The changing atmosphere in 2005
The Shine Dome, Canberra, 21 February 2005
Nobel Laureate Professor F. Sherwood Rowland
Donald Bren Research Professor of Chemistry and Earth System Science
University of California, Irvine
I first came to Australia about 20 years ago. The reason that I came here was a very simple one, that the atmosphere is shared by everybody but there is a northern hemisphere and a southern hemisphere, and the premier atmospheric group in the southern hemisphere is CSIRO, operating at Cape Grim in Tasmania and out of Melbourne. So it became very important to work together with the scientists here in Australia and that has continued, so that this is probably my seventh or eighth visit to Australia. The starting point for my lecture is simply to point out that the concern about the atmosphere is not totally new. This is a picture taken, or an etching made, of an event that happened just about 200 years ago, when two French scientists, Mr Gay-Lussac, who has a gas law named after him, and physicist Biot took an evacuated flask up to, eventually, 7000 metres altitude, filled it, brought it back, analysed it, and found that the composition of the air at 7000 metres had the same ratio of nitrogen to oxygen that is at the surface of the Earth. But basically at that time even such brilliant scientists as Gay-Lussac and Biot were, they had limitations that came about because the science itself hadn't advanced to the point at which we now have, from the combined efforts of hundreds and hundreds of scientists, much greater precision, much greater sensitivity, in the measurements that we make, and that allows us to see much more of what is in the atmosphere. This slide is a list of the compounds that are in the atmosphere, if you are able to measure down to one part in 108. There are 14 of them there, so it is a very simple picture. And at that level, things don't change very much. Carbon dioxide is changing, but on a logarithmic scale it is changing only a little more than the width of that line. Methane is changing, with the same reason. But down at the lower level there are many other important materials. For me the starting point in the atmosphere of, let's say, the current understanding, started at the International Geophysical Year, which was in 1957-58. A scientist named Dave Keeling, from the Scripps Institution of Oceanography, started to measure carbon dioxide in the atmosphere at the same location repeatedly. It had been measured before, but nobody did it with such persistence and in the same location, so it could tell you that actually it was changing. You see what happened in the first year that Dave was making his measurements: up until May the carbon dioxide was increasing and then until October it was decreasing, and then it started up again. And in May it topped off and then it started down, and October it changed and went back. And it's only after a few years that you can definitely see that there was more carbon dioxide all along. The seasonal effect was the same, but it's superimposed on a rising amount of carbon dioxide. In here it had gone from 315 ppm to 325. Now it is almost up to 380 ppm, and Keeling still has the premier record of what that carbon dioxide is doing. I got into atmospheric chemistry by hearing about some measurements made by scientist named Jim Lovelock, from Great Britain, who had invented a very sensitive detection device which allowed him to sense certain molecules, the trichlorofluoro (CCl3F) compounds, with perhaps a factor of a million increase in sensitivity. Lovelock's data were published a little bit later, but the thing that struck me as a chemist was that it would be interesting to know what was going to happen to these molecules. And so, with Dr Mario Molina, who joined my research group as a postdoctoral and shared in the Nobel Prize 25 years later, I went after that to ask the question of what will happen to molecules like that in the atmosphere. This slide shows you now where the concentrations are. You have to get down below 10-9 to see these chlorofluorocarbons, and some of the other reactive species are down at even much lower concentration. If we ask the question of what is going to happen to a molecule that goes into the atmosphere, then there are three answers of which, usually, one or the other is the answer for any molecule. I have illustrated it here with three chlorine compounds. Molecular chlorine is a green gas. The fact that it is green means it is intercepting sunlight. When it does, it breaks apart and gives you two chlorine atoms. That takes about an hour. Hydrogen chloride is a transparent gas, but hydrogen chloride can dissolve in rainwater and rain out as hydrochloric acid, and that gets rid of it. That takes about a month or two. And then other molecules like methyl chloride can react with hydroxyl radicals, which will attack the hydrogen atoms. So the thing about trichlorofluoromethane is that none of these things happen to it. It is a transparent gas, it doesn't dissolve in water and it doesn't react with the oxidising agents. So that means you have to ask some further questions, and perhaps it is not going to go away rapidly. In fact, the key point to it is that they have long lifetimes in the atmosphere. I