It’s been recognized and, as I discussed in the previous chapter, that several different compounds–carbon dioxide being the most important, but several other compounds all three atoms or more because of this infrared trapping property play a role in the Greenhouse Effect, and when the UN Framework Convention on Climate Change was introduced and then the Kyoto Protocol, the focus was on the so-called anthropogenic, well-mixed greenhouse gases.
These are greenhouse gases that are caused by human activity, so not water vapor which is a huge greenhouse gas but not directly caused by human activity, and a kind of gas that mixes in the atmosphere. that’s extremely important also to understand when carbon dioxide is emitted from a power plant in China or in New York State or in South Africa or in Indonesia within about a month the carbon dioxide that is emitted from any of those particular sites is pretty much uniformly distributed in the whole world. In other words, the atmosphere is well mixed.
The carbon dioxide that I’m inhaling right now or exhaling isn’t going to be specific to New York City where I’m speaking right now; it’s going to have an effect on the global carbon dioxide concentration in a uniform way. We could add as a footnote that if each place on the world emitted its own greenhouse gases that have stayed there over their own heads and their own responsibility in their own local climate, we’d probably reach a resolution of this crisis a lot more quickly because a place that was experiencing warming or climate disruption would say look what we’re doing to ourselves, but since when we emit carbon dioxide or methane or nitrous oxide has effects globally, we don’t pay so much attention to our local actions causing global effects spilling over all over the world. Well, the Kyoto Protocol identified these well-mixed anthropogenic greenhouse gases and the focus has been on the major 6 which are shown in this table.
Carbon dioxide, the most important of all, and it will be our focus through most of this course because it’s so important, it is so long-lasting in its effect, it is so deeply part of the world economy because the core of these anthropogenic emissions are from our use of fossil fuels for use of modern energy that it is appropriate to call the whole challenge decarbonization, but you can see there are other very important greenhouse gases as well.
Methane or CH4 is emitted by human activity in the atmosphere in many many ways that we’ll discuss these briefly: in agriculture, in landfills, in escaped methane from natural gas pipelines as gases piped to our cities, and into kitchens in many parts of the world.
Nitrous oxide is another greenhouse gas that comes from ways that we burn fossil fuel that comes from fertilizer use of nitrogen-based fertilizers that comes from other industrial processes, and then there are these long complicated so-called “F gases” fluorine-based gases: the hydrofluorocarbons or HFCS the perfluorocarbons or PFCs and sulphur hexafluoride or SF6. These are also very potent greenhouse gases. Fortunately, they’re used in such limited amount that they’re still quite small part of the overall anthropogenic process.
Now, in order to aggregate across the six anthropogenic greenhouse gases, we have to ask the question how powerful are they in their greenhouse effect, how much of that infrared radiation emitted from the earth does each molecule of these 6 gases absorb and by comparing that absorption of infrared radiation by these different gases we can give a weighting to the different gases in terms of their overall greenhouse effect.
Everything is scored relative to carbon dioxide, and so we want to understand the greenhouse effect of each gas relative to carbon dioxide so carbon dioxide is given a warming potential of one that you can see in the middle column top row of this table.
Then, each of the other gases molecule for molecule has even a more powerful warming effect than a molecule of CO2; methane 23 times molecule for molecule taken on a hundred-year timescale more greenhouse warming than carbon dioxide. Nitrous oxide, you see in the table, 296 times more powerful.
Perfluorocarbons 5,500 times more powerful, so why is CO2 so important because there’s so much human emission of carbon dioxide compared to the others that even adjusting for the per-molecule warming potential taking the number of molecules that humanity is emitting of each kind of greenhouse gas multiplying it by the warming potential per molecule, CO2 comes out way ahead and that you can see in the final column this is for the year 2000 and of course it keeps changing as the relative proportions of emissions change but in the year 2000, carbon dioxide accounted for about 77 percent of the human-induced greenhouse effect of missions that year.
Methane number two 14 percent, nitrous oxide number three and the sum total of the fluorine-based gases under 2 percent, so that’s the summation; that’s why decarbonization is number one three-quarters of the total effect but there’s another factor that’s also extremely important to keep in mind, and that is that when we put these molecules into the atmosphere through human activity whether it’s burning coal, oil, and gas or whether it is farming in a way that emits methane from a rice paddy or from livestock that are being grown for meat production whatever it is that molecule in the atmosphere has a certain lifetime expectancy of being in the atmosphere before it is reabsorbed onto the earth or into the oceans.
Part of the carbon dioxide that is emitted each year is immediately absorbed by the ocean, part goes into the photosynthesis on the planet and is stored biologically rather than staying in the atmosphere, but the fact of the matter is that when carbon dioxide molecules enter the atmosphere at least a lot of that input of CO2 into the atmosphere is gonna stay there for a long time.
