The fourth international workshop of the Palaeoclimate Modelling Intercomparison Project was held June 22-27th 2002 at New Hall, Cambridge to define research priorities for the next phase of the PMIP project.
PMIP (PMIP: Joussaume and Taylor, 2000) is an international project involving members of all the major climate modeling groups worldwide and endorsed by the World Climate Research Programme (WCRP) and the International Geosphere-Biosphere Programme (IGBP). The project was launched in 1994 with the dual aims of:
In its initial phase, designed to test the atmospheric component of climate models (atmospheric general circulation models: AGCMs), the project focused on the last glacial maximum (LGM: ca 21,000 years before present, 21 ka BP) and the mid-Holocene (6000 years before present, 6 ka BP). The LGM simulation was conceived as an experiment to examine the climate response to the presence of large ice sheets, cold oceans and lowered greenhouse gas concentrations. The extent and height of the ice sheets were prescribed from Peltier (1994, the extent of sea ice and the temperature of the surface ocean was either prescribed from the CLIMAP (1981) reconstruction or computed interactively using a sea-ice model and a simple thermodynamic mixed-layer ocean model, and the atmospheric CO2 concentration was lowered to represent the decrease from pre- industrial to glacial levels. The mid-Holocene simulation was conceived as an experiment to examine the climate response to a change in the seasonal and latitudinal distribution of incoming solar radiation (insolation) caused by known changes in orbital forcing. The 6 ka BP experiment differed from the control simulation in only two respects: orbital parameters were changed and atmospheric CO2 concentration was lowered to pre-industrial levels.
Many features of the PMIP experiments, including global cooling at the LGM and the expansion of the northern hemisphere summer monsoons during the mid-Holocene, are robust in that they are shown both by all models and by palaeoenvironmental observations.. However, differences in the magnitude of the response between individual models are large. AGCMs forced by CLIMAP SSTs, for example, fail (as expected) to produce the magnitude of cooling in the tropics shown by palaeoenvironmental observations. However, although some of the atmosphere-mixed-layer ocean models produce tropical cooling of the right magnitude, others produce no greater cooling than the AGCM simulations (Harrison, 2000). Similarly, the simulated expansion of the African monsoon at 6 ka BP is considerably less than shown by palaeoenvironmental observations: some models underestimate the precipitation required to sustain vegetation at 23°N in the Sahara by 50% while others fail to produce an increase in precipitation this far north (Joussaume et al., 1999). This sort of analysis formed a crucial part of the evaluation of climate models in the Third Assessment Report of the Intergovernmental Panel on Climatic Change.
PMIP has not confined itself to analyzing and evaluating the benchmark LGM and mid-Holocene experiments. Complementary experiments, examining the role of the ocean and of the land surface in past climate changes, were also carried out by several of the participating groups. Perhaps one of the most important conclusions emerging from the first phase of PMIP was the importance of including ocean and vegetation feedbacks in model simulations in order to simulate the regional patterns and magnitude of past climate changes correctly. Largely as a result of this realisation, PMIP created a working group to design protocols for palaeo-experiments using fully coupled models at its last workshop (La Huardière, Canada, October 1999). These protocols were presented at the Cambridge workshop. Although some of the details remain to be decided, the basic experiments and experimental design were agreed upon by participants (see below). The focus on evaluation of coupled models in the second phase of PMIP is timely because coupled ocean-atmosphere models (OAGCMs) have, in recent years, become the basic tool for projections of future climate change. Coupled ocean-atmosphere-dynamic vegetation models (OAVGCMs) have now been developed by several modeling groups and are likely to be used for future climate simulations in time for the next IPCC assessment exercise.
At the Cambridge Workshop it wad decided that Phase II of PMIP will have five modeling foci:
Evaluation of the PMIP experiments is crucially dependent on the existence of spatially-explicit data sets which can be compared with output from the model simulations. Although the construction of palaeoenvironmental data sets for model evaluation began earlier, PMIP has played a key role in stimulating the continued development and improvement of such data sets and has been instrumental in the creation of new data sets (e.g.: the BIOME 6000 data set and the 21ka Tropical Terrestrial Data Synthesis: see Harrison, 2000 for details). The need to evaluate new aspects of the climate system in coupled models - not only aspects of the simulation of the ocean or the land surface but also the simulation of climate variability on timescales ranging from years to centuries - means there is an urgent need for the creation of new, global palaeoenvironmental data sets. The PMIP data-model comparison committee (contact: Sandy Harrison, MPI-Biogeochemistry, Germany, Click for email) plans to sponsor a series of workshops in the next few years to stimulate the creation of such data sets and to facilitate their use for model evaluation in the second phase of PMIP.
The workshop was attended by nearly 40 scientists, representing each of the major climate modeling groups worldwide. Financial support for the workshop was provided by the Max Planck Institute for Biogeochemistry, Jena.
CLIMAP, Seasonal reconstructions of the Earth's surface at the Last Glacial Maximum, Geological Society of America Map and Chart Series, MC-36, New York, 1981.
Harrison, S.P., Palaeoenvironmental data sets and model evaluation in PMIP, in Paleoclimate Modelling Intercomparison Project (PMIP). Proceedings of the Third PMIP workshop - edited by P. Braconnot, pp. 25-42, WCRP, La Huardiere, Canada, 4-8 October 1999, 2000.
Joussaume, S., and K.E. Taylor, The Paleoclimate Modeling Intercomparison Project, in Paleoclimate Modelling Intercomparison Project (PMIP). Proceedings of the Third PMIP workshop., edited by P. Braconnot, pp. 9-25, WCRP, La Huardiere, Canada, 4-8 October 1999, 2000.
Joussaume, S., K.E. Taylor, P. Braconnot, J.F.B. Mitchell, J.E. Kutzbach, S.P. Harrison, I.C. Prentice, A.J. Broccoli, A. Abe-Ouchi, P.J. Bartlein, C. Bonfils, B. Dong, J. Guiot, K. Herterich, C.D. Hewitt, D. Jolly, J.W. Kim, A. Kislov, A. Kitoh, M.F. Loutre, V. Masson, B. McAvaney, N. McFarlane, N. de Noblet, W.R. Peltier, J.Y. Peterschmitt, D. Pollard, D. Rind, J.F. Royer, M.E. Schlesinger, J. Syktus, S. Thompson, P. Valdes, G. Vettoretti, R.S. Webb, and U. Wyputta, Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP), Geophysical Research Letters, 26 (7), 859-862, 1999. Peltier, W.R., Ice age paleotopography, Science, 265 (5169), 195-201, 1994.
Peltier, R. W., 1994: Ice age paleotopography. Science, 265, 195-201.