Models of past climate conditions, including the position of sea level and the advance and retreat of land-based glaciers and ice sheets, rely on numerous sources of information. Tree rings, fossils, and sediment layers are only a few examples. These data sources preserve the past in different ways and with differing degrees of precision. Constructing an accurate, coherent model requires accessing and integrating these various types of data, as well as understanding the strengths and limitations of each type.
The exquisitely exposed last interglacial (LIG) fossil coral reef in the Yucatán Peninsula, Mexico, served as the backdrop to the fifth and final meeting of the Paleo Constraints on Sea Level Rise 2 (PALSEA2) working group. The workshop highlighted current research on ice sheet and sea level reconstructions; addressed critical gaps in field observations; and assessed the current knowledge of causes, rates, and mechanisms related to sea level and ice sheet dynamics during past warm periods. The 5-day program included more than two dozen presentations, a poster session, and field excursions to a fossil coral reef at Xcaret and a coastal limestone cave at Rio Secreto.
A major theme throughout the conference was the need to combine existing paleodatabases into a centralized global paleo–sea level/ice sheet compilation with a streamlined user interface. Participants discussed standardizing, interpreting, and assessing the quality of field data as key components for integration and application by the modeling community. For instance, some presenters considered how sample elevation does not necessarily equate to relative paleo–sea level and must be viewed in the context of indicative range, whereas others discussed complications of interpreting sea level from coral reef structures where accretion is often determined by storm deposition of coral rubble.
Glacial isostatic adjustment (GIA), how land deforms under the decreasing weight of melting glaciers, was another key topic; participants discussed ways to improve quantifying the uncertainties of GIA in global sea level signals. These uncertainties can have profound effects on calculated local sea levels that must be accounted for during the course of an interglacial period.
Participants examined GIA uncertainty stemming from ice sheet configuration and 3-D Earth model parameters, and they emphasized the need for additional paleo–sea level elevation data. They also stressed the need to understand glacial ice sheet volume and spatial coverage prior to the last glacial cycle, especially for the penultimate glacial maximum. This understanding is essential for determining peak LIG sea level reconstructions. Discussion included how cosmogenic nuclide techniques have the potential to help constrain these glacial ice extents over million-year timescales.
Dynamic topography caused by mantle convection is another parameter that could have substantial effects on the elevation of sea level indicators from the LIG to the Pliocene. Determining these effects currently involves substantial uncertainty in terms of vertical errors. Thus, participants recommended larger uncertainty bars for reported estimates of the peak sea level reached during the LIG, currently assessed at 6–9 meters above the present global mean sea level.
Attendees also stressed that Pliocene sea level remains to be of key interest because atmospheric carbon dioxide concentrations during this period were similar to those today. Thus, information from that time could offer insights into the relations between greenhouse gas concentrations in the atmosphere and climate-induced sea level changes.
Attendees agreed that long-term objectives for the paleoclimate modeling community should include creating broader consistency across the various scientific disciplines to define model error and standard results and endorsing transparency and open-source records for modeling and data acquisitions.
We thank the organizers of the PALSEA2 Workshop, namely, Andrea Dutton, Anders Carlson, Glenn Milne, Antony Long, and Paul Blanchon, as well as the funding organizations: Past Global Changes (PAGES) and the International Union for Quaternary Research (INQUA).
—Michael R. Sandstrom (email: [email protected]), Lamont-Doherty Earth Observatory, Columbia University, New York, N.Y.; Alexandra Skrivanek, University of Florida, Gainesville; and Jeremy Shakun, Boston College, Chestnut Hill, Mass.