In this chart, it says five to two hundred years but the fact of the matter is a certain fraction of that carbon dioxide is gonna stay in the atmosphere for thousands of years maybe 20 percent in total of the co2 in the atmosphere will stay for hundreds or thousands of years–what’s called the residence time in the atmosphere will be very very long what we’re doing to change the atmospheric composition is not going to be reversed very quickly.
On the other hand when methane is put into the atmosphere the residence time for methane is very short through chemical and physical processes the residence time methane in the atmosphere is much much smaller than for carbon dioxide, which means that if we control the methane emissions, the methane that we have historically emitted into the atmosphere isn’t gonna stay there for decades or centuries like carbon dioxide it’s going to fall much more quickly so when we think about the role of each of these greenhouse gases, we need to think about the molecule for molecule warming potential, the total amount of the molecules that human activity is putting into the atmosphere and the residence time of those molecules in the atmosphere we need all three of those dimensions to shape an appropriate response.
It’s even worse because of course the greenhouse gases are not the only changes caused by humanity that affect the climate and on top of the human-induced effects there are multiple feedbacks and multiple natural forcings as well. No one said this is easy and no one said that it’s an absolute simple matter so when we look at the total net radiative effect of all these greenhouse gases and other chemicals that humanity’s putting into the atmosphere and other effects of humanity on the climate system were led to a rather complicated chart of the kind that you are looking at right now.
Now what this is is an attempt to add up across not only the greenhouse gases but other kinds of factors that changed the net radiative balance of the planet the watts per meter squared of net radiation that determines the earth’s eventual balance in temperature. At the top of this chart are the long-lived, well-mixed anthropogenic greenhouse gases, and the biggest bar in red that means warming, net radiative warming, of the planet is CO2. That’s what is our main focus.
Then, as you see in the next bar not as important as CO2 roughly 1/4 of the total CO2 effect are the other greenhouse gases: methane, nitrous oxide and the fluorine-based gases or here called Halocarbons. Then there is an ozone effect which is a kind of feedback because many of our chemical processes change the composition of ozone in the troposphere–that’s the lower atmosphere– and in the stratosphere–that’s the upper atmosphere.
On balance as ozone increases for example in the troposphere through various chemical effects and warming itself you get another bit of radiative forcing to warm the planet that’s the third red bar that you see not as important as carbon dioxide or the other greenhouse gases but still a major warming effect. Well, you can go down category by category. Let me draw your attention to surface albedo that remember is the reflectance of the earth’s surface.If you have forests, they absorb a lot of sunlight; clear the forest, you get more reflection of the sunlight not absorption.
The sunshine comes in, goes straight out to space and doesn’t get absorbed as much as if you have forest cover so as humanity changes the surface of the earth: ice, forests, cities and so forth we changed the reflectance or the albedo of the earth. The net effect of that is mixed. Land use change has been having a net cooling or negative radiance effect Some kinds of surface changes such as pollution which makes the ice darker because soot falls on the ice means that what normally would be reflected is absorbed by the ice so you see a little bit here called black carbon on snow meaning that even the snow was not reflecting because it’s a it has this pollution on it and it absorbs more of the radiation.
Then comes a major category called aerosols. Aerosols are a set of small particles also partly driven by nature such as a volcanic eruption which huge amounts and of sulfates into the stratosphere or by human activity when we burn coal and coal has sulfur pollutants and we put sulfates into the atmosphere and these sulfates are tiny little particles which we call aerosols. They are often disastrous for you in health when you have those huge smog attacks in Beijing Beijing or in Indian cities in recent years. That’s aerosol pollution. Now, there are many many kinds of aerosols, unfortunately. There are so called white aerosols like sulfates, there are black aerosols like soot, there are organic aerosols that come from burning certain kinds of chemical compounds and certain kinds of biomass. They each have their effects.
The sulfates, the white aerosols, tend to dim the sunshine and they have a net cooling effect even though they’re very polluting There are even some very bad ideas called geoengineering ideas of putting sulfates deliberately into the air to dim the sunlight as a kind of remedy for our greenhouse gas emissions. Bad idea we’lll come back to it but in any event it’s out there.
Other kinds of these aerosols especially the soot tend to warm the atmosphere so we have to add up all these effects and then there are the direct effects of dimming the sunshine and what are called the indirect effects or the cloud albedo effects. When we put aerosols into the atmosphere, they change the cloud formation they sometimes provide the nucleus for clouds to form those clouds then reflect incoming solar radiation and thereby have a cooling effect on the planet so aerosoles not only diffuse sunlight and dim sunshine and cool the planet that way but they also change the cloud cover and can have an indirect cooling effect as well.
