The Atmosphere, the Ocean, and Environmental Change

GG 140 - Lecture 29 - Global Warming III

Chapter 1: Review of Exam 3 [00:00:00]

Professor Ron Smith: OK, so the first question was one of the most difficult ones on the exam, I believe. And the point is that you were given, first the Gulf Stream coming up along the east coast of the United States. At a particular latitude, 35 degrees North, you are given information that this is 50 kilometers wide, has a depth of 1 kilometer, and the pressure difference across it ΔP, is given by 4000 Pascals. And you are asked to compute how much water is flowing, the volumetric flow rate, in the in the Gulf Stream.

So the way I did it was first to use the formula for geostrophic speed, which is the pressure difference divided by the length over which the pressure changes. That's the so called pressure gradient. And then down below goes 2, the density of water, times the rotation rate of the earth, times the sine of the latitude.

That formula is given to you at the top of the second page. You just need to cancel out the volume and solve for U, and you'll get that formula for geostrophic flow.

So then what goes into this is a ΔP of 4000 Pascals, a length of 50 kilometers, 50,000 meters, 2, the density of seawater is about a 1025. The rotation rate of the earth is 7.27x10-5. And the sine of the latitude, it's going to be the sine of 35 degrees. When you put in all those numbers, I got 0.9 meters per second for the speed of flow in the Gulf Stream.

To get the volumetric flow rate, that's going to be the rate of the product of the area and the flow speed. The area is given by the product of the depth and the width. So it's going to be 50,000 meters times 1000 meters. So that's going to be 5 times 10 to, what the seventh? So when you put this value into there and this value into there, I got about 47x106 cubic meters per second. That's the volume, how many cubic meters of water per second are passing through some line you draw across the Gulf Stream. I asked you to express that--I asked you to express that in sverdrups. A sverdrup is 1,000,000 cubic meters per second. So this is 47 sverdrups. Any questions on that? Yes.

Student: What is the—why is that the same as the length?

Professor Ron Smith: This is the, well sorry. So this is a pressure gradient. So it's how pressure changes with distance. So I may have messed up my symbols here, but that would be that distance. I gave you the pressure difference across the Gulf Stream. I gave you the width of the Gulf Stream. And so the pressure difference divided by that length over which the pressure changes is the pressure gradient. That is units of Pascals per meter. How much does this pressure change per meter as you walk across or swim across the Gulf Stream? Questions on that then?

Student: So is the pressure gradient force not pushing the air in the direction it’s moving, it’s actually pushing it across--

Professor Ron Smith: That's right. So presumably there'd be a high pressure over here, low pressure over there. So if you were to do a little force balance on a piece of water in the Gulf Stream, the pressure gradient force would be off in that direction from high to low. Because there's higher pressure on that side of the parcel that on that side. And that would have to be balanced by a Coriolis force equal and opposite to that. And then in order to get that Coriolis force, the fluid would have to be moving in that direction, that's you, so we have that right angle relationship between the velocity of the material and the Coriolis force.

So I didn't ask you about the direction, but it would be off to the Northeast, the way I've drawn everything in this diagram. Other questions on that?

OK, question two. Now for that you needed this diagram on the next page. And what I recommended--Are you all set? I'm going to withdraw it because I'm not ready to use the board yet. But that's terrific. Thanks so much. I appreciate that.

AV Guy fixing projector: I would just blank it at the moment if it's--

Professor Ron Smith: That's fine. How do I blank it? Blank it. Yes. Yes, terrific.

So you were given the temperature and salinity for two water masses and you could plot them on this S-T diagram. And A plots up here somewhere and B plots down there somewhere. And from the lines of constant density, which I'm not going to be able to draw very accurately, you can find that B is more--so I've got the numbers there. So the density for B was 1027.5 and the density for A was 1026.5 units kilograms per cubic meters.

