Skip to 0 minutes and 1 secondSo welcome to this week three of the Causes of Climate Change MOOC; - and today, as we had the two other weeks, we will have a feedback video on some of the questions that you had. And I would like to thank you all for participating. This is the last week of the MOOC, and we’re really satisfied with the amount of people that has carried it through - all the three weeks. So, this week we have had some questions about the use of isotopes to say something about the past climate. So, Anaïs, maybe you can try to explain sort of the basis for this? Yes.

Skip to 0 minutes and 37 secondsSo there was quite some discussion about how we can use oxygen to actually infer temperature in the past. That is what we are going to talk about today. One important information is that oxygen has... there is actually different types of oxygen. We have the oxygen 16, the oxygen 17 and oxygen 18. There is a fixed ratio on earth on how much you have of these different oxygens. And they have different weight. This one I will refer to as a light one and it’s also the most abundant, 99.76%. And then the second one most abundant is this one, which is only 0.2%. And then what matters... so okay you have the ocean here, and a lot of reactions...

Skip to 1 minute and 37 secondsSo when you have reactions that imply oxygen, the ratio of how much you have of this one and this one, varies..... You have small animals in the ocean, and they build... carbonate shells. This animal integrates Oxygen to build its shell, which is this element (CaCO3). You have oxygen here, so it will use oxygen (oxygen 18 and oxygen 16) during his lifetime. But how much it takes of these two elements depends of the temperature, basically.

Skip to 2 minutes and 22 secondsAnd so...when it dies and sinks to the bottom of the ocean, it becomes part of the sediments layers. When you will make a sediment core and find some of these animals in every layers, you can measure the ratio of oxygen 18 and oxygen 16. Then, knowing the relation with temperature, that you can measure for example in laboratories, then you can find what was the temperature at this time or at this time. You can measure oxygen in there, in the animals shells. And then you have the same kind of...of processes that happen -not exactly- at the ice caps.. so you have a small clouds with water, that you know, is also made out of oxygen, of course (H2O).

Skip to 3 minutes and 26 secondsHere it is water vapor and then when it becomes... in this case snow, this reaction of precipitation is also, would also....the ratio would also depend of temperature. So again if you do an ice core here... then you will find the temperature of the cloud actually, that constitute this layer of snow. The temperature of the cloud and by 1 extension the temperature of the atmosphere. At this time and then this time.

Skip to 4 minutes and 3 secondsBut then there are of course other factors that will change a bit this ratio and one of them would be that if you have big ice caps, it will actually change the ratio in the ocean because there is more O16 ...when you have this reaction of liquid water to water vapour, one of the results is that it extracts O16 mostly, so if you have a lot of this water traveling toward the North and becoming snow at the poles the snow accumulates in these ice caps and it removes a lot of O16 from the ocean. So you should also take this into account if you want to get a more precise temperature.

Skip to 5 minutes and 5 seconds- Does this also mean that it matters where you have the evaporation and the transport towards this cloud ? - Yes that’s one of the thing that you have to take into account as well - so how accurate is this? - I don’t have numbers.. - No, right, I guess this is one of the key ways of finding.. - Yes. I guess, so the best would be to combine different methods. There are different ways of finding temperature. An other one would be to measure the temperature directly in the borehole. Although it probably seems a bit surprising, the temperature is somehow conserved.

Skip to 5 minutes and 53 secondsYou have to apply some mathematical expressions of course but just by measuring the temperature right now in the ice cap you can find the temperature back in time. So if you combine all these information, hopefully you get a more accurate temperature record. But that's the main principle behind the oxygen isotope thermometer. Well, thanks for the explanation. So, I hope that you got a better feeling for this, at least I did! So I would just say thank you to everyone that has sort of been with us these three weeks. And keep on having these good discussions on the discussion forum, that’s what’s really is the key of this course. Thank you very much.

Week 3: feedback

In this video the educators will be going through central questions from this week’s lessons.

Questions from all learners - posted through the “FutureLearn - Comments” have been considered for the feedback session.

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This video is from the free online course:

Causes of Climate Change

University of Bergen