Greenland
The Potential for Melt Water Pulses from Greeland Ice Sheet Ice Saddle Collapse
About 20,000 years ago, most of Canada and the northern U.S. was covered by the large Laurentide and Cordilleran ice sheets. This was part of an era called the “Last Glacial Maximum” where Earth’s ice sheets were at their greatest southern extent and sea levels were at their lowest due to a much cooler global climate. At the end of this era when global temperatures were rising, global sea levels rose periodically and rapidly in several distinct events called melt water pulses (MWP). The first event, called MWP-1A, rose global sea levels roughly 10-20 meters (32-65 ft) in only 500 years which is among the fastest recorded in geologic history. Although the origin of MWP-1A is controversial, some of this newly added water has been attributed to the melting of the Laurentide and Cordilleran ice sheets. Specifically, it has been suggested that the topographic low point where the two ice sheets connect called an ice saddle accelerated the melting causing it to collapse and raise sea levels rapidly.
The current Greenland Ice Sheet (GrIS) is multi-domed, with a large dome in center and a smaller one in the south, connected in an ice saddle. Evidence also suggests that past configurations of the GrIS resembles the one in present day. With present day anthropogenic climate change and past times of rising temperatures this leads us to the questions: has a saddle collapse occured in the Greenland Ice Sheet in the past? What contribution has it made to MWP? What does this mean for the future? In this project, we used the Greenland Ice Sheet as a test case to understand its potential to generate a MWP under idealized conditions. Specifically, we aim to find how ice dynamics (i.e., how ice flows and changes) control saddle collapse by using numerical simulations of the GrIS.
We find MWP events occurs in every simulation, associated with the collapse of the pre-existing saddle and a much smaller saddle that forms and collapses within a simulation, each contributing roughly 1/2 m (nearly 2 ft) of sea level. When either of these saddles collapse appears to be controlled largely by how fast we make melting occur at the surface and deep, fast-moving outlet glaciers. These outlet glaciers control how much ice from the interior (where most of the ice resides) is added to the ocean, ultimately controlling sea level rise from the GrIS. Some of these outlet glaciers are more senstive to faster melting which cause that original saddle to collapase relatively quickly (within 2,000 years).
Below is an animation from one of our simulations showing how the ice thickness of the GrIS changes as we melt it from the margins very slowly (very slowly, it takes about 100,000 years before something “exciting” happens). The colored lines point on the smaller plots on the right which show three of these outlet glaciers that are important to saddle collapse.
