Dryas flower - namesake of the Younger Dryas
Dryas flower – namesake of the Younger Dryas

By Claire

By now, it’s no secret to our regular readers that I love the movie “The Day After Tomorrow“. It’s the reason I became a climatologist and an awesome tool for talking to people about what I do for a living. When I was doing my undergraduate degree I did a course, which required me to research a climatological topic of my choosing and write up an essay. As many other people had done before me, I chose to look in to the science of “The Day After Tomorrow” to see if the plot was in any way realistic.

As it turns out, it is.

About 20,000 years ago, Earth was in an ice age. North America and Northern Eurasia were covered in large ice sheets and global temperatures were 3° – 5° C cooler on average than today (as a minimum estimate). From about 18,000 years ago, the Earth began to warm up and the large ice sheets began to melt.

This gradual warming trend was punctuated by a number of abrupt climate events – the most famous of which is the Younger Dryas.

(For those of you wondering about the name, it actually refers to a flower that grows in cold climates that was seen to resurge in Europe during this period, as recorded by pollen in sediment cores).

The Younger Dryas occurred between approximately 12,900 and 11,700 years ago. Just before the Younger Dryas, temperatures were only around 2°C cooler than today. However, in a period, as little as a decade, the Northern Hemisphere moved back into an ice age, with temperatures dropping back near glacial conditions, and ice sheets beginning to grow again.

The cause of the Younger Dryas remains a huge area of debate. What scientists generally agree on though, is that the meridional overturning circulation rapidly slowed or even collapsed during this time, reducing the amount of warm water that is moved north in the Gulf Stream, causing temperatures to rapidly cool in the Northern Hemisphere.

If you’ve watched “The Day After Tomorrow”, this idea should sound familiar. In the movie, global warming induced melting of the Greenland ice sheet lead to large volumes of fresh water being dumped in the North Atlantic. This caused the Gulf Stream to shut down, and the Northern Hemisphere to move into an ice age (via some awesome continent sized storm cells).

Now, we don’t know the details of how the Earth moved in to the Younger Dryas, but I’m going to suggest that it wasn’t via awesome super cooled storms that can flash freeze marble. However, what we do know is that over the period of a decade or so the Earth moved back into an ice age. As far as climate is concerned, this is fast!!

If you can filter through all the Hollywood-isms in the movie, you actually get a really interesting depiction of a known historic climate event (again, before I get stoned for this comment, I’m talking some pretty serious filtering).

Ice core record of the Younger Dryas. Alley et al, 2000.
Ice core record of the Younger Dryas. Alley et al, 2000.

As part of my PhD, I will be running some model simulations, looking for a cause for the Younger Dryas. There are a number of competing theories, but the two main ones are 1) that there was a catastrophic flood event that dumped a large volume of water into the North Atlantic very quickly, or 2) the catchments of the rivers flowing into the North Atlantic grew during the period as the Laurentide Ice Sheet (the large ice sheet over North America) melted, causing more water to be routed into the North Atlantic. (There is actually a third theory that the Younger Dryas was caused by a comet impact, but it is not widely accepted).

So far, I have modelled scenario 1 by dumping large volumes of fresh water in to the North Atlantic using the CSIRO Mk3L model. Much to my delight, I managed to destroy the overturning circulation and dramatically change the climate around the North Altantic (yay!). Although this scenario could provide an explanation for the cause of the Younger Dryas, there is very little geological evidence for a large flood event at this time.

Which brings me to hypothesis number 2: river routing. This hypothesis is one of the scenarios I will be running on the NASA GISS ModelE while I am in New York (and continuing once I get home to Australia). I have essentially expanded the catchment of the St Lawrence river 3 fold, in order to see if simply routing water in to the North Atlantic is enough to trigger a Younger Dryas type event.

This work is based on a paper by Anders Carlson and collaborators published in 2009 in PNAS. It turns out that there is a number of lines of evidence supporting the river routing hypothesis. It is really important that any mechanism you use to make the model behave in a certain way is backed up by geological evidence. This adds another level of support to your theory and means it’s more likely to be taken seriously by your peers.

I’m still working on getting this model scenario up and running (any day now!) and hopefully I will have something interesting and worthwhile to add to the Younger Dryas debate at the end of it.