Bette Otto-Bliesner Dr. Bette L. Otto-Bliesner National Center for Atmospheric Research USA If you would like to interview this scientist, please contact Leah Christen. > biographical information > abstract > paper

Biographical Information:

I am a scientist and head of the Paleoclimate Group at the Climate and Global Dynamics Division of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. My current research concentrates on understanding climate variability and climate sensitivity during the Quaternary with coupled atmosphere-ocean models, particularly for the present interglacial (the Holocene), the Last Glacial Maximum (21,000 years before present) and the Last Interglacial (120,000 to 130,000 years before present). I am a lead author for Chapter 6 Paleoclimate of the Intergovernmental Panel on Climatic Change (IPCC) Fourth Assessment Report (AR4) and a Steering Committee member for the second phase of the Paleoclimate Modeling Intercomparison Project (PMIP-2). I have been co-chair of the Paleoclimate Working Group of the NCAR Community Climate System Model (CCSM) since its inception in 1996. I also chair the American Geophysical Union Paleoceanography and Paleoclimatology Focus Group.

Abstract:

Climate Sensitivity Derived from PMIP-2 Model-Data Intercomparisons for the Last Glacial Maximum and Mid-Holocene

Climate models are tuned to reproduce the current mean climate state. Their ability to predict the large climate changes of the future can be critically assessed with the large forcings of past climate states. The Paleoclimate Intercomparison Project (PMIP) is a long-standing initiative endorsed by the World Climate Research Programme (WCRP; JSC/CLIVAR working group on Coupled Models) and the International Geosphere-Biosphere Programme (IGBP; GAIM and PAGES). The second phase of PMIP (PMIP-2), launched in Autumn 2003, is evaluating fully-coupled ocean-atmosphere and ocean-atmosphere-vegetation models in simulating the climates of the mid-Holocene (6,000 years before present) and the Last Glacial Maximum, LGM (21,000 years before present).

In this talk, I will present results of coupled simulations of the mid-Holocene and LGM that have been done at modeling centers worldwide using a standard set of forcings and boundary conditions for these two time periods. Model-model intercomparisons address climate sensitivity, the role of ocean, land-surface and sea-ice feedbacks, and interannual to multi-decadal variability that can be compared to predictions of future climate change of these coupled models. Model-data comparisons provide benchmarks of the model simulations, particularly the ability of these coupled models in capturing shifts in vegetation associated with past changes in the mean climate and its variability, and in reproducing changes in the surface and deep ocean and associated thermohaline circulation.

PMIP results formed a crucial part of the evaluation of climate models in the Third Assessment Report of the Intergovernmental Panel on Climatic Change (IPCC) and will contribute importantly to the IPCC Fourth Assessment Report (AR4). The extensive model-model and model-data comparisons for the LGM and mid-Holocene of PMIP-2 will provide another metric for constraining climate sensitivities of the models included in AR4 assessment of future change. Key results and diagnostics from PMIP-2 that are relevant for future climate change and that will help us evaluate models used for IPCC-AR4 scenarios will be discussed.

Paper:

Climate Sensitivity Derived from PMIP-2 Model-Data Intercomparisons for the Last Glacial Maximum and Mid-Holocene

Why is it important to model past climates?
The Working Group 1 component of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) will include projections of climate into the 21st and 22nd centuries from many participating coupled atmosphere-ocean models. Climate models are first judged on how well they reproduce the present-day climate. Comparisons are made to average climate of the Earth such as temperature and precipitation, sea-ice extent and ocean currents, and to year-to-year and decade-to-decade variability of this climate associated with El Niño and the North Atlantic Oscillation. Climate models are then tested for past climates. Comparisons for past climates, such as being done by the Paleoclimate Modeling Intercomparison Project (PMIP), give us confidence that climate models can give us useful predictions of what our future climate may be as land uses change and greenhouse gases increase.

What is a climate model?
Climate models are based on models that have been developed to predict our weather. They have been expanded to include the circulation of the atmosphere and ocean and interactions with vegetation and the atmospheric trace gases, such as carbon dioxide and methane. The climate system is described in terms of basic physical laws. Scientists use computers to solve a series of equations representing these laws on a three-dimensional grid that covers the Earth and extends from the ocean floor upward into the atmosphere. Climate models can be costly to use and require supercomputers to produce numerical solutions. To recreate a single day of the world’s climate, some models, such as the National Center for Atmospheric Research (NCAR) Community Climate System Model (CCSM), must perform over 700 billion calculations.

What do the Last Glacial Maximum and Holocene tell us about climate change?
The magnitude of climate changes for the Last Glacial Maximum (21,000 years ago) and the mid-Holocene (6,000 years ago) are larger than changes observed over the last few millennia. As such, simulations for these periods by climate models provide rigorous tests of our understanding of the climate system under changed mechanisms that drive climate change, and of proposed explanations for numerous proxy records extracted by scientists from the geological archives. For the mid-Holocene, changes in the Earth’s orbital character resulted in a 5% increase of summer solar radiation in the Northern Hemisphere. Climate models respond by generating more intense monsoons in Africa and Asia. Lower levels of atmospheric greenhouse gases and the presence of large ice sheets over North America and Eurasia produced a colder and drier climate at the Last Glacial Maximum. PMIP-2 models give a range of global cooling of 3-5°C and polar amplification of this cooling.

What are the next steps?
Rapid climate changes associated with freshening of the North Atlantic by increased precipitation or melting of Greenland are of current interest. A recent cold and dry abrupt event in the Northern Hemisphere about 8,200 years ago, generally attributed to an outburst of glacial water into the North Atlantic, provides a valuable test of climate models. Another test of climate models will be their ability to predict past periods of glacial inception and thus their validity for predicting the next ice age.

Figure 1.