illustrate here the visible spectrum, violet to red. On this side [left] with higher energy we have the ultraviolet, on this side [right] we have the infrared energy, which really you can think of as heat. And then we have the ultraviolet attacking molecular oxygen, producing oxygen atoms which make ozone. So it is this radiation over here that makes the ozone layer. I should point out that we are talking about two things. One is the question of stratospheric ozone depletion and the other is global warming from greenhouse gases. What happens is that the chlorine compounds destroy some of the ozone, making it easier for ultraviolet radiation to penetrate to the surface. The greenhouse gases are absorbing energy way over here I will come back to that and making it harder for infrared energy from the Earth to escape, and because it's hard for it to escape it tends to be warming of the planet. So it is less ozone letting through more ultraviolet and more infrared being intercepted, leading to a warming circumstance. This is a picture from Corona del Mar, California, where we live. It is just pointing out what all of us know but do not necessarily think about, that red light goes through the atmosphere better than blue light. If you have a long path length at sunset, then the blue light has scattered out and you see it up in here [on next slide, showing blue ocean]. So that is something that we know, and as you go into the ultraviolet then it is scattered even more. With the chlorofluorocarbons, what one finds is that there is ultraviolet light that can be absorbed and will break them apart, split up and give chlorine atoms. And up to that point everything is very straightforward. The key that changed this from what was an interesting scientific problem for Mario and myself was to find out that there is a chlorine atom chain reaction in which one chlorine atom attacks an ozone and makes chlorine oxide, which intercepts an oxygen atom that would otherwise have formed ozone and gives you the chlorine back. And it only took one minute for both of those processes to happen. So what you are doing is getting rid of an ozone and a would-be ozone, and you still have the chlorine atom. This can go on and on and on, and what one gets is about a factor of 100,000 as the number of ozone molecules destroyed by an individual chlorine atom. And that factor of 100,000 put ozone depletion from the chlorofluorocarbons on a par with the natural processes that were already there. All of this was calculation done with some laboratory measurements, but it hadn't been verified out there in the atmosphere. So the experiment that was one of the important ones here was an experiment that was done by two research groups in Boulder, Colorado very much like Gay-Lussac's experiment 175 years earlier, because what the Boulder people had was spheres that were evacuated in the laboratory. They were sending up to higher altitudes and so the spheres were not accompanied by men; they went up by balloon. Down here [below balloon] we have those going up in the Colorado morning. What you see is that when the analysis was made, for both of those research groups there was good agreement between what we had calculated they would find and what they found: that the chlorofluorocarbon compounds do get into the stratosphere and do decompose at the altitude which we had expected. This was the starting point for what has become the major part of the experimental work that we do. This was before we were funded for it. I have a gas canister here in the West Indies the photographer was my wife and this was the starting point that we started making measurements of. Now we have done, oh, I think, 50,000 different canisterfuls of air from various locations. This is a typical analytical system that we see. What we are looking at is from a US aeroplane which took off from Hobart and was headed out toward New Zealand. In fact it was south of New Zealand there. All I want to indicate is that not only do we see the trichlorofluoromethane here but there are about 20 different man-made compounds that were down here south of New Zealand, a long way from any of the places where they had been released to the atmosphere. It shows that the atmosphere mixes very thoroughly and in great detail. This was an experiment when we were working with a Japanese group. The aeroplane put down at Darwin and they saw black smoke on the horizon, and so they rented a helicopter and went there. The technician, Murray McKechnie, was the person who actually collected the samples there. We were looking at it to see all of the detail of what was happening the chemistry of the bushfire that had occurred near Katherine. Well, our measurements that we were making of trichlorofluoromethane in 1978-79 showed that there was a lot more trichlorofluoromethane eight years later than when Lovelock was there. And what that did was convince us that the lifetime of this molecule was very long. The best current estimates say that trichlorofluoromethane lasts 45 years in the atmosphere, and another compound, the dichloro, lasts about 100 years. So the chlorofluorocarbons now have measured half-lives in the atmosphere which are very long. It means that once you put them in, it will be a while