Mario Molina (1998) - Environmental Challenges for the 21st Century

Thank you very much Professor Wüthrich for your kind introduction. Being the last speaker on behalf of my fellow laureates I would like to thank Countess Bernadotte and the organisers of this very nice meeting for inviting us and for giving us the opportunity to interact with you, the students. I must add, being the last speaker, I have very many tough acts to follow. And I am going to be using view graphs. I am going to talk about environmental challenges for this coming century but I won’t be talking about all of them. I will focus particularly on ozone depletion, perhaps say a few words also about the greenhouse effect. But I would like to put this in perspective on the types of problems that we are facing for the next century. Many of them have the real basis in this very large population that we now have in our planet. Close to 6 billion people, you heard that already in some of the other talks. And this is just one way to sort them out. We have problems that have to do with depletion of natural resources. You just heard the end of Professor Schmidt’s lecture, some thoughts on these types of problems. This is by no means a complete list, there are other issues here, one important one being the depletion of fisheries in the world’s oceans. Besides stressing natural resources with so many people on our planet, we have a lot of waste, waste from human activities. This waste can be solid or they can be water soluble. They can lead to water pollution or they can be volatile and that leads to air pollution. What we also have and this is what is relatively recent. We have only realised that in this really recent decade or two, that we have truly some global environmental problems. I’m listing three here, the greenhouse effect or climate change and ozone depletion, which are the two I will stress. And tropospheric ozone which is what Professor Crutzen and Professor Rowland talked about at the beginning of this meeting. I want to say a few more words about the difference between these types of global problems which have to do with time scales. And to better appreciate this, let’s just look, take a look at our planet from space. Let’s consider atmospheric motions. If you release some gases, say in the United States, they can reach Europe in a matter of months. So mixing is relatively rapid, particularly in the longitudinal direction. But it’s also very fast north to south. In fact it takes something of the order of a year or two for gases released in the northern hemisphere to be well mixed in the southern hemisphere as well. So whether you have a global problem or not, or the type of global problem that you have, depends on how stable are these gases that you release. Gases like carbon dioxide or the CFC’s that I will be talking about have residence times in the atmosphere that are many years long. And that’s why no matter where they are released, they can be found throughout the planet. In contrast there are other types of pollutants that are much shorter lived. They are removed from the atmosphere by various cleansing processes. And hence you have a more localised pollution. Perhaps the best known example is local air pollution in cities, urban pollution. What happens is even that type of problem can become global because it’s happening now with so many people in the planet. It’s happening now in many large cities all over the world. And we also have for example forest fires, bio mass burning in many different places, in many continents. That’s why that’s another way to get a global problem, even though you have species that do not remain in the atmosphere long enough to be well mixed. So I will talk about in particular the problems with the long lived species. I want to call your attention to another feature of the atmosphere of our planet. And that’s just to stress how really thin it is. The distance between the two poles is about 20,000 kilometres. The farthest distance you can go on earth from where we are now is about 20,000 kilometres. But the atmosphere is such that the first 20 kilometres above the earth surface contain roughly 95% of the mass of the atmosphere. So it’s really like the skin of an apple, very thin, very fragile layer looking at this from this cosmic perspective. And it’s remarkable how these mixing processes can be so efficient in spite of having such a thin atmosphere. The issue that I’ll be talking about, one of these gasses that is long lived enough, or series of gases. Again to put it in perspective I want to go back in time towards the beginning of the century with development of technology. Refrigerators, in the 1920’s they became very popular appliances in rich countries, in the United States and in Europe. And that’s because it was possible to produce electric refrigerators. These refrigerators essentially replaced the earlier ones that were a little bit cumbersome. The earlier ones all functioned with blocks of ice that had to be imported from northern frozen lakes. But what happened here is that if you have a motor you can compress a fluid called a refrigerant, convert it from a vapour to a liquid, as a liquid evaporates, it cools and it can be recycled. And that’s how refrigerators essentially function this way nowadays. The problem with those early ones, for which by the way something like a million dollars just in advertisement was invested earlier to sell these types of machines, but they had a problem, namely the refrigerant. The fluid that was compressed, liquefied and then evaporated, was rather toxic, was ammonia or sulphur dioxide. So a number of rather severe accidents happened where small families were sort of sleeping close to these types of refrigerators. And so eventually a solution had to be found and was found and the