean Dipole (IOD). Very briefly, the Indian Ocean Dipole is measured as east-west temperature differences in the Indian Ocean just like the El Nino for the Pacific. During a so-called “positive IOD event”, the Indo-Pacific is much cooler and receives less rain, leading to droughts, while the East African coast is much warmer and receives more rain, which can cause floods (see below).

Diagram showing the temperature anomalies during a Positive IOD event, with potential natural disasters on both sides of the Indian Ocean (Image Credit: Australian Bureau of Meteorology)

As you may recall, in January and February Australia suffered multiple massive wildfires that devastated the country. Many climate scientists point to the extreme positive IOD event in late 2019 as a major contributor to the lack of rainfall and terrible wildfire season that followed.

Wildfires raging in Bairnsdale, Australia in December of 2019 (Image Credit: Glen Moery via AP)

In recent years, scientists have theorized that 21st century global warming is increasing the frequency of extreme positive IOD events, which would have huge implications for the people of the Indo-Pacific region. To prove this hypothesis though, we need to get an accurate sense of long-term natural IOD variability and see if the number of positive IOD events in recent years has been unprecedented.

Corals Provide the Answer

Because accurate records of Indian ocean sea surface temperature extend back only a few decades, we need to look into the past to reconstruct the long-term signal of IOD. This is why Dr. Abram and others recovered samples of fossilized Porites corals from the Mentawai Islands off the coast of Sumatra to better understand the IOD. The authors chose to sample corals from this location specifically because it’s been shown that sea surface temperatures off the coast of Sumatra are very strongly correlated with the IOD. If we can get a historical record of sea surface temperatures from corals then we can see how IOD has varied in the past.

Living Porites corals from the Caribbean. Corals are like logbooks that record sea temperatures of the past. Geochemists can analyze them to reconstruct past environmental conditions (Image Credit: Wikimedia Commons)

Results: Climate Change Increasing IOD Frequency

Dr. Abram and her colleagues measured the oxygen isotopic ratio of multiple fossil corals from the Mentawai Islands region and were able to reconstruct a semi-continuous record of IOD variability over the last 1,000 years.

They found that extreme positive IOD events occurred 4 times in the last 60 years.  Given that the same extreme events had occurred only 6 times in the entire preceding 1,000 years, Abram and others take this as strong evidence that extreme positive IOD events have been becoming more frequent in recent decades.  This increase is likely the result of global warming.

An unexpected finding from their study came when the scientists compared the coral record of IOD variability to similar coral records of El Nino variability in the Pacific Ocean. They found that over the last millennium, IOD and El Nino variability covaried to a remarkable degree. While the coupling of the two climate signals is not airtight—in many cases there are positive IOD years without a strong El Nino, and vice versa—this finding supports theories that propose a strong interconnectedness between the Indian and Pacific Ocean climates.

Looking Forward for the Indo-Pacific

Dr. Abram and coauthors presented a key work of paleoclimate research that has impacted the way we understand the IOD. The research shows that the Indian Ocean Dipole is sensitive to human-caused climate change, leading to more positive IOD events as a result. In addition, the data shows a long-term covariance between the IOD and the El Nino-Southern Oscillation, a relationship that could transform the way we understand tropical climate.

Droughts in Indonesia are potentially becoming more frequent with IOD intensity increasing due to climate change (Greenpeace Southeast Asia

While the paleoclimate community will continue to conduct research to validate these conclusions, government leaders, policy researchers, and agricultural industries should heed these findings as a warning of the potentially devastating effects climate change could induce on this populous area of the world.

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