The Science of Climate Change - Helen Johnson

[00:00:00] Speaker 1: It's probably worth saying from the start that I'm a climate scientist rather than a climate change scientist so I'm interested in the way the climate system works and quite frankly I'd be interested in the way the climate system works even if we weren't messing with it you know the climate system is an interesting and interesting beast with lots of complex interactions that affects the way we live but of course there's the backdrop of human influence on climate which is a major motivation for me so I'd say that this doesn't represent what I research but it's there in the background all the time and we have Miles here who of course is more closely tied to this in a research sense if we get any really tricky questions. Do feel free to stop me anytime I'm quite happy to be interrupted I'd rather things were clear and especially as it's quite diverse group and it's possible I'll have made assumptions about what some of you know and equally I don't want to passionate you so tell me if it's all old news then I'll move off. My first slide is blatantly stolen from some of Miles' lecture notes as are many those who sit in geography will perhaps know but I think you know for the people who are writing up the summary this might be a good start it's even got the same name climate science 101 as to your course handbook yes surface well mean surface temperatures are warming yes it's us we're sure it's us hopefully I'll show you these things today these are really significant changes that we're talking about in geological context and then hopefully end on a bit more of a positive note about you know the prospects for fixing it in a purely scientific sense so here's roughly where I'm going then so the first third of the talk will be some basic climate physics how do rising greenhouse gas concentrations cause warming then we'll have section on global warming today try and show you how we know that human influence is the dominant cause of the recent warming over the last few decades and then we'll move to the future and talk about what science suggests in terms of mitigation what we expect and what science suggests we can do with it so you have to bear with me here you have one slide of pure physics which I think is all the physics the pure physics you really need to know to understand this problem basic physics and that's that the amount of radiative energy that anything commits everything above absolute zero above zero Kelvin is emitting radiative energy and the amount of energy that that thing emits and the wavelength at which it's emitted depends on temperature and that's the key physics that underpins all this and so there's this graph over here so we have intensity of radiation on the y-axis and wavelength on the x-axis and each of the curved lines shows you the radiation that energy emitted at a given temperature and these are the temperatures in Kelvin and you can see that as the temperature increases not only do we emit more radiative energy it also happens at a lower wavelength okay and so if you understand that then that's that's the key to understanding the the greenhouse effect the total amount of radiation emitted depends on the constant times temperature to the power of four the sun's surface temperature is about 6,000 Kelvin so the sun emits radiation that's very much what we call shortwave radiation so it occurs in the visible part of the electromagnetic spectrum is everyone familiar with an electromagnetic spectrum like electromagnetic radiation which spans very short wave length waves gamma rays right through to long wavelength radio waves and the sun emits in the visible part of the spectrum and that's no coincidence and we've adapted to for our eyes to work in that part of the spectrum a little bit in the ultraviolet and a little bit in the infrared okay and that's because of where of its temperature where it sits on this curve okay and the earth is bathed in that radiation okay and the earth as a system absorbs much of that radiation some of it is reflected a lot of it is absorbed and the earth as a consequence warms up okay and the earth as a consequence warms up okay and the earth as a consequence warms up okay and the earth as a system as a system warms up okay and the earth as a system warms up okay and the earth has to be emitting as much energy back to space as it's receiving from the sun okay and so it warms up until it reaches temperature where the radiation that it's emitted is the same as what's coming in from the sun okay so sigma t to the four okay warms up till sigma t to the four is equal to the basically is equal to the basically is equal to the the radiation coming in okay and we can work out the temperature that that that that it has to warm to to do that and that's 255 kelvin or about minus 18 degrees celsius okay so that's what we call the planetary emission temperature okay that's the temperature that the planet has to be at to stay in equilibrium or that the planet as a whole has to radiate energy at in order for us to stay in equilibrium now that's noticeably less than the surface temperature of the planet and that's because the radiation that's emitted to space is not coming from the surface okay it's coming from somewhere high up in the atmosphere okay so to understand why the radiation that's emitted to space from the earth system is coming from high up in the atmosphere and not from the surface of the planet you need to understand something about the atmosphere and this is where greenhouse gases come in and this is where the solar radiation is emitted in the atmosphere and this is where the solar radiation is emitted in the visible part of the spectrum this is a um the electromagnetic magnetic spectrum ultraviolet visible and infrared radiation we're going to go this way um so a warm body like the sun emits here and a colder body like the earth at 255 kelvin emits in a very different part of that spectrum okay emits less