October 2005
Climate Change
http://www.researchmatters.harvard.edu/
story.php?article_id=807§ion=earth
"We are performing an experiment on a planetary scale that hasn't been done for millions of years," Schrag said.
"Nobody knows what's going to happen and there will be surprises."
Climate scientist Daniel Schrag says that human-caused climate change is inevitable, though scientists don't know exactly how severe or even exactly what its effects will be. Schrag said the public health effects related to climate change would probably be most severe in poorer nations.
Though climate effects will be experienced in richer nations, those countries have the resources to adapt and protect the health of their citizens. Schrag pointed to Hurricane Mitch, which devastated Honduras in 1998, compared with 2004's series of hurricanes that slammed into Florida, with relatively low loss of life. Though some researchers expect that a warmer environment will mean a spread of infectious tropical diseases, like malaria, into cooler latitudes, Schrag said he thought developed nations' public health systems are up to the challenge. Though poorer nations may be hardest hit by climate change, Schrag said it would be a mistake to divert attention from global warming to pour resources into developing those nations.
Though there is much uncertainty about global warming, Schrag said, it appears clear that the effects will be dramatic and widespread.
"We are performing an experiment on a planetary scale that hasn't been done for millions of years," Schrag said. "Nobody knows what's going to happen and there will be surprises."
Director, Laboratory for Geochemical Oceanography
Education
B.S., 1988, Yale University
Ph.D., 1993, University of California at Berkeley
Research Interests
Isotope Geochemistry, Paleoclimatology, Geochemical Oceanography
Research Description
My research applies geochemistry to problems in paleoclimatology and oceanography on a variety of timescales. To attack these problems, I combine a variety of modeling techniques (mineral-fluid interaction, chemical transport, and ocean circulation models) with analytical chemistry and mass spectrometry.
One of the goals of my research program has been to develop new ways of extracting information about past climates. I have worked with deep sea pore fluid to reconstruct past changes in the isotopic composition of seawater. Because the sediment-pore water system behaves like a non reactive, diffusive porous medium over timescales of tens of thousands of years, it is possible to reconstruct the amplitude of oscillations in the seawater boundary at the top of the sediment from depth variations in modern pore fluids. Using a similar model of oxygen isotope transport in deep sea sediment, we can calculate the
d18O of seawater during the last glacial maximum, essentially the ice volume component of the foraminiferal record, resolving a problem that has plagued workers since oxygen isotope paleothermometry was first described by Urey in the late 40's. New research continues to develop this approach, including new measurements such as chloride and deuterium (in collaboration with Jess Adkins, Kate McItyre, and Jane Alexander). Deuterium in deep sea pore fluids can also be used to measure the flux of mantle water releaased to the ocean during low-temperature alteration of the crust. In an additional project involving pore fluids, I am studying the isotopic composition of dissolved inorganic carbon. Graduate student Brian Fehlau is collecting data from deep sea sediments to measure the rates of methanogenesis and methanotrphy in anoxic sediments, and provide evidence for chemoautotrophy in carbonate sediments, hundreds of meters below the sea floor.Heather Stoll (former graduate student) and I have been exploring the geochemical cycling of strontium. We speculated that weathering of coral reefs at times when sea level dropped rapidly could dramatically alter the strontium concentration in seawater, introducing variability into a geochemical system most workers have considered to be at steady state. Our initial focus was on Cretaceous limestones, exploring the question of whether large sea level changes suggested by the Exxon curve could be global and caused by ice accumulation on continents. More recently, we have been studying variations in the Pleistocene. We have calculated that changes in the Sr/Ca ratio of seawater due to reef weathering are small but significant over Pleistocene glacial cycles, and may bias coral-based reconstructions of past ocean temperatures by approximately 1 degree. Much more work remains to be done on Cretaceous climates, and our efforts are now focused on land sections from Italy and Spain. In addition, post-doc Ros Rickaby and I are exploring biomineralization effects on incorporation of strontium into coccoliths, with the hope that we can constrain calcification rate and relate that to growth rate more generally. The work on strontium has led to an investigation of other minor elements in calcite shells. Former post-doc Katharina Billups is working with me on using Mg/Ca in foraminifera to reconstruct ocean temperatures on a variety of time scales in the Cenozoic.
A large portion of my current research effort uses corals as recorders of information on past and present climates. Research on corals from the Red Sea, Indonesia and the western Pacific is directed at characterizing seasonal and interannual climate variability through geologic time. Gidon Eshel and I have used modern corals from the Red Sea to study surface ocean processes and climate dynamics of this hyperarid region. Modern corals from the Pacific are being used to reconstruct El Niņo variability over the last few centuries, and to assess the reliability of coral records. A critical development has been the invention of a new technique for measuring high-precision Sr/Ca ratios in corals much more rapidly than previous efforts, so that construction of data sets with several thousand analyses is now routine. I have also applied this approach to the geologic past. Using giant corals from uplifted terraces around Indonesia, former post-doc Konrad Hughen and I examined seasonal variations in the oxygen isotopic composition of coral aragonite for windows of time over the last several thousand years. Recent results from a coral that grew during the last interglacial period (123,000 years ago) show that the El Niņo variability was very similar to the modern period (with the exception of the last 30 years which seem to be anomalous). Work on fossil corals will continue, with the goal of determining how El Niņo varies in frequency and intensity at times when the mean climate was different from today.
