Data-Model Comparisons

North American Climate Model Maps

Climate Model Comparisons
The maps above represent the differences between Growing Degree Days (GDD) for 6,000 years ago and present-day. The upper nine maps were produced by climate simulations from different models, while the map on the lower right represents the same GDD difference inferred from pollen. Click to enlarge. (62 KB)

Global climate models are now being enlarged to earth-system models in which the vegetation and ocean models are coupled to the climate models. These models are powerful tools for simulating past changes in climate, vegetation, and hydrological conditions. The simulations show how the changing controls on climate from seasonal insolation, carbon dioxide concentrations, and ice sheet size may have affected regional and continental climate and environmental patterns. Large sets of pollen and lake-level data now exist for checking the results of these models, and Brown researchers have pioneered in interpreting and analyzing these data and using them to test the model simulations. One of the first tasks of COHMAP (Cooperative Holocene Mapping Project) was to assemble a global set of data for the last glacial maximum (LGM, which was 18,000 radiocarbon years ago or 21,000 calendar years ago) to check the initial climate modeling simulations for LGM by Gates (1976, Science) and Manabe and Hahn (1977, JGR).This Peterson et al. (1979, Quaternary Research) paper was the start to the international, interdisciplinary, interinstitutional effort that COHMAP researchers headed up, over the next decade, to assemble a global data base for the last 21,000 years and to use it to test the modeling simulations by Kutzbach and others with the NCAR (National Center for Atmospheric Research) Community Climate Model (CCM, version 0) for the time series of climates from 21,000 year ago until present. COHMAP researchers published their results in Science (COHMAP Members, 1988) and later in the book Global Climates Since the Last Glacial Maximum (Wright et al., 1993) and illustrated the combined impacts of orbital forcing, ice sheet size, and increases in carbon dioxide and sea-surface temperatures on the global climates, vegetation, and lakes. A follow-up to these pioneering studies has recently appeared in Quaternary Science Reviews (v. 17, nos. 6-7) in 1998 on Late Quaternary Climates: Data Synthesis and Model Experiments with Tom Webb as the editor. Kutzbach and other ran a new set of modeling simulations with the NCAR CCM1, an interactive mixed-layer model for the oceans. I.Colin Prentice and Sandy Harrison used a global biome model, BIOME1, to simulate the global patterns of biomes and several colleagues assembled data sets from Africa, Europe, and North America to compare to the new model results. Ben Felzer also published a part of his PhD research at Brown in a study describing a series of climate model sensitivity experiments to separate out the impact of ice sheets from that of carbon dioxide and orbital insolation changes on late Quaternary climates.

North American Biome Distribution Maps

Biome Distributions
Maps of biome distributions for 6,000 years ago (6 ka, upper left) and 500 years ago (0.5 ka, upper right) inferred from pollen evidence. The biome difference map (lower left) displays the 6ka biome assignment for the grid cells that experienced a vegetation change between 6ka and 0ka, present-day.
Click to enlarge. (30 KB)

Current research involves use of climate estimates derived from pollen data to test the climate-model simulations for 6,000 years ago by PMIP (Paleoclimate Modeling Intercomparison Project). Former PhD student Jack Williams, Pat Barlein, and Tom Webb have collaborated on that study (Williams et al., in press), which includes the two figures on this page. Researchers at Brown have also been involved in the BIOME 6000 project to organize global sets of pollen date for mapping biomes at 6,000 and 21,000 years ago. These paleovegetation maps are being used to test the results of climate models coupled asynchronously to BIOME3, a global biome model developed by Colin Prentice and co-workers.

Coastal Studies     Geoarchaeology     Paleoclimates     Vegetation Dynamics

Data Model Comparisons     Paleohydrology