Abrupt Climate Change

A special area of interest of MIT paleoclimatologists is rapid climate change that occurs on decadal-to-millennial timescales.  Research groups led by Ed Boyle and Julian Sachs are developing high-resolution paleoclimate records from throughout the world ocean to determine the persistence in space and coherence in time of rapid climate events.  Close collaborations with researchers at the Woods Hole Oceanographic Institution are an integral part of paleoclimate research at MIT.

Long sediment cores  can be recovered from research vessels almost anywhere in the world ocean.  In special locations, where the sediment accumulates very rapidly, highly detailed paleoclimate records can be obtained.  One such location is the Bermuda Rise in the subtropical Northwest Atlantic Ocean (Fig. Color).  By using a technique known as alkenone paleothermometry, precise sea surface temperature (SST) estimates can be made by measuring the alkenone unsaturation ratio along the length of a sediment core.  The alkenone unsaturation measurements shown below (Fig. SST) were performed every 1-2 cm from a section of sediment recovered 21-33 m below the seafloor.  The overlying water is 4500 m deep.  The blue line is oxygen isotope data from the GISP2 Ice core (Grootes and Stuiver (1997), Journal of Geophysical Research 102(C12): 26,455-26,470) recovered from the summit of the Greenland ice sheet.  The ratio of ¹⁸O to ¹⁶O in glacial ice is a strong function of air temperature over the site.

Fig. Color:  

Color variations in core MD95-2036 from 35 to 45 meters. This interval covers the period from about 70,000-140,000 years before present. The color changes represent changes in the calcium carbonate content (lighter and darker) and hematite content (red).  These sedimentological changes are climate controlled and indicate the extreme variability of climate change on time scales of hundreds of years and less.

Sea surface temperatures (red) at the Bermuda Rise and a Greenland air temperature proxy (blue) 30,000 to 60,000 years ago.  The very large and abrupt temperature changes suggest the climate system can respond rapidly (decades to a few centuries), and that the amplitude of this response is large in both polar and warm subtropical latitudes (Sachs and Lehman (1999), Science 286: 756-759).

The abrupt SST changes observed at Bermuda Rise most likely occurred in response to changes in the North Atlantic thermohaline circulation, which is responsible for transporting heat from the warm tropical latitudes to the cold northerly latitudes.  Measurements of the cadmium content of bottom-dwelling Foraminifera allow the reconstruction of past circulation changes since cadmium is depleted in waters from the North Atlantic and enriched in waters from the South Atlantic.  For instance, at the end of the last interglacial period, ~117,000 years ago, the cadmium content of benthic Foraminifera at the Bermuda Rise increased abruptly in a period of ~400 years (Fig. MIS5e), ushering in the onset of cold glacial conditions throughout the globe.

Paleoceanographic data from Bermuda Rise sediment core MD95-2036, showing variations in accumulation rate of CaCO3 and non-carbonate material, and surface and deep water properties recorded in the shells of fossil foraminifera (Adkins, Boyle, Keigwin, and Cortijo (1997)  Nature 390: 154-156).