Now I mentioned in class that quite a number of you have forgotten that you need to put a 10 in front of the numbers that are given here. For example these things are written something like 27.5. You need to make that into 1027.5 in order to have proper units of kilograms per cubic meter. To remember that, you could look back at the properties of water on the front page, where I give a typical density for seawater as 1025 kilograms per cubic meter. A number like 27.5 would be ridiculous for water. That would be a factor of 100 too light.

The other thing is then A turns out to be less than B. And therefore because density must increase going down, water mass A must be the higher one in the water column. And that was the second part to that question.

Question three, I used the formula, change in salinity is given by the initial salinity minus d over capital D plus little d. And the problem said the heavy rain adds a half meter. So d is plus 0.5 meters. And that fresh water is mixed down to a depth of 50 meters. So the effect of that added fresh water is felt all the way down to 50 meters. That means I put in 35 parts per thousand for S0 and then it's minus 0.5 over 50 plus 0.5 and that comes out to be minus 0.346 parts per thousand. And because the original salinity 35, the new salinity is then 34.65 parts per thousand. I just subtracted that from the 35 to get the new salinity. Questions on that?

Question four was about El Niño and in the eastern tropical Pacific, so in the east there, the SST would be higher than usual, the air pressure would be lower than usual, precipitation higher than usual, biological productivity lower than usual. And explaining the relationship between A and D, that would be like this. So if you have warm water near the surface and cold water beneath, it's going to be very difficult for nutrient rich waters to reach the surface because of that stability in the water column. If the nutrients can't reach the euphotic zone, then you're going to have low biological productivity.

Question five was about the last glacial maximum. CO2 in the atmosphere was low, the isotopes in fresh snow on Greenland would be lighter than normal, because in that cold condition there would be more precipitation between source and Greenland. More water would have been rained out, the heavier isotopes rain out, you end up with lighter ones on Greenland. The isotopes in deep sea sediments would be heavy because with ice stored on land, it would be isotopically light. The remaining water in the ocean would be isotopically heavier and the sediments would have picked up and retained that signal.

Sea level would be low because water is stored on land. So then the relationship between C and D, oxygen isotopes in deep sea sediments in sea level, well I've just said that, so sea level low means that water is being stored on land in the ice sheets. And because that is light isotopes, because of the evaporation process, that's going to make the oceans heavy. Questions on that?

In recent centuries we have a perihelion in January. Explain how the climate would be different if due to procession-- All right, so let me put on the board this side review of the plane of the ecliptic. The sun will be off center andearth will be here and here. And this will be today. But this will be say 10,000 years from now. And today the perihelion is in January, which means the tilt of the earth is like that. If it was going to be in June, then we know that the tilt of the earth would be like this, because this is northern hemisphere summer, which is June. So the tilt is like that.

So then what I wanted you to explain was basically how these two climates would be different. And one thing is that in this season the northern--in this situation, the northern hemisphere summers, being perihelion, would be warmer. The northern hemisphere winters would be colder because of that distance. So the northern hemisphere seasons would be stronger.

The southern hemisphere seasons would be weaker, because in the winter tilted away, you’re closer to the sun, tilted towards, you’re further from the sun. So the proper answer would be that the intensity of the seasons would be changed, but oppositely early in the northern and southern hemispheres. Questions on that?

Sea ice is frozen seawater. Thickness, let's say 1 to 4 meters. Salinity, it starts out as seawater. It loses quite a bit of its salt when it freezes, but not all. So a typical salinity for sea ice is between 5 and 20, somewhere in that range, whereas seawater is 35. Icebergs on the other hand are compacted snow, an entirely different origin than sea ice. And their salinity is essentially zero, since it's fresh water snow has fallen on the glacier. Whatever formed the iceberg. Questions there?

Student: You said that the salinity of sea ice is what?

Professor Ron Smith: 0. Now it may be that if it's been floating in seawater for a while a little bit of seawater has kind of worked its way into some of the cracks. But if you find a chunk of pure ice in the iceberg, it'll have 0 salinity because it came from fallen snow some years or centuries before.