before they go away. When we started having these samples, then we started measuring other gases. Methane was one that we looked at. If you look at the amount of methane here, you see it is about 1.6 ppm in the north and 1.5 in the south more of the methane comes out in the north than in the south. But it comes from rice paddies. It comes from cattle. It comes from a whole set of different things. The ones shown in red on the slide are the ones which mankind has a big effect on; the ones in yellow are the ones that would have been there anyway. But it indicates that mankind is having a big effect on methane emissions. If you continue to follow that, then you see it was down here at 1.6 ppm before, whereas at that point it was at 1.8. So methane was growing very rapidly in the atmosphere, as well as carbon dioxide. This takes our methane numbers up into the 21st century. It was very straightforward for a while, going up about 1 per cent a year, and then it's become interesting because of the variation that has been taking place. Here we have Keeling's carbon dioxide measurements, now taken out into the 21st century. What you can see, starting off here, is that the first of it showed they got to 325 ppmv and now you see it up here [at top right]. If you look here, you see it's moving at a rate of a little bit less than 1 ppm per year, and up here it's almost 2. So much more carbon dioxide is being emitted to the atmosphere now than it was when Keeling started making his measurement. So now we come to looking a little bit more at the greenhouse effect. The molecules which are important for the greenhouse effect are the molecules that have more than two atoms. That means that it is methane, it is carbon dioxide, nitrous oxide, ozone and water vapour, and then the CFCs. Those are the key molecules that are involved in the calculations of greenhouse effect. Let me say a little bit about the background on this. First, what I am just about to explain briefly is the only explanation that is around in the scientific community for what happens with these molecules in the atmosphere. It is the only explanation for why the temperature of the Earth is what it is now. That is, let's say in 1900 and 2000, there is a natural greenhouse effect and it amounts to 32°C or 33°C. What I put here is part of that calculation. I finished the calculation here, but nobody is arguing about this. The question is that if you didn't have the greenhouse gases present the molecules with three or more atoms and if all of the infrared radiation that is coming from the Earth were given off, out through the atmosphere, then what you would expect is a temperature of about -18°C. What you observe, let's say in the year 1900, was 14°C+. And that difference is because there is infrared radiation that is failing to get out. Here is a graph of what you find in the laboratory with two molecules, methane and ozone. The chemists describe the infrared as a fingerprint on these molecules. There are regions where the radiation of the Sun is not affected at all by these gases; then there are other regions that interact very strongly. So you have transparent regions and you have regions that are absorbed by the greenhouse gas molecules. Because the greenhouse gas molecules do absorb some radiation, the temperature has to rise to give off enough energy to balance that that's coming in. That is the basis of the greenhouse calculation. As I say, everybody agrees that it exists; the question is how much more effective the greenhouse gases will be by having 380 ppm of CO2 instead of what was there about 200 years ago, 280 ppm of CO2. This is an infrared measurement taken from outside Space, and again what you see is that there is a transparent region, there is a place where carbon dioxide is absorbing, a place where ozone is absorbing, and so on. You can look at it in a different fashion here. If you are looking down again from Space, you can see [on the sphere labelled 'Visible'] where Africa ends and the Atlantic begins, and that is true also in the infrared, over here [at right of slide]: you can see where Africa ends and the Indian Ocean begins. It means some of the infrared radiation is getting out, but here is another wavelength where it is not getting out at all and what you are seeing is just the motions of the tops of clouds. There are two additional things that go into the greenhouse calculation. There are feedback processes which strengthen the greenhouse effect. One of them is simply that water evaporates faster as the temperature goes up, and so if you raise the temperature of the ocean then you increase the amount of water vapour that is going into the atmosphere. This is particularly important in the northern hemisphere in the polar regions, if you melt ice. Ice reflects solar radiation very well; water absorbs most of it. So if you have some of the ice melt and it is replaced by water, then there is a lot less radiation being reflected back because you have replaced the albedo of ice with the albedo of water. That means more absorption, and tends to warm up where the ice is melting. That [slide] is a North Polar reaction, and all the models show up that when you say 'greenhouse warming' you don't mean everything goes up at the same