Unit 3: Global Sea-Level Response to Ice Mass Loss: GRACE and InSAR data
What is the contribution of melting ice sheets compared to other sources of sea-level rise? How much is the sea level projected to increase during the twenty-first century? In this unit students will use Gravity Recovery and Climate Experiment (GRACE) ice-mass loss time series from Greenland and Antarctica to calculate sea-level rise due to the addition of freshwater inputs from melting ice sheets, and use Interferometric Synthetic Aperture Radar (InSAR) ice-velocity data to extrapolate which regions of the ice sheets are losing the greatest mass. Sea-level rise from melting ice sheets is then contrasted to the other dominant causes of sea-level rise, including thermal expansion, melting glaciers, and changes in land water storage. Lastly, students will extrapolate how much sea-level rise will occur by year 2100 based on recent observed rates of sea-level rise and compare these values to sea-level rise projections from the Intergovernmental Panel on Climate Change.
Unit 3 Learning Outcomes
Students will be able to:
- Create time-series graphs of GRACE ice-sheet mass loss and calculate how much the ice loss contributes to sea-level rise.
- Infer which regions of the ice sheets are losing the greatest mass by examining maps of ice velocity constructed from InSAR data.
- Evaluate the sources of sea-level rise and rank the sources from largest to smallest contributors to sea-level rise.
- Extrapolate how much sea-level rise is projected by 2100 based on current sea-level rise rates.
Unit 3 Teaching Objectives
- Cognitive: Enable students to evaluate the contribution of melting ice sheets to sea-level rise.
- Behavioral: Promote skills in making and interpreting graphs and using linear equations to estimate future sea-level rise.
- Affective: Facilitate the students' appreciation that numerous processes must be considered when assessing the causes of sea-level rise.
Context for Use
The content in Unit 3 is appropriate for advanced Climate, Cryosphere, Geology, Geoscience, or Environmental-related courses conducted at the junior and/or senior level in which geodesy data can be introduced. Students should be cognizant that the Earth's climate is warming and have familiarity with the causes of the warming. The student exercise in Unit 3 requires access to computers to create Excel (or similar) graphs and calculations, so the exercise is best executed in a computer lab setting or could be assigned as an out-of-class activity. Students will need basic proficiency in creating graphs from time-series data set, including conducting linear trend analysis. Unit 3 is designed to follow Unit 2: Global Sea-Level Response to Temperature Changes. Some preparatory lecture time may be required to review how GRACE and InSAR data are acquired, and to review ice sheet anatomy and factors that influence ice sheet melt rates. If the entire two-week module will not be utilized, we recommend pairing Unit 3 with Unit 2: Global Sea-Level Response to Temperature Changes to give students an opportunity to investigate the dominant causes of current sea level.
Description and Teaching Materials
In this unit students work through an exercise with GRACE and InSAR-derived ice velocity to better understand how the ice sheets are responding to the changing climate. The student exercise takes 1–2 hours that could be done in a lab period, stretched across several shorter class periods, or done partly/largely as a homework assignment. It works well to have students start the exercise in a one-hour class and then finish the remainder at home.
Background information for Unit 3 includes three PowerPoint presentations, several videos, and some readings. Instructors can decide what if any elements they want to assign as homework prior to class (such as reading or watching a video). The presentations include an overview of ice sheets and information on how GRACE and InSAR data are collected. The presentations are each fairly short and could be done in sequence.
Student Exercise Elements
The Unit 3 student exercise requires that students have access to a computer and Excel (or similar). Students can complete the exercise either during class or lab time, or as a take-home activity. However, Part 2 of the student exercise requires the student to acquire a data graph from the instructor, so if the assignment is given as a take-home activity the instructor will need to make sure they know how to access it.
In Part 1 of the student exercise, students use GRACE data to calculate mass change in the Antarctic and Greenland ice sheets, and examine ice-sheet velocity patterns using InSAR data. In Part 2 students calculate how much sea-level rise has occurred due to ice-sheet mass loss, and evaluate how the ice mass contribution compares to observed sea level. In Part 3 students evaluate sources of sea-level rise and rank the sources from largest to smallest. Lastly, in Part 4 students project how much sea-level rise will occur by year 2100 based on recent observed sea-level rise rates and compare this to sea-level rise projections from the Intergovernmental Panel on Climate Change. In Part 2 there are two graphs that are best given to students after they finish Questions 1–8. These are provided as separate handouts.
- Introductory and background information
- Unit 3: Ice sheet background information (PowerPoint 2007 (.pptx) 10.1MB Nov13 19)
- Overview of using InSAR to measure ice velocity (PowerPoint 2007 (.pptx) 105.2MB Nov13 19)
- Overview of how GRACE satellites measure mass changes (PowerPoint 2007 (.pptx) 10MB Nov8 19)
- Unit 3: Ice sheet background information (PowerPoint 2007 (.pptx) 10.1MB Nov13 19)
- Videos on GRACE (Gravity Recovery and Climate Experiment)
- How GRACE works (Acrobat (PDF) 244kB Jun2 15)
- Student exercise files
- Unit 3: Student Exercise Global Sea Level Response to Ice Mass Loss (Microsoft Word 2007 (.docx) 6.7MB May21 21) PDF (Acrobat (PDF) 2.8MB May21 21)
- Unit 3: Student Exercise Data File (Excel 2007 (.xlsx) 43kB May21 21)
- During the student exercise, the instructor will need to hand out additional data figures to the students for Questions 9 and 10. We strongly recommend waiting until students have completed the work in the unit up to Question 9 and 10 before distributing the figures, as much of the earlier work requires students to make calculations to create some of the time series in the figures.