Student: I think he asked sea ice.

Professor Ron Smith: I'm sorry you asked me about sea ice. Sorry. I answered the wrong question. Sea ice is fresher than ocean water, but has some salt in it. Yes?

Student: Can I talk you after class about--

Professor Ron Smith: Of course. OK. Now question eight. What was the question? OK. Recent trends in sea ice. So you may recall that it's conventional to judge both of these in September. Now in the northern hemisphere, September is the minimum in sea ice and in the southern hemisphere that's the maximum in sea ice. But that makes sense because in the northern hemisphere the maximum in sea ice, which occurs say in March or April, fills the entire basin. So it's not a question of cold conditions giving us more sea ice in winter in the Arctic Ocean, it's already coast to coast. Now a little more may spill out into the Pacific and the Atlantic, but in terms of the Arctic Basin, it's full. So that would not be a sensible way to measure changes in the arctic sea ice.

And for the southern ocean, at the end of the warming season, there's very little sea ice left. It retreats mostly right back to the coast. And so that wouldn't be a sensible way to measure. Instead we measure it at its maximum in the southern hemisphere, which is in September. Anyway in the arctic, sea ice is rapidly decreasing. In the southern ocean it's approximately constant by the measure I've just described.

Question nine is computing the mass of salt in the world ocean. What I thought you would do there was to estimate the depth of the ocean at about 5 kilometers. Estimate the surface area of the ocean as about 2/3 of the global. Multiply the two together to get a volume of ocean water. Multiply that times density to get a mass of ocean water, and then use the salinity of 35 parts per thousand to get how much salt. And I ended up--and your number may be slightly different than mine--but I ended up with about 60x1018 kilograms of salt. Was the answer I got. Yes?

Student: When you're converting from volume to mass are you using density of water or seawater?

Professor Ron Smith: I think I used sea water, but the difference is very slight. It's only 1025 versus 1000. So your answer would be off by--if you chose one versus your other, the answer would be 2% different, which I don't think for a rough calculation like this is very significant.

Finally question ten. The Little Ice Age is the cool period--I'm sorry what was question 10a? Antarctic bottom water. Antarctic bottom water is--I changed the exams. It's an old version. The Antarctic bottom water is that cool, cold water mass in fact, formed at the bottom of the ocean, formed near the shores of Antarctica. A terminal moraine is a pile of rock and soil deposited at the tip of a moving glacier.

Equatorial upwelling is rising water from the diverging Ekman Layer, flow at the equator, due to a reversal in the Coriolis force. Mid-ocean ridge is the shallow region of water, of shallower depth in the ocean connected with where ocean crust is being created by solidifying material from the mantle and then it's a spreading center for ocean crust. And the Ekman Layer is the ocean flow driven by wind stress at right angles to the wind. You could also have talked about, well the fact--what kind of a force balance it has, but some kind of a definition of the Ekman Layer there was needed. OK. So that's exam three.

So we're going to finish up the discussion--Question. Julia.

Student: What was the average of the exam?

Professor Ron Smith: I'm not sure. Do you guys know what the average is on this? 82. Better than last time. Was that the other question too? Yes.

Chapter 2: Greenhouse Gas Emissions Scenarios [00:20:45]

So we're going to finish up the global warming discussion today by talking about emission scenarios. Now again, the primary reference here is the IPCC reports. So everything in the diagrams I'm showing are almost entirely from the IPCC reports which you have, or you can get very easily. But there's another report as part of it, SRES. It also is on the IPCC website which stands for--What does it stand for? Special Report on Emission Scenarios. And I'll be talking about that today. It should have been easy to remember, given the title of the slide.

