The Climate System History and Dynamics Program - CSHD

W.R. Peltier
Department of Physics
University of Toronto
Toronto, Ontario
Canada M5S 1A7

Over the past decade the Canadian Earth Science Community has been involved in two large- scale national projects. The first of these was LITHOPROBE, an endeavor based primarily upon the application of techniques for deep crustal reflection seismology to the understanding of near surface structure on a series of traverses across geologically interesting segments of the Canadian landmass. The second such project, CSHD, has now been functioning for five years and the papers in this special issue of the Canadian Journal of Earth Sciences are intended to represent examples of some of the work accomplished in CSHD-Phase 1, which lasted three years, having begun in 1995 and ended in 1998, jointly sponsored as a Collaborative Special Project by NSERC and the Atmospheric Environment Service of Canada (AES). Just as Lithoprobe must be seen as a joint program linking Canadian university and government based scientists to private sector companies active in the resource extraction industries, so CSHD must be seen as a joint program linking Canadian university and government based scientists to our national endeavor to better understand the climate system so that we may improve our knowledge of what the future might bring by way of change to the global environment.

In Phase 1 of CSHD, recently refinanced and jointly sponsored by NSERC and AES under the Research Networks Program, ten groups of scientists, including groups interested in the geological reconstruction of past climate states and groups interested in the use of models to predict these states, combined in a focused effort to provide a Canadian contribution to the international Paleoclimate Model Intercomparison Project (PMIP). PMIP is itself an activity of the PAGES (for Past Global Changes) core project of the International Geosphere-Biosphere Program. The primary focus in CSHD-Phase 1 was upon two "time-slices" of the late Cenozoic, namely the Last Glacial Maximum (LGM) of the Pleistocene epoch at 21,000 years before present and the mid-Holocene warm period at 6000 years before present. In the collection of papers contained in this issue of the journal, various analyses of these fiducial epochs feature prominently.

The latter period is of course the least challenging from both the modelling and proxy climate data reconstruction perspectives. However, the period also provides an opportunity for detailed model-data intercomparison, as there is especially abundant data available that may be employed for paleoclimate reconstruction. At this time the coastline separating continents from oceans was essentially identical to present, surface ice cover was also essentially equivalent to modern, and sea surface temperatures are also thought to have been similar to today's. The sole significant difference in the factors responsible for determining climate state therefore involved the solar insolation regime. At that time the Earth's orbit around the sun was such that increased radiation was received during northern hemisphere summer, leading to enhanced temperature contrast between land and sea during this season and thus to an enhancement of the monsoon circulations. Although all existing general circulation climate models have been found to fail to predict the "greening of the Sahara" that characterized this epoch of Earth history (see Joussaume et al., 1999), it was discovered during PMIP that the Canadian Climate Centre Atmospheric General Circulation Model (AGCM) was the most extreme outlier among the PMIP set of models in terms of its northern hemisphere summer surface temperature predictions over land. Intercomparison of PMIP model predictions was shown to provide an effective means of identifying the reason for this behaviour, as demonstrated in the first of the Vettoretti et al. papers in this special issue of the Journal. Since it was in part on the basis of the expectation that such intercomparisons would lead to significant model improvements that CSHD-Phase 1 was funded, it has been very satisfying to the group to be able to demonstrate so clearly that this expectation was fully warranted. Insofar as the 6 ka model-data intercomparisons are concerned, and as is made clear by the papers contained in this collection, there remain very significant misfits between the climate model predictions for this epoch (again, as discussed in the first of the Vettoretti et al. papers), and the paleo-reconstructions, both those based upon palynology (see the paper by Gajewski et al.) and those based upon peatland distribution (see the paper by Vitt et al.). One encouraging result of this study is that the different proxy climate interpretations give similar results at the broad scale. Detailed analysis of the pollen record ensures that the quantitative results that have been obtained make sense in the context of our knowledge of vegetation dynamics (Richard 1994, 1995). Another encouraging result is that shallow water marine and terrestrial proxy data have led to similar paleoclimate reconstructions (Sawada et al., 1999), suggesting that our goal of producing high quality global-scale data-model intercomparisons will be achievable.

One geographical region that has not been analysed adequately to date is the Canadian arctic, even though climate model simulations are especially sensitive to feedbacks associated with the location of the forest-tundra boundary (e.g. Foley et al. 1994). Other work in progress under CSHD therefore has as its goal the quantification of Holocene climate change in this region of the country (Gajewski, 1995; Gajewski et al. 1995), though the lack of sufficient data is a significant impediment. Indeed, one of the results of the reconstruction of 6ka climate discussed in this volume has been a clear demonstration of the sensitivity of empirical calibrations to the modern pollen site density. Further work is ongoing during Phase 2 of the program to improve both the model for the 6 ka epoch and the proxy data reconstructions. An especially interesting archive of such paleoclimate information consists of peatlands. Northern peatlands currently sequester about one- third of the total world pool of soil carbon in regions that were largely glaciated in the recent past. Holocene increases have been suggested to have impacted atmospheric carbon levels, but regional studies linking changes between both spatial and temporal scales are limited. In Vitt et al. (this volume) the temporal and spatial dynamics of carbon storage in continental western Canada are examined through the Holocene and related to climatic and edaphic (permafrost development) factors. This paper represents the first attempt to quantify where and when carbon was and is sequestered in this terrestrial component of the array of reservoirs involved in the global carbon cycle. Vitt et al. provide evidence that, while relative increases in storage were largest during the early Holocene, net increases were greatest in the mid-Holocene. Net increases in carbon storage have declined over the last 3,000 years as permafrost developed and peatlands reached their current southern limits. However, these ecosystems remain a net sink for carbon today, with continental western Canada estimated to sequester carbon at a rate of 19.4 g m-2 yr-1.

