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Abrupt tropical cooling ~8,000 years ago M.K. Gagan,

Abrupt tropical cooling ~8,000 years ago

M.K. Gagan, L.K. Ayliffe*, H. Scott-Gagan, W.S. Hantoro, B.W. Suwargadi, D. Prayudi, M.T. McCulloch

Understanding the sensitivity of tropical temperatures to climate change is a fundamental goal in climate change research. Establishing the relative timing and magnitude of abrupt climate change in the tropics and polar regions provides an important means for evaluating the potential role of the tropics in global climate change. The largest abrupt climate change in the Holocene occurred between 8200 and 8000 calendar years ago, when the temperature dropped by 4-8 C in central Greenland and 1.5-3 C around the North Atlantic region. However, little is known about the nature of the so-called 8.2 ka cold event in the tropics.

 

We drilled a sequence of exceptionally large, well-preserved Porites corals within an uplifted palaeo-reef in Alor, Indonesia, with Th-230 ages spanning the period 8400 to 7600 calendar years before present (Figure 2). The corals lie within the Western Pacific Warm Pool, which at present has the highest mean annual temperature in the world's ocean. Measurements of coral Sr/Ca and oxygen 18 isotopes at 5-year sampling increments for five of the fossil corals (310 annual growth increments) have yielded a semi-continuous record spanning the 8.2 ka event. The measurements (Figure 2) show that sea-surface temperatures were essentially the same as today from 8400 to 8100 years ago, followed by an abrupt ~3°C cooling over a period of ~100 years, reaching a minimum ~8000 years ago. The cooling calculated from coral oxygen 18 isotopes is similar to that derived from Sr/Ca. The exact timing of the termination of the cooling event is not yet known, but a coral dated as 7600 years shows sea-surface temperatures similar to those of today.

 

The cooling of ~3°C was very rapid (~0.3°C per decade) and is nearly synchronous with abrupt cooling in the North Atlantic region, as indicated by the decrease in oxygen 18 isotope values of ice from the GISP2 ice core (Figure 2). This new finding supports the hypothesis that abrupt climate change at high latitudes can propagate rapidly to the tropics. Initial cooling at high latitudes could serve to enhance the equator-to-pole temperature gradient and strengthen both the meridional atmospheric circulation and the tropical tradewinds. Today, strong tradewinds in the vicinity of Alor, and elsewhere in the tropics, drive upwelling that locally cools the ocean surface. Our results indicate that the Warm Pool region is unexpectedly sensitive to climate change at high latitudes, suggesting that ocean-atmosphere feedbacks involving the tropics may play a role in propagating abrupt climate changes between the northern and southern hemispheres.


Figure 2: (A) Summary of decadal-average coral Sr/Ca (red symbols) and delta 18OPDB (blue) temperatures calculated for Holocene (Th230 ages) Porites from Alor, southern Indonesia (8°17'S, 124°25'E). For comparison, the upper blue curve shows the 5-year average delta 18OSMOW values (smoothed with 25-year running mean) of ice in the GISP2 ice core, interpreted to reflect the temperature of precipitation over Summit, Greenland (Stuiver et al., 1995). Horizontal bars for coral Sr/Ca and delta 18O indicate time-span of coral cores (40-75 years) plus the 2 sigma uncertainty in the Th230 age determinations. Vertical bars indicate standard error of temperature estimate. Records for cores 3 and 4 have been divided into sections to show timing of cooling event relative to that observed in the GISP2 ice core. Red line shows modern mean sea-surface temperature (28.6°C) at Alor.