Skip to 0 minutes and 5 secondsWelcome to the second week of our course causes of climate change. During this feedback video we'll answer some of the questions you have during the course. I'm Kerim Nisancioglu, and together with me I have Asgeir Sorteberg. Yeah and this week we have two of our Ph.D. students here. It's Sunil and Omar, and they have been monitoring the discussion forums. Like last week there's been an enormous amount of questions, and they have tried to pick out a few questions that we're going to try to answer in this video. So Omar do you have a question? During this second week our students realised how important is the Gulf Stream to keep Norway and Western Europe warmer than Greenland, for example.

Skip to 0 minutes and 49 secondsAnd many of our students are wondering how the current climate change might affect this trend of the Gulf Stream. So that's a great question. And it turns out that the Gulf Stream has been there as long as the Atlantic has been in existence. And as long as there's winds. And of course there's always winds, and so the ocean circulation, which depends on the winds, is going to have been in existence for a very long time. And the Atlantic Ocean has been open for at least a hundred million years, or it started opening a hundred million years ago. So, therefore, also you have the circulation in the Gulf Stream existing very, very long time.

Skip to 1 minute and 28 secondsNow why is there a circulation in the ocean as we know it today, and what is the effect of the wind on the circulation? This goes back to the work of Harold Sverdrup, who was a Norwegian oceanographer. And he's known for what's called Sverdrup Balance, which basically describes the effect of the wind and wind stress on the ocean circulation. So if we very draw an Atlantic basin here where this is the Antarctic continent. This is now Europe, and this is North America.

Skip to 2 minutes and 6 secondsThis is-- so this is now the equator. And here you have the trade winds, and further north you have the westerlies. So that's kind of the dominant wind pattern. Now what Sverdrup found is that the circulation in the ocean interior follows a pattern like this where the currents are going in this kind of clockwise fashion. These are called ocean gyres, and there's another one up here in the higher latitudes. This is the Arctic. So you can see that they have this circulating gyres that transport most of the water in the ocean. And these are by far the dominant feature in general circulation in the ocean. To balance this you also have to have currents along the Western boundaries.

Skip to 2 minutes and 53 secondsSo these are Western boundary currents. And-- so these are partly responsible for why you transport also heat to high latitudes, and water circulation is really dominated by the system. But in addition to this you have the Gulf Stream which actually is more general circulation in this manner. Where it brings a lot of warm water from the tropics up to the high latitudes and up to Europe. So the Gulf Stream is a small part, or a small component, of this large scale circulation where-- which is dominated by the gyres. But they still-- there is a transport of heat and water by the Gulf Stream. And part of this, as it goes from the tropics to the high latitudes, this cools.

Skip to 3 minutes and 36 secondsBecause it ventilates a lot of heat to the atmosphere and eventually it sinks. And that creates a vertical circulation. So it's not-- this is the horizontal circulation. Now if we look from the south to the north in the section with depth. So this is now the North Pole, this is the equator, and this is depth. So we have Z for depth. Then you will see that the gulf stream transports heat from the tropics to the high latitudes. And it's losing heat to the atmosphere continuously. At some point it sinks, and it returns back. So this is what's called the Atlantic Meridional Overturning Circulation, which also contributes to some heat transport.

Skip to 4 minutes and 12 secondsBut it's far inferior of the gyre circulation or the horizontal circulation. Now what happens if you start melting the green ice sheet, or in the global warming scenario you also have a lot more precipitation at high latitudes. So we have extra precipitation in the high latitudes in the North Atlantic and the Arctic. And you also have fresh water from melting glaciers, and the green ice cap. So this fresh, very, very light, fresh water will actually start stratifying the upper ocean. So you will have this layer of fresh surface water, and that will make it much harder for the warm, salty Atlantic water coming from the tropics to sink.

Skip to 4 minutes and 54 secondsSo if you have a sufficient amount of freshening at the surface in the high latitudes in the north Atlantic you will actually prevent the sinking. So this deep overturning branch will be shut down. So part of this Gulf Stream system will be weakened, but you still will have this horizontal gyre circulations. And because you have a very strong westerly winds, you will still pick up a lot of heat from the ocean, and place it on land, meaning heating the continents, especially Europe. So even if the Gulf Stream system is weak, or shutdown, you will still have a relatively warm Europe. And not only will you have the effect of the ocean, you will also have the global warming.