The second of the PMIP fiducial epochs, namely the LGM at 21,000 years before present, is one on which the CSHD group has had an especially intense focus. Several of the primary data sets employed to define the boundary conditions that obtained during this epoch for the purpose of the PMIP experiments, in particular global topography and land-sea distribution, had in fact been produced by CSHD members at the time of the launch of CSHD-Phase 1 (Peltier, 1994; 1996; 1998; see the Figure on the cover of this issue of the Journal) and continue to be used by all groups in the international PMIP consortium. At LGM, sea level was lower by approximately 120 m on average, the water removed from the oceans being trapped in the large continental ice sheets which covered all of Canada and much of northwestern Europe at that time. Over both Canada (centred on Hudson Bay) and northwestern Europe (centred on the Gulf of Bothnia and the Barents and Kara Seas) these ice sheets reached thicknesses of approximately 4 kilometers (Canada) and 2-3 kilometers (Europe). With the lowering of sea level there were of course vast tracts of continental shelf that became exposed land, in some locations creating "land-bridges" which joined continental masses that are now separated by the sea (examples of such bridges would include the Beringian bridge that connected Alaska and Siberia, the bridge that connected Australia to New Guinea, and that which connected Britain to France across what is now the English Channel; graphics depicting these regions were provided in Peltier (1994)).

Although there remain many unanswered questions concerning the climate of the LGM period, in CSHD-Phase 1 several important preliminary analyses of this epoch were completed, involving both proxy data based reconstructions and climate model simulations. Probably most important among these efforts, and one that is well documented in the paper by de Vernal et al. in this collection, is the work that has been accomplished in reconstructing the sea surface temperature and salinity of the North Atlantic ocean during this epoch. This work is leading to significant revisions of understanding compared to that based upon the CLIMAP (1976) reconstructions that were produced approximately 25 years ago. These CSHD analyses strongly suggest that high latitude SST's were somewhat warmer and sea ice much less abundant than had been inferred on the basis of this previous work. Tropical SST's on the other hand, are now thought to have been significantly colder than inferred by the CLIMAP group. General circulation model reconstructions of LGM climate based upon the AGCM of the Canadian Climate Centre (see the second paper by Vettoretti et al., this volume) demonstrates the model to be in close accord with the de Vernal reconstruction so long as net ocean heat transports were not significantly different during the glacial period than they are in the modern circulation. This raises an extremely interesting question that is currently under investigation in the context of CSHD-Phase 2. Since the thermohaline circulation (THC) is thought to have been significantly reduced in strength under full glacial conditions, the northward heat transport by the wind driven circulation must have increased to compensate. Although this is entirely plausible, given that the strength of the atmospheric circulation was significantly enhanced at that time (e.g. see the second Vettoretti et al. paper, this volume), much remains to be investigated in this connection. Further revisions of the LGM surface boundary condition data sets for both land and sea are continuing during CSHD-Phase 2 and the results are contributing to the EPILOG program of PAGES which is attempting to compile a complete revision of the maps of the LGM world originally produced by the CLIMAP group.

The LGM period is perhaps especially interesting from a glaciological perspective since the ICE-4G model (Peltier 1994) of the continental ice sheets, which has been inferred on the basis of geophysical inversion of the relative sea level data from glaciated regions, includes a Laurentide (North American) ice sheet component that is significantly thinner, given the surface area that it covered, than conventional ice mechanical analyses would predict. This issue has been explored at some length by Tarasov and Peltier (1999, 2000) and is further reviewed in the paper by Marshall et al. in the collection to follow. When the conventional Glen (1955) flow law is assumed to describe the rheology of ice, then models of the 100 kyr ice age cycle of the late Pleistocene inevitably predict LGM ice sheets that are too thick and which cover too little surface area. Although the conventional way out of this impass has been simply to accelerate the flow rate predicted with the Glen model, appealing to sub-glacial processes to justify this ad hoc modification, Peltier (1998) and Peltier et al. (2000) demonstrate that, when the recently proposed Grain Boundary Sliding rheology of Goldsby and Kohlstedt (1997) is employed in a three dimensional thermo- mechanical model of ice-sheet evolution, the problem is significantly ameliorated. In Phase 2 of the CSHD program this issue is under continuing investigation. As will be clear on the basis of the recent paper by P.U. Clark et al. (1999), these ideas have apparently "touched a nerve" in the glaciology community. In Phase-2 of the CSHD program, much further work on fundamental issues in glaciology is planned, in particular to build on the innovative ideas concerning glacial inception (Marshall and Clarke, 1999a) and freshwater run-off from, and formation of, the proglacial lakes that formed during Laurentide ice-sheet retreat (Marshall and Clarke, 1999b). These ideas are expected to be extremely important to the further development of our understanding of the Younger-Dryas event (see the paper by Rutter et al. in this volume) that occurred during the most recent glacial- interglacial transition.

