COHMAP and paleoclimate modeling

The international COHMAP project represented a major paradigm shift in Holocene climate research with its close integration of paleoclimate "proxy-data"

scientists with paleoclimate modelers. Although COHMAP started life in 1977 as Climates of the Holocene - Mapping based on Pollen Data, it quickly became global in its research area and in the climate proxies it considered. It changed its name to Co-operative Holocene Mapping Project. It was masterminded by John Kutzbach, Tom Webb, Herb Wright, and others (Wright et al. 1993). The basic idea of COHMAP was to simulate past climates at 18 000, 15 000, 12 000, 9000, 6000, 3000, and 0 years ago using the Community Climate Model (CCM) and to compare the model simulations with the available paleoclimate data, particularly terrestrial pollen, lake-levels, and pack-rat midden data and marine plankton data (COHMAP Members 1988; Wright et al. 1993).

Prior to COHMAP, paleoceanographic studies (Hays et al. 1976) had established that orbitally induced variations in insolation as proposed by James Croll (1821-1890) and Milutin Milankovitch are the pacemakers of the ice ages (Imbrie and Imbrie 1979). Variations in insolation are the result of the 22 000-year precession cycle, the 40 000-year tilt cycle, and the 100 000-year eccentricity cycle in the geometric relationships between the Earth and the Sun. COHMAP investigated in detail the role of such orbital variations in insolation on Holocene climate history. The 22 000-year precession cycle controls the time of year when the Earth-Sun distance is at a maximum or minimum and hence determines seasonality. The 40 000-year tilt affects the latitudinal distribution of solar radiation (COHMAP Members 1988).

Input to COHMAP CCM simulations of past climate at 3000-year time-slices were external orbitally determined insolation conditions and internal surface boundary conditions of mountain and ice-sheet orography, atmospheric trace-gas concentrations, sea-surface temperatures, sea-ice limits, snow cover, albedo, and effective soil moisture (COHMAP Members 1988). The boundary conditions used in the COHMAP simulations are summarized in Figure 2.11. These conditions provided a major new paradigm for Holocene climate research at the global spatial scale and at broad millennial temporal scales. The seasonal and latitudinal distributions of solar radiation at 18 000 years ago, the time of the Last Glacial Maximum, were similar to conditions today. The atmospheric conditions simulated for glacial times must therefore largely be a result of the very different internal surface boundary conditions (COHMAP Members 1988). Between 15 000 and 9000 years ago climate seasonality increased in the Northern Hemisphere and decreased in the Southern Hemisphere due to the precession and tilt cycles. As a result, continental ice sheets and sea-ice retreated and the oceans warmed. By about 9000 years ago, solar radiation over the Northern Hemisphere was, on average, 8 percent higher in July and 8 percent lower in January than today. After 9000 years ago, the extremes in solar radiation decreased towards modern values (COHMAP Members 1988).

COHMAP's model-data comparisons showed that orbitally induced changes in insolation explain, in broad terms, the history of Holocene climate and landscape development. Clearly climatic events of short duration in the late-glacial or Holocene were not considered in the modeling experiments or data syntheses because COHMAP considered 3000-year time-slices only (Wright et al. 1993).

Figure 2.11 Boundary conditions for the COHMAP simulations of past climate for the past 18 000 years. External forcing is shown for Northern Hemisphere solar radiation in June-August (Sjja) and December-February (Sdjf) as percentage difference from present-day radiation. Internal boundary conditions include land ice (Ice) as percentage of ice-volume at 18 000 years ago, global mean annual sea-surface temperatures (SST) as deviations from present-day values (degrees K), excess glacial-age aerosol (arbitrary scale), and atmospheric CO2 concentration (parts per million by volume). In the COHMAP simulations only the 18 000-year model used a lowered CO2 concentration (200 ppm). All the other simulations used a CO2 concentration of330 ppm. Aerosol loadings for the 18 000- and 15 000-year simulations were not used. The pre-Holocene part is shaded in blue. (Modified from COHMAP Members (1988). Reprinted with permission from AAAS.)

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COHMAP's modeling experiment results showed how orbitally induced changes in insolation and changes in surface boundary conditions affect regional climates and hence the observed broad-scale changes in vegetation, marine plankton, and hydrology (COHMAP Members 1988). Variations in insolation changed seasonality at low- and mid-latitudes. By enhancing the thermal contrast between oceans and land, increased seasonality resulted in strong summer monsoons from 12 000 to 6000 years ago in the Northern Hemisphere tropics and sub-tropics and warm and dry summers in the continental interiors of northern mid-latitudes (COHMAP Members 1988). The model results also showed how internal surface boundary conditions (land and sea-ice extent, sea-surface temperatures) could have influenced atmospheric circulation patterns and thus patterns in temperature, precipitation, and wind. The climate of North America in the early Holocene was unlike any today and this may have led to biotic assemblages without modern analogs. The insolation maximum occurred before 9000 years ago and resulted in high temperatures south-west of the Laurentide ice sheet, whereas in eastern North America, the slow retreat of the ice sheet there delayed the summer thermal maximum to 6000 years ago (COHMAP Members 1988).

COHMAP was a major turning point in Holocene climate research for several reasons. First, it used what were then state-of-the-art climate models to simulate paleoclimate under specified boundary conditions. Second, it resulted in detailed compilations and syntheses of paleoclimate proxy data that were then used in data-model comparisons. Third, it considered the global climate system as a whole and revealed the strong regional interconnections between different components of the Earth's climate system. Fourth, it revealed the remarkable spatial and temporal variation in circulation patterns and climate at 3000-year intervals during the

Holocene. This variation helps explain the complex spatial and temporal patterns of tree spreading and of no-analog biotic assemblages in the early Holocene (Jackson and Overpeck 2000; Jackson and Williams 2004).

COHMAP Members (1988) recognized the implications of their work for understanding not only past climate but also future climates. They concluded their 1988 paper by suggesting:

"Climate has influenced human activities. Two great cultural developments emerged at about the time of major environmental change between 12 and 10 ka - the earliest appearance of agriculture in the Old World, and the cultural changes accompanying extinction of the Pleistocene megafauna in the New World. Both developments may have been caused at least indirectly by the types of climatic change examined here. Now we may be faced with the reverse: human modification of climate and of related aspects of the physical environment. Application of a well-tested climate model is at present the only method for predicting these climatic and environmental changes and can help in planning a response to them. COHMAP research is contributing to the testing of these models and to the understanding of past climates." (COHMAP Members 1988, p. 1051)

Many of the researchers involved in COHMAP then attempted a unified global synthesis of paleovegetation data at the level of biomes for 6000 radiocarbon years ago, the so-called BIOME 6000 project (Prentice and Webb 1998). Such a synthesis was designed to be comparable to other global syntheses of sea-surface temperatures and ice sheets by CLIMAP Project Members (1976) and of lake-level changes by Street-Perrott and Harrison (1985). The BIOME 6000 data synthesis aimed to provide basic paleodata that could be used to evaluate simulations from coupled climate-biosphere models and to assess the extent of biogeophysical (vegetation-atmosphere) feedbacks within the global climate system (Anon 1994).

Climate models and paleoclimate and climate-biosphere modeling have made enormous advances since the pioneering CLIMAP, COHMAP, and BIOME projects with the development and application of box models, energy balance models, earth system models of intermediate complexity, and general circulation models. These advances and their contribution to our understanding of Holocene climate history are reviewed by Valdes (2003).

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Responses

  • mandy
    What is the Cohmap model?
    8 years ago

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