Paleoclimate Research: Earth's Climate in Deep Time

Earth's Climate from the Pliocene to Holocene

There are still many gaps in our understanding of future climate change, including the stability of the polar ice sheets and consequences for sea level, the regional responses of the hydrologic cycle and vegetation, and feedbacks of carbon dioxide and methane with the ocean and land as climate changes. We can inform the future on these questions by understanding the past. NCAR paleoclimate modeling in close coordination with proxy data reconstructions integrates Earth as a system, currently looking at the interactions among the atmosphere, ocean and land surface, but progressing now to include interactions with biology, chemistry, and ice sheets.

Glacial-Interglacial Simulations

Building on successful "snapshot" simulations with CCSM, glacial-interglacial simulations being done by Bette Otto-Bliesner and Esther Brady are now focusing on transient climate changes. Our CCSM3 simulation for the Last Glacial Maximum (LGM) included many successes, including a realistic reproduction of the deep ocean temperature and salinity structure and thermohaline circulation, with sea ice controls on water mass formation in both hemispheres important; and tropical cooling matching that recorded by planktonic foraminifera in the Indian and Western Pacific Oceans indicating that CCSM3 has about right the tropical climate sensitivity to CO2. With these successes we received a large computing grant from DOE INCITE to run the first set of synchronously coupled transient ocean-atmosphere-dynamic vegetation GCM simulations of the past 21,000 years (TraCE-21) using CCSM3. The TraCE-21 simulation, in collaboration with Zhengyu Liu and Anders Carlson (U. Wisconsin), Peter Clark (Oregon State), and Rob Jacobs and Dave Erickson (DOE), marks a new era in paleoclimate model-data comparison by allowing for a direct comparison of time series between model and data. It also provides a strong test on CCSM for its climate sensitivity to various forcings, especially, the greenhouse forcing, as well as its capability for the simulation of abrupt climate changes. Otto-Bliesner and Nan Rosenbloom, collaborating with Carrie Morrill and Amy Wagner (CIRES), are also looking in detail at the 8.2ka event, an abrupt climate change associated with anomalous freshwater flow into the North Atlantic.

Workshops are being developed together with our university colleagues to connect scientists, worldwide and across disciplines, to allow intercomparisons and synthesis of these simulations with proxy data. A series of two workshops, SynTraCE-21000, have been planned to prepare a three-dimensional synthesis and database of the transient evolution of the Earth system over the last 21,000 years. The first workshop with over 20 university researchers was held 10-13 August in Madison; the second workshop will be held in summer 2009 at NCAR. We are also active participants in PMIP with our simulations for LGM and mid-Holocene available on the PMIP database for access worldwide. Bette Otto-Bliesner is also organizing the next PMIP2 Workshop, to be held 13-19 September in Estes Park, where over 70 international participants will discuss future model-data intercomparison projects and develop a White Paper of proposed paleoclimate simulations for AR5.

Mid-Pliocene Simulations

In 1999 the United States Geological Survey (USGS) created PRISM2 (Pliocene Research, Interpretation and Synoptic Mapping datasets, version 2), a set of gridded datasets for sea surface temperatures, sea ice, geography, and vegetation for the mid-Pliocene (~3 million years ago). This period is possibly the closest paleo analog for the equilibrium climate with current CO2 levels. A CAM/CLM simulation has been run for this time period by Bette Otto-Bliesner and Nan Rosenbloom and intercompared to similar simulations with the GISS and HadAM models. In addition, working with SOARS student Zi Zi Searles of San Francisco State University, sensitivity simulations have been run warming sea surface temperatures even more, based on new proxy observations, for the California, Peruvian, North African, and South African margin. These simulations show the importance of warm sea surface temperatures off the coast of California for correctly simulating proxy indicators of a wetter western US during the mid-Pliocene. These simulations fit closely with future themes of the Paleoclimate Modeling Intercomparison Project (PMIP).

PMIP and IPCC

The Paleoclimate Modeling Intercomparison Project (PMIP) is a long standing initiative endorsed by both WCRP and IGBP, coordinating paleoclimate modelling activities that provide valuable information on the mechanisms of climate change, the identification of key feedbacks operating in the climate system and, through model evaluation, the capability of climate models to reproduce climates different from today. PMIP results have been used extensively in past IPCC assessments. At its next workshop in September 2008, paleo scientists will identify key climate targets for model simulations and data synthesis that can help reduce uncertainties in future climate projections. Specific simulations of key past time periods will be proposed to the WCRP WGCM as potential contributions of PMIP to the IPCC Fifth Assessment (AR5).