So the idea is here that some economists and some industrial engineers got together to imagine how the emissions of carbon dioxide in the atmosphere might proceed over the next 100 years, based on certain population and economic assumptions. And they tried quite a variety of different things, as you will see. And then for each of those, they handed those off to the climate modelers. And the climate modelers ran their climate models with these different carbon dioxide concentrations. And the result is a set of projections into the future of how both CO2 and Earth's climate will change. And that's what we're going to talk about today.

So here are most of the IPCC emission scenarios. Time is on the x-axis from 1990 to 2100. Emission rate is on the y-axis in units of gigatons of carbon per year. Gigatons of carbon per year. Remember that's not the mass of CO2, that's the mass of the carbon in the CO2. So if you want to compute the math of carbon dioxide, just correct for the ratio of the molecular weight of a carbon dioxide molecule to the carbon by itself. 44/12 would be that ratio, right? Carbon dioxide is 44, carbon 12. So yes. Just multiply these times 44/12.

And the A1 is broken up into some subcategories. Generally the As have quite a bit of increasing emissions over the next 100 years. The two B scenarios are a little more optimistic. they climb and then declined for B1, or climb and then increase at a very much slower rate for B2.

These documents were published using data from about 2000 and projecting it from about 2002 and we have a few years now to look and see which of these lines we've been on the last five years. It's a little bit hard to tell, because they don't diverge so strongly in the first few years. They're all pretty similar. But from the articles I've read recently, it looks like we're a bit closer to the higher projections than we are to the lower ones, if you look what's happened over the last five years.

Now, this may have changed a bit since 2008 when we began to have these economic difficulties. So you're going to want to read the literature carefully, but as of about 2008, 2009, it looks like we were on some of the more discouraging trajectories, in terms of CO2 emissions.

Now from these, with a little bit of understanding about how carbon is put back into the biosphere, you can come up with total cumulative--Well sorry, this is just summing them up. Total carbon dioxide cumulative emissions, so just adding those together to get the different scenarios expressed in a different way.

From that, with a bit of understanding of how some of that carbon dioxide will be recycled back into the biosphere, you can come up with carbon dioxide concentration projections over the next 100 years. I don't know why this artist has put the two up on top there. That's not a conventional way to write CO2. So don't be misled by that. I think this diagram is still accurate in spite of that loss of credibility given by putting the two in the wrong place. But again, you see that the B1 scenarios are leveling off, whereas the A scenarios are climbing very rapidly, especially the A2 scenario, which has us reaching over 800 parts per million by volume by the year 2100.

Now are you familiar with this organization called 350.org? So this is what's his name, McKibben's organization. With some scientific basis, I'm not sure everyone would agree, but with some scientific thought, they've decided that 350 should be the limit we should strive for on CO2. But remember, we've already passed that. We're at 397 already. So but just for record, you could put that on this diagram, 350.org would have you put the limit right there. It helps you to understand how far in exceedance of that number we are and will be in the future. Any questions on this diagram? Yes.

Student: What are the different criteria they used to create different scenarios?

Professor Ron Smith: I don't have those on the tip of my tongue. They have to do with the way certain economic sectors will develop and the way--which countries will dominate production of certain items. It's rather detailed, and that SRES report goes into that. It makes rather interesting reading. I apologize for the fact that I don't have those different economic definitions prepared.

Student: But it’s based on the economic growth?

Professor Ron Smith: Economic growth, economic--where production occurs. Not only a total gross, but in what country production shifts to and things like that.

Student: So it takes into account shifts to different forms of energy?

Professor Ron Smith: Exactly. Some of that's in there too. That's right. That's right. So now the climate modelers perform their magic. And as you know, there's about a dozen or so of these climate models run by different groups around the world that do these projections. So you get a lot of different projections. The number of things gets multiplied because we now have all these different scenarios, and we have all the different models running on all the different scenarios. So you get a lot of different output. It's a little hard to manage sometimes.

But I want to show you this diagram. Again, this is from IPCC report. It shows the surface warming based on a pre-industrial reference and versus time, 1985. This just only goes to 2025. This is a short time scale here. It shows something called the commitment curve. That is code for constant composition. In other words, that essentially says no further CO2 emissions starting in the year 2000, essentially.