radiation that's not clear from these curves at the top here they've been normalized so that we can see them together there's a lot less radiation emitted by the earth because it's much cooler than the sun but it's also emitted a much longer wavelength so we talk about short wave radiation emitted by the sun and long wave radiation emitted by the earth and the important thing on this figure is this panel here okay which tells you the fraction of the radiation at each wavelength that's absorbed on a single pass through the atmosphere so that might be short wave radiation coming down or it might be long wave radiation coming up from the earth and it shows you um how much is absorbed and there's a few key things to notice the first key thing to notice is that the atmosphere is really really um quite transparent in the visible part of the spectrum so the sun's radiation is able to pass through the atmosphere relatively unhindered in the visible part of the spectrum in the ultraviolet the atmosphere is really opaque okay so the sun's radiation in the ultraviolet in the ultraviolet in the ultraviolet in the ultraviolet in the ultraviolet in the ultraviolet part of the spectrum really doesn't get very far at all and then in the infrared the atmosphere has very variable um opacity at some wavelengths it's completely opaque to long wave radiation and in others it's completely transparent okay the lines at the bottom show you what it is that's doing the dominant um absorbing at the different uh wavelengths and the first thing to notice about this is what's missing from this list 78% of the atmosphere is nitrogen that doesn't feature in this list 20% of the atmosphere is oxygen we're up to 98% now and oxygen absorbs only in the ultraviolet down here the solar ultraviolet light high up in the atmosphere and so the dominant components of the atmosphere that make up 98% of the atmospheric composition play no role in this absorption of radiation at all what's doing it is minor constituents in the atmosphere trace gases if you like um like carbon dioxide ozone methane and water vapour okay and these are what we call the greenhouse gases many of them many of them tri-atomic molecules they've got three atoms in them and that gives them rotational and vibrational modes that that make it easy for them to be excited by infrared radiation so they can vibrate faster if you like um they can easily absorb take on energy um at um infrared wavelengths okay so that's just a summary of uh of what i've said there um so what that means for the for the atmosphere this is the um a kind of recent steady state global and annual mean energy balance the numbers here are in watts per meter squares so energy per unit time and area um what it means basically is that the incoming solar radiation from the sun while some of it is reflected okay from clouds and from the earth's surface very little bit is actually absorbed by the atmosphere okay the bit that is absorbed is absorbed by um oxygen very high in the atmosphere in contrast the um in contrast the um radiation from the earth's surface is almost all absorbed by the atmosphere except for in very narrow wavelength bands we call the atmospheric window um at almost wavelengths all of the energy emitted from the sun's surface is absorbed by the atmosphere in a relatively thin layer of the atmosphere actually um and then re-emitted uh up and down to be absorbed again by another layer of the atmosphere re-emitted up and down absorbed again by a higher layer of the atmosphere such that only the only radiation that can escape back into space is radiation that's emitted from so high up in the atmosphere that there's not much atmosphere above that level there's not much greenhouse gases to actually do the absorbing above that level the atmosphere gets thinner as you go up we'll just look at that in a second and so as you get as the as you get higher there's less and less um concentration of uh greenhouse gases and eventually you get to an altitude where there's not enough greenhouse gas above you to reabsorb that radiation that's emitted and that's where radiation escapes to space from and that's the level where the temperature has to be 255 kelvin so as a consequence when we look at um the planets in the infrared wavelengths what we see is something like this okay we don't see the surface we see somewhere several kilometers up in the atmosphere and then just just to to go through this again and make this perhaps a little bit more rigorous this curve has got this graph has got two separate bits of information on it one is the temperature structure in the atmosphere so this is height on the y-axis um and on the up here we've got temperature marks and so as you go up through the atmosphere through the lower 10 or 15 kilometers of the atmosphere the temperature drops off linearly with height okay for good physical reasons that we understand the other curves here show you the concentration of any absorber any gas like co2 above each particular altitude so if you just focus on the dotted line for now okay so if we know that radiation can only escape to space from an altitude where there's sufficiently thin atmosphere above okay we might say okay radiation can escape to space if we have less than 0.5 on this axis and so it can escape to space when we get to a height of about eight kilometers okay at that height the temperature of the atmosphere is 255 minus 18 degrees okay is everyone okay with that provided the temperature at that height is 255 kelvin then the radiation emitted that's getting out to space is equal to the solar radiation that's coming in and we have a balanced system okay but what we do when we add carbon dioxide or other greenhouse gases to the atmosphere is we increase the concentration of the absorber throughout the whole atmosphere so you can imagine stepping up to this dashed curve here okay so it still decreases with height but what it's done is it's pushed up the level from which radiation can escape