I am also using geochemistry of corals to understand recent patterns of ocean circulation. Tom Guilderson (former post-doc) and I are investigating seasonal radiocarbon variability in modern coral material to understand seasonal and interannual changes in ocean circulation, in collaboration with scientists from the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory. We have mapped out the post-bomb radiocarbon variability at a variety of locations in the tropical and subtropical Pacific. The radiocarbon project involves a substantial modeling effort in collaboration with Keith Rogers (now at the Max Plank Institute in Hamburg) and Mark Cane at Columbia. Our goal is to use the radiocarbon data to test the ocean circulation model and improve its skill. In the course of this project, Tom Guilderson and I discovered an unusual shift in the radiocarbon content of a coral growing in the Galapagos Islands in 1976 that we think is related to a deepening of the thermocline in the eastern tropical Pacific. This shift may be related to the increase in intensity and frequency of El Niņo since 1976.
A new effort on reconstructing recent climate variability at sub-annual resolution involves isotopic measurements on trees. Post-doc Michael Evans and I are developing a rapid method for analysis of oxygen isotopes in cellulose. We hope to apply this method to trees from many different locations that are particularly sensitive to interannual climate variability. Graduate student Pascale Poussart is working with me and Bill Jenkins (Univ. of Southampton) to measure the tritium content of seawater using mangrove trees, as these trees get their water (and hence tritium) from seawater but the wood grown in the last three decades can be dated with radiocarbon.
A new research interest, thanks to collaborations with colleague Paul Hoffman, involves the unusal climatic conditions that occurred at the end of the Proterozoic, just before the first evidence of multicellular animals. Based on observations from years of work by Paul and his colleagues in Namibia, we suggested that the Earth froze over from equator to pole (i.e., a Snowball Earth). The hypothesis we proposed suggests that, once sea ice covered the world's oceans, the high albedo of ice locked the Earth into this unusal climate state for approximately 10 million years until volcanic activity released enough carbon dioxide to raise the Earth's natural greenhouse atmosphere to the point where the ice could melt. The deglaciation would have occurred very rapidly (probably less than a few hundred years), and the Earth would end up in an ultra-greenhouse state, with surface temperatures at the equator in excess of 50°C. These conditions would drive rapid weathering of fresh silicate material, producing carbonate deposits ("cap" carbonates) that are observed globally on top of Neoproteroic glacial deposits. We proposed that this extreme climate fluctuation led to the emergence of metazoa, and ultimately to the Cambrian explosion that followed. We are continuing to work on Namibian rocks, looking for new ways to test our hypothesis, as well as looking at other similar deposits on other continents. We are also collaborating with Ray Pierrehumbert from the University of Chicago to understand the climate dynamics of this unusual time interval.
Classes
EPS 8: Earth History (with P. Hoffman)
EPS 107: Environmental Geochemistry
EPS 242: Biogeochemistry of Light Stable Isotopes (with J. Hayes)
EPS 281r Snowball Earth Phenomena (with P. Hoffman)
ESPP 90: Population and the Human Condition (with J. Holdren)
H.M. Stoll and D.P. Schrag, Glacial control of rapid sea level changes in the Early Cretaceous, Science, 272, 1771-1774, 1996.
Schrag, D.P., G. Hampt, D.W. Murray, The temperature and oxygen isotopic composition of the glacial ocean, Science, 272, 1930-1932, 1996.
Moore, M.D., D.P. Schrag and M. Kashgarian, Radiocarbon seasonality in the Western Pacific and Indonesian seaway from coral AMS measurements, JGR-Oceans, 102, 12,359-12,365, 1997.
Rodgers, K.B., M.A. Cane and D.P. Schrag, Seasonal variability of sea surface
D14C in the equatorial Pacific in an ocean circulation model, JGR-Oceans, 102, 18,627-18,639, 1997.Stoll, H.M. and D.P. Schrag, Effect of Quaternary sea level cycles on the Sr concentration of seawater, Geochim. Cosmochim. Acta, 62, 1107-1118, 1998.
Guilderson, T.P. and D.P. Schrag, Abrupt shift in subsurface temperatures in the Eastern Tropical Pacific associated with recent changes in El Niņo, Science, 281, 240-243, 1998.