Now the temperature does continue to climb on that, because even with constant CO2 emissions, you still have to warm up the oceans. Remember the oceans are putting a lag on all of this, because of their enormous heat capacity. So this continued rise is due to mostly trying to warm up the oceans, even though the greenhouse effect is kind of fixed. Question, yes.

Student: Is that only anthropogenic on the outside, or is it all across the outside?

Professor Ron Smith: Well it's constant atmospheric composition, is the way it's defined. [Correction: The assumed carbon dioxide concentration is about 370ppmv).]

And then these different scenarios follow each other pretty closely over this time frame. But if you remember back to the emissions scenarios, or to the composition one, your past 2025 before they really diverge very much. So it's not surprising that even though these scenarios are wildly different, you don't see much of that up to the year 2025. But you do see a lot of rise. I mean now you're up to a degree or so of warming and the rate is rather impressive.

Now when you go out to a much longer time scale, that's when you see the big differences. So here is up to the present day, this is actual data, and here's the constant composition commitment curve that you saw beginning to peel off in the previous one. While the other ones continued together, but by the time you get to 2050 now, they're beginning to diverge quite strongly. And by 2100 they really are quite different. I don't want to project how long each of you will live, but I expect that a lot of you in the classroom will be around maybe in 2080. And so that is the world that you will probably live to see, with some variability. But I think if the last five years is any guide, probably you'll be up in this upper range if things continue as they are going. So that's a warming of about--again this is based on this reference, not pre-industrial--but that's a warming of about 3 degrees Celsius from the current day.

Then they begin to level out. All of these scenarios begin to level out, except for A2 perhaps, because of the assumptions that have been made. And in fact by the time we are removing fossil fuels at a high rate for the next 100 years, we will have depleted a fairly significant fraction of the fossil fuel. So this turning over is not all our choice. Some of this will turn over simply because the remaining amount of fossil fuels to be burned is getting to be so small. Questions on this? Again, you'll find this in the IPCC report.

Now when you plot the same data on a larger time frame, going back 1000 years, so here we are today. So what they've done is taken that historic proxy data that I've shown you before, with a little bit of a hint of a medieval warming, and a little bit of a hint of a little ice age in here, and then our current kind of two phased warming in the 20th century, and then put these IPCC scenarios tacked on to that, it helps you to put in perspective as to how the changes relate to what we've seen over the last part of the Holocene period. It's quite a steep and dramatic rise compared to the flat climate we've had recently. Any questions on that? Yes.

Student: How about relative to periods significantly prior to--

Professor Ron Smith: Yes. So that's important. Now. I don't have the diagram here, but they're loaded in the previous presentations. You can go back and see that. And of course what will happen is when you get 10,000 years back--so here's 1000 years back--when you go 10 times more, then you're back into the Pleistocene, the LGM, the Last Glacial Maximum, and then this temperature drops about 5 degrees. So take that distance and put it down here and that'll give you a different sense. Right? That'll give you a sense of well, OK, this is higher than any of that, but as an absolute change it is comparable to what we had going in and out of the ice ages. It was all down here however, so this is unique in its warmth, but not unique in its magnitude of the fluctuating. That's a good point to keep in mind. So you can be fooled by just what period of geologic history you've used here to form a basis for comparison. Other questions on this?

And of course it won't be uniform, the warming. Here's the warmth they anticipate under three of the scenarios, B1, A1, B and A2 up from 20 to 29, most of the warmth is in the northern hemisphere. Up to the end of the century then, much more warming, but again concentrated in the northern hemisphere, high latitudes. Values as high as 7 degrees Celsius. My god, that's a lot of warming. That's an amazing amount of warming. Certainly there would be no arctic ice. Certainly there'd be no glaciers in the northern hemisphere, mountain glaciers, under that climate. OK, any questions on these IPCC projections? Yes.