Well you see in this that the total aerosol effect of anthropogenic aerosols that’s human-caused aerosols is deemed to be cooling on balance but there are certain kinds of aerosols that are definitely warming kinds of aerosols. Well you can add up all of these and get the total anthropogenic effect that is the effect of the net energy balance caused by human activity but of course that’s not the only thing going on even on a relatively short scale we have changes though modest in the amount of incoming solar radiation. The sun has a natural cycle of more radiation or less; it’s a very small margin but it does have a very small effect on earth’s temperature.
We have long long cycles of changes of solar radiation that come from changes in the Earth’s orbit over tens of thousands of years that’s what gives us the long fluctuations of up and down of CO2 that we saw in an earlier lecture that are also part of the ice ages of the interglacial periods of the Pleistocene epoch and those are very long changes of dynamics so we have the human caused greenhouse gases, we have changes of the earth’s reflectance, we have pollutants that through aerosol effects both direct and indirect indirectly change the climate, we have changes in solar radiation in the short term solar cycles, and we have long-term changes that depend on earth’s orbit.
It’s a complicated story. it is the role of climate science to parse these various effects, to use advanced physics, both in theory and through many different kinds of observation, to create a graph like the one that you’re looking at that is able to add up across all these different factors to ask: what is the human effect? Now if we turn to the next graph, we can get overtime two crucial facts: first that the total amount of emissions is rising that we know because the world economy is growing, more fossil fuels are being used, more nitrous oxide and methane a fluorine-based gases are being used and emitted into the atmosphere.
We can also allocate the total net greenhouse effect across these different gases by using and CO2 equivalents multiplied by the number of molecules of each kind of greenhouse gas put into the atmosphere year by year. If you look all the way to the right hand side of this rising curve you can see the allocation of the total greenhouse effect according to key categories.
The big base in beige at the bottom is the carbon dioxide emitted through fuels and through other industrial processes there are a few industrial processes like cement manufacturing that emit CO2 not by burning fossil fuels but by other kinds of material transformation.
Then the next bar darker maroon color is the carbon dioxide that comes from deforestation and other land-use change and this is also notable but a lesser contribution of carbon dioxide emissions than the energy use.
Then the next major category of the greenhouse effect is methane emissions. Methane emissions come from many different kinds of industrial activities come from so-called fugitive gas that’s being released by drilling for gas and oil or being piped in pipelines. Methane is released by ruminant animals such as cows from the anaerobic digestion in their multiple stomachs. Methane is released from rice paddies again by anaerobic respiration of bacteria in the flooded rice paddies so humanity introduces a lot of methane. Methane next to carbon dioxide is the second most important of the greenhouse effects an then the next bar up in light blue is the nitrous oxide, the N2O.
That is again a side effect of combustion processes, of industrial activities, of chemical changes to nitrogen based fertilizers and other agricultural activities and then the small amount of about one to two percent of the total greenhouse effect are the fluorine based industrial chemicals used as refrigerants and for other industrial processes.
Take the total picture CO2 from energy & industry sixty-five percent add in the CO2 from land-use change that’s another 11 percent we’re up to 76 percent 3/4 of the total greenhouse effect. Add in methane and you’re at ninety-two percent of the total add in the nitrous oxide and basically you’re at about ninety eight to ninety-nine percent of the total.
Our focus in most of these lectures will be on carbon dioxide mainly on carbon dioxide from energy and industry but any real agreement that is meaningful next year in Paris at COP 21 is going to have to pay attention to all of the anthropogenic greenhouse gases. If we turn to the next graph, we get yet a different way to view this issue and that is by asking what sectors are the source of emissions of these various greenhouse gases and so we add up across all the greenhouse gases and ask what’s responsible for this and of the total emissions we can then allocate them through direct actions in various sectors and indirectly from electricity generation that is then used by these various end use activities so what are the end use activities that are shown here if you go around the circle starting at the top in green is something called AFOLU which is agriculture, forestry, and other land-use, thank you, and that is ll the emissions that come from the land use sector including from agricultural activity.
Now agriculture emits not only carbon dioxide through energy used in agriculture but emits methane from the bellies of our livestock and our ruminants and from rice paddies it emits nitrous oxide from chemical changes to urea and other nitrogen-based fertilizers it turns out that this AFOLU sector agriculture, forestry, meaning deforestation, and other land-use change is the single biggest sector of all in terms of anthropogenic greenhouse gas emissions if you go around counterclockwise this time you have the dark blue of the building sector and that means greenhouse gases released within buildings. What is that? Well, the direct emissions are from our boilers and furnaces and our stoves and gas, natural gas, cooking and so forth.