Skip to 5 minutes and 28 secondsWhich will affect the continent more than the reduction in the Gulf Stream. So that's the answer to that question. And now we're going to move on to Asgeir's part, and I think you Sunil have picked out a question or two for Asgeir. So I'll give the word over to you. Yes. There are a lot of questions in the feedback topic, and I picked up some of them. The first question is about lapse rate feedback, and the second question about cloud feedback. So, Asgeir, can you explain briefly about lapse rate feedback, and its connection with the water vapour feedback. And the second question is under how the cloud feedback process works, and how different types of cloud affect atmospheric radioactive forcing.

Skip to 6 minutes and 10 secondsSo I'll try to explain the lapse rate feedback first. So the lapse rate feedback is maybe the most difficult feedback to understand. And I'll have to use some mathematics to try to explain this for you. So if we start with just a schematic of-- so we have temperature here. And you have the height of the atmosphere here. Let's say we have 10 kilometres here. The lowest 10 kilometre of the atmosphere. And in the lowest 10 kilometre of the atmosphere the temperature is decreasing with height like this. And that's what we call the lapse rate. It is the change in temperature with height. So if we now have water vapour feedback.

Skip to 6 minutes and 57 secondsYou are putting more water vapour in the whole column of the lowest 10 kilometre of the atmosphere. And so originally have a lot of water near the surface because you have evaporation from the ocean. And you have less water up here. So that means that if you put more water into the system. Let's say we put one extra molecule of water here, and we put one extra molecule of water here. The effect of that molecule will be different because there's so much water already here so putting some extra water in here is not going to make a big water vapour feedback.

Skip to 7 minutes and 39 secondsBut if you are able to put some water up here, that's going to have a big warming effect because there's so little water up here already, so adding a little bit more water is actually going to make the atmosphere much warmer. So if you now have a climate change scenario you are changing the temperature. But what happens is, let's say that in the future the temperature will be plus two degrees at the surface. But lower-- the higher up maybe you have a change which is three degrees.

Skip to 8 minutes and 14 secondsBecause of the effect of putting water vapour up here is larger than putting more water down here. So then your lapse rate is going to go like this, right? So now there's a three degree difference here, and there's a two degree difference here. So the whole-- you have changed the whole temperature profile of the atmosphere. So and that has an effect on the greenhouse effect, and I'll try to explain that now. So, the greenhouse effect. That is basically that the Earth is trying to get rid of energy from the surface. And the amount of energy that it try to get rid is-- can be calculated as a constant times the temperature in the fourth power.

Skip to 9 minutes and 3 secondsSo this is the temperature at the surface in the fourth power. So that's the amount of energy that the surface is trying to get rid of. And up here let's say we have a layer in the atmosphere which are absorbing this. So this amount of water is not going to-- now this amount of radiation is not going to go all the way to the top of the atmosphere. It's going to be absorbed and re-emitted here. So let's say that this layer is in five kilometre. And it absorbs all the energy and it re -emits, but this time it re -emits things to the top of the atmosphere. And so back to the surface.

Skip to 9 minutes and 43 secondsBut with the temperature, which is the temperature we have up here. Maybe, let's say that is in five kilometre. right? And you have-- and some is going down. Five kilometre to the fourth. So-- and that's the greenhouse effect. So the surface is trying to get rid of some of the energy, but it's absorbed, and some is emitted from the atmosphere itself. But since this temperature is colder than-- in fact it is colder than the surface. There's less energy that is going out than is trying to escape from the surface. And this is basically the greenhouse effect. So what is happening if you have a temperature increase at the surface that is lower than higher up in the atmosphere?