rate, but there tends to be concentration in the northern hemisphere. One would like to have examples of the atmosphere from the past, for comparison, and after a certain amount of looking around for where you can get atmospheric samples of that kind, what one found is that the place where air is preserved for a long time is glaciers, either in the Antarctic or in the Arctic and Greenland, or in the tropical regions like the Quelccaya Glacier of Peru. This one is very interesting. This is a man named Lonnie Thompson, from Ohio State. In Peru the winds blow from the Amazon for part of the year, and they bring a lot of water that falls as snow and ice. And then for part of the year it comes from the Bolivian Alta Plano and Chilean desert and it doesn't bring water at all, it just brings dust. So what you see is dust and ice, dust and ice, and you just count back as you go down. At this location they had 1500 years of counting back and looking at what was trapped in bubbles in there. You take an ice core that looks like this, and looking under a microscope you see that about 10 per cent of the volume of that ice is air. And in that air you find the molecules that were present in the atmosphere when the ice closed up. Here I am pointing out when the carbon dioxide concentration started up because of combustion that is, the burning, first of wood, then of coal, then of oil, then of gas and if you look at 1800 you see the emissions of carbon dioxide were only about eight metric tonnes on a scale that at 2000 was 6600. So what you can see is that the carbon dioxide and the same thing is true of methane started to go up around the beginning of the Industrial Revolution and have gone faster and faster since that time. Here are some measurements that have been made by various groups. These are southern hemisphere measurements on the methane concentration, going back 1000 years. Right here, up at the top, we have the measurements they were measured by canisters by the CSIRO people at Cape Grim . You see that these measurements here really started up about 1800 and then took off. And it is the activities of humans since the Industrial Revolution that have caused the molecules to go up like that. Now let's go to a different location, to a place called Vostok, which is up on the Antarctic Plateau, 700 or 800 miles from the South Pole. There isn't very much snow or ice forming there every year, and when you start going down you are going back not 100 years or 200 or 1000 years; you are going back hundreds of thousands of years. This graph shows the measurements that were made at the Vostok core, going back 160,000 years. From this one can see several things. Temperature is measured by the isotopic composition of the water. If you look, you see that the last 10,000 years have been pretty stable in the temperature. In fact, it is very hard in the last 500,000 years to find a 10,000-year period that stable. Here their measurements don't come to any closer than the last 1000 years, because it takes a while, you have to accumulate a certain amount of ice, before you prevent air from moving back and forth. So this takes us up to the last 1000 years or so, going back then to the Ice Age 20,000 years before that. During all of this time period here, what you will see is that methane varied from around 300 ppbv to around 700, and now it's over 1700. And carbon dioxide, up here [at top of slide], varied from 190 ppmv to 280, and now it's around 380. So the measurements in the ice core show that nothing like what is happening now has occurred over the last few hundred thousand years. Going back to the Ice Ages, in the northern hemisphere, the thing to do is to pick out Greenland here, and then you see how much more ice and snow there was there. There was about a mile of ice on Scandinavia, on Canada and so on. That was 18,000-20,000 years ago. This demonstrates the ice coverage that has been reconstructed for where it was over Scandinavia. Here you see the United States, and the Great Lakes were all covered. Then up in the area around Siberia and Alaska, because you had so much water tied up as ice, then what that meant is that you had a lot less water in the ocean itself, and about 350 feet shallower ocean at that time. At that time, then, between Alaska and Siberia was a landbridge. Here in the southern hemisphere there is a landbridge north from Australia, and over there. They are really entirely different physical circumstances, simply because the water was tied up over the northern hemisphere in the continents and to some extent the southern hemisphere as well. Now I am going to go back to some consideration of the chlorofluorocarbon problem. This is the station of the British Antarctic Survey at a place called Halley Bay . The British Antarctic Survey was set up also in the International Geophysical Year. At the same time that Keeling began making measurements of CO2, they started measuring ozone in the Antarctic, and Halley Bay proceeded to make those measurements. October, as you know, is the first month of spring, and also it is the first time that Halley Bay starts seeing sunlight after a long winter. What they saw for the first decade or more after the IGY, which is back here [at left of graph], is that the