refrigerators that came about looked very much like the ones we use nowadays, very similar, still with a compressor. The only significant thing that changed is the refrigerant itself. This is a picture from the 1930’s and what happens is that the refrigerant that became available was invented by Thomas Midgley, specifically to replace the toxic refrigerants. These are the so called CFC’s. Very simple chemical compounds, derivatives of methane and ethane. We have here an example of a 2-carbon one, in which you replace all the hydrogen atoms by halogens. And what you achieve is great chemical stability, lack of toxicity. So that was the dominant feature in terms of chemistry. And by choosing the number of halogens, the size of the molecule, you could also choose the physical properties that make them useful as refrigerants. They boil close to room temperature and hence they can be readily converted from a vapour to a liquid. These were the first two that were used for refrigeration. But they became so successful that they found many other applications. A much more recent one, CFC-113 is used, you see it has a higher boiling point, or was used essentially as a solvent to clean electronic boards of the type that are used to make computers. But there was one particular application that also led to a very significant industrial production of these compounds, relatively easy to make. Application which all of you are familiar with, spray cans. And what happens is that what you want to do in a spray can is essentially to be able to put a fluid, so called propellant. You want to put a large amount of it, so in compressed form it’s a liquid. And when you press a button, the liquid evaporates and you can spray whatever you want. This turns out to be a very efficient way to deposit large amounts of CFC’s in the environment. It turns out that refrigerators do the same thing because the CFC’s are so stable that they outlast the refrigerators. So when you eventually dispose of your refrigerator, the CFC’s also end up in the atmosphere. So what do they do? This is a question that Sherry Rowland and I posed. This was in early 1970, 1973. We were aware that these compounds could be measured at that time throughout the globe. Because we know, as I explained a minute ago, how mixing occurs, the concentrations were rather small. And the question that we asked is: what happens to these compounds that are entirely of industrial origin, certainly not natural? What processes remove them from the environment and what are the consequences if any of their presence in the environment? To explain that let me just review with you some properties of the atmosphere, Professor Crutzen already showed you something of this. This is just a typical temperature profile. Temperature varies with latitude, with season, but this is a typical one. You can see first of all how pressure decreases very rapidly and this leads to most of the atmosphere being close to the surface. But the property of the atmosphere, it’s mostly heated from below. That’s why we expect temperature to drop but eventually it increases again. And this is what gives rise to different layers and this is what gives rise to these very efficient motions down here in the troposphere that I talk about. Motions in the stratosphere are much slower, particularly in the vertical direction because this inverted or constant temperature profile gives the stratosphere large stability. But there is another very important difference between these two layers besides speed at which gases mix. And that’s the way the atmosphere functions, the cleansing mechanism. Professor Crutzen described very clearly in his lecture on Monday how the hydroxyl radical, OH, works cleaning the lower atmosphere. It’s like the detergent of the atmosphere. In that it oxidises for example hydrocarbons. But the rest of the cleansing mechanism consists of clouds, rain. The hydrocarbons themselves are not water soluble, but this oxidation converts the hydrocarbons into water soluble compounds, aldehydes, alcohols, acids and so on. And rain removes anything that is water soluble, extremely efficiently, just a matter of weeks. But that doesn’t happen in the stratosphere. The stratosphere is very dry and the reason is that between these two layers the temperatures are so low, could be -80, -70 degrees centigrade or -60, that most of the water condenses, it came down already. So you end up having only parts per million of water in the stratosphere. It doesn’t rain there. So we have this equation then to repeat, slow motions, no efficient cleansing mechanism. That’s what makes the stratosphere very vulnerable if somehow or other you can deposit pollutants at those altitudes. Before I discuss the pollutants any further, just briefly describe again, Professor Crutzen mentioned that already, the origin of the temperature rise in the stratosphere. And that’s a presence of ozone which is a very unstable molecule as all of you chemists know. And that’s why it’s again only present in parts per million level, levels comparable to those of water vapour, most of it being in the stratosphere, but pollution close to the surface capable of generating significant amounts down below. And this also comes about, just to discuss very, very briefly, essentially by the action of sun light on molecular oxygen, wavelengths around 200 nanometres are capable of breaking the O2 bond. And the free oxygen atoms pretty rapidly make ozone. This process, absorption of radiation by ozone, is what heats the stratosphere and also what shields the earth surface from ultraviolet radiation that is harmful to biological systems. Essentially wavelengths shorter than 290 nanometres are taken out by this parts per million of ozone that exists at those altitudes. And in this mechanism, which is what you