30 April 2012


Some islands in the bulls'-eye of climate change may dodge the worst thanks to heretofore unknown dynamics between deep currents, upwelling, and rising temperatures.
Gilbert Island archipelago in the Pacific island nation of Kiribati. Credit: NASA/Aqua Satellite.
A new paper in Nature Climate Change reports on an unexpected refuge of cooler water around the equatorial Gilbert Islands—one of three archipelagos of the Pacific island nation of Kiribati (say: KEER-uh-bus).
This is good news at a time when when scientists predict that rising ocean temperatures in the equatorial Pacific will wreak havoc (coral bleaching, coral disease) on coral reef ecosystems by the end of the century
The new study shows that climate change could cause ocean currents to operate in a surprising way to mitigate warming near a handful of islands on the equator... which may then become isolated refuges for corals and fish.
The three archipelagos of the Pacific island nation of Kiribati straddle the Equator. Map based on: TUBS via Wikimedia Commons.
The dynamics of this cooler-water refuge include:
  1. Equatorial trade winds pushing a surface current, the Equatorial Countercurrent, from east to west
  2. A swift Equatorial Undercurrent flowing below the Equatorial Countercurrent in the opposite direction, west to east, at 100-200 meters (328-656 feet)

Where the Equatorial Undercurrent encounters an island, its flow is deflected upward on the island's western flank, carrying its cooler nutrient-rich water to the sunlit surface and creating localized areas of greater ocean productivity.

You can see the dynamics of this in the map and graph below showing chlorophyll levels—a marker of phytoplankton and hence marine productivity—across the equatorial Pacific. The red in the map view indicates highest chlorophyll / phytoplankton / marine productivity.

Credit: Kristopher B. Karnauskas and Anne L. Cohen. Nature Climate Change. DOI:10.1038/nclimate1499.
Clearly, the most productive waters occur in the eastern tropical Pacific, where the Equatorial Undercurrent drives up against the Galapagos Islands to create huge upwelling.
Chlorophyll quantities then dwindle rapidly as you move west... until the anomalous red signature marking the outlines of the Gilbert Islands of Kiribati seen inside the white-dotted rectangle. The islands are not shown in the map view, just their chlorophyll signatures.
Co-author Anne Cohen at WHOI says:
"Global models predict significant temperature increases in the central tropical Pacific over the next few decades, but in truth conditions can be highly variable across and around a coral reef island. To predict what the coral reef will experience in global climate change, we have to use high-resolution models, not global models."

Coral reef of the equatorial Pacific. Credit: USFWS.

The models predict:
  • That as air temperatures rise and equatorial trade winds weaken, the Pacific surface current will also weaken by 15 percent by the end of the century.
  • But the then-weaker surface current will also impose less friction and drag on the EUC, so this deeper current will actually strengthen by 14 percent.
The high-resolution models developed by  Kristopher Karnauskas, also at WHOI, and Cohen, suggest the amount of upwelling will actually increase by about 50 percent around the Gilbert Islands, to reduce the rate of warming waters there by about 0.7°C (1.25°F) per century.
From the paper:

In the central Pacific, home to one of the largest marine protected areas and fishery regions in the global tropics, sea surface temperatures are projected to increase by 2.8 °C by the end of this century. Of critical concern is that marine protected areas may not provide refuge from the anticipated rate of large-scale warming, which could exceed the evolutionary capacity of coral and their symbionts to adapt. Combining high-resolution satellite measurements, an ensemble of global climate models and an eddy-resolving regional ocean circulation model, we show that warming and productivity decline around select Pacific islands will be mitigated by enhanced upwelling associated with a strengthening of the equatorial undercurrent. Enhanced topographic upwelling will act as a negative feedback, locally mitigating the surface warming. At the Gilbert Islands, the rate of warming will be reduced by 0.7±0.3 °C or 25 ± 9% per century, or an overall cooling effect comparable to the local anomaly for a typical El Niño, by the end of this century. As the equatorial undercurrent is dynamically constrained to the Equator, only a handful of coral reefs stand to benefit from this equatorial island effect. Nevertheless, those that do face a lower rate of warming, conferring a significant advantage over neighbouring reef systems. If realized, these predictions help to identify potential refuges for coral reef communities from anticipated climate changes of the twenty-first century.

A bathymetric view of the Phoenix Islands group of the Pacific islands nation of Kiribati. Credit: Phoenix Islands Protected Area.
As an interesting aside, in 2006 Kiribati created the Phoenix Islands Protected Area (PIPA) to the east of the Gilbert Islands. In 2008 they doubled its size to make it the world's largest marine protected area. (Since then the Chagos Marine Reserve in the Indian Ocean has surpassed PIPA in size.)

410,500 square kilometers (158,453 square miles), about the size of California, PIPA preserves one of the Earth's last intact oceanic coral archipelago ecosystems, complete with eight coral atolls, two submerged reef systems, underwater sea mounts, and abundant marine and bird life.
This is a truly phenomenal accomplishment. 

But if the Gilbert Islands are destined to become one of the few places where coral reef biodiversity is able to hang on in the coming century, then maybe we should begin thinking about giving those waters stronger protections too.
An atoll of Kiribati. Via Flickr.

The paper:

  • Kristopher B. Karnauskas & Anne L. Cohen. Equatorial refuge amid tropical warming. Nature Climate Change (2012) DOI:10.1038/nclimate1499
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