AP Environmental Science Notes 9.3 - Greenhouse Effect

hey everybody it's mr smeeds and welcome to video notes for topic 9. 3 which is the greenhouse effect our objective for the day is to be able to identify the most important greenhouse gases from an environmental standpoint but then also to be able to describe the difference in their potency or their ability to warm our climate so before we talk about the actual greenhouse effect we need to understand a little bit about solar radiation and the different things that can happen when the sun's rays are headed towards earth so first i want to point out that not all solar radiation will actually reach earth's surface so this diagram is really helpful for helping us understand again what are the different possibilities or the things that can happen with the radiation that's emitted by the sun so first of all about a quarter of all incoming radiation is going to be reflected right back out into space by the atmosphere and by the clouds and so we can see that demonstrated here again about a quarter of that incoming radiation will hit a cloud or hit some sort of gas molecule in the atmosphere and it'll just be bounced right back out into outer space and we have about 20 percent that's going to be absorbed by the atmosphere by clouds and when it's absorbed think of it as the energy kind of entering that cloud or that molecule it absorbing or taking it in momentarily and then it's going to radiate it right back out it's going to give it off and so we can kind of see that effect happening here where the radiation that's hit clouds again or gases in earth's atmosphere will take it in momentarily and then it will radiate some of it out to space and some of it back down to earth and then what's left of that incoming solar radiation is going to reach earth's surface and then there's two things that can happen when it hits earth's surface so it can hit earth's surface and be absorbed or it can hit earth's surface and be reflected and so if it's absorbed it's going to be given off as infrared radiation if we look at this portion right here we can kind of understand this idea solar radiation hits earth and it hits a surface with low albedo meaning that it absorbs a lot of it it will give it off as infrared radiation we'll be focusing more on this on the next slides as well but the amount of sunlight from earth that is either absorbed or reflected depends on albedo so remember that if we have darker low albedo surfaces so these could be things like dark deep ocean waters or exposed soil or blacktop if we're talking about the urban heat island effect they're going to absorb sunlight and then give off infrared radiation lighter albedo surfaces on the other hand which could be things like snow or polar ice caps they're going to reflect sunlight so they're not going to absorb very much and they're going to send those rays right back out into space and again if we look at our handy diagram here we can kind of see this idea that a high albedo surface will reflect light right back out into outer space and so it's going to leave earth's atmosphere so now we'll take a look at the greenhouse effect the greenhouse effect is this idea that there are gases in earth's atmosphere and those gases trap heat and radiate it or send it back down to earth now the greenhouse effect gets kind of a bad wrap in environmental circles sometimes but it's important to point out that without the greenhouse effect there would be no life on earth the planet would just be too cold we wouldn't be able to support water in a liquid state on much of earth's surface and so we wouldn't have any life on earth without it so it's really important and it's really critical the problem is that it's a really delicate balance and we'll talk about what i mean by that coming up here let's take a look at how it works though like i mentioned earlier you can kind of think of it as this blanket of gases around earth's surface but we have to understand first of all where does the heat that the greenhouse effect traps come from so the heat's going to come from the sun so if we take a look at this diagram there are solar radiation solar rays excuse me coming in and so that's going to be uv light it's going to be visible light and what happens is when it strikes or surface it's converted to infrared radiation now we can't see infrared radiation but we can experience it as warmth there is heat and so the earth's surface when it gets hit by sunlight is going to give off that infrared radiation if we look at this part of the diagram we can see these red light waves which represent infrared radiation they're radiating or coming off of earth's surface and this is where the greenhouse effect comes into play now the greenhouse gases just refer to gases that can absorb that radiation so here's a molecule co2 it absorbs infrared radiation it basically takes that energy on briefly for a second but then it's going to radiate it right back down towards earth's surface now it's important to point out that it also radiates out away from our surface but when we talk about the greenhouse effect what we're referring to is specifically the portion of infrared radiation which remember humans feel this as warmth or heat we can't see it but it's going to be radiated back down towards earth and that portion of the infrared radiation is what we refer to as the greenhouse effect so as a really quick recap because this is so critical visible and uv light comes in from the sun strikes earth's surface it changes to infrared radiation and earth's surface gives off that infrared radiation which we feel it as heat and that infiltration infrared radiation hits greenhouse gases carbon dioxide water actually is a greenhouse gas we'll talk about that shortly methane among some others and the portion that's radiated back down towards earth is what we refer to as the greenhouse effect and this is why earth is warm enough to support life but it's also why adding more greenhouse gases can make earth a little bit too warm so we'll get into that here shortly now we'll take a look at different greenhouse gases and their sources so we have a diagram here the same one as before to help us remember what those greenhouse gases do the first one we'll talk about is carbon dioxide carbon dioxide is probably the most critical because it's being admitted in by far the greatest amount but we'll talk about how it actually kind of serves as a benchmark or the gas by which we measure all other greenhouse gases relatively here on a second major sources of carbon dioxide are fossil fuel combustion of course decomposition of organic matter gives off some carbon dioxide deforestation is going to lead to net carbon dioxide release remember that trees can't sequester carbon dioxide after we cut them down then we have methane so methane comes from natural gas extraction it comes from natural gas combustion it's also going to come from something called anaerobic decomposition remember that's the breakdown of organic matter especially in low oxygen conditions like underwater so bogs and wetlands can give off some natural methane through their decomposition but especially in the permafrost as well so that's this permanently frozen area you know really high latitudes on earth and so that's going to basically lead to the breakdown of organic matter underneath pools of water as the permafrost thaws so that's a big methane contributor then we have nitrous oxide so don't confuse this with nox it's different this is not nitrogen oxide this is nitrous oxide so that's n2o remember the big source globally about three quarters of all nitrous oxide emissions come from the management of agricultural soils so what do we mean by the management of agricultural soils what that really means is basically adding nitrate fertilizers to soils and then the natural denitrification that happens especially in over-fertilized and over-watered soils so again in those anaerobic conditions nitrifying bacteria are going to take those excess nitrates and convert them in some cases into n2 but in other cases into nitrous oxide and so that is the main contributor globally to nitrous oxide then we have cfcs so think back to 9. 1 and 9.