would predict if you had an atmosphere consisting only of nitrogen and oxygen, you would have a formation mechanism. And this is the destruction reaction, the oxygen atoms instead of making ozone, occasionally react directly with ozone. But we know this mechanism is incomplete because there is less ozone than one would predict from these reactions alone. And the reason is that there is an additional set of reactions. An important set of reactions which were actually first discovered or postulated by Paul Crutzen, again in the early ‘70’s. And this is a catalytic cycle. Here is what happens. You have nitrogen oxides that are naturally present in the stratosphere in rather small concentrations, parts per billion levels, very much less abundance than ozone. This is a classical catalytic effect, a chain reaction, you recycle the reactants and that’s how you can explain this amplification factor. The net effect is speed up ozone destruction and explain the actual levels of ozone that you find there. From a chemical point of view these catalysts, we can explain how they function because they are free radicals. Free radicals have an unpaired electron. In fact all of these free radicals have an odd number of electrons. So one of them has to be unpaired. And that’s why they are so reactive and so efficient. And we and others realise that there are other free radicals that can function in very similar ways. For example chlorine atoms are extremely efficient in achieving also ozone destruction. So with all this information I can explain now in a nutshell the hypothesis that Sherry Rowland and I put forward in 1974 which is as follows. You have the CFC’s released at the earth surface, they mix essentially throughout the globe, before they reach the upper stratosphere. Not much happens to them at these lower altitudes, rain doesn’t affect them. The hydroxyl radical, the detergent of the atmosphere is inefficient because they don’t react with OH. But what we know is that they can be photolysed, they can absorb short wavelength radiation. But by looking at the spectrum in the ultraviolet, we realise that they have to move, essentially above the ozone layer. And then they are photolysed at wavelengths around 200 nanometres, comparable wavelengths to those that make ozone by breaking molecular oxygen. And when they break, that's when you get the free radicals. The free radicals are so reactive that only a fraction of the time, normally 2% of the time they are in this form. But they go back and forth making more stable species that are not free radicals. These species or the free radicals themselves, if they were to be formed at lower altitudes, this cleansing mechanism would remove them in just a matter of weeks. Instead they remain a matter of several years in the stratosphere. And that’s through these catalytic cycles, how one can explain that the human activity which initially was depositing of parts per trillion and eventually parts per billion of CFC’s in the atmosphere could have such significant effects. Now I won’t explain this in detail but what I want to point out is that of the species that exist in the stratosphere, this atmospheric chemistry being an experimental science. Many of the predictions of the theories that we’ve put forward, we and others in the community, were tested by measuring the presence of many of these species in the stratosphere. Another field which is the one that my research group is engaged in is measuring in the laboratory the rates of the reactions that interconvert species from one product to another. You notice that the sources are, except for water itself, mostly reduced form. We have some natural halogen sources like methyl chloride. In the stratosphere they decompose, give these free radicals which go back and forth between radicals and these so called temporary reservoirs that are oxidised. These are water soluble, these are the ones that eventually bring these compounds to the surface of the planet and in the troposphere are very rapidly removed. So there is a very rich field here, gas space chemical kinetics. And all sorts of interesting findings connected perhaps more with the thoughts you heard earlier this morning, that I could spend a lot of time talking about, for example many of these radical-radical reactions. One recent one we have studied is OH + ClO that gives Cl + HO2 but occasionally you get HCl as a product. Some of these reactions speed up at lower temperatures. They have negative temperature dependencies. There are some others not listed here, like SO3 + water to make sulphuric acid that has a very large negative temperature dependency, something like 30 kilocalories per mole. And we have yet to explain clearly how that comes about. But what I’m stressing right now is there’s a large interplay between fundamental chemistry, fundamental chemistry from the point of view of gas space chemical kinetics or kinetics on surfaces and atmospheric chemistry. And this interplay has made the field very exciting. But one of those fields is measuring the species in the atmosphere and what was missing for many years is measuring of actual effects on ozone. Although many of those were found to be there, what remained to be proved is that something was happening to ozone. Until 1985 when it became clear that something quite dramatic was happening to ozone over Antarctica. I am not showing here the regional measurements that led to the discovery of the ozone hole by Joe Farman of the British Antarctic Survey by doing measurements from the ground in Antarctica. But these are results of measurements carried out by satellite, looking at the amount of ultra violet light reflected by the atmosphere. And what we can see here in early years, this equation, you can see the different colours here representing different amounts