Professor Antony Long, one of the lead researchers in WP1 (Rapid Neoglacial-transitions in the North Atlantic) of the Tipping Points project, presented some interesting research relevant to the project at the last American Geophysical Union meeting, the world’s largest conference on geophysical science. The study concerns the Jakobshavn ice stream (pictured above), one of the largest ice streams in Greenland, located in the central western region of the country. Ice streams are areas of fast flowing ice embedded within the slow-moving ice of an ice sheet. Recently, Jakobshavn has displayed unstable behaviour. In the past five years it has been retreating very rapidly making researchers question whether the Greenland ice sheet is crossing a ‘tipping point.’ Studying the behaviour of ice streams reveals that ice sheets aren’t merely big pieces of ice, but complex systems that interact with the land and sea on multiple levels. The Jakobshavn ice stream is important to Greenland because it drains six to seven percent of the ice sheet into the ocean. Scientists have been paying close attention to large ice streams like the Jakobshavn because they have such a large influence on the stability of ice sheets.
In order to investigate the recent behaviour of the Jakobshavn ice stream, Long and his team used sea level data recorded from the past that reveals whether or not the land is rising or falling relative to the sea. “When you lose a lot of ice, you take weight off the land and the land bounces up, so sea level falls locally. If the ice is building up the opposite happens, its weight loads the earth and locally sea level rises. So we can use evidence from past sea level changes to infer changes in ice load through time,” said Long.
Long and his team used a computer model to simulate how sea level would change geographically if a lot of ice was lost from the Jakobshavn ice stream. The model predicts that sites close to the point of ice loss (the snout of the ice stream) will experience faster rebound than sites further away. “A critical question for us is to understand whether or not the behaviour we’ve been seeing in the last decade or so with Jakobshavn is natural variability which has been going on all along or if it is something quite exceptional,” he said.
For the study they looked at three different sites, one in Pakitsok located close to the Jakobshavn ice stream in the Greenland ice sheet, one in Aasiaat located further away from the ice sheet and one further down the west coast of Greenland close to the town of Sisimiut. In order to measure the difference in land movement for each site researchers used salt marshes as ‘natural tide gauges’ for measuring how the land moves relative to the sea. They extracted microscopic single-celled plants (diatoms) from marshes located at all three sites to track changes in how salty the marshes were and thus how often they are flooded by tides. Some of the diatoms used for the study prefer freshwater habitats and grow in higher parts of the marsh that aren’t flooded much, while others like it lower down where it floods a lot. This method is valuable in finding out how the land moved over decadal to century timescales, including during what is known as the ‘Little Ice Age’ that occurred around 1300-1870.
To try to get information from the last century or so, Long has targeted the analysis of sediments laid down in the upper part of salt marshes deposited in the recent past. By radiocarbon dating plant remains, especially the seeds of sedges that grew on the high marsh, Long is able for the first time to provide a record of sea level changes over the last 100 years or so. When combined with older records, what this new work shows is unexpected; it shows that sea level was rising at Pakitsoq, near to the Jakobshavn ice stream, but suddenly slowed after about AD 1800. But at the Aasiaat site, further from the ice stream terminus, Long found that the rise in sea level slowed down 200 years earlier, at about AD 1600. This suggests that the ice stream has been behaving differently than the broader ice sheet. “The signal of land movements we see around 1800, close to Jakobshavn, and the signal along the outer coast is very strong indeed and we never knew about that, that’s a ‘tipping point’ if you’d like, that no one has ever seen before,” said Long.
The behaviour of the Jakobshavn ice stream observed recently by researchers may be influenced by climate change, but it’s difficult to say to what degree as recent research shows that the retreat of this and several other big ice streams have slowed in the last few years. Long’s research reveals that the relationship between an ice sheet and ice streams is perhaps more complex than what has been understood before and that sudden change events or ‘tipping points’ may play a significant role in how they behave over time.