to space so if we're still looking for this tick here i should have drawn some lines on here then it's pushed up the level from which radiation can escape to space so it's now happening at a higher altitude and at that higher altitude because of this decline in temperature with height the temperature is colder and if you remember the amount of radiation that's emitted sigma t to the four is now less because we're a colder temperature so we've got the same amount of solar radiation coming in but our emission back to space from the planet as a whole is happening higher in the atmosphere where it's colder and so there's less of it and that gives us an imbalance less outgoing radiation than we have solar radiation net solar radiation coming in a bit after we've had some reflected okay so what that means is that we've got more energy coming in than going out and the planet is is destined to to warm so i'm going to um show you that slightly differently this is another of miles's lovely um schematics this graph shows you what happens to the height that radiation escapes to space from as we increase carbon dioxide in the atmosphere and so there's a red line at the top that shows you the total amount of energy escaping to space and there's a red you'll see some red lines appearing here that shows you the energy escaping to space from each level and so as we increase carbon dioxide concentrations you can see that the levels from which things are reaching space move up through the atmosphere and the total goes down okay so to summarize again high cold air radiates less energy to space um generating a surplus of incoming over outgoing radiation this is what we call a radiative forcing okay so i've summarized that i'll make the notes available so you've got that there can we go back helen to the [00:15:33] Speaker 2: yes way the the spectra just because it's uh this is a nice explanation to draw attention to one of the sort of skeptic arguments that's often thrown out the skeptic the other one before then no one before then there we go yeah so um i just just wanted to sort of draw attention to something which people if you look at this one which people are often fond of pointing out well look the water vapor is interrupting everything where the carbon dioxide is absorbing you see how where the carbon dioxide is doing its absorption there's also a lot of absorption by water vapor so this if you go on blogs and ask if you google the carbon dioxide you'll find helen we were just chatting beforehand it's remarkable how quickly you get from sort of like normal useful information into complete garbage if you google anything related to climate change anyway so so one of the favorite lines is well the water vapor is absorbing everything so it doesn't mean you don't need to be adding more carbon dioxide because it's a fake anyway and if you look at that diagram you see that there's it looks like there's some there's something in there but the reason for that is that this is looking at the whole depth of the atmosphere but the story home has just told you is about what's going on up high in the atmosphere which is well above where all water vapor is so again it's just one of those things where yeah um it's sort of it's it's one of those lines of argument that people quite often like to throw around just to sort of chuck a little bit of grit in the wheels and yeah you feel a bit confused about this um and uh and it's it's not helped by the fact that actually the sort of conventional greenhouse analogy people get taught in schools is actually wrong um and uh and so it sort of makes it easy to sort of mess around with people's understanding of the issue in this way yeah [00:17:22] Speaker 1: i think it's true that this is it seven seven and ten centimeters of water will absorb all the outgoing long wave radiation so if you have you know so first if you have a bag of water that's got seven centimeters thickness of water in and you shine in infrared camera through it you can't see the other side that means if you've got cumulative seven centimeters of water in your column of air above you then that outgoing long wave radiation can't make it through it's absorbed again that's actually satisfied in the bottom a few kilometers of the atmosphere usually yeah but as myles says that's that doesn't change any of the argument for higher up in the atmosphere i have a slide on the saturation argument in a minute so yeah yeah so is water vapor trapping then is that yes water vapor is trapping as well yeah yeah okay let's carry on um where did we get to okay so um so we said that as we lifted the level from which radiation is escaping to space we've got less outgoing radiation than we have incoming radiation from the sun and that's given us a positive radiative forcing what we call a positive rate of forcing so surplus of energy which is going to warm the planet up and that's going to continue until the temperature at the new emission level if you like is back at 255 kelvin so what we're emitting to space is again the same as what's coming in from the sun and because the temperature the rate at which temperature changes um through the lower part of the atmosphere the lower 10 15 kilometers is constant and we know roughly what that is in order for the temperature up here to increase then the temperature at the surface has to increase too okay questions no happy so this is making a point similar to um to miles's point um so this is so felix suggests no it's fine that's fine that's fine thank you no please do dive in um so felix suggested that we try and do a bit of debunking of myths as we go through um and so this you know miles has done one and this is a similar argument that the atmosphere is saturated either with co2 or water vapor or whatever you like in some wavelength bands and adding additional co2 won't really have any major input uh impact and but hopefully you can see from my argument that there's always scope to push up that level um at which uh