of ozone. Not at a particular altitude but total ozone integrated from the surface to the top of the atmosphere, a unit such as 300 units, Dobson units. This is a typical value. This is such that if you bring all the ozone in the atmosphere where it’s yellow, you would get a layer of about 3 millimetres if you bring all that ozone to the earth surface, this is how much ozone there is. Since ozone is made predominantly in the upper tropical stratosphere, you don’t get as much over the holes but you get rather significant amounts because normally it’s rather protected there, it’s not destroyed. But this is what has been happening just in recent years, very significant, very low levels of ozone over Antarctica. We had not predicted specifically this ozone depletion over Antarctica when we first started doing this work. What became clear eventually is that what no one in the community had considered is that clouds actually do form in the stratosphere, specifically over Antarctica. Because in spite of the stratosphere being so dry, over Antarctica the temperature drops sufficiently, And these then facilitate different types of reactions that we had originally not taken into account. The reactions that we considered initially for chlorine species can be perhaps summarised schematically here. These are the free radicals, these are the stable reservoirs. And the way the species interconnect to each other is in the gas spaces always through the free radicals. The green species do not react with each other, this is a catalytic cycle. For example chlorine when it finds methane makes HCl and so on. What happens in the presence of the clouds is that you open the possibility of new types of reactions. What the ice clouds provide is surfaces for aqueous type chemistry to take place. And in particular what happens is that HCl has an affinity for the surface of ice and it can react with species such as chlorine nitrate or HOCl to make not a free radical but the element chloric. This is actually close to a free radical because being a green gas absorbs lights very efficiently. And when the light comes out in Antarctica after the long polar night, you get the radicals and that’s when ozone starts being depleted very rapidly. There is another effect that the clouds have and what the clouds do is they scavenge. They clean the atmosphere but predominantly from nitrogen oxide, let me see if I can describe this. The nitrogen oxides are converted to nitric acid and the clouds actually clean that nitric acid from the atmosphere. So this has the following effect. I showed initially the two sets of reactions, nitrogen oxides and chlorine and chlorine monoxide depleting ozone. When you mix the two radicals you don’t get the sum of the individual effects, you get less and that’s because the radicals interfere with each other. NO2 for example scavenges ClO to make a stable species. If somehow or another you remove NO2, you get much faster chemistry. And that’s indeed what happens. When you remove the ClO for example, it can begin to react with itself. We showed this around 1987 in some laboratory works actually carried out by my wife Luisa who is here in the audience. So we showed that you can make this type of species, chlorine peroxide offering a way to explain this very fast ozone depletion that occurs over Antarctica. Just one more view perhaps of what happens there in these spring months. This is an ozone profile as measured by balloon, just when the light is beginning to come out for the free radicals begin to pile up. So in a time scale of the order of a month, you can see how this normal looking ozone profile, most of the ozone being present in the lower stratosphere there, that most of the ozone actually disappears over this altitude range. More than 99% of the ozone is gone over this altitude range. So it’s a very spectacular phenomenon. And just one more view of an experiment. This is again a satellite measurement of ozone depletion, the different colours, here the blue colour representing low levels of ozone. But what's shown in the lower panel here, measurements from a satellite of chlorine monoxide. So much, the concentration of the free radical is so large that a large fraction of the chlorine is in the form of chlorine monoxide that it can be measured remotely by microwave emission. And these types of experiments are the ones that indicated very clearly that the chlorine is really the cause of this depletion. And ozone is disappearing not just over Antarctica, that’s where we could see this very spectacularly thing. What we have here also in France at mid-latitudes, between 60 degrees north and south and we can clearly see how in recent years the amounts have been decreasing. What an important event happened, which has to do with the response of society to this problem. And that is an international agreement called the Montreal Protocol which was originally signed in 1987. And to explain what the implications of this agreement are, let me show you this view graph where we have chlorine in the atmosphere, particularly in the stratosphere as a function of the year, calendar year. There is a certain natural level which comes mostly from methyl chloride that we don’t expect to change. We have here a rapid increase which is based on measurements. But of course from here on, these are just predictions for difference based on different scenarios. If we had done nothing, chlorine perhaps would have continued to increase along these lines. The original Montreal Protocol was relatively weak. But eventually it was strengthened because the scientific evidence became very clear. And so in Copenhagen the agreement was to phase out the CFC’s completely, but only in the industrialised countries by the end of 1995. Developing countries are allowed to continue producing