A maximum in global glacier extent during MIS 4

The most recent maximum in global ice volume occurred around 23,000 to 19,000 years ago, during Marine Isotope Stage 2 (MIS 2; ∼29-14 ka) according to benthic δ18O and sea level records. However, evidence from cosmogenic surface exposure dating indicates that world-wide many glacier systems of different sizes as well as portions of some ice sheets were more extensive during MIS 4 (∼71-57 ka) and MIS 3 (∼57-29 ka) than they were during MIS 2. This discrepancy between global ice volume and ice extent must be explained in order to understand Earth’s recent paleoclimate history. Here, we review MIS 4 moraine chronologies based on 10Be exposure dating, and we describe additional paleoclimate proxy records that indicate similar magnitudes of cooling during MIS 4 and MIS 2. While certain regions may have benefited from a wetter MIS 4 relative to MIS 2, it is unlikely that precipitation alone can explain more extensive glaciation on a global scale between 71 and 57 ka. Our review supports the hypothesis that the discrepancy between ice volume and ice extent during MIS 4 can be attributed to the growth of the North American ice sheets (and perhaps other northern ice sheets). Glaciers ultimately respond to changes in climate, however, large northern ice sheets also were affected by factors involving topography, isostacy, and glaciologic and mass balance dynamics. Given these feedbacks, the North American ice sheets’ dominant role in global ice volume, sea level, and benthic δ18O signals might therefore result in a skewed picture of global climate. If maximum global ice volume during MIS 2 is mainly a function of North American ice sheet volume and not necessarily directly connected to global temperatures, then other records with extremes during MIS 2, such as dust and CO2, could be primarily reflecting ice volume change.