Another obscure rapid climate change event: the Younger Dryas Cooling

We have talked about two distinct repetitive abrupt climate changes that are evident in the ice core records over the last 60,000 years: Dansgaard-Oescher (D/O) cycles and Heinrich events. 

However there is one particular cold event in the recent geological past that scientists are not able to identify as either one of them. This is called the Younger Dryas cold event (YD).

(Rahmstorf, 2002)

The YD cold event appears as an interesting rapid climate change event, since it has been detected during a phase of long-term warming. So why did climate all of a sudden drop to nearly ice-age like conditions? The interesting part is that geological evidence strongly suggests first a cooling, then a warming of roughly 4-8 degrees in Greenland within less than 20 years (Barber et al., 1999; Alley et al., 1993; Mayewski et al., 1993; Dansgaardet al., 1989)! 

The YD does indeed deserve the name RAPID climate change…

The most obvious mechanism known so far to drastically change northern hemispheric climate on such a short timescale is again the Atlantic circulation. In 1989, Richard Fairbanks researched coral cores from Barbados and calculated melt water discharge into the Atlantic Ocean from oxygen isotope records in the corals (see Science Fact for more information). He found two distinct events during the YD where freshwater entered at a maximum rate of 14 000 and 9 500 km3/year! To put that in relation: during Heinrich events models had calculated 1.25 million km3 of freshwater release, but in more than 250 years (see last post). That is a maximum rate of only 5000 km3/year. Today, the two big rivers flowing into the Atlantic (Mississippi and St Lawrence rivers) release barely 900 km3/year (Fairbanks,1989).

Hence, it is extremely likely that such a flush event of freshwater would have an effect on the thermohaline circulation in the Atlantic.

The question is what could cause such a sudden release of freshwater in such a short time?

In the late 1980s, Broecker and colleagues concluded that most likely a large freshwater lake emptied into the North Atlantic Ocean (Broecker et al., 1989; Broecker et al., 1988). They knew that the big Laurentide ice sheet on North America had been melting for a while (remember we are actually in a time of climate warming). Due to big moraines in the landscape it was possible to form two large lakes, Lake Agassiz and Lake Ojibway, located near the Great Lake area of today (Barber et al., 1999). Broecker and colleagues noticed that the normal outflow of the lake went southwards through the Mississippi river basin into the equatorial Atlantic. However at the same time of the YD onset, the outflow of the lakes changed, most likely due to a moraine breakdown that had served as a dam. Large amounts of water now catastrophically raced through the Hudson Straight into the North Atlantic (Broecker, 1989). A new study soon to be published in 2015 by Li &Piper shows that the Labrador Current (flowing along the North American coastline) did indeed speed up during the YD event probably due to all the extra freshwater input.

These findings led to a great increase in research about the YD and possible routes which the water could have taken. In 2010, Murton et al. presented evidence for old fluvial layers deposited in the Arctic Ocean. They argue that there was a second important outlet which formed during the same time and enabled large freshwater floods straight into the deep water formation sites. Hence, the Hudson Straight outlet might have been of only secondary importance in providing catastrophic freshwater flushes.

Outflow via Mississippi River and via Hudson Straight (Eastern Outlets) during the Younger Dryas cold event. notice that y-axises are inverted! (Broecker et al., 1988)

Both outflows from Lake Agassiz and Lake Ojibway (eastern & northern red arrows) 

(Murton et al., 2010)

In general, scientists are sure that this event led to a shutdown of the Atlantic circulation (Clark et al., 2001; Rahmstorf, 2002; Barber et al., 1999). Especially the release of freshwater directly into the areas of deep water formation, as it is the case with the Arctic outflow (Murton et al., 2010) will lead to fast reorganisations of  the circulation and stop new deep water formation if the freshwater force is strong enough.

Yet, there are many unresolved questions that scientists are still trying to work out.

One question deals with the mechanisms that can make a rather local disaster a global climate event. Here Rach et al.(2014) just found evidence that there was an obvious delay between climate cooling in Greenland and in Western Europe. This delay may show that not only cooling temperatures, but also changes in wind and rain patterns have influences on terrestrial climate (Rach et al., 2014).

Another question looks at CO₂ evolution during the YD. With the YD being a cold event, one would have expected low CO₂ concentrations. However, Steinthorsdottir et al.(2014) show an abrupt increase, then decrease of CO₂ at the beginning of the YD period suggesting that something had changed in the ocean circulation and forced it to “burp out” a cloud of CO₂. Did that change in ocean circulation maybe influence the YD event? We don’t know yet.

Short review:

After looking at three different distinct abrupt climate change events (Daansgard-Oescher cycles, Heinrich events, the Younger Dryas cooling event), we see that ocean circulation is not necessary the driver, but the amplifier of rapid climate changes. It almost seems like the Atlantic Ocean is operating in several modes... and what about today....?

Keep reading the blog ;)