Deep Time to Our Time: The Scale Factor in Climate Change A paper (with illustrations) presented at: Global Warming – Scientific Controversies in Climate Variability KTH, Stockholm, Sept. 11-12, 2006 R.M. Carter Marine Geophysical Laboratory James Cook University, Townsville, Qld. 4811 Email: email@example.com Climate change is a geological as much as a meteorological phenomenon. Yet contemporary public discussion of the issue - influenced by the short-termist approach of the Intergovernmental Panel on Climate Change (IPCC) - is concerned, first, with the minutiae of temperature change over the last two decades of the 20th century; and second, with the claimed abnormality of late 20th century warmth as compared with the preceding, trivially short, 2000-year long, Christian era. At best, our instrumental record of climate extends back for about 140 years, and that only for a small number of locations worldwide. It is only since the deployment of satellite sensors in the late 1970s that a high quality, genuinely global meteorological dataset has become available. The ensuing 25 year-long data series is shorter than even one “climate normal” interval, and therefore of inadequate length to say anything very useful about climate change. Geological records of climate change offer the great advantage of covering adequate time spans to reflect natural climate change on all scales. However, they have their own inadequacy in being inescapably based upon the measurement of proxy indicators. For example, past temperatures are estimated from oxygen isotope data from polar ice cores or oceanic mud cores. Consideration of the last 16.000 year part of the well-dated, totemic Greenland ice- core record exemplifies the ambiguity of using naive linear curve-fitting to answer the apparently simple question “is warming occurring”, for the answer is controlled entirely by the choice of start and end points (Davis & Bohling, 2001). In Greenland, at least, warming has taken place since 16,000 years ago, and also since 100 years ago. Over intermediate time periods, however, cooling has occurred since 10,000 and 2.000 years ago, and temperature stasis characterizes both the last 700 years and (globally, from meteorological records) the last 7 years. Both the 7 and 100 year - long intervals are too short to carry statistical significance regarding long-term climate change. Therefore, the most useful comment that can be made about such short-term data is that neither the rate nor the magnitude of temperature change in Greenland during the late 20th century exceeds the natural levels recorded in earlier instrumental and geological records. The IPCC concentrates heavily on a radiative model of climate change. This approach has been criticized by Kininmonth (2004) because it under-estimates the influence on climate of the major meriodional heat flows transported within the world’s atmosphere and ocean. The importance of oceanic heat flows is reinforced by Lyman et al. (2006), who report that the global surface ocean heat anomaly has decreased over the last two years, a finding which should lead us to reflect on the imbalance between the time constants for heat turnover in the atmosphere (1 year) and ocean (1000 years). A better understanding of climate change must involve a more accurate treatment of the coupling of oceanographic and atmospheric heat flows, and, in this regard, climate records from the mid-latitudes are of great importance. Ocean drilling in the New Zealand region, Southwest Pacific Ocean, has yielded important information about climate change, including the magnitudes and rates of meridional heat transfer between south polar and tropical regions. DSDP Sites 277 (520 S; 1210 m water depth) and 279 provided the first extended Southern Ocean oxygen isotope record reflecting ocean temperature since about 60 Ma (Shackleton & Kennett, 1975). Subtropical warmth in the Eocene declined gradually towards the 33.5 Ma Eocene-Oligocene boundary, where a sharp step- cooling of several degrees corresponds to Antarctic glaciers first arriving at the coast, with the subsequent formation of cold deep water. Temperature then fluctuated spasmodically through the Oligocene and Miocene, to reach a warm peak again at about 15 Ma, after which the temperature declined into the worldwide Pliocene- Pleistocene glaciations. The resolution of this record is only about 1 My, but that is adequate to suggest that the major climatic fluctuations seen were forced by regional tectonic and global oceanographic and atmospheric controls. ODP Site 1123, located at latitude 420 S and 3290 m water depth, lies beneath the Pacific Deep Western Boundary Current (DWBC). The ~20 Sv flow of the DWBC represents about 40% of the input into the global ocean of cold, deep water, and is thus a major agent of heat transfer. The oxygen isotope record from this site is typical for the world ocean, and implies water temperatures that were significantly warmer than today’s during past interglacials MIS 5, 9 and 11 (Hall et al., 2001). Parallel