Unit 3: Distinguishing between natural climate variability and anthropogenic forcings

Alia Khan (Western Washington University)
Twila Moon (University of Colorado, Boulder)
Spruce Schoenemann (University of Montana Western)

Initial Publication Date: October 11, 2023

Summary

Distinguishing between natural climate variability and anthropogenic forcings

Unit 3 begins with student reading and exploration of satellite imagery to discuss differences between natural and human-induced glacial change and conceive of methods for investigating this question. Developing measurements of terminus retreat rate and using a simple glacier model, students test their ideas. Findings should ultimately lead to insight on the current human-caused retreat of the ice sheet.

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Learning Goals

Driving Question: Are the ice sheet/glacier margins indicating significant climatic changes in temperature and precipitation trends?

Learning Goals: 1) Evaluate the stability of the GrIS based on current rates of retreat 2) Compare how different glacier basin systems respond to a perturbation (natural or man-made)

Context for Use

The content in Unit 3 is appropriate for upper-division geology, environmental sciences, meteorology, and other geoscience courses; junior/senior-level courses in which geomorphology, climatology, or glacial geology studies are building on prior introductory material. Unit 3 activities can easily be adapted to serve small- or large-enrollment classes and can be executed in lecture and lab settings as an interactive lecture activity, an in-class activity in which students work in small groups, a short lab exercise, or as part of a ~three-week investigation incorporating GIS and Interactive Environments to understand a glacier basin system and cryosphere using the entire Exploring the Glacier Basin System module. This is the third Unit in the Long-term Spatial Transformations of the Glacier Basin System module and explores how scientists determine the range of natural glacier length variability and detect if anthropogenic climate disruption is causing glacier retreat beyond historic ranges.

Description and Teaching Materials

Part 1: Historical Glacier Terminus Positions

Pre-Class Reading: Roe, Journal of Glaciology, 2011 (first 4 pages), https://doi.org/10.3189/002214311796905640

Students review aerial maps/imagery of Greenland outlet glaciers (see selected glaciers from Leclercq_The_Cryosphere_2012.pdf (Acrobat (PDF) 1.2MB Oct11 23)) to identify terminus position since the end of the Little Ice Age (~1850) using GoogleEarth.

Leading prompts - What do you notice about the glacier termini? Have they advanced, retreated, or remained stable. What do you think might be the explanation for the changes?

Focus question - Are these observations outside the natural/expected glacial length variations? Students come up with possible methods to answer this question.

Part 2: Glacier Length Change- Natural vs. Anthropogenic?

Students use satellite imagery of specific outlet glaciers to determine the length of retreat (if any) using the Arctic DEM Explorer. Then, based on the historical observations (Greenland glacier length change dataset), calculate the rate of terminus retreat per decade since the Little Ice Age and compare to the Early-Mid Holocene ice margin retreat  (this builds off Mod 2, Unit 1).

  • Image Slide Deck: Revisit Natural glacier length variability vs. forced (anthropogenic) from Mod1 Unit2 Part3 Mod 1 Unit 2 Part 3 Basics Glacier Length Variation.pptx (PowerPoint 2007 (.pptx) 30.4MB Dec12 22)
    • Discuss how the timing of the glacier's characteristic response time (Tau = h/b) is linked to average thickness (h in m) and accumulation (b in m/yr), which can produce lagged responses to climate forcing based on the glacier's size?

Students use the Greenland glacier length data and rates of change to determine how much of the observed changes are within natural variability vs. anthropogenic warming (assuming pre-Industrial changes are a benchmark for natural rates)?

What we learned:

  • Differentiate between Natural vs. Human forcings that cause glaciers/ice sheets to advance and retreat
  • Determine that the response of glacier/ice sheet margin retreat in Greenland is outside of the natural/expected variations
  • Determine that the rates of retreat and thickness change have accelerated beyond historical/proxy levels with continued carbon emissions

Assessment

Assessment: Mod 2 Unit 3 Assessment (Microsoft Word 2007 (.docx) 503kB Sep8 22), Mod 2 Unit 3 Rubric (Microsoft Word 2007 (.docx) 20kB Sep8 22);

Figure and graph interpretation: Students use cross-sections and graphs to evaluate whether the changes to glacier length are within the range of natural variability or are a result of anthropogenic forcings.

References and Resources

PowerPoint: Natural Glacier Length Variation, Gerard Roe

MATLAB: Flowline Glacier Model, Gerard Roe

Reading: Huybers, K., & Roe, G. H. (2009). Spatial Patterns of Glaciers in Response to Spatial Patterns in Regional Climate. Journal of Climate, 22(17), 4606–4620. http://doi.org/10.1175/2009JCLI2857.1

Reading: Roe, G. H. (2011). What do glaciers tell us about climate variability and climate change? Journal of Glaciology, 57(203), 567–578. http://doi.org/10.3189/002214311796905640