Abstract
The cyclical pattern of glaciations (about every
~100,000 years in the past million years in isotope
records) suggests an external forcing, such as changes in the amount of
sunlight (insolation) reaching areas on the cusp of glaciation (e.g.,
~65° latitude) during the season when ice is most likely
to melt (summer). Alternatively, ice ages could be caused by insolation
triggering changes within the Earth’s climate system that alter carbon
dioxide and ice sheet size, leading to globally synchronous climate
change. The key to testing these hypotheses is knowing the timing and
magnitude of past temperature change. Mountain glaciers tell a slightly
different story about past climate change. Strand et al. (2022,
https://doi.org/10.1029/2022PA004423) contribute a new moraine
chronology from an understudied region in Mongolia showing that the
glacier reached a maxima 10,000 years prior to the lowest
CO2 level and peak in global ice volume, and that
massive glacier retreat was underway millennia prior to any of the
proposed climate drivers. Their moraine chronology is in agreement with
those from the Southern Alps of New Zealand, which is located in the
opposite hemisphere and maritime climate rather than continental. Here,
I review the results from Strand et al. (2022) in the context of ongoing
advances in the methods and moraine chronologies from the last glacial
cycle. The issue remains that we do not have a clear understanding of
the cause of ice ages and the strength of feedbacks within Earth’s
climate system.