Skip to 10 minutes and 30 secondsWell, the difference between what is trying to get out than what's actually going out to space it's going to be reduced. And because it's reduced it means that the effect of the greenhouse effect is less efficient. And that means that we will have a cooling. And this is what we call a negative feedback. That's why the lapse rate feedback is a negative feedback. So I hope that helped explain the lapse rate feedback. So, Sunil, could you repeat the second question? Yes. The second question is about the cloud feedback. So can you explain about how cloud feedback works, and how different types of clouds effect the atmospheric radioactive forcing. Right. So this is actually a very difficult question.

Skip to 11 minutes and 20 secondsBecause clouds is maybe the thing that we know least about in the whole climate system. But I'll try to explain a little bit about the different type of clouds. So clouds basically have two effects on the radioactive deduction. First of all, they reflect solar radiation. So if we have a cloud. Let's say this is the cloud. If the solar radiation is coming in, it's going to be reflected out, right? So that's the first-- and that's a cooling effect. So this is cooling.

Skip to 11 minutes and 51 secondsBut then they have a second effect. And that's that they are absorbing the radiation that tried to escape from the surface and the atmosphere into space. So the same clouds will also-- if you have radiation coming into it, it will absorb this radiation. And let's say it comes from the surface. Then we know that the energy is sigma times temperature to the fourth power. And this cloud has a different temperatures than the surface. And it will re -emit both ways with-- but this time, with the temperature of the cloud itself, to the fourth power. And in the atmosphere as we saw in the last question we know that the temperature is decreasing with height.

Skip to 12 minutes and 39 secondsSo usually the cloud temperature is colder than the surface temperature. And, yes, this means that this has a warming effect.

Skip to 12 minutes and 52 secondsSo this-- and the reason is that a certain amount of energy is trying to escape to space, but is absorbed by the cloud. As some is going up to space, but a smaller part then that's actually trying to escape from the surface. So that's the warming effect. And so these are two competing effects. And which one of them that actually wins is depending on the cloud properties. So I'll take an example where I have two different clouds. One is a cloud down here. Now this is the height, and this is temperature here. So this is a-- let's say it's a thick warm cloud. So it's thick and it's warm.

Skip to 13 minutes and 31 secondsAnd then we have another cloud up here, and this one is thin and cold. So it's thin and cold. So if we take this one first. Since it's thick, it's going to reflect a lot of the solar radiation. So it has a huge cooling effect. But and then it's warm, so the temperature of this cloud is not so different from the temperature at the surface. So that means that the warming effect is actually going to be small. So the cooling effect of the reflection of solar radiation is going to out compete the warming effect by absorption of the terrestrial radiation. So this type of clouds, if you have more of them, it's going to cool the earth system.

Skip to 14 minutes and 22 secondsBut if we look on this thin one here. Since it's thin, a lot of the solar radiation is just going to go through it, and very little is reflected. So the cooling effect here is quite small. But on the other hand, the warming effect is going to be big because the temperature of this cloud is very different from the temperature at the surface. So it's going to have a big warming effect. So for this type of clouds, the warming effect is going to out compete the cooling effect. So more of this type of clouds is going to give a warming. And this is how complicated they are.

Skip to 14 minutes and 57 secondsSo different types of clouds is going to either cool the system, or warm the system. And we have to sum up all the differences to get the total cloud feedback. And this is maybe one of the feedbacks that is most uncertain in climate models today. All right. So I hope we answered some of your questions. And we also are looking forward to next week when we're going to talk about past climate variability and? Well I'm going to talk about heat transport into the deep ocean and how that is affecting the climate. So keep on asking all the good questions, and we'll come back with the feedback video in the end of next week.

Week 2: feedback

In this short video the educators meet a group of University of Bergen students, discussing some central questions from this weeks lessons.

The main questions discussed in this video are the following:

  • What is the lapse rate feedback and what is its connection to the water vapour feedback?

  • How does the cloud feedback process work and how do different types of clouds impact atmospheric radiative forcing?

  • How may the current climate change we are experiencing, affect the strenght of the Gulf Stream?

Questions from all learners - posted througt the “FutureLearn - Comments” are also considered for the feedback sessions.

Subtitles and transcript will follow later.

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This video is from the free online course:

Causes of Climate Change

University of Bergen