the emission to space is is happening because the carbon dioxide concentrations are always going to fall off with altitude you know they're dependent on the density of the atmosphere which decreases exponentially as we go up in the atmosphere and so um so certainly not saturated in the lots of things the density of the atmosphere but also the concentration of all these gases that we're talking about the carbon dioxide [00:20:29] Speaker ?: yeah [00:20:33] Speaker 1: and because of this you know exponential decrease in the concentration of the greenhouse gases and this linear um decrease in temperature with height then every doubling of co2 has the same impact on the radiative forcing this imbalance at the top of the atmosphere as every other doubling of co2 okay so wherever you start from you double co2 you have the same impact on the range to forcing and that's about four um watts per meter squared 3.7 watts per meter squared um that's relatively well pinned down okay what's a lot less well pinned down is the amount by which the climate system will warm up okay and that's due to feedbacks in the response okay so as we start to increase greenhouse gas concentrations we start to warm the planet lots of other things change too which also affect that radiation budget so we've already talked a little bit about water vapor any um physical scientists amongst you will know that a warmer atmosphere can hold more moisture and so there's a feedback right away right there um we also changed the amount and the brightness of clouds which changed the reflectivity of the um of the earth and consequently the amount of solar radiation that's reflected um and changed the surface albedo so the reflectivity of the surface itself and also this rate at which temperature decreases with height we've said that's relatively well known and and fixed um uh it's possible that that will change too it's called that the lapse rate that that will will change too and so there are feedbacks in the system uh which mean that well we know um what to expect in terms of the imbalance in radiation at the top of the atmosphere we don't know what to expect in terms of the surface temperature response okay even the global mean surface temperature response so what we call the equilibrium climate sensitivity the long-term response to a doubling in carbon dioxide after which you hold the concentrations of carbon dioxide constant for a long time um estimates of climate sensitivity is quite a broad range kind of 1.5 to 6 um calvin roughly here's the um figure from the intergovernmental panel on climate change report in 2013 and i know there's been other estimates since but i don't think they changed the [00:22:55] Speaker 2: story particularly well there was an estimate published today in literature i believe um uh or was it what those what they did yes it was today um which actually gave it sort of right down the middle and i just thought it was quite amusing that this was heading in the times as um alarmist predictions about climate change shown to be wrong because of course it had this new estimate had trimmed down the top end of the range it had also trimmed down by an equal amount the bottom end of the range but for some reason the times didn't [00:23:30] Speaker 1: um okay um so uh it might be worth you might think that you know we know how much carbon dioxide we've emitted into the atmosphere and we know roughly speaking how much the planet is warm so one question is why we don't know the climate sensitivity why can't we just work it out um from what's happened so far and that is actually reasonably um complicated answer to that but the simple story is that there's a delayed response um because the heat doesn't ultimately stay in the atmosphere the extra the extra heat in the earth system most of it ends up in the ocean and transferring it into the deep ocean actually takes a long time it's not to say that it takes a long time for the temperatures at the surface to change this may be offset by other feedbacks but it does make it difficult to estimate the climate sensitivity we don't understand the details of the feedbacks enough and how they interact with each other and how how um how much of the warming that we how much of the warming we should expect we've already encountered and so it's uh estimating the climate sensitivity is a tricky thing to do okay so what do we know then so um we know that atmospheric carbon dioxide concentrations are rising so the latest value on the um website that reports the the more and lower observations in hawaii so these are made at the top of the volcano in hawaii um it's 408 parts per million you can see the time series here from the 60s um and we know uh what's causing that okay we know it's not volcanoes there's another myth debunked volcanoes produce more co2 than the world's cars and industries combined um volcanoes produce i think it's it's less than 0.5 gigatons of carbon per year we we produce 10 gigatons of carbon per year at the moment and you can't as you look at the calm dioxide concentrations increasing you don't see any influence of these major um volcanic eruptions in that time series of carbon dioxide and so um so it's [00:25:35] Speaker 2: certainly not volcanoes i feel strongly about them because my my eight-year-old kid came home from school and told me that oh yeah dad no i heard that actually and i was like if an eight-year-old is being told you this i found that worrying anyway very worrying [00:25:54] Speaker 1: um so where is it coming from uh so it's coming from fossil fuel burning as we all know um so this using this point this um breaks it down in terms of um coal oil gas and cement uh but mostly from the what jumps out from this figure is the rapid increase um you know especially since the kind of 50s um 40s 50s uh you may see that huge um steep curve there's also a contribution that comes from land use chain shown in the yellow here that's things like deforestation um yeah and then on the bottom