amount of ozone was about the same every year, about 300 Dobson units. A Dobson unit is about 1 part in 109, one part in a billion, and it is approximately the average value for the world. And so what one gets here is that for a long time it was just an average situation, and then in the late 1970s it began to fall, and when it dropped below 200 instead of being 300, then they said, 'Something strange is going on over the southern hemisphere.' There was a satellite measurement which started in November 1978; displayed here is a measurement of ozone for the entire southern hemisphere. The white spots are places where there is no data. This is about 100,000 measurements shown here. The low value in here [above the centre], shown with the colour code for 250 [Dobson units], is the low value in 1979. In the first week in 1983, the low value over here [to right of the centre] is around 175 [Dobson units]. In 1987 the low value is actually here [at the centre], down around 125 [Dobson units]. This was the onset of the 'Antarctic Ozone Hole'. The question of what was causing this was answered by some experiments done both on the ground at McMurdo in Antarctica and by flying out of Chile in the southern hemisphere with an ER2, the ex-spyplane that would fly at 18 kilometres altitude. What you see here, from the first successful flight, is the chlorine oxide, which tells you because it is formed by chlorine reacting with ozone that there is a reaction taking out ozone. Chlorine oxide jumped way up when they got out over the Antarctic Peninsula and further there. Ozone hadn't changed very much, but the date was August 23, so it was still winter and the sunlight is just beginning to hit in here. Now you move ahead, three weeks later, and what you see is that chlorine oxide again was high but now two-thirds of the ozone at that altitude was gone. It is measurements of that kind, plus the ground-based measurements, that went into activating controls on ozone. This was a meeting in 1989 in Great Britain about the ozone problem. The Montreal Protocol had been passed in 1987, and that was going to cut the amount going in the atmosphere by a factor of 2, but shortly after that the meeting in 1990 in London and in 1992 in Copenhagen called first for a 100 per cent decrease and then a 100 per cent decrease by January 1, 1996. And so we are about eight or nine years into this region where in the developed world you are not supposed to be producing these molecules any more and should be following this orange line. The measurements that are being carried into 2001 by the NOAA group in Boulder, Colorado, show that of the two fluorocarbons here, CFC-11 peaked back about 1995; CFC-12 is just about level. What these measurements show is that of the amounts of chlorofluorocarbons in the atmosphere, one of them is going down, the other one is about stopped. It means the Montreal Protocol is working extremely well. But these compounds have very long lifetimes. This one is the one with the 100-year lifetime. So even though there isn't any more going in, or a very little amount that is going in now, there are going to be effects lasting for a long period. Some good measurements have been made at the southern tip of Argentina in a comparison of the amount of ultraviolet radiation coming to the surface when the ozone hole is overhead and when it is rather normal. At 295 nanometre wavelength is the place of most sensitivity for human skin. Between the measurement with a lot of ozone and the measurement with a low ozone, there is about a factor of 100 change in the intensity of that ultraviolet radiation. So it is no wonder that people say that you should be using sunscreen during those time periods when the ozone hole is overhead. Here are some comparisons, also of ultraviolet radiation, measured by an index which responds to the sensitivity of human skin. What you see there is that in fact the intensity at Palmer, in the Antarctic Peninsula, is greater than the intensity that is observed in San Diego, California, at its most intense. So it says that Antarctica is sometimes, when the ozone hole is overhead, seeing levels of ultraviolet radiation that are not seen in southern California. And even at Barrow, Alaska, the value is about 40 per cent of what it is that you are getting at San Diego. Here are some ozone hole measurements from this last year, in the last few months. This is a time when, at Punta Arenas and Ushuaia, both in the southern hemisphere, the tip of South America, the ozone hole is not anywhere near it, but then five days later the ozone hole has swung over it. The amounts of ozone overhead dropped by about 40 per cent, and during that time they would be exposed to high ultraviolet radiation. To come back now to global warming, these were the best measurements of the temperatures of the surface of the Earth. You need to have measurements by thermometers, and having thermometers spread widely around the Earth really didn't happen until 1860 or thereabouts. So taking 1860-1910 as the average value, which is this period through here, the temperature started to go up and then it fell for a while, and then it has been going up rapidly since. This explanation here is generally attributed to the use of high sulphur