CFC’s for some time. I should point out also before I remove these that because the CFC’s are so long lived, we don’t expect them to be removed from the environment fast. In fact the Antarctic ozone hold will remain until the middle of next century according to these predictions. But there’s definitely the levelling off predicted. And in fact one can already measure that, although we cannot measure effects on ozone yet, we cannot measure the recovery yet because of this time scales. These are measurements from various surface stations of one of the CFC’s and industry of course knew ahead of time that they had to phase out the CFC’s by the end of 1995. And so they began to level off. And there’s one more compound, a shorter lived one, not a CFC but one that is used as a solvent and that is called industrial methyl chloroform , trichloroethane. And this one has a residence time of only a few years as opposed to 50 or more years for the CFC’s. And in this case we can clearly see on a global scale how the Montreal Protocol is already working. Concentrations of chlorine are beginning to decrease. What I want to do in the remaining minutes, I hope I have a few, is to now talk about, make a connection with global warming, with the greenhouse effect. I’ll try to do that very briefly. First of all let me just remind you, most of you are aware of this, how the greenhouse effect comes about. The sun emits radiation that we can mostly see visible and the earth receives this radiation and essentially loses the same amount of energy that it receives but in the form of infrared radiation. And our atmosphere is such that it is mostly transparent to this radiation but not to this one and that’s the origin of the greenhouse effect. If you do an energy balance, you can realise how easy that, you have to consider the colour of our planet, not all the energy that we receive from the sun penetrates to the surface. Some 30% or so is reflected by clouds. But the energy that the earth loses, a significant fraction is absorbed in the atmosphere itself. Not by nitrogen and oxygen that as you know are transparent in the infrared. These diatomic molecules have no dipole moment. But these are very good infrared absorbers even though they are not very abundant. They have a very important consequence which is to increase the temperature that the surface of the earth has by something like 30 degrees, 33 degrees. Without the greenhouse effect the average temperature on our planet would be something like - 18 degrees centigrade. And the oceans would be frozen. In fact it is about + 15 degrees centigrade. I won’t have time to discuss many of the details but you’ve already seen some of the earlier talks showing that indeed temperature is increasing, it’s not a large increase, not by the degree since measurements really started. And we have other incomes that show us that the climate is indeed changing. For example the glaciers are receding all over the world. This is just an example of a glacier in the United States, in Montana, measurements last century in the red, versus measurements in this century. You can see a much smaller mass. Let me just summarise what I believe the situation is with respect to this climate issue effect by showing you the results of some calculations carried out by some colleagues of mine at MIT: Calculations in which the economic models were put together with models of the climate and of the chemistry of the atmosphere. And what this does is predict temperature changes, particularly for the next century. And the yellow shading here represents, or is an attempt to represent natural variability. So we don’t know yet for sure that this temperature increase that I showed before is indeed a consequence of human activities. But there are many indications that it is. But we don’t know for sure because the signal is not yet large enough. We have here seven different lines and the point here is that there are very large uncertainties. Here are some of the large uncertainties which come from the economic models themselves. What will society do? But the very significant factor of the uncertainty is the climate system. What do the oceans do, how will the clouds change? That’s very complicated. The point however is that all these predictions are plausible. We’re not sure which line we’re on. If we are on the green line the effect will not be perhaps very dramatic. But if we are on the red one, we’re indeed in trouble. this begins to approach the types of changes that we have between a glacial and an interglacial age, very large changes. All sorts of consequences such as sea level rise, more flooding and drought and so on. I won’t discuss the consequence anymore. But I just want to point out that we are actually carrying out an experiment. We are contributing to the increase of greenhouse gases in the atmosphere. And just remember the quote from James Watson that Professor Schmidt showed in the previous talk. As a scientist I would like to see the outcome of the experiment. It would be interesting to see which line are we on? That however is very risky. As an individual I think since there is certainly no good evidence that we are not on this red line, it’s just a very risky thing to do. So we don’t need to know for sure where we are in order to decide whether society should take some action or not. I should clarify that for these statements I am speaking as an individual, not as a scientist. Science can only tell us what the range of our certainties are. Science cannot tell us what to do. But we do have the responsibility of making decisions. And certainly in my opinion we have enough information to begin to take some action. One of the significant problems here is that a very large fraction of the problem will come in the future from developing nations. So it’s