The next in red is the transport sector. Under the hood, what do almost all vehicles have? Internal combustion engines; a few now have batteries that are running electric motors but most of our vehicles until now over the last century have been internal combustion engine burning diesel or gasoline or other petroleum-based fuels and a few of these internal combustion engines burn biofuels as well but that transport sector which includes not only automobiles and trucks but also rail and shipping, ocean shipping and aviation, is a very substantial part of total emissions about 14 percent of the total greenhouse gases.
The industrial sector obviously a major emitter, major user of energy, a major transformer of materials, such as turning calcium carbonate to calcium oxide on the way to cement production thereby releasing carbon dioxide into the atmosphere. Iron and steel, pulp and paper petrochemicals–they’re all major emitters of carbon dioxide. It’s why the heavy industrial economies are in a way structurally major emitters of greenhouse gases. Now all of those shown around the circle until we get to the right hand side are direct emissions of carbon dioxide.
What’s called here the indirect CO2 emissions are the emissions that come from generating electricity at a power plant, meaning perhaps by burning coal or by burning natural gas and then that electricity is used in one of these other sectors. It can be used in industry, it can be used a little bit in transport like electric vehicles, it could be used of course in buildings to heat, cool, and ventilate buildings and so this is another piece of the action it’s the power sector you see the power sector is a big deal.
It is a major source of emissions decarbonizing the electricity generation will be one of the keys to decarbonizing the world economy and we see that the industry in the building sector are the two big users of electricity and therefore they are indirect, this is an indirect source of the emissions now I just love the next diagram I want you to go cross-eyed looking at it.
It’s a beautiful, artistic schematic of almost the same thing but is a way to track where these emissions come from on the right hand side are the greenhouse gas emissions with that big brown section being the CO2 and then just below it, the “F gases” and just below that the methane and then at the bottom the nitrous oxide and then if you go all the way to the right hand the left hand side of this diagram you ask what sectors are the sectors that are emitting these greenhouse gases found on the right hand side so it’s another way to go from a basic sector allocation to the greenhouse gas emissions and let’s just take an example.
Start at the top on the left hand side you have the transport sector and the transport sector is responsible for in this characterization of about a quarter of the total greenhouse gas emissions. Go to the middle and ask what kind of transport? Well, roads are most of it but then also air, rail, shipping, and other transport are also there and what kind of greenhouse gases does the transport sector emit? Almost entirely carbon dioxide the transport sector doesn’t emit methane; it doesn’t emit nitrous oxide much; it doesn’t emit any of the other fluorine-based gases and so in the end it’s a carbon dioxide emitter. The next big block is the energy sector for producing electricity and heat or for direct fuel combustion for boilers and furnaces and buildings or for combustion for industrial processes such as iron and steel production or petrochemical production.
If you go towards the bottom you have a Purple Line which is agriculture. What does agriculture do? Well agriculture is responsible for changes of nitrous oxide emissions for instance through fertilizer use. Agriculture includes livestock raising and the livestock as I’ve noted emit methane through digestive fermentation processes and the like.
This is a very detailed I think rather ingenious rendering of the complexity of where all of the greenhouse gases come from. In essence to get greenhouse gas concentrations under control, we are going to need to move logically and systematically across this graph.
What to do with transport what to do with agriculture what to do with the energy sector That’s the topic of lectures to come and if we finally just look at the spaghetti of the energy sector alone and in just one country we can see the remarkable complexity of different sources of energy and different uses of energy and so in this final graphical rendering made by Lawrence Livermore National Laboratory in the United States for the US energy system we are primary energy sources on the left hand side of this table at the base is petroleum you can see carrying along it goes mainly into the transport sector.
Then comes coal and if you track that along some goes to industry a lot goes to electricity generation and the like. We have many forms of primary energy. They are transformed partly into electricity for end use, partly through direct use up the primary energy in buildings or in automobiles or in industrial processes and then in the end, it is the carbon use in particular as opposed to alternatives like solar, nuclear, hydro, wind, and so forth that contribute to the greenhouse gas emissions.
A chart like this is extremely important because two-thirds of the total radiative forcing of anthropogenic greenhouse gases is carbon dioxide from the energy sector and therefore reforming the energy sector so that there’s more reliance on low or zero carbon sources and more energy efficiency will be absolutely central to our ability to stabilize carbon dioxide in the atmosphere in the next section we’re gonna talk about carbon dioxide more because it’s the relentless increase of human-induced carbon dioxide that is really at the core of the drama.