coal which would produce sulphuric acid which acts to scatter radiation and ends up cooling, but then when the regulations were passed that said you needed to get rid of the sulphur in the high-sulphur coal, it had the corollary effect that it no longer masked the greenhouse gases. This is sulphate being taken out and now it is going up in a direct fashion of that sort. The total change of temperature there is, as you can see, about eight-tenths of a degree Centigrade since what was the average for 50 years there. If you take the individual years and now I have got 2004 on this list here the 10 warmest years are all from 1991 on. And the cool years that are in there, 1992, 1993 and 1994, were depressed by the Pinatubo volcano, which put a scattering layer of sulphuric acid in the stratosphere. It took a couple of years to go away. Well, there has been something called the Intergovernmental Panel on Climate Change, which was put together first in 1988. They made their first report in 1990 and said: 'The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more.' And, of course, we are 15 years down, one and a half decades. The primary energy production of the world, for all of the uses of mankind, comes from processes which involve burning carbon in some form oil or coal or natural gas. Hydro power and nuclear are important, but about 85 per cent is here. And even the traditional types are sources of carbon dioxide as well, so that the increase in energy use in the world has been very closely tied with the increase in carbon dioxide. In 1995 the IPCC said: 'The balance of evidence suggests a discernible human influence on global climate,' and their report in 2001 said that there is much more substantial evidence now. One of the things that you hear regularly about the climate is that there are all sorts of uncertainties involved, and that is true. But just saying it in that way obscures the fact that there are uncertainties that are big and there are uncertainties that are rather small. A decade ago, climatologist Jerry Mahlman tried to put it in his estimate of the numerical odds, and so I have here: Human-caused increase in greenhouse gases; Increase in greenhouse gas causes heating effect; Large cooling in stratosphere and so on. All of these are virtually certain if you were at Las Vegas you wouldn't take bets on this, because there wouldn't be anybody on the other side willing to bet. Those things are happening. The last one there is: Natural variability adds confusion. And there are a lot of people who make a living out of adding to that confusion as well. And then there are some things that aren't quite as certain, but still global increase in rainfall, reduction in northern sea ice and rise in global mean sea level, he indicates, rate at 9 out of 10. And then there are some, like whether tropical storms will increase in intensity, that are 'plausible' but not really proven there. These are the top 10 countries in producing carbon dioxide or the greenhouse equivalent from methane and N2O. And, of course, the United States and Canada have the highest emissions per capita there. If we put it in just as per capita these are 1995 measurements then Australia is right up there with Canada and the United States. Coming from the United States, I know that we are the prime culprit in putting greenhouse gases in the atmosphere. The cumulative emissions over the last half of the 20th century put the US as one quarter, the other OECD countries as another quarter, Eastern Europe and the Soviet Union as another quarter, and the developing countries as the fourth quarter. So it's everywhere, but in different amounts of per capita contributions from the various countries. This is the 2001 climate change report that came out. The model calculations show that if you were to double carbon dioxide, then you would get a lot more warming in the North Polar region. The reason that the North Pole is more affected than the South Pole is that the South Polar region is mostly at 10,000 feet altitude and much colder. It is so cold that the ice can't melt very easily, so you don't get the change that you get in the northern hemisphere when you melt floating Arctic ice, which has to be very close to the freezing point of the water. Well, what difference does it make if we change the temperature? For the Glacier National Park in the northern part of Montana, what one has here is where the glaciers were in 1850, with the red; then the yellow shows where the glaciers were in 1979. And they have retreated even further since that time. This is the Quelccaya Glacier, in Peru, that I showed you before. Everything that you see here has already melted. This glacier has other parts to it that still exist, but the 1500-year record here has melted and gone away, and the tropical glaciers are melting all over the world. Well, as glaciers melt and ice that had been on the land is now water, that raises sea level. Warming of the water raises the sea level. And a one-metre sea level rise is shown here for the area round the mouth of the Nile, at Alexandria. In Bangladesh, one metre would have a very severe effect. They get a lot of storm surges already. In the United States, in Florida, one metre would have a great effect on