Okay, welcome to Week Two of our online course. This week we’ve had quite a few questions and good discussions. And one of the points is that we wanted you to discuss among yourselves, so we don’t go in and start correcting… But we picked out some topics we think are important and we’re going to explain one today, that relates to the concepts of feedbacks and heat transport by the ocean. 0:24 As many of you know there was a film some years ago called “The Day After Tomorrow” where there is this scenario, where you have a glaciation basically, with ice covering large parts of the Northern Hemisphere.
And that was a lot of discussions relating to this, are also relevant in terms of what we’ve talked about in the course. 0:44 Basically you’ve learned about the ocean transport. The ocean brings heat from the tropics up to the high latitudes. This is called the Gulfstream or the North Atlantic Drift. It brings heat up to the higher latitudes, in particular in Western Europe, and makes the climate here quite warm and then sinks and returns back south. The question
IS: can this weaken because of changes for example in climate? And that’s what Anaïs will talk about. Whats the consequence of…IF it were to weaken, let’s say if Greenland started melting even more than it is today and you start freshening in the surface Atlantic… Could you reduce the amount of heat transported by these currents to the north. So Anaïs? 1:27 So that is what is suggested in the movie: you get suddenly a large amount of fresh fresh water in the north hemisphere which shuts down the circulation. Then, because of this other concept you learned about, the albedo Feedback…
you have a change in temperature. (I will write it like this “T”) A change in temperature, would…. in this case a decrease in the temperature, would enhance the ice formation. So the ice area increases. We know that the ice has a very large albedo so it will actually increase the reflectivity of the surface.
And the effect of this increased reflectivity (= less solar radiations are absorbed) is a cooling of the North Hemisphere. This is a positive feedback. So you would have this again and again and this is what triggers a new ice age in the movie. 2:53 This is kind of a runaway in the sense that you get a cooling, and you get more ice, you get more solar radiation reflected back to space and then it gives more cooling. So it’s kind of unstoppable in some sense. And so there’s been some discussion about this. What if this circulation in the ocean (especially the Gulf Stream) were to weaken and the temperature started to decrease at high latitude? Will this happen?
3:17 What they talk about in the movie… there is this one researcher who notices that this could happen by mentioning an event that happened ten thousand of years ago. 3:So This is the temperature reconstructed from an ice core in Greenland (-100ky to today). And you can see very well our interglacial time here, and here is the last glacial period. You can see that there are large variations of temperature… but what is interesting is this event, corresponding to the younger Dryas. There while we were going from the glacial to the interglacial, so the temperature is increasing, suddenly the temperature increases abruptly. But this is followed by an abrupt cooling as well.
It was suggested that maybe, this large increase in temperature reduced the ice cap and the resulting fresh water flux would have then triggered the abrupt cooling. 4:26 Normally you would expect temperature to just rise like that, coming out of an ice age. You just expect a warming (and this is today, on Greenland). And then what you observe here though is this really rapid cooling… 4:38 So now the question is: Why couldn’t it happen today as well, since it happened in the past? 4:45 This has been debated a lot in the literature. It’s a hot topic in the science as well. Why can we have a very abrupt reversal of the warming?
For example you’re going into a warm climate because Greenland starts melting. You have a lot of freshwater. The Gulf Stream is weakened. And then you get the cooling, a bit like what happened we think during the glaciation. We’ll talk more about the glaciation next week. 5:10 Now in reality the ocean is much more complex than just thinking of a Gulfstream that sort of transports heat north into the northern hemisphere. In the Nordic seas and into the Arctic. In reality there’s a lot of circulation that’s more in the horizontals dimension, meaning you have gyre circulations. You learned about this during this week.
So most of the ocean water is circulated in the upper layers and you basically have a return flow like this. And you also have a flow in the other sense. These are the gyres, the big circulation gyres and the oceans. So that is where most of the warm water, and also heat, are transported. So even if you reduce the amount of sinking, which is this gulf stream which goes north, gets dense, and sinks.. if you reduce that, you still have this large circulation gyres, which are wind driven, and they will transport, still, quite a bit of heat.
And the ocean will be quite warm compared to land, so when you have the strong winds (you have what is known as the westerlies for example) you will still transport this warm temperatures from the ocean onto land. Therefore Europe will remain warm even if you reduce what is known as the gulf stream, because the wind driven circulation will prevail. 6:26 The other thing is that we know now that Greenland is melting and we have quite a bit of of meltwater coming off the Greenland continent into the ocean but that water mostly just transports itself along the coast.
It id isolated from the large interior basin so it doesn’t affect as much the ocean in the interior, where you would grow sea ice. So it doesn’t matter as much for the Gulf Stream, as we thought ti would before. So for now it looks like the effect of this change in ocean heat transport is going to be minor, and that the global warming is what is prevailing. Also locally here in terms of climate changes. 7:04 Also, compared to the younger Dryas, the volume of ice available to actually shut down the AMOC (Atlantic Meridional Overturning circulation) is much smaller. The simulations show that you need a very large amount of fresh water (to shut down the circulation).
7:19 It’s a good point. In the glacial all of this area here and here was covered with ice. So it’s huge amounts of ice and a lot of it melted in a short time. Now we are left with Greenland and the fact is that it is much smaller. So that’s it for now. Stay with us. We’ll come back with some highlights next week!