Student: What about in comparison to the Pliocene, the period that we said was comparable--

Professor Ron Smith: The Pliocene, right. So that would then be comparable--the Pliocene also seemed to have much higher temperatures at the high latitudes than we have today. So this kind of scenario is one of the reasons why there is a lot of research on the Pliocene, because they too had this kind of warmth in the northern high latitudes. And we don't understand why that is exactly, but it may well be something similar to this. Except that it didn't seem that the CO2 values were as high back then during the Pliocene.

But it'd be worth reading a couple papers on the Pliocene to see to what extent they--if you just Google Pliocene climate, you can quickly--Just use Google Scholar for example. If you want to get the peer reviewed literature, go into Google Scholar and search for Pliocene climate and you'll find a lot of recent papers that are trying to deal with just this issue.

Chapter 3: Problems Connected with Global Warming [00:36:48]

So a lot of problems then we perceive could be connected with this global warming. And these are all pretty obvious. I just list them here. And it may not be complete. We expect increasing drought, which will--and some human populations as well as animal populations will be forced to migrate. They'll be some extinctions probably. The one that's most talked about will be the polar bears.

My strategy for global warming is that if I just buy a house 300 miles north of New Haven, that'll pretty much account for the global warming that'll take place in my lifetime. Pretty clever, right? A lot of deep thought went into that.

But imagine the polar bears, right? They live in this arctic environment. It's going to warm up. They're going to lose the sea ice very quickly from which they hunt. They aren't going to mind the warm so much. They can probably handle that, but they normally do their hunting off the sea ice. Without sea ice, they won't to have a way to eat. And therefore we're probably going to lose the polar bear pretty quickly. That ecosystem will be gone.

There will be frequent heat waves. For example, a few years ago we had a heat wave in Europe. I believe that was in 2003 that killed some tens of thousands of people. And we had one just a year and a half ago in eastern Europe. And the projections are that these will occur very frequently as we get towards the middle and the end of the current century. And of course, if you're living up north that's not too bad, or if you have air conditioning. But air conditioning is a problem because that uses energy which may require fossil fuel burning, which would put more CO2 in the atmosphere. So that's kind of a downward spiral.

The ice on land will melt. The mountain glaciers we spoke about, and the ice sheets of Greenland and Antarctica we talked about. And because that ice on land is supported by the land, when it melts, that lifts sea level. Remember if ice is already floating in the ocean and you melt it, that doesn't change sea level. But if ice is supported by the land, as a mountain glacier would be, or a large ice sheet would be, that will cause sea level to rise. And it could be the order of several meters, which would have a big impact on coastal development.

Today many rivers flow all summer because they get--for example, in California the rivers that come down out of the Sierra Nevada range, they do decrease their flow in summertime because there's not much rain. But they keep it going because there's enough glacial ice melting through the summer to provide those rivers with water even in July and August. Well, that will certainly change. And there's all these rivers coming down out of the mountains.

If there's not rain in that season, those rivers will certainly go dry. Because without the ice and snow to store water at high altitudes--and this will be a big difference between today. Many, many rivers around the world are flowing in summer only because of snow pack melting, that stores that water until late summer.

Tropical diseases will move forward. And I hesitated to put this in there but I did, because it's not likely that would happen, but it's constantly discussed in the literature, in the scientific literature, as a possibility. That is, if you look at the planet Venus with its unusually warm climate--surface temperature for Venus is 460 in Celsius, 735 in Kelvin--it has the solar system's strongest greenhouse effect, has a rather high albedo, it reflects a lot of sunlight. But nevertheless it as a very high surface temperature because of its high greenhouse effect.

And the idea is that it probably wasn't always like this, but some kind of process amplified itself. Probably it started to warm up, that for some of this carbon that was in the surface of the planet to come off the planet and form carbon dioxide which warmed the planet further.

