Durham Geographers settle dispute over source of historic sea level rise

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Research led by Durham’s Department of Geography and published in the journal Nature has settled the dispute over the source of a major surge in sea level at the end of the last ice age. The 500-year event known as Meltwater Pulse 1A (MWP-1A) resulted in a sea level rise of 18 metres. Understanding the triggers of past sea level changes like MWP-1A is critical to informing projections of future sea levels.

Between the end of the last glacial period 20,000 years ago and a period of stability 2,500 years ago, the Earth’s sea level gradually rose by more than125 metres. This period was punctuated by a series of rapid rises called meltwater pulses, which were caused by the climate change induced disintegrations of ice sheets.

The largest of these pulses was the Meltwater Pulse 1A, which began around 14,600 years ago.

During MWP-1A, sea levels rose by 40 to 60 mm per year, much faster than the current rate of 3 to 5 mm per year. Along with the thermal expansion of seawater due to increasing temperatures, the melting of land-based ice is one of the major causes of contemporary sea level change. Understanding the ice-ocean-climate interactions of these historic meltwater pulse events is vitally important in understanding how the climate will respond to the melting of ice sheets today.

The new data allowed researchers to refute the Antarctic Ice Shelf as the main source

Scientists have long been divided over the source of the meltwater that caused MWP-1A. Some researchers pointed to evidence that implicated Northern Hemisphere Ice Sheets in North America and Scandinavia, whereas others believed the massive Antarctic Ice Sheet to be the main culprit.

This disagreement was fuelled by limitations in the sea-level fingerprinting technique, which aims to match ice sheet melting with sea-level change records from multiple geographically distributed sites. The primary problem is the lack of sea level records for the period during MWP-1A. Commonly, just three sites are used – coral reef data from Tahiti and Barbados, and sedimentary indicators from Sunda Shelf in Southeast Asia. By using these three sites it is impossible to determine whether meltwater arrived from the Antarctic or Scandinavian Ice Shelves.

The new study by Durham researchers used an additional three sites, including data from northeast Scotland, to solve this problem. It identified the North American Ice Sheet as the source of most of the meltwater. The research also indicates that a significant portion of the sea water input came from the Scandinavian Ice Sheet. The new Scottish data also allowed researchers to firmly refute the Antarctic Ice Shelf being the main source of meltwater.

Researchers now need to determine what triggered the North American Ice Sheet to melt so suddenly – and why the Antarctic Ice Sheet didn’t also melt. This will allow better models of how the Earth’s last two remaining ice sheets – the Greenland and Antarctic Ice Sheets – will respond to climate change.

While MWP-1A was massive, roughly equivalent to melting an ice sheet twice the size of Greenland in just 500 years, the Antarctic Ice Shelf could cause a sea-level rise over three times greater.

Researchers now need to determine what triggered MWP-1A

Research into sea levels past and present is particularly relevant, because the Greenland Ice Sheet is rapidly melting. This massive discharge of cold freshwater into the North Atlantic is a symptom of climate breakdown and is disrupting ocean circulation. The Atlantic Ocean circulation that underpins the Gulf Stream is already at its weakest for a millennium, and further weakening could lead to more extreme weather across Europe.

And extreme weather isn’t the only consequence. A large proportion of the global population currently live in areas vulnerable to sea level rise, meaning many millions of people may be forced to migrate over the coming decades. Projections suggest that by the end of the century, 630 million people could live on land lower than the annual flood levels.

The most pessimistic future sea level predictions rely on large amounts of melting in Antarctica, but there is a lot of uncertainty surrounding this. Therefore, understanding why the Antarctic Ice Sheet didn’t contribute significantly to the MWP-1A is extremely important in predicting whether we are headed towards a worst-case scenario, or whether we will be lucky enough to avoid the worst wrath of the oceans.

Image: Alex Peace via Flickr

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