Stephen Burns Dr. Stephen J. Burns Department of Geosciences University of Massachusetts USA If you would like to interview this scientist, please contact Leah Christen. > biographical information > abstract > paper

Biographical Information:

I received my PhD from Duke University, Durham NC, USA, in 1987, where I studied the origin and geochemistry of dolomite together with Paul Baker. From there I went on to a two-year post-doctoral position at the University of Miami, and in 1989 I moved to the University of Bern, Switzerland. In Bern, I headed the Stable Isotope Laboratory and began working on climate problems, particularly in the region of southern Arabia. Our initial studies using stalagmites to study climate change in this region produced such informative results that using speleothems to study climate is now easily the main focus of my work. I have done speleothem-related climate studies and fieldwork in Oman, Yemen, Austria, Costa Rica, Panama, the southwestern US and Brazil. In 2001, I took a faculty position at the University of Massachusetts, where I am in the Department of Geosciences, oversee the Stable Isotope Laboratory and am a member of the Climate System Research Center.

Abstract:

The Tropics and Rapid Climate Change - Records of Changes in Rainfall and Atmospheric Circulation from Speleothems

Precipitation in the tropics is dominated by seasonal movement of the intertropical convergence zone over the oceans and associated development of the monsoons on land. On geologic timescales, the mean location and intensity of tropical rainfall can vary greatly. These changes are reflected in the oxygen isotope ratio of rainfall, which is recorded in the oxygen isotope ratio of speleothem calcite. Studies of the timing and pattern of changes in precipitation can help identify causal mechanisms.

During the last glacial period, speleothem records of precipitation from the Northern Hemisphere tropics show a remarkable correspondence to changes in temperature at high northern latitudes. The millennial-scale Dansgaard/Oeschger cycles of slowly decreasing and rapidly increasing temperature recorded in Greenland ice cores appear as slowly decreasing and rapidly increasing monsoon rainfall in stalagmites from Socotra Island in the Indian Ocean. Results from a speleothem from Brazil in the Southern Hemisphere, however, show very little variation in tropical climate associated with the D/O cycles. Instead, variation in precipitation and circulation follow the precessional cycle of solar insolation. Taken together, these records suggest that the northern limit of annual ITCZ migration was strongly influenced by ice cover and temperatures in the high northern latitudes, while the southern limit of ITCZ migration remained primarily under the influence of solar insolation.

During the Holocene, rainfall and ITCZ migration in both the Northern and Southern Hemisphere tropics appear to respond in a nearly linear fashion to insolation. Speleothems from both Oman and Brazil indicate a slow southward migration of the mean ITCZ location, which resulted in decreasing Indian Monsoon precipitation and increasing South American Monsoon precipitation. Superimposed on these trends, at least in the Northern Hemisphere, are rapid climate events associated with temporary decreases in NADW formation at 8.2 ky BP, and less prominently at 9.2 ky BP.

Paper:

The tropics and rapid climate change – records of changes in rainfall and atmospheric circulation from speleothems

The tropics cover 40% of the globe, receive 47% of the earth’s potential solar insolation and contain 35% of the earth’s population. Tropical climate is dominated by seasonal migration of surface low-pressure systems that follows the locus of maximum solar insolation. Over the ocean this low-pressure region is the inter-tropical convergence zone, and over land surface low-pressure drives the monsoons of India, East Asia, Africa and South America. Ocean and atmospheric circulation also link the tropics to mid- and high-latitude climate. More solar energy is received than is returned to space in the tropics, resulting in a net excess of heat, which must be exported to higher latitudes via the atmosphere and oceans. The export of heat and moisture from the tropics links climates of low and high latitude regions. In turn, high-latitude processes such as deep-water production, sea-ice, and snow-cover can affect the sinks for exported tropical heat and change global and tropical climates.

Speleothems, calcium-carbonate cave deposits, are a nearly ideal archive for studying tropical climate change and the relationship between high and low-latitude climate. The oxygen isotope ratio of speleothems records changes in the intensity and location of tropical rainfall, and speleothems can be accurately dated. Past changes in rainfall amount and source can be reconstructed for the region above a cave at up to annual resolution and for time periods as long as 400,00 years. Figure 1 shows recovery of a stalagmite from a cave in Brazil.

My colleagues and I have used speleothems to reconstruct tropical climate over much of the past 120,000 years principally in two areas, southern Arabia and Brazil. Our results show that during the Last Glacial Period, from about 120,000 to about 10,000 years ago, when large ice sheets covered much of northern North America and Europe, climate change at high latitudes was remarkably similar to that of the northern Indian Ocean tropics. The millennial-scale, repeated pattern of gradual cooling and sudden warming, called Dansgaard/Oeschger cycles, observed in the North Atlantic region was mirrored in the tropics by gradual decreases and sudden increases in precipitation. We have interpreted decreasing precipitation to be the result of an increasingly more southerly position of the ITCZ during the Boreal summer. It is not yet certain whether the D/O cycles originate at high or low latitudes.

Figure 2 shows the similarity of climate as recorded by ice in Greenland and by a stalagmite from Socotra Island in the Indian Ocean.

One hint that these events might originate in the high latitudes, however, comes from comparison of high and low latitude climate records of the Holocene. The generally warm climate of the early Holocene, after the end of the Last Glacial Period, was interrupted in the North Atlantic region by a short (<100 years), intense cold event at ~8.2 ky B.P. This event was triggered by an outburst of glacial melt water into the North Atlantic Ocean. Just as with the D/O cycles, the cold event is reflected by a sharp decrease in tropical precipitation in the Indian Ocean as recorded by stalagmites in northern and southern Oman. Other, smaller meltwater events in the North Atlantic also seem to have resulted in decreases in precipitation in the Indian Ocean. A further hint may be that the D/O events are only weakly recognized in stalagmites that reflect climate change during the Last Glacial Period in the Southern Hemisphere from Brazil.