Water may have played a role too, but now most of the water is gone. Water may have played a role in getting Venus to its hot state, but most of that's gone now. Anyway, either a water vapor feedback or a CO2 feedback probably took Venus from an earth-like state to its current state. And so there is some worry that this could happen to earth. We could get to some point where suddenly these two feedbacks, carbon dioxide and water vapor feedbacks, might then take control of the climate and run away and give us something that's much, much higher than any of these IPCC projections. It's not likely, but you can read about it in the literature.

Chapter 4: Possible Advantages of Global Warming [00:42:29]

There are advantages however. There are vast regions in the northern hemisphere, especially Canada and northern Asia, where agriculture is mostly limited by lack of summer warmth. And so you would find greatly increased agricultural productivity in Canada and Asia under these IPCC global warming scenarios. Also at the present time, many more people die from cold every year than from warmth. And of course, my heating bill will be less, so I'd enjoy that.

And it's now known and well documented in the literature that when CO2 concentrations rise, plants grow more quickly because of what's called CO2 fertilization. And so crops will grow generally more quickly. Some would argue that the crop—the nature of the plant structure however changes and makes that plant material less nutritious. So be careful. It's not only the mass of the plant that you grow, but whether--if you're going to eat it, whether or not it's nutritious for humans. So be a little bit careful on that one. But there's no doubt that CO2 fertilization is already being seen in forests and in agriculture. So that's a real factor, a real positive factor.

Chapter 5: Actions that May Reduce Global Warming [00:44:03]

So these again are pretty obvious. If we wanted to reduce global warming what would we do? Well none of these are easy. Many of these are impossible. But I list them anyway, being the eternal optimist. Reduce human populations, reduce per capita use of energy. One way to do that would be to increase energy costs so that each of us would work harder to reduce our per capita use of energy.

Reforest the continents, because when you grow a tree, you sequester a certain amount of carbon dioxide. There are a couple of problems with that. I've mentioned one of them already. A tree typically only lives 60 or 100 years and then it will die and that carbon dioxide will be returned. Within 20 years, it'll be back in the atmosphere. So it's not a permanent way to store carbon dioxide.

And also recently in the literature, it's been pointed out, and it's really quite obvious when you think about it, forests are very dark in their coloration. Their albedo is very low. And so if you add more forest, you decrease the average albedo of the planet, which would warm the planet. So be careful about that trade off.

Fertilize the oceans. For a while we were talking about putting iron into the oceans, because that turned out to be a limiting nutrient for phytoplankton growth. And phytoplankton draw in CO2 just like plants on land do. The question once again though, how long would it stay in the oceans? Would it fall to the bottom to be covered over, or would it just return back into the atmosphere.

Stop third world economic growth. Well that's kind of a joke, because how in the world would you do that? Of course in the first world we use much more carbon dioxide, we emit much more carbon dioxide per capita than the third world does. And that's because we have a higher standard of living and the third world aspire to have the same standard of living that we do. And so that's going to be where a lot of the increasing CO2 emissions will come from.

Shift to nuclear energy. Nuclear energy does not emit any carbon dioxide. Shift to renewable energies of various types, wind, solar, geothermal. I'm going to be talking about these, by the way, in the last week of the course. We're going to talk a bit about renewable energy.

The big thing that's talked about these days is CCS, Carbon Capture and Storage. It's removing carbon dioxide from the atmosphere and burying it down deep in the crust of the earth. A lot of research is being funded, including a big grant here in the geology department at Yale to work on some aspects of this. The question is--one of the questions is would it stay down there? I mean it's light material. You'd like to combine it or condense it in some way that it's stable and would stay down where you put it. But after all, it is a material that would like to gasify and come back out. And so there again I would worry about how long it would stay buried down there. And then various geoengineering hypotheses have been made, such as constructing some kind of a shade over the earth to prevent some of the sunlight from reaching the earth.

We're out of time. I've got a few more comments about this for next Monday, but enjoy your weekend.

[end of transcript]