Probably the most important lead-in to CSHD Phase-2, however, has been the work undertaken in observational paleoceanography, specifically in the groups led by Claude Hillaire- Marcel at UQAM and by Larry Mayer at UNB. A substantial commitment of funds in CSHD Phase- 1 went to support the effort to acquire a number of long deep-sea sedimentary cores from the western Atlantic in the context of the first IMAGES (the International Marine component of the IGBP core project PAGES) cruise of the French vessel the Marion Dufresne. An example of the exceptional oxygen isotopic records that have been obtained from these cores is described in the paper in this collection by Hillaire-Marcel and Bilodeau. Further CSHD coring in the Fjords of eastern Canada was completed in summer 1999, also aboard the Marion Dufresne, and a major extension of this component of the CSHD effort will commence in 2001, also in the context of the international IMAGES program, to core along the eastern Pacific margin, a component of IMAGES that is being led by CSHD Principal Investigator Tom Pederson of UBC. The data sets being collected in this paleoceanographic component of CSHD are stimulating intense efforts among the modelling groups to understand more fully the high frequency Dansgaard-Oeschger oscillations and Heinrich events which are such prevalent components of climate system behaviour during Oxygen Isotope Stage 3 (e.g. Sakai and Peltier 1995, 1996, 1997, 1999; G.K.C. Clarke et al. 1999), according to the European GRIP and American GISP2 records from the deep ice-cores drilled at Summit, Greenland. Equally interesting is the recent demonstration by Bond et al. (1999) of millennial-scale variability in the ocean during the Holocene that seems to be a continuation of these events. This Holocene climate variability is, of course, well known from the terrestrial record, and Fisher (1983) and Gajewski (1987) had previously pointed out the scale interaction between the century-scale climate variability such as the Little Ice Age and the slower Milankovitch-caused Holocene climate changes.

Complementing this work, and extending the analyses in CSHD to much longer timescales, are intercomparisons and correlations between the long nearly continuous, Chinese loess-paleosol sequences with North Pacific ocean marine records, reaching back in time to about one million years (see Rack et al., this volume; Lui et al., 1999; Rokosh et al., 1998). During the course of this program we have expanded the investigation of the long, terrestrial record (2.5 Ma) to include central Siberia and the Russian Plain in order to reconstruct climate changes during various time intervals over a wider area (Chlachula et al., 1996, 1998; Little et al., 1999). An important by-product of the loess work, has been a better understanding of the origins of magnetic susceptibility variations in such records and the limitations of such records as a climate proxy. It has been found, through analyses of the records from China, that MS signals suggesting the existence of a milder climate are opposite to the signature found in records from central Siberia (Evans and Rutter, 1999). Intensive investigation of the near continuous loess-paleosol deposits of China and Siberia have provided high- resolution climate records that have enabled us to identify climate changes to the millennium time scale, such as the Younger Dryas, as well as to trace the maximum extent of desert margins during the LGM in China (Rutter et al., 1995, 1996; Ding et al., 1994, 1995). We have also tested a coupled general circulation model with proxy climate records for the Younger Dryas event and found them compatable (see Rutter et al., this volume).

Future developments of the CSHD Program will continue to be focused primarily on issues raised by the observed climate variability in the late Cenozoic period, although significant enhancements are being undertaken that will explore the extent to which models of climate variability developed in CSHD to explain such observations may be invoked to help understand climate system evolution on much longer timescales. Efforts of the latter kind have already begun with the application of the UofT model of the 100 kyr ice-age cycle of the late Pleistocene (Tarasov and Peltier, 1997, 1999) as an aid to understanding the glaciation events that occurred during both the Carboniferous (Hyde et al. 1999) and Neo Proterozoic periods (Hyde et al. 2000). The latter epoch is perhaps especially important as it has been taken by some to be one in which the surface of the Earth was entirely ice-covered (the so-called "snowball" phase, e.g. see Hoffman et al. 1998). In Hyde et al. (2000) it is demonstrated that this scenario is not entirely implausible, although the models predict the existence of substantial equatorially confined refugia unless atmospheric CO2 concentration is more significantly reduced than most geological reconstructions view as plausible.

As the Canadian earth science community works towards the development of future large scale programs that might serve as follow-ons to the successful and continuing efforts in Lithoprobe and CSHD, it may well be that the most fruitful avenue we might follow is one that attempts to bring together the solid earth sciences on the one hand and the fluid earth sciences (atmospheric and oceanic sciences) on the other. The theme of "Earth Evolution and Climate" suggests itself (to me) as one that would be able to mobilize our community in a way that would bring us closer together and by so doing perhaps increase our collective productivity in a very healthy way.


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