Boundary conditions

Some consider changes in solar radiation pattern and timing associated with Milankovitch cycles an important factor in maintaining a glacial climate. These changes are routinely included in GCM simulations of the LGM, however, they have minimal impact, as they were actually very similar to today. The continental configuration was also very similar to today, and changes need not be considered. The conditions at the LGM were the culmination of 100,000 years of cooling, thus models of the LGM must include an estimate of certain features in the climate system that hold memory of that cooling. These features include massive continental ice sheets, cooler oceans, lower sea levels, and lowered levels of atmospheric greenhouse gases.

The massive ice sheets that spread across the polar reaches of the continents during the 100,000 years of cooling prior to the LGM were imposed when modeling the LGM. Antarctica was glaciated, as it is today. The other major ice sheets are called the Fennoscan-dian, which covered much of Europe, and the Laurentide, which covered much of North America. Both the change in surface type (to ice) and the increase in elevation are included. The bright white surface of the ice will reflect more sunlight, encouraging cool conditions. The height of the ice sheets (up to 2.5 mi., or 4 km.) also alters the track of weather systems, particularly in the Northern Hemisphere.

With so much water locked up in the extensive continental ice sheets, the sea level is estimated to have been 393 ft. (120 m.) lower at the LGM. This exposes more land and alters the pattern of land-sea contrast. This is particularly important in regions where landmasses are surrounded by shallow seas. One such place is to the north of Australia, where a land bridge connected Australia to Papua New Guinea and Indonesia to its north, limiting the oceanic warm pool, which is of great importance to the monsoon and rainfall in the region. The exposure of this land bridge also had consequences for human migration.

Oceans circulate on timescales much longer than the atmosphere. Conditions at the surface are slowly transferred to the deeper ocean, thus the ocean would have a memory of the cool conditions that were experienced for thousands of years prior to the LGM. The ocean could also be seen as forcing glacial conditions. As in the first phase of PMIP, sea-surface temperatures can be prescribed, which provides a strong constraint on the global climate. Models forced by prescribed sea-surface temperatures provide a test-ground for a range of experiments, including the consistency between proxy evidence from the land and ocean.

They also require reasonably low levels of computing power. However, prescribing sea-surface temperatures neglects interactions between the atmosphere and the ocean, which are important to the climate and variability of both.

More recent efforts at modeling the extreme climate of the LGM include full models of the ocean. The LGM ocean has a temperature structure that has been exposed to thousands of years of cool atmospheric conditions during the preceding glacial cycle, and would be expected to be much cooler than the modern ocean, thus modeling the LGM ocean requires an appropriate initial state. There are a variety of methods used to achieve an appropriately cool ocean. Due to the high computational expense, approximations or accelerations are required. One method is to run the ocean model alone, intermittently inputting atmospheric information.

Variations in sea-ice extent alter the ocean-atmosphere interactions that can occur. If open ocean is exposed to the atmosphere, the relatively warm water has the opportunity to impart heat, moisture, and energy. However, a cover of sea-ice cuts off this interaction. There is evidence that sea-ice was more extensive at the LGM. Models with prescribed sea-surface temperatures also prescribe sea-ice, while models that include full oceans usually also include models that determine the extent of sea-ice from the oceanic conditions.

Levels of atmospheric greenhouse gases are very important to the climate, and these are generally imposed in models of the LGM. For well-mixed gases such as carbon dioxide, levels are reasonably well known from the LGM air bubbles trapped in polar ice. Some EMICs include a module of interactive atmospheric chemistry as part of the carbon cycle. For PMIP GCMs, the level of atmospheric carbon dioxide is set at 280 parts per million by volume (ppmv) for the present day (pre-industrial) and 185 ppmv for the LGM. This change is estimated to reduce the radiative forcing of the troposphere by 2.8 Wm-2.

Vegetation changes and atmospheric particulates are believed to have influenced the global and regional climate of the LGM. EMICs have generally included a crude estimate of vegetation; some of the most recent attempts to simulate the LGM include vegetation, its changes and feedbacks. At the LGM, the generally drier environment meant that there was more potential for dust from drier, less-vegetated surfaces to enter the atmosphere. The result of increased particulate levels in the atmosphere is to cool the surface—it is estimated that the increased dust at the LGM reduced the radiative forcing of the troposphere by about 1 Wm-2. Particulates and dust are not currently included in the GCMs in PMIP, but it has been flagged as an important component to be considered in the future.

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