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High Asian Icy Landscapes during warm interstadials (HAIL): coupling between monsoon intensity and past glacier culminations?

Periodic Reporting for period 1 - HAIL (High Asian Icy Landscapes during warm interstadials (HAIL): coupling between monsoon intensity and past glacier culminations?)

Reporting period: 2022-01-03 to 2024-01-02

Mountain glaciers are among the most sensitive indicators of climate change. They typically respond to changes in climate by expanding when the climate cools (less melt) or retreating when the climate warms (more melt). Understanding glacier change beyond the historical record has great societal relevance because it allows us to untangle how glaciers respond to non-human induced climate change.

For example, the growth and retreat of the mid-latitude ice sheets was caused by 100 thousand year-long-cycles of orbital changes in insolation intensity, and they culminated in size about 22 thousand years ago (global Last Glacial Maximum, LGM). However, superimposed on this cycle, between 29 and 57 thousand years ago, are much more rapid millennial-scale transitions between cold and warm intervals, which could also drive glacier change.

The mechanisms behind these rapid climate shifts are not fully understood but their fingerprints can be traced all the way to high-mountain Asia (HMA), which includes the Tibetan Plateau, the Himalaya, Pamir, Karakoram, Tian Shan, and Altai. Here extensive glacier culminations have been shown to broadly overlap with the period of these rapid shifts, out-of-phase with restricted mid-latitude ice sheets.

A large uncertainty hindering explanation of this out-of-phase behavior includes the role of increased precipitation intensities caused by variations in the Mid-Latitude Westerlies and the Asian Monsoons. As current climate change is rapid, understanding the spatial and temporal patterns of glacier change during past periods of rapid climate shifts becomes increasingly important. The out-of-phase behavior is also regionally complex. Not all ice sheet sectors culminated concordantly and similarly there are potentially large regional variability in the timing and extent of glacier culminations across HMA.

HAIL was therefore designed to create a bridge between empirical and numerical glacier-climate reconstructions to systematically evaluate existing empirical data on past glacier change in HMA and address one of the long-standing questions in the fields of paleoglaciology and paleoclimatology: what made High Asian glacial systems so exceptional that they responded out of-phase with the rest of the world’s ice masses to globally warmer conditions between 29 and 57 thousand years ago?

The research objectives of HAIL are to (1) produce a database of empirical paleoglacier reconstructions, which can be used to constrain and interpret glacier models; (2) combine paleoclimate proxy data with model-based simulations of empirical glacier reconstructions to evaluate climate controls on spatial and temporal glacier variability; and (3) decouple regional from global climate drivers of documented glacier culminations.
Work performed during the first part of the HAIL project has focused on producing a database of empirical glacier reconstructions. This means we have come up with estimates of how long glaciers where in the past during different time periods for specific valleys across HMA. This process has included systematic re-mapping of ice-marginal limits (e.g. terminal, and lateral moraines or till and glacial boulder limits) associated with sites where a rich research legacy has collected geological samples for dating glacier culminations.

High-mountain Asia Paleoglacier Inventory
An extensive meta-analysis of over 120 publications has aided our effort to constrain past ice flow and ice margin limits, together with reinterpretation of a range of satellite and elevation datasets. The main data type of HAPI—High-mountain Asia Paleoglacier Inventory—is paleoglacier flow lines stored in a geographical information system. A paleoglacier flow line is a glacial metric describing how long the former glacier was (i.e. glacier length) at a specific point in time.

HAPI connects to the first research objective of HAIL: to produce a database of paleoglacier reconstructions, which can be used to constrain and interpret glacier models. Additionally, it links to two deliverables 1) a database disseminated though a project web page and 2) a scientific publication (under preparation) on observed patterns of glaciation across HMA. Together this work sums up HAILs first milestone: completion of the HMA-wide empirical reconstruction.

Main result
If we consider only robustly dated maximum glacier limits across HMA, we find that a majority of paleoglaciers reached their maximum overlapping with the global LGM. However, each sub-region present temporal variability, either through individual outliers or through separate paleoglacier populations which reached their maximum earlier during the last glacial cycle. Well-dated sites where paleoglaciers reached their maximum during the subperiod 29—57 thousand years include Kunlun Shan, central and eastern Himalaya, and south and East Tibet. Timing of these maximum expansions broadly coincide with intensified precipitation rates through correlation with monsoon records, as well as restricted mid-latitude glaciations.

Exploitation activities
Examples of potential exploitation activities includes, using HAPI to: interpret and evaluate numerical glacier models, evaluate paleoclimate proxy reconstructions, aid surficial geological mapping (i.e. providing estimates for limits of glacial sediment) and communicate locally about past glacier change. Information about HAPI is currently being disseminated to the international research community and relevant stakeholders through planned conference participation, through our web site and via various list servers. The website features information about HAIL and HAPI.

Additional work
Additional work being completed for the project period has been to set up steady state simulations using the Sicopolis glacier model. Sicopolis is a 3d model that simulates the evolution of ice sheets and ice caps and outlet glaciers. This work includes defining model domains and preparing input climate data. HAIL has for example prepared PMIP and CESM climate simulations for 21 and 35 thousand years ago which will be used to simulate paleoglaciation across HMA.
HAIL publishes the first systematically produced, open source and high resolution paleoglacier database for HMA. There are equivalent databases for North America, Patagonia, and Eurasia but this is the first one for this part of the world. HAPI is therefore an extremely important tool for which the paleoglacier and paleoclimate community can use, share, and adapt. In the future HAPI will be used to evaluate paleoclimate datasets, as climate models need to replicate past climates which can sustain comparable glacier culminations.