southern Florida. The Thames River is a little bit more complicated in its effect. They get storm surges, where water is going against the current. The current is running out to the North Sea, but when they get big storms up in the northern part of the North Sea, sweeping south it tends to drive the water down into a shallower and narrower area, and pile up and come as a surge up the Thames. When they built this tidal barrier, what they expected was that they would be using it several times a decade. And in 2003, as you can see, they used it 18 times. The estimate of the cost of the failure of the Thames tidal barrier is that if it allowed the water to then get into central London, that would be £30 billion as the immediate damage level from having this fail. So it has become very important to the British government to start thinking what they can do about controlling the greenhouse effect. This is something in the Kenai peninsula, in Alaska, where four million acres of spruce died. It is a warming effect, the warming being in the winter in that region not being as cold as before, and the spruce bark beetle didn't winter-kill. When it didn't winter-kill, then it flourished very rapidly, and when it did that it killed all the trees. So the question of what the effect of global warming will be is one of many things, some of them biological, a lot of them having to do with water distribution. This was a break-off in the Antarctic, where an ice shelf collapsed. And when the ice shelf collapsed, it also essentially removed a wall that was holding back ice and snow that were trying to move down toward the ocean and now can move more freely. The motion of the ice is faster than it was. This shows the polar ice in the north in 1979 and 2003. There are some differences that are not totally consistent, but by and large the ice coverage up there has been shrinking. This brings us to talking about the thermal haline circulation in the North Atlantic. When the Gulf Stream goes northward it is getting colder and it is evaporating. And as it evaporates the water is getting salt, because only the water evaporates and the salt stays there. So it is getting saltier and heavier, and getting colder gets it heavier still. And then it sinks, up around Greenland, and then flows back 1000 metres down, coming back here. So this is a big circulation factor here. And if you melt ice up here, then that's fresh water that is being added, and the measurements do show that the water in the Arctic Ocean is freshening, getting a little less salty, because of the melting of ice. So that there is at least a concern about whether or not the thermal haline circulation and the distribution of heat will be affected. Well, some of this has to do with population. This [slide] is something that was calculated in 1995, with very big uncertainties, and now it looks like it's following right along about in here: they are saying that maybe at the middle of the century there will be of the order of 9 billion people. Those are the most populated cities in the world in 1900. London had four and a half million people, there were 13 cities with a population of a million or more. These are the most populated cities in the year 2000. There are about 350 cities in the world now with a population of a million. And there clearly are problems of the energy use for these populations, and particular problems having to do with urban areas, because there are so many urban areas that are being produced now. I want to read here what Tony Blair said in September. I won't read it all, but just near the end of the first paragraph:
I have just one more thing. I haven't said anything about what you do to replace the energy. I have windmills in here, you have nuclear power as a possibility, you have conservation on a major scale there are a whole lot of things that can be done, once you decide that you are going to do it. I am going to close by referring to a conversation I had with a man from British Petroleum when we were in India together about three weeks ago. He was asking me about whether there would be some sort of control process that the Earth had to keep climate in hand.And so as my last slide I will show you the 420,000-year record from Vostok, with carbon dioxide in the red here [in upper graph] and methane in the red here [in lower graph]. The blue is the temperature. And the striking thing, I think, to those of us who look at this and then look back at it again and think about it a bit, is that every time it got cold it went down to about the same temperature: this is the last Ice Age, and in the one before it, the temperature was also around -9º. And every time it got warm it went to the same place too. And so you say to yourself, 'That really looks as though there was something that was controlling these.' When you got warm, you didn't continue to get warm; you stopped at that point. Something had kicked in. And so you ask what is happening now. At the present time we are not at 280 ppm with carbon dioxide; we are at 380. We are not at 700 ppv, which is what was happening with methane; we are at 1700. So whatever this controlling process was that was acting here is not applicable any more, because we have overwhelmed it with the greenhouse gases.