2 where we talked about cfcs and really the entire class of compounds that they encompass they're going to be greenhouse gases as well then one final thing that we need to point out here is that water is by definition a greenhouse gas so water absorbs infrared radiation when it's in the atmosphere and it emits it back down to earth but it's not a greenhouse gas that we're particularly concerned about and here's why with all of these other greenhouse gases carbon dioxide methane cfcs and nitrous oxide they are non-condensable so what that means is as the temperature and earth's atmosphere changes they don't condense into liquid and leave the atmosphere but water does so remember that water is dependent or i should say the amount of water the atmosphere holds is dependent on a temperature not the other way around so again what i mean by that is as earth gets cooler as the atmosphere gets cooler as you rise in the troposphere it's eventually going to get too cool to support more water vapor that water vapor is going to condense and fall down to earth so we can't just pump more and more water into the atmosphere because water in the atmosphere is dependent on its temperature whereas all these other gases as you add more and more of them to the atmosphere they stay there and they influence earth's atmospheric temperature not the other way around so one final time with respect to water here the earth's atmospheric temperature influences the amount of water it holds not the other way around with carbon dioxide nitrous oxide cfcs and methane they influence earth's atmospheric temperature you add more of them they don't condense and leave they stay there and they increase earth's atmospheric temperature all right and the final thing we'll talk about today is the potency of each greenhouse gas or what we call sometimes its global warming potential potential or it's gwp so this is basically a measure of how much a given molecule of gas could contribute to the warming of earth's climate or atmosphere specifically over a 100 year period now there's something important to point out here it's all relative to co2 so we've basically taken the global warming potential of co2 set it equal to one and then we're measuring every other greenhouse gas as a fraction or really as a multiplier i should say of carbon dioxide and so global warming potential really comes down to two factors or two characteristics of a gas the first one is residence time so how long does it stay in the atmosphere and the second one is infrared radiation how good is it at absorbing infrared radiation and re-emitting that radiation back down to earth different molecules have different structures due to the way that their atoms bond and so this is going to give rise to this property where again some molecules absorb a lot of infrared radiation and radiate it back down to earth and some don't absorb nearly as much so let's take a look at each of these four major greenhouse gases now so remember that we have carbon dioxide and we're going to set its greenhouse or its global warming potential equal to one so there's not really anything to dive into here we just should know that carbon dioxide is equal to one next we'll take a look at methane which remember ch4 methane stays in the atmosphere for around 12 years so that would be its residence time and then it's going to absorb a fair deal more infrared radiation than carbon dioxide and so depending on the time span you measure it methane can have anywhere from a 23 times the global warming potential to 84 times the global warming potential of carbon dioxide so remember what that means is that one molecule of methane would warm the earth about 23 times as much as one molecule of carbon dioxide over a 100 year period next we have nitrous oxide nitrous oxide is going to have a pretty long residence time which is on average 115 years and so that's going to contribute to its its global warming potential but we also have the fact that it absorbs a lot more infrared radiation than carbon dioxide and so we give it a global warming potential of 300. so again that means one molecule of nitrous oxide we would expect to warm earth's atmosphere about 300 times as much as one molecule of carbon dioxide and then finally we have cfcs and cfcs are going to be really greatly ranging in their global warming potential because they are so diverse as a class but what we should know is that they stay in the atmosphere for 50 to 500 years so they can have an extremely long residence time and their global warming potential because they absorb so much infrared radiation is going to be anywhere from 1 600 to 13 000 and so you don't need to memorize these numbers but you do need to be aware of how much more warming a molecule of cfcs would be compared to a molecule of carbon dioxide and finally for our practice frq for 9.