here this shows you where that carbon dioxide ultimately ends up so the previous graph was where the heat in the system ends up but this is actually the carbon dioxide okay and you can see that you know some of it goes into the ocean you can see the dark blue um part portion here and the green is parts that are ultimately locked up on land but what's left in the atmosphere is about 50 percent of what we emit um yeah so these carbon dioxide uh changes are large in the you know the grand scheme of things i sit in an earth sciences department i work very much on um modern day processes but i'm surrounded by people who worry about um the much longer um time frame um so this shows you the um co2 concentration in the atmosphere has been measured from bubbles in uh ice cores uh taken from the greenland and antarctic ice sheets um over many glacial cycles so during um glacial periods it's cold and we have low carbon dioxide concentrations you know 180 200 during warm interglacials um it's warm and we have um 260 280 so there's a range there of 100 or so over glacial cycles which take place over you know tens of thousands of years you can see this goes back 800 000 years and here's where we are today up at 400 so we're well outside and i think in this recent um glacial interglacial cycle we're also um starting to look special in terms of the longer climate record so here we are 408 um increasing quickly and you can see that um you know certainly when we get to kind of 500 600 700 we're um uh it's starting to look like 20 million years ago that we had that we know that we had carbon dioxide levels that uh high um the earth's climate does vary on all timescales it has long natural rhythms you know you've we've seen some the glacial interglacial cycles on the previous um slide and even longer rhythms but um we're certainly uh seeing something that's unusual in geological terms and is happening very quickly compared to geological changes and what would have been the geological cause for that like that kind of similar level 20 million years ago would it have been just like the interglacial uh i don't know that far back i'm more comfortable on the glacial interglacial cycles but even there we don't understand we we can't sorry we can't um fully explain this change of about 80 to 100 uh parts per million over the glacial interglacial cycle we don't know there are lots of contenders but we don't know um concretely what causes that change yet and i'm not sure do you know about the longer time [00:29:28] Speaker 2: uh on on the on the very long on on the glacial cycles the language cycles play a big role but and they're part of those no on these very long time scales it's subduction into the earth's crust because that's the only place it can go and to store it away and and i think i mean this is where your colleagues would know but i think the big transition sort of 30 whatever it is 20 something years ago was a reconfiguration of the continents which meant that um a lot of carbon which was in the atmosphere then got squirreled away down back into the earth's crust it's not the formation it's not when the fossil carbon was formed that was way before then back in the carboniferous um so that this so so that carbon got subducted down into the earth's crust i guess in the form of [00:30:19] Speaker 1: some sediments carbonates yeah so generally speaking the the carbon dioxide that comes from volcanoes into the atmosphere is balanced by carbon dioxide returning into the earth's interior at subduction zones where the sea floor becomes subducted into the interior and the sea floor has often has a layer of carbonate sediments on it which contain a lot of that carbon and that's in balance and generally but [00:30:43] Speaker 3: there are times when that balance changes um the main theory for why the carbon dioxide concentration was so rapidly reduced about 30 million years ago was um when india crashed into eurasia and formed himalayas and then you had a lot of fresh rock being made which caused very rapid ruthering of the rock in which the carbon dioxide was incorporated into the rock so it's something as big as the formation of [00:31:06] Speaker 2: himalayas but the crashing makes it sound i mean you should bear in mind this didn't happen overnight and and that's why it's so different from what's going on at the moment and we are talking about that i don't know how fast that crash happened but um it's a lot slower than what's going on right now okay sorry one question about the graph what is represented by the vertical width of the blue is [00:31:35] Speaker 1: that uncertainty yeah like physical flash it's uncertainties yeah all of the vertical bars are uncertainties i'm not sure actually what the blue is the blue's modeling i believe isn't that yeah [00:31:47] Speaker 2: doesn't say actually um okay i think it's a it's a that's a range of models of what's going on but [00:32:00] Speaker 1: obviously it gets less well pinned back the pill down as we go back in time okay so um so condo upside variations are not new for the planet but um we are um but the changes in the recent um century have been very significant um and i think the change of the speed is new yes i think [00:32:23] Speaker 2: it's very hard to imagine a mechanism that would make a change of the stocks yeah [00:32:29] Speaker 1: yeah um this is a graph i imagine many of you have seen before so this takes the same record of carbon dioxide now just over 450 000 years and um plots alongside it temperature change measured over that same period so this uh temperature is measured using the oxygen isotopes in the water that's melted from the ice core data so drill far enough down under greenland the ice at the bottom is really old melt it and look at the oxygen atoms in the water molecules and you can back out the temperature at which the temperature of the atmosphere when that water precipitated out and it's quite an integral measure so um yeah and you can see that co2 levels and temperