During the Last Glacial Maximum (LGM), about 21,000 years ago, an area encompassing the Indian and westernmost Pacific Oceans experienced big shifts in temperature and rainfall.
In a new study, scientists have melded field data with climate modeling to uncover past drivers of these large climate swings—work that could be useful in predicting what will happen amid future climatic changes.
During the LGM, the prevailing winds in the Indo-Pacific region were reversed compared to what they are today, and there were unusual changes in ocean temperatures. The researchers used both terrestrial and marine proxy data—including microfossils, geochemical indicators of sea surface temperatures, isotopic ratios, and carbon plant typing—from sites in Africa, Asia, and Australia to reconstruct the dramatic and dynamic climatic changes.
“The geologic record tells us that Indonesia and the monsoon regions of the east Indian Ocean became drier and cooler while the west [Indian Ocean] became wetter and remained warmer,” said study coauthor Jessica Tierney, a paleoclimatologist at the University of Arizona, in a statement.
Well-established dating of the LGM and the large amount of data collected from the time period allowed the team to “infer robust patterns of hydroclimate change and ocean-surface cooling,” they wrote.
To tease out what drove the LGM climatic changes in the region, the team ran a variety of simple and complex climate models, testing different factors—such as greenhouse gas concentrations, ice sheet topography, land cover, and sea levels—to see which best reproduced the climate indicated by the geologic record.
They found that two major factors were likely at play: land exposure and albedo changes.
As ice sheets grew over Canada and Scandinavia, sea levels dropped up to 120 meters, exposing large areas of ocean shelf, including continental land bridges from Thailand to Australia. The increased land cut off ocean throughflow passages and altered tidal mixing, thereby changing regional temperatures, rainfall, and wind patterns.
Increased ice albedo altered monsoon patterns around the world. During the LGM, the changing air masses from albedo weakened the Indian monsoon, cooling the Arabian sea and decreasing moisture supplies to the region.
Fresh Approach to Climate Modeling
The “multipronged approach” and the variety of climate models of different complexity used in this study are strengths of the work, says Katie Dagon, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., who was not involved in the research. That approach “can be a very effective way of [studying] different mechanisms,” she says.
Dagon notes that it was the uncoupled model, a simpler model that didn’t link the full ocean and atmosphere together, that worked best. She adds that this study is a good example of simple models being effective and that it helps in discussions about approaches to future modeling work.
“The most complicated model is not always the best choice,” Dagon says.
Dagon says the study, by focusing on the LGM and trying to tease apart driving mechanisms during that time period, could help scientists better understand how climate could change in the future.
“The [researchers] talk about the response of greenhouse gases and the fact that it was cooler during that time period,” Dagon says. “It’s interesting to think about how these mechanisms apply to a potentially warmer future climate.”
The study authors noted that uncovering the drivers of Indo-Pacific regional climate during around the LGM is important for understanding how hydrologic cycles might change in the future.
“A big climate shift like this could have a huge impact on water availability over the heavily populated Indian Ocean rim,” said lead author Pedro DiNezio of the University of Texas at Austin in a statement.
This article is part of a series made possible through the generous collaboration of the writers and editors of Earth magazine, formerly published by the American Geosciences Institute.
Derouin, S. (2019), Ice drove past Indo-Pacific climate variance, Eos, 100, https://doi.org/10.1029/2019EO119709. Published on 02 April 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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