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An international research team led by the University of Bremen has investigated what influenced the expansion of the Patagonian ice sheet during the last ice age. The scientists found evidence that the advances and retreats of glaciers in South America over the past 120,000 years were primarily influenced by changes in summer solar radiation and the duration of the summers.

These energy fluctuations followed a regular cycle similar to changes in the Earth's tilt angle, but were additionally amplified or suppressed by short-term climate variations over thousands of years. The results of the study have been in Nature Communications.

Patagonia is an impressively diverse region with a wide variety of landscapes. Expansive forests, open grasslands, arid deserts and imposing mountain ranges characterize the region. But it was not always so diverse, and even today the relics of a completely different landscape can be found: the Patagonian ice fields.

In the western part of the region, compact glacier areas stretch for hundreds of kilometers along the Andes in Chile and Argentina. The northern and southern Patagonian ice fields are remnants of a much larger ice cover that reached its maximum extent around 35,000 years ago. At that time the central Andes Mountain chain was covered with ice between 38 and 55 degrees southern latitude.

"The aim of our study was to investigate the spatial-temporal history of the Patagonian ice sheet during the entire last glacial cycle—from around 120,000 years ago until today—and thus to gain a better understanding of the causes of the temporal sequences and dynamics of the advances and retreats of the glaciers," explains Dr. Andrés Castillo-Llarena, first author of the study.

Castillo-Llarena is an Earth-system modeler at MARUM—Center for Marine Environmental Sciences, and the Geosciences Department of the University of Bremen. A primary focus of the international team was the role of climate variability on a millennial timescale as a driving mechanism for changes in the Patagonian ice sheet.

The last ice age had an impact on environmental conditions and landscapes around the world. North America, northern Europe and Patagonia in particular were heavily covered by gigantic ice sheets that subsequently disappeared after the ice age. Because the growth and shrinking of the ice masses was largely controlled by fluctuations of temperature and precipitation, the ice sheets of the past provide important information about past climate changes.

The present paleoclimatic findings from Patagonia and New Zealand indicate that the maximum glacier expansion in the middle latitudes of the southern hemisphere occurred almost simultaneously, but was not synchronous with the glacial history of the northern hemisphere.

In order to study this more closely, Castillo-Llarena's team carried out computer simulations. Their results indicate that the ice cover of the Patagonian ice sheet underwent periods of expansion and contraction rather than having a uniform ice-age history, which is contrary to earlier assumptions that were based on geological reconstructions.

They further show that the Patagonian ice sheet underwent two main periods of glacial advance during the last ice-age cycle: At the beginning of marine isotope stage (MIS) 4 about 71,000 years ago and at the end of MIS 3 around 35,000 years ago. Between these there was a temporary shrinking of the ice sheet at about 60,000 years ago.

The team of researchers was able to identify the driving force for the long-term fluctuations as the combination of changes in the duration of summer seasons with the intensity of summer solar radiation, which is known as "integrated summer energy." This fluctuates with changes in the Earth's tilt on a time scale of around 40,000 years.

"We suspect that the integrated summer energy modulated not only the behavior of the Patagonian ice sheet, but also that of the other ice masses throughout the mid-latitudes of the southern hemisphere," said Castillo-Llarena. In addition, the team discovered that, superimposed upon the long-term fluctuations, there were short-term variations in the Patagonian ice sheet at millennial time scales that can be linked to abrupt climate changes in the northern hemisphere.

"These findings are of particular significance because there are relatively few data on past climate fluctuations from the southern hemisphere. For a better understanding of future climate change, however, it is important to understand how the northern and southern hemispheres interact with one another," explains MARUM scientist Dr. Matthias Prange, a co-author of the study.