Results

The modeled LGM surface temperature is cooler compared to the modeled pre-industrial temperatures. The global cooling ranges from 5.9-8.4 degrees F (3.3-4.7 degrees C) when the temperature of the sea surface is prescribed. When the LGM ocean is also modeled the cooling is generally greater, ranging from 6.5-10.2 degrees F (3.6-5.7 degrees C) cooler. The Northern Hemisphere has greater cooling than the Southern Hemisphere due to the massive ice sheets, with temperatures up to 54 degrees F (30 degrees C cooler). The height of the ice-sheets also modifies weather systems, contributing to greater cooling right across Eurasia. In the tropics, the cooling is generally less pronounced, while over the ocean there is a large range, with cooling ranging from only 3.6-9 degrees F (2-5 degrees C).

The U.S. Geological Survey National Ice Core Lab in Denver, Colorado, stores and studies ice cores from all over the world.

A major shift in the LGM climate compared to the present was the reduction in atmospheric water vapor, leading to a much drier environment. This drying has implications for the energy of weather systems across the globe. The greatest drying is over the ice sheets and sea ice. Water vapor is also a strong greenhouse gas, and the reduction of water vapor, carbon dioxide, and methane in the atmosphere is the main driver of the cooling poleward of 54 degrees F (30 degrees C) in these modeled glacial climates. In the Northern Hemisphere, reflection off the massive continental icesheets and extensive sea ice were also important to the cooling at high latitudes. This effect was particularly important in summer, when the sun is higher in the sky, contributing to about half of the modeled cooling.

The westerly winds in the Southern Hemisphere impact the southern reaches of the three mid-latitude continents. The prescribed ocean-surface temperatures in the first phase of PMIP drove a southward shift of the westerlies at the LGM, particularly in the Australian region. The results from models that include full oceans are not in agreement. The models generally show no latitudinal shift in the westerlies, with most showing a reduction in the intensity of the winter winds, particularly in the Pacific Ocean sector. In the Northern Hemisphere, there was an equator-ward shift in the winter jet-stream in most models.

The models involved in PMIP can clearly simulate a cooler climate for the LGM, although the prescribed ocean temperatures used in phase one have known errors that lead to erroneously warm conditions in some regions. Results are similar across models in those regions, where the models are strongly-constrained by boundary conditions such as ice sheets. However, away from these regions, such as in the Southern Ocean, there is great variability.

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