Discussion Question While we are getting organised, I would you like to ask you about your last comment. We have virtually come into control over the chlorofluorocarbons, but we have the long lifecycle ahead of us for those compounds. But, with respect to carbon dioxide and methane, and some of the other gases, if we really are able, on a global basis, to put in perhaps downward controls, maybe several times greater than the Kyoto levels, are we going to achieve a re-entry to that natural cycle, or even better, and how many years are we going to need? Sherwood Rowland If you choose the 60 per cent cutback in carbon as a source of energy which was the figure being quoted by Sir David King, the science adviser to Tony Blair then you may control carbon dioxide at around 450 to 500 ppm, but not at 280. And you would be hitting that at some time around the end of the 21st century. So you are not at all clear that you would be able to get back in any reasonable time. Carbon dioxide has a lifetime in the atmosphere that is controlled by the mixing of the upper ocean waters into the deep ocean, and that's of the order of a century. And so it is going to take two or three centuries of not putting in much carbon dioxide to drive it back down there. So no, there is no guarantee that if you stop at double the carbon dioxide, which is one of the other things that people are talking about, there is no guarantee that you will be in the control regime that the planet was before. Question I was wondering if you could comment on the latest Michael Crichton book, State of Fear. Sherwood Rowland Michael Crichton writes science fiction even when he thinks he is writing non-science fiction. Let me give you a specific example. There is a big discussion in there where he criticises Jim Hansen's calculations in 1988 of what the temperature rise would be, a decade ahead. Crichton put this as 0.35ºC. Hansen didn't make a prediction of what the temperature would be. What he did was make three calculations on various hypotheses. One of them, the middle calculation, included a volcano. And in that 10 years there was a volcano. So that's the one that is actually most appropriate, and Hansen had that nailed almost on the head. But the one that is being quoted is the calculation that was volcano-free and had carbon dioxide contributions going up at one of the most rapid rates. So what Crichton did was to chose an inappropriate part of Hansen's model and then hammer him for it. What we think of that might depend on whether he knew he was doing it or whether he just read somebody else's summary there were a number of people who had summarised Hansen's results by including only the one that was volcano-free, just to get at him. Question Is the solar constant changing? Sherwood Rowland The question of the input of energy from the sun is a very important one. What one knows is that you have measurements from beryllium-10 and carbon-14 that allow you to make some estimates of how much the solar constant may have changed in the past. Over the last 25 years there have been a number of satellites that are up, and they show that the solar constant is not quite constant. It varies over an 11-year cycle, which was known. But if you average over the last 11 years and then the 11 years before that, there is essentially no difference. So the solar constant effect in the last 25 years has not been important at all.It was clearly important in the past, in the 1700s, when sunspots disappeared for a while. The solar constant, that measurement shows up in the beryllium-10 and carbon-14. But if you look at it for the last 25 years or so, there is a very rapid temperature rise and it is not a change in the output of the sun.Question The fact that the temperature and CO2 levels track each other so well in the Vostok core is, of course, one of the most powerful pieces of evidence pointing to future warming. There are good physical reasons why increases in the CO2 will elevate the temperature. But of course there are feedbacks, in terms of increased decomposition of peaty soils at high latitudes. Are there models now, or analyses, that can determine or at least analyse what fraction of the change is the CO2 dog wagging the temperature tail as opposed to the temperature dog wagging the CO2 tail?Sherwood Rowland When you make a comparison of temperature which is based on isotopes of oxygen and deuterium in ice, versus the gases, then you get into the fact that the ice molecules don't move much and the CO2 and the methane can move through the ice until it gets to 50 or 75 metres of depth. So the timing of when things close off for the gas molecules has some uncertainty in it of a couple of thousand years when you are dealing with something like Vostok. And that is why you don't look at Vostok to see any evidence for the upsurge in the last 200 years, because that is just so recent that it hasn't closed off.And as far as I can see there is no strong evidence that in any of the instances the temperature change was way ahead of the gases. They seem to be in synch, or very close to it, and within the limits, I think, of the era of when the gases actually did close off.Question You mentioned positive feedbacks from human-induced global warming, such as from water vapour and ice melting. Are there any prospects of negative feedbacks from human-induced greenhouse warming, such as increased cloud cover?Sherwood Rowland The question of cloud cover is an important one. When I look at it, the ratio of the amount of water vapour that is in the air to the amount that it could hold comes very close to 50 per cent, in air that is around 98ºF and in air that is around 30ºF. That is, the relative humidity seems to be around 50 per cent over a very wide range of temperatures. That may suggest to me that it is a reasonable hypothesis that when things start to warm up, the relative humidity will still be distributed about the way it has been before, because it is over such a wide range. And that is certainly entered in calculation of the strengthening of the greenhouse effect by the water vapour, that the relative humidity would remain the same. I think that is a very hard thing to get a firm conclusion on, but what the modellers are doing seems to me to be in the right ball park for what the changes will be.Question I was just wondering if you could comment a little on the issue of increased CO2 uptake in the oceans of the world, and perhaps how that might affect the buffering capacity of the oceans.Sherwood Rowland When you uptake carbon dioxide in the ocean, you are slowly making it more acidic, and any biological process that is going on there would have to be examined for the change. So far it is a change of the order of a tenth of a pH unit, and there are some things that will be responding to that and others that will not. It will be a long-term concern. As carbon dioxide continues to pile up in the atmosphere, it will continue to become bicarbonate and carbonic acid in the ocean, and there will be effects of that on some of the biological systems that are specially sensitive to pH.