- Student exercise answer keys
Teaching Notes and Tips
- The unit requires use of computers. Ideally each student will have their own computer or laptop. At least there should be one computer per work group. If it is not feasible to use computers during the class/lab period, the instructor should take extra steps to make sure that students understand what is required of them to do the exercise outside of class.
- The provided data files are in Excel. Students may actually prefer to use another spreadsheet program such as Google Sheets if they are working on their own computers. Google Sheets, while less powerful than Excel, has the capabilities to do the analyses for this unit. The Excel files should open fine in Sheets, but you may want to double-check this process yourself if you are not familiar with it. If you have students working on their own computers and thus have different software and different versions, it is better to emphasize the processes and how to query Help rather than try to show exactly the on-screen steps for the different programs.
- The assignment is currently set up so that students work through the first 8 questions, and upon completion are given graphs with additional data to interpret for Questions 9 and 10. The assignment was set up this way so that the students weren't given graphical representation of all of the data initially. If the instructor chooses to have students complete this assignment outside of class, the students will need to be provided with the data for questions 9 and 10 ahead of time.
- At the completion of this project, the instructor should consider having a class discussion of how much sea-level rise is expected by 2100, and the causes of uncertainty, which complements questions 15 and 16. While the upper projection of sea-level rise by 2100 is 1 m in the 2014 IPCC report, more recent studies indicate that sea level rise could be in excess of 1 m. Bamber et al, 2019, Ice sheet contributions to future sea-level rise from structured expert judgment addresses the potential range and sources of uncertainty.
- If students will be completing the Stakeholder Report in Unit 5, it is useful for the instructor to remind the students that they may be incrementally working on Unit 5 by beginning to incorporate their findings from Units 1, 2, and 3.
- One approach that can work is to spend one hour of class time on the exercise, making sure, during that time, of the students' understanding of each of the different calculations that need to be made. Then they can finish it at home using the same methods without the same oversight.
- The Teaching with Spreadsheets across the Curriculum site provides support for teaching with programs such as Excel. If your students need supporting math practice, The Math You Need site provides an opportunity to brush up on skills such as graphing and unit conversion.
Formative assessment of student learning may be done through conversations with individuals and groups during the exercise if it is done during class time. The student exercise is the summative assessment for the unit. The Unit 3: Student exercise assessment rubric (Microsoft Word 2007 (.docx) 23kB Nov8 19) PDF (Acrobat (PDF) 73kB Nov13 19) provides an example of how the exercise may be evaluated. We suggest including the rubric with the student exercise so the students know the criteria ahead of time. The discussion provides a less formal form of summative assessment and is also helpful for encouraging student reflection.
References and Resources
- GRACE Mission webpage provides detailed information about the Gravity Recovery and Climate Experiment which ran 2002–2017.
- GRACE Follow-On Mission webpage provides detailed information about the Follow-On mission that was launched in 2018.
- GRACE FollowOn Mission Brochure (Acrobat (PDF) 4.9MB Jan16 19) gives a summary of GRACE accomplishments as well as an overview of GRACE Follow-On additional capabilities
- NPR news article about GRACE-FO: NASA Launching New Satellites To Measure Earth's Lumpy Gravity
- GRACE data: Bevis et al. 2019, Accelerating changes in ice mass within Greenland, and the ice sheet's sensitivity to atmospheric forcing, PNAS.
- GRACE data source used: CU Mascon Visualization Tool
InSAR Ice Velocity
- Greenland: MEaSUREs Greenland Ice Sheet Velocity Map from InSAR Data, Version 2 from the
Greenland Ice sheet Mapping Project (GIMP)
- Antarctica: MEaSUREs InSAR-Based Antarctica Ice Velocity Map, Version 2 from the Antarctic Ice Sheet Velocity and Mapping Data
- Technical information on the methods used:
- Joughin, I. (2002). Ice-Sheet Velocity Mapping: A Combined Interferometric and Speckle-Tracking Approach. Annals of Glaciology, 34, 195–201.
- Joughin, I., Smith, B., Howat, I., Scambos, T. Greenland Ice Mapping Project 2 (GIMP-2) Algorithm Theoretical Basis Document Version 2, A NASA MEASURES PROJECT.
Ice sheets/Ice streams
- The National Snow & Ice Data Center provides an overview of ice sheets, including descriptions of Greenland and Antarctica ice sheet structure and flow; climate; and how ice sheet mass balance is measured: https://nsidc.org/cryosphere/sotc/ice_sheets.html
- An overview of ice streams is provided at: http://www.antarcticglaciers.org/modern-glaciers/types-of-glacier-2/ice-streams/
- Is Ice Sheet Collapse in West Antarctica Unstoppable? is a perspective article published in Science magazine that discusses the mechanisms and potential of collapse of the West Antarctic ice sheet.
- This National Oceanic and Atmospheric Administration (NOAA) website provides an excellent overview of why rising sea level matters, the causes of sea-level rise, how sea level is measured, and future sea-level rise: https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level
- The instructor may want to include a conversation about the role of melting sea ice in sea-level rise. https://www.yaleclimateconnections.org/2014/11/loss-of-land-ice-not-sea-ice-more-sea-level-rise/ While melting sea ice has a minimal direct effect on sea level, the effect is not completely negligible as explained at https://www.realclearscience.com/blog/2015/04/the_biggest_myth_about_melting_sea_ice.html
- Intergovernmental Panel on Climate Change Synthesis Report: Climate Change 2014