Part of the PMIP effort was focused on the LGM to understand the impacts of extreme cold conditions and to study the feedbacks associated with a decrease in [CO2]atm and ice sheet elevation of 2 to 3 km above North America and northern Europe. The LGM was simulated by 17 models through the PMIP program that prescribe a series of set boundary conditions (Pinot et al., 1999). An update of the ice sheet extent and height was provided by (Peltier, 1994). As a result of the Laurentide and Fennoscandian ice sheets being 1,000 meters lower than the previous reconstruction (CLIMAP, 1981), [CO2]atm was prescribed to be 200p.p.m.V, as inferred from Antarctic ice cores (Raynaud, et al., 1993), and Earth's orbital parameters were changed according to their values at 21,000 yr bp. Over the oceans, two sets of experiments were defined: one with SST changes prescribed from estimates (CLIMAP, 1981), the other with SSTs computed using coupled atmosphere-mixed layer ocean models and assuming no change in ocean heat transport. Each approach has its advantages and problems:
1. Computing SSTs permits an evaluation of models used for future climate prediction, but does not account for any changes in oceanic heat transport, despite evidence for thermohaline circulation changes.
2. Prescribing SSTs should yield better results over land; however, SST estimates are subject to substantial uncertainty (Duplessy et al., 1988).
Indeed, a limitation of model simulations has been the role of ocean dynamics, which is crucial for understanding changes in tropical upwelling (Bush and Philander, 1998; Ruter et al., 2004) and resultant rainfall regimes. Model simulations still under-represent tropical cooling at the LGM, apart from over Eastern Africa where most of the models indicate a cooling (relatively minor) similar to the observational data. Following the debate on the degree of tropical SST cooling raised by Rind and Peteet (1985), Farrera et al. (1999), and Pinot et al. (1999) it can be shown that: (1) all PMIP simulations using the relatively warm tropical SSTs given by CLIMAP (1981) tend to be too warm over land, except over equatorial Africa; (2) computed SSTs are colder than CLIMAP, especially over the tropical Pacific where the erroneous warm pools of CLIMAP are not reproduced (Pinot et al., 1999); (3) models with computed SSTs show a range of terrestrial cooling strongly related to the intensity of tropical SST
cooling. Some models produce a strong terrestrial cooling consistent with the paleodata, but this is associated with SST cooling that is too large when compared against recent alkenone data on SST. However, one model gives reasonable results over both land and oceans: "CLIMBER", a model of intermediate complexity, reconstructs a tropical land cooling of 4.6°C with SST cooling of 3.3°C in the Atlantic, 2.4°C in the Pacific and 1.3°C in the Indian Ocean (Ganopolski et al., 1998). This is in broad agreement with the data that show tropical SSTs were 5°C colder in Barbados corals (the coldest throughout the tropics), although 2-3°C is a more common value (Guilderson et al., 1994). Terrestrial temperatures simulated by 17 models within PMIP are relatively similar showing temperature was reduced by 5-6° C about the LGM. For some models run for both fixed and computed SSTs (UGAMP and GEN2), the annual mean change in temperature remains very similar on global average, although regional differences can be substantial (Dong and Valdes, 1998) but are comparable with data-based reconstructions that also document considerable variation.
According to PMIP simulations, vegetation change in the tropical realm is primarily driven by precipitation changes. According to these models, the LGM was relatively dry apart from East Africa and throughout high elevations of South America and Papua New Guinea (Pinot et al., 1999). Reduced precipitation, particularly in mid-latitude western South America, is likely to result from a reduction in the intensity of westerly climate systems. In a comparison of two models within the PMIP suite (CCM3 and CSM) differences are quite small in most measurements of atmospheric circulation, with one exception that involves tropical precipitation (Joussaume and Taylor, 1995). Moisture changes tend to be associated with changes at the regional scale when model simulations are characterized by a number of common features, including a reduction in the strength of the Afro-Asian monsoon and increased inter-tropical aridity, corroborated by various paleoindicators. Climates of all the continents have monsoonal climates; change in insolation, such as that occurring about the LGM due to changes in Earth's orbital parameters, would cause changes in the monsoonal climate (Joussaume et al., 1999) and the associated feedbacks. For example, application of a coupled ocean-atmosphere model (FOAM) indicate that SST feedbacks produce a much larger enhancement of precipitation in Central America than direct radiative forcing alone (Harrison et al., 2003).
Discrepancies between LGM model runs and comparison with paleoenviron-mental data sets are likely to result from missing feedbacks—for example, all simulations omit possible influences of vegetation change due to climate-induced shifts, and CO2-induced changes in vegetation and leaf conductance. Indeed, numerous factors are not included—for example, mineral aerosol (dust) concentrations were many times higher than today at the LGM, especially in the polar stratosphere, and this could have provided an extra cooling effect. Taking into account climate and CO2-induced vegetation changes to infer the dust distribution (which was in fairly good agreement with proxy data), computations indicate a small positive change in radiative forcing in high latitudes, but a larger negative change in the tropics. Ocean dynamic changes are also likely to be important at the LGM, as demonstrated by models of intermediate complexity (Ganopolski, et al., 1998; Weaver, et al., 1998); indeed, dramatic changes in ocean circulation are likely to be responsible for abrupt climate change during the last ice age and transition to the Holocene (Stocker and Marchal., 2000).
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