Hoffman, P.F., A.J. Kaufman, G.P. Galverson and D.P. Schrag, A Neoproterozoic snowball earth, Science, 281, 1342-1346, 1998.
Guilderson, T.P., D.P. Schrag, M. Kashgarian and J. Southon, Radiocarbon variability in the western equatorial Pacific inferred from a high-resolution coral record from Nauru Island, JGR-Oceans, 103, 24,641-24,650, 1998.
Schrag, D.P., Rapid determination of high-precision Sr/Ca ratios in corals and other marine carbonates, Paleoceanography, 14, 97-102, 1999.
Guilderson, T.P. and D.P. Schrag, Reliability of coral isotope records from the western Pacific warm pool: A comparison using age-optimized records, Paleoceanography, 14, 457-464, 1999.
Rodgers, K.B., M.A. Cane, N.H. Naik and D.P. Schrag, The role of the Indonesian Throughflow in equatorial Pacific thermocline ventilation, JGR-Oceans, 104, 20,551-20,570, 1999.
Hughen, K.A., D.P. Schrag, S.B. Jacobsen, and W. Hantoro, El Niņo during the last Interglacial recorded by fossil corals from Indonesia, Geophys. Res. Lett., 26, 3129-3132, 1999.
Schrag, D.P., Effects of diagenesis on late Paleogene equatorial sea surface temperatures, Chemical Geology, 161, 215-224, 1999.
Stoll, H.M., D.P. Schrag and S. Clemens, Are seawater Sr/Ca variations preseerved in Quaternary foraminifera?, Geochim. Cosmochim. Acta, 63, 3535-3547, 1999.
Hughen, K.A., D.P. Schrag, S.B. Jacobsen, and W. Hantoro, El Niņo during the last Interglacial recorded by fossil corals from Indonesia, Geophys. Res. Lett., 26, 3129-3132, 1999.
Hoffman, P.F. and D.P. Schrag, The Snowball Earth, Scientific American, 282, 68-75, 2000.
Billups, K. and D.P. Schrag, Surface density of the tropical and
subtropical oceans during the Last Glacial
Maximum, Paleoceanography, 15, 10-123, 2000.
Stoll, H.M. and D.P. Schrag, High resolution stable isotope records from the Upper Cretaceous of Italy and Spain: Glacial episodes in a greenhouse planet?, G.S.A. Bulletin, 112, 308-319, 2000.
Gagan, M.K., L.K. Ayliffe, J.W. Beck, J.E. Cole, E.R.M. Druffel, R.B. Dunbar and D.P. Schrag, New views of tropical paleoclimates from corals, Quaternary Science Reviews, 19, 45-64,2000.
Schrag, D.P., Of ice and elephants, Nature, 404, 23-24, 2000.
Eshel, G., D. P. Schrag and B. F. Farrell, Troposphere - Planetary Boundary Layer Interactions and the Evolution of Surface Density: Lessons from Red Sea Corals, J. Climate, 13, 339-351, 2000.
Stoll, H.M. and D.P. Schrag, Coccolith Sr/Ca as a new indicator of coccolithophorid calcification and growth rate Geochem., Geophys., Geosyst., 1, Paper # 1999GC000015, 2000.
Rodgers, K.B., D.P. Schrag, M.A. Cane and N.H. Naik, The bomb-14C transient in the Pacific ocean, JGR-Oceans, 105, 8489-8512, 2000.
Alexander, J.L. and D.P. Schrag, Mantle influences on deuterium in deep sea pore fluids, Earth Planet. Sci. Lett., in press 1999.
Guilderson, T.P., D.P. Schrag, E.A. Goddard, M. Kashgarian, G.M. Wellington, and B.K. Linsley, Southwest subtropical Pacific surface water radiocarbon in a high-resolution coral record, Radiocarbon, in press 1999.
O’Brien, D.M., D.P. Schrag, Alloction of nectar nutrients to reproduction in Amphion Floridensis: A novel quantitative method using stable carbon isotopes, Ecology, in press 1999.
Adkins, J.F. and D.P. Schrag, Pore fluid constraints on deep ocean temperature and salinity during the Last Glacial Maximum, Geophys. Res. Lett., in press, 2000.
Stoll, H.M. and D.P. Schrag, Sr/Ca variations in Cretaceous carbonates: Relation to productivity and sea level changes, submitted to Palaeogeography, Palaeoclimatology, Palaeoecology, 1999.
Reuer, M.K., D.P. Schrag, L.D. Keigwin, The oxygen-18 content of glacial
seawater:
Pore fluid constraints from the western North Atlantic, submitted to Paleoceanography,
2000.
Laboratory for Geochemical Oceanography
Department of Earth and Planetary Sciences
Harvard University
20 Oxford St.
Cambridge, MA 02138
(617) 495-7676 (phone)
(617) 496-4387 (fax)
October 5, 2005