Freshwater Injections

Hello MOOCers!

It has been a very interesting week 2 on the MOOC, and we have seen a lot of debate about tidal power, and how structures like the Severn Barrage would affect the estuary and wildlife, versus the potential “green power” benefits.

It has also been quite a different week with the content, a little bit of maths, congratulations to everyone who gave it a go, and don’t forget you can download the PDF with the answers from FutureLearn.

One of the most commonly asked questions has been about the circulation system, and what would happen if a large volume of freshwater was introduced to the system. This is a particularly fascinating area of oceanography, and the subject of continued debate. Fortunately, we do have some answers, and as this forms a significant part of my PhD, I thought I would try to explain what we know about this phenomena, and what happened the last time, some 8200 years ago.

After the end of the last ice age, much of what is now North America was covered in a large ice sheet: the Laurentide Ice Sheet (LIS). This ice sheet was between 2400m-3000m high at is thickest point, comparable to Greenland today. When the planet started to warm, this ice began to melt, and the process took several thousand years to complete. This is an image from the excellent book by William Ruddiman, many of these images can be found here. 


The Laurentide Ice Sheet

When ice sheets melt, they often form what are known as Pro-glacial lakes, large bodies of cold, fresh water surrounding the edge of the ice. This is normally a result of the land rebounding once the weight of ice is removed (much like a cushion you have been sat on, the indent remains for some time, but it slowly returns to a neutral level, this is called isostatic rebound). The sediments pushed forwards by, and deposited at the front edge of the ice sheet form moraines, large mounds that often dam the water behind them. There are many such lakes in existence today, though none of the same size as that which surround the LIS. This lake is known as Lake Agassiz.

This lake suffered a catastrophic outburst at around 8.47 thousand years before now, and as a result, a large volume of freshwater entered the North Atlantic, by some calculations this happened in less than a year. This has the effect of reducing sea surface salinity and temperature in one of the most crucial regions of the world for deepwater formation. This outburst is believed to be the cause of a slow-down in the strength of the Thermohaline Circulation System that you have been reading about this week.

Over the next 300 years, there is a near global signature of change. Over Greenland, it was between 5.7-11 degrees celcius cooler, and most of this change happened in less than a decade. The monsoonal regions of the world show a near contemporaneous decrease in the strength of the monsoon, and rainfall patterns changed substantially over much of Europe. In addition, rapid sea level rise occurred, though there are widely varying estimates of this, the most commonly reported figures are between 1.2-2m of sea level rise.

This is not the only time that this type of event has occurred. Around 12.7 thousand years ago, when the LIS was still a significant size, an outburst cause more significant cooling, and for those still following, there are older events known as “Heinrich Events” where armadas of icebergs entered the North Atlantic following rapid deglaciation. I won’t go into details here, but here is a good link.

 Here is a more technical overview from NASA

A summary from the IPCC report on rapid climate change 

Feel free to ask questions!




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