grainsize studies reveal that fluctuations of intensity of the DWBC are closely coupled with the climate signal, with stronger current speeds during glacial intervals. Even small fluctuations in the strength of DBWC, or overlying AAIW, flow will of course cause significant variations in the world surface ocean heat anomaly (cf. Lyman et al., 2006). ODP Site 1119, located at latitude 440 S, 395 m water depth, and beneath Subantarctic Mode Water (SAMW), contains a 4 My-long record of shallow intermediate water flow. Grainsize fluctuations at this site suggest that stronger intermediate water flows occur primarily during warm interglacial periods (Carter et al., 2004a). Natural gamma ray (NGR) measurements from the site reflect the delivery of K-rich mud from the nearby New Zealand Southern Alps, and yield a detailed local ice-volume record. This (inferred atmospheric) signal corresponds closely at millenial scale with the Antarctic plateau temperature record reconstructed from the Vostok ice core, and demonstrates a close integration of the changing climate system across at least 450 of latitude (Carter et al., 2004b). DSDP Site 594 lies a little seawards of Site 1119 and, at a depth of 1204 m, under AAIW. The site has yielded a classic climatic record back to about MIS 19 and beyond, and contains similar climatic lithological layering as that at 1119 (Nelson et al., 1985; 1993). High resolution (1 mm spacing) colour reflectance measurements provide a detailed record of changing carbonate content on a decadal time-scale (Holland et al., 2005). Over the cold period between 20 and 30 ka, the 594 reflectance scans exhibit two 5,000 year climatic cycles that are modulated by continually varying decadal climate fluctuations of similar wavelength and magnitude to those seen in the 20th century global average temperature record. Advancing our knowledge of climate change requires the collection of more and better extended climate records from the marine realm. Alley (2003) has argued that there is a strong need “to generate a few internationally coordinated, multiply replicated, multiparameter, high time resolution type sections of oceanic (climate) change”, using similar scientific protocols to those applied to polar ice coring. Because of the energy flows that pass through them, the southern mid-latitudes are a critical place where to locate one such type section. ODP Proposal 590-Pre proposes to create the SnowMELT climate transect, a line of drillholes along latitude 450 S that supplement the historic sites 594 and 1119 with new multi-cored sites located in the South Island glacial lakes and on the west side of South Island. Cores from the New Zealand region carry the unique advantages of high sedimentation rates (i.e. high resolution climate signals), the availability of proxy measures for both atmospheric and marine climate change, and the ability to study the extended time periods required to elucidate the “deep time to our time” record of natural climate change. REFERENCES Alley, R.B. 2003 Raising paleoceanography. Paleoceanography 18, 9-1 to 9-2. DOI 10.1029/2003PA000942 Carter, R.M., Gammon, P.R. & Millwood, L. 2004a Glacial-interglacial (MIS 1-10) migrations of the Subtropical Front (STF) across ODP Site 1119, Canterbury Bight, Southwest Pacific Ocean. Marine Geology 205, 29-58. Carter, R.M. & Gammon, P. 2004b New Zealand maritime glaciation: millennial-scale southern climate change since 3.9 Ma. Science 304, 1659-1662. Davis, J.C. & Bohling, G.C. 2001 The search for patterns in ice-core temperature curves. In: Gerhard, L.C. et al. (eds.), Geological Perspectives of Global Climate Change, American Association of Petroleum Geologists, Studies in Geology 47, 213-229. Hall, I.R., McCave, I.N., Shackleton, N.J., Weedon, G.P. & Harris, S.E. 2001 Glacial intensification of deep Pacific inflow and ventilation. Nature 412, 809-811. Holland, M.E., Schultheiss, P.J., Carter, R.M., Roberts, J.A. & Francis, T.J.G. 2005 IODP's untapped wealth: multi- parameter logging of legacy core. Scientific Drilling 1, 50-51. Kininmonth, W. 2004 “Climate Change: a Natural Hazard”. Multi-Science Publishing, Brentwood, Essex, 207 pp. Lyman, J.M., Willis, J.K. & Johnsopn, G.C. 2006 Recent cooling of the upper ocean. Geophys. Res. Lett., in press. Nelson, C.S., Hendy, C.H., Jarrett, G.R. & Cuthbertson, A.M. 1985 Near-synchroneity of New Zealand alpine glaciations and Northern Hemisphere continental glaciations during the past 750 kyr. Nature 318, 361-363. Nelson, C.S., Cooke, P.J., Hendy, C.H. & Cuthbertson, A.M. 1993 Oceanographic and climate changes over the past 150,000 years at Deep Sea Drilling Project Site 594 off southeastern New Zealand, southwest Pacific Ocean. Palaeoceanography 8, 435-458. Shackleton, N.J. & Kennett, J.P. 1975 Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: oxygen and carbon isotope analyses in DSDP Sites 277, 279, and 281. Init. Repts. DSDP, vol. XXIX, Washington, U.S. Govt. Printing Office, pp. 743-755.
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