3.13

## University of Bergen

Skip to 0 minutes and 5 secondsWelcome. This is the feedback video of week 3. My name is Kerim Nisancioglu and together with me I have Professor Asgeir Sorteberg. And we're going to try to answer a few of the questions that you had on the discussion forums during the previous few days. And together with us we have the two mentors of the course, Ida Marie and Nadine. And Ida Marie, do you have some questions for this week? Yes. Hi, my name is Ida, and I have a master's degree in metrology, and now I'm a Ph.D. Student in renewable energy. So Asgeir, one of the questions from week 3 was regarding the heat release from humans, and also the direct heat release from actually burning fossil fuels.

Skip to 0 minutes and 45 secondsSo how large are these contributions compared to other forcings. Well that's a good question. And we haven't really touched upon that in the course. But we of course have direct heat from fossil fuels, and I'll write down the equations. So if you have some burning of fossil fuel, so you have a hydrocarbon, depending on what kind of fossil fuel you have, that will react with the oxygen, and produce CO2. It will produce water, and finally some heat.

Skip to 1 minute and 20 secondsSo the question is, how big is this component, the direct heat release, compared to, for example, increasing the CO2 levels? So I have some notes here of some numbers and I tried to break a back of the envelope calculation of that, so you get some idea of the numbers here. So the global energy consumption is around 3.9 time 10 to the 20th joules per year. So if you start with that, energy consumption that's 3.9 time 10 to the 20th and that's joule per year. So that's the total amount of energy we use. Around 80% of that energy is due to fossil fuels. And if we take the Earth area...

Skip to 2 minutes and 10 secondsEarth area, that's around 5.1 time 10 to the 8th square kilometre. So what we can do now to actually get how much energy is released per square metre, we can calculate the flux, which is just calculating the energy consumption, which is this number. And we-- divide it by, sorry, we multiply by 0.8, because 80% is fossil fuel. And we divide it with the Earth area. And we end up with a flux which is 0.02 watts per square metre. So that's the total energy released directly from the heat component. And this can be, for example, compared to the changes in the radiative forcing since the industrial era, which is in the order of 2 watts per square metre.

Skip to 3 minutes and 0 secondsSo this is a factor a 100 smaller. So globally averaged, the direct heat release gives very little extra energy into the climate system compared to the release of CO2 and other greenhouse gases. Then the second part of the question, what about humans? We emit energy because we are warm and we are emitting heat. And that is following the metabolic rate, more or less. And the metabolic rate is depending on if you sit down or if you're jogging or whatever. But it's in the order of somewhere between 60 and 120 watts per square metre. So and then the population on Earth is around 5.4 is it, no 7.4 billion peoples.

Skip to 3 minutes and 51 secondsSo if we just take the metabolic rate and we multiply it by the area of a human, which is typically 1.5 square metre, and the number of persons on Earth, and then divide it by the Earth area, we can get the flux coming directly from humans. And if we do that, we get that the flux in the order of 0.002 watts per square metre. So it's even smaller. It's a factor of 10 smaller than the heat released from fossil fuel, and it's a factor of a thousand smaller than the radiative forcing over the last few hundred years. So globally speaking, these two components are not very important for the energy budget.

Skip to 4 minutes and 33 secondsBut of course, this one can be important on a local scale. So these are numbers that are global averages. So what about the local effect in, for example, a compact city? Right. So that's a great question, and this is part of what we call the urban heat island, the fact that urban areas are a bit warmer than the surroundings. I'll try to give you an example of that. So I'm going to use New York as an example.

Skip to 5 minutes and 5 secondsAnd the energy consumption in the US, the average energy consumption for-- is around 9000 watts per person during a year. And if we think of New York as sort of being this area here, so you have the city here, and you have the surroundings here. And there are-- the population of New York is around 8 million people. So you can sort of easily calculate-- and the size of New York City is around 1000 square kilometres. So then you can calculate the amount of energy coming from the city because of the energy consumption from the humans there. You will then end up with a flux which is in the order of 70 watts per square metre.

Skip to 5 minutes and 57 secondsWe remember the global average was 0.02, but in the city, it's much, much larger. So it's an important contributor, but, there is a but here. And that's that, this air, since it's of course not going to stay in the city forever, you're going to have winds coming in. So you release heated air, but that air is also going out of the city. So how much this will have an effect on the temperature is depending not only on the amount of energy that we put into the system, but also how fast it is released, or moved out of the city, and how much of the air column is going to be heated.

Skip to 6 minutes and 38 secondsSo let's say that we have-- you heat the first 500 metres, for example, and you have a wind speed, typical wind speed in a city is maybe 3 metres per second.

Skip to 6 minutes and 50 secondsSo that's the wind speed, which is around 10 kilometres per hour. If the city now is a square, it will have the-- it will be around 30 kilometre wide. So you will end up with the residence time of the heated air being around 1.5 hours before it is moved out of the city. And if you use all that information, you can actually calculate the temperature effect of this extra 70 watts. And you will end up that the temperature change due to this is in the order of 0.65 degrees Celsius.

Skip to 7 minutes and 32 secondsSo that will be the extra heating, but as we see, it depends on the wind speed, it depends on the size of the city, the energy consumption, how many people lives there. And it's really-- and it's only part of the urban island heat effect, because you have other parts, like you've changed the reflectivity, and you've changed the amount of evaporation inside the city, which will also give rise due to changes in the temperature inside the city. But this is the direct effect. Typically a direct effect of the fact that you are consuming a lot of energy inside the city. So many of the observations are located near these compact cities.