clear that we will have to work together with these developing nations in order to prevent very large damage. I have some more material which I won’t show but developing nations have a very severe local pollution problem and we have to couple the two in order to make things easy. But let me just start my, to finish here, let me show again this view of population growth. You have seen already several this week early on. But I am taking here the longer view. What is very clear is that it’s the rate at which population is increasing that has changed very dramatically and that’s just happening since the industrial revolution. Fortunately it looks like we are levelling off. This rate of course cannot continue for much longer. But we have so many more billions of people now than we had in the past. This is really putting all the stress in the planet that we have to live with. But there’s a second consequence of this very large population. The second consequence has to do with how vulnerable our society is. It’s no longer the case as we had before that if the climate gets, degrades, you can just move somewhere else. We have people all over the planet. There is no place to move. To feed all these people you need high yield agriculture, you need stable climate and you need a non-polluted planet in the end if there’s going to be stability. Particularly if you consider that all these people in developing worlds want to increase their standard of living as well. So from my point of view it’s not going to be possible for them to increase their standard of living the same way we have done it in the industrial countries. We simply have to learn to do it in different ways. I must stress I am not talking about not letting developing countries develop, they just have to do it in more clever ways. And we have to collaborate with them from the industrial country’s point of view. So let me just finish with this view graph coming back to the ozone issue, just a symbol of high technology, very high indeed and the earth’s atmosphere which shows how thin it is and how fragile it is. And if you want to use again the example of the ozone and CFC problem, what that issue has shown us is first of all very, very clearly that human society is quite capable of effecting the environment truly on a global scale. There is really now no doubt about it. What the CFC ozone issue has also shown us is that society is capable of solving these problems. The CFC issue is in some sense a success story, the solution is not perfect, we have to keep working at it. But largely we can see that the solution is beginning to work. The CFC’s are decreasing. What it took is a lot of work. The industrial segments had to work with scientists, also with environmental organisations, with diplomats. But we showed that there is a way, that this brings about possibilities. Now it’s not going to be easy. So a great deal of hard work will be required to address all these very serious issues that we have in the environment facing us in the next century. And this is really a challenge particularly for you bright students of the younger generation. Thank you for your attention. Applause.

Mario Molina (1998)

Environmental Challenges for the 21st Century

Mario Molina (1998)

Environmental Challenges for the 21st Century

Abstract

There is now strong scientific evidence that human activities have led to the accumulation of a variety of trace gases in the Earth’s atmosphere, particularly in the past few decades. These global scale changes in the chemical composition of the atmosphere are expected to influence the climate, and have already led to significant depletion of the ozone layer of the stratosphere, which shields the Earth’s surface from harmful ultraviolet radiation from the sun. The ozone layer is particularly vulnerable to emissions of chlorofluorocarbons (CFCs), which are industrial chemicals used mainly as refrigerants, solvents, propellants for spray cans, etc. After their release to the environment, these very stable compounds slowly diffuse into the stratosphere, where they decompose, releasing chlorine atoms and leading to ozone destruction. In the past decade a very significant depletion of the ozone layer has been observed over Antarctica and in recent years also at northern latitudes. Laboratory and field experiments have very clearly pointed to the CFCs as the dominant cause for this depletion.

Climate change may result as a consequence of the enhancement of the greenhouse effect. The energy which the Earth receives from the sun is balanced by an equivalent amount of energy emitted by the planet in the form of infrared radiation. There are some gases in the atmosphere which trap a portion of this outgoing radiation; the most important one is water vapor, followed by carbon dioxide, methane and nitrous oxide and the CFCs. The net result is that the average temperature of the Earth’s surface is higher than it would otherwise be; this is the process known as the “greenhouse effect”. The science of climate change is at present rather uncertain, but there is little doubt that the levels of some greenhouse gases -- notably carbon dioxide and methane – have increased significantly in recent decades as a consequence of human activities, and that these gases will affect the Earth’s climate; the question is when, and to what extent. It is also clear that the climate is changing: the average surface temperature has increased a fraction of a degree in this century; mountain glaciers are receding, and sea level is rising. The question is whether or not human activities are largely responsible for these changes; the consensus among many climate experts is that “the balance of evidence suggests a discernible human influence on global climate.”

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