Skip to 8 minutes and 8 secondsAnd we notice that this number is comparable to the global average temperature increase. So how is this-- Right, so-- but this is only the temperature difference from what you would have in the city and the surroundings. And of course the temperature change can also be different in the city if the city grows fast, it will be warmer. And this is in the global temperature records, this is corrected for. So you don't use the urban observations directly. You actually try to do your best to correct for the heat-- for the heat island effect. So that the temperatures you see on the web and other places, on the global temperature changes, then this effect is corrected for.

Skip to 8 minutes and 50 secondsThank you, Asgeir, for your very good answers. And Nadine, you also had some questions, right? Yes, exactly. That question was related to the other topic of this week about past climate change. And we've learned that at the moment, we're in an interglacial state. And Kerim, can you explain to us how long you're going to stay in the state, and if we are getting into an ice age again? This is a very interesting question, and it's something that actually scientists have been working on over many years. As Nadine said, we are now in the interglacial. It lasted about 10,000 years. We talked about it this lecture and also previous lectures.

Skip to 9 minutes and 31 secondsSo this is basically a very stable period where there's not much ice on land. There's the Greenland ice sheet, of course, as on Antarctica, but there was none of these ice sheets that you had over North America and Eurasia during the last ice age. So the question is, how long will this interglacial period, warm period, last? And then if you look at the insulation today, and you learned during the first week that the insulation on Earth is governed by the changes to the Earth's orbit around the sun, and the tilt of the Earth's axis in relation to the fixed stars.

Skip to 10 minutes and 6 secondsSo if we just look at that for today, to see, OK, what are the indications given the insulation on the surface of the Earth in terms of the kind of next ice age? And as you remember, we have the sun and the Earth's orbit is an ellipse, exaggerated in this figure. And depending on where the seasons are relative to the Earth's orbit, you will have more or less insulation, for example, in the Northern Hemisphere. And we often think about the Northern Hemisphere, because that's where we have most of the land masses, and that's also where the glaciers grew during the last ice age. So actually, in today's situation, we're actually very close to the full equinox.

Skip to 10 minutes and 53 secondsAnd if you look at this drawing, we'll put on the Earth-- the full equinox, which is actually today here. And as I talked about in the first week, because of the procession of the Earth's spin axis in relation to the fixed stars, the full equinox and all the other solstices will be moving around this elliptical orbit. But today, which is very close to full equinox in September, this is what the situation looks like. This is summer solstice then. And this is winter solstice. And this is how the rotation and direction will be. And then this is the spring equinox. So fall equinox, spring equinox, that means basically, have equal amounts of daylight and night time.

Skip to 11 minutes and 44 secondsAnd so summer solstice, at least in the Northern Hemisphere, That's When you have a lot longer days in the high latitudes of the Northern Hemisphere. So if you look at this, and if you remember what you learned in the first week, when you have summer here, this is where you are quite far from the sun. This is the sun compared to winter. So this means you have relatively cool summers today. And the last time this occurred was 20,000 years ago. And that was during the last ice age. So if you look at this you would say, well, we should have been on our way into another ice age. And obviously, we're not.

Skip to 12 minutes and 25 secondsSo then this has been a puzzle for a lot of scientists for a while because normally the interglacials will last not much more than 10,000 years. There are some cases when they're longer, especially when the eccentricity, so the ellipticity, ellipse is weaker, so it's more of a circle orbit. Then you might have a situation where you kind of skip a few cycles, and you stay in interglacial. That's also partly true today, but there's still decreasing insulation at high northern latitudes so you would expect we would go into a ice age. So what's happened?

Skip to 12 minutes and 59 secondsSo apparently, over these 6, or 5,000, 6,000 years ago, because of the development of modern agriculture, humans have already made an impact on the surface of the planet. In particular, we've started cultivating rice, which emits a lot of methane. And then we also have contributed in a big way in terms of deforestation. So already probably 3,000, 4,000, 5,000 years ago, because of human population increase and the start of modern agriculture, we've actually influence-- impacted the amount of greenhouse gases in the atmosphere.

Skip to 13 minutes and 37 secondsSo this is one theory, and it's also now more recently verified by looking at the ice core record of methane, that shows that yes, you do have an increase due to human impact over the past at least 3,000 years. So that could have to some extent counteracted the decrease in insulation that we expect. So basically, one theory for not being in an ice age over the longer thousand years, is because we've increased the amount of methane, CO2 in the atmosphere, already thousands of years ago. And then more recently, in the last 150 years, now we're way off the scale in terms of CO2 because of the Industrial Revolution. So that's a whole other story.

Skip to 14 minutes and 18 secondsGiven today's atmospheric CO2 levels, we may never go into a new ice age. And that's something that is, of course, hard to know. But at the current rate of CO2 release, the amount of CO2 in the atmosphere and the time it takes for that CO2 to be removed from the atmosphere is extremely long. So we'll basically be staying in this situation for a very long time, without being able to-- the insulation to give us a new ice age.

Skip to 14 minutes and 50 secondsThank you for following our course. We hope you've learned a lot about the causes of climate change, and that you'll also be inspired to pursue this topic further. We also want to thank our team of mentors who've been with us throughout the course, Ida Marie and Nadine. Yes, and we would also like to thank you all for your contribution to the discussion forum. It has been very interesting and also challenging to answer all your questions and comments.

# Week 3: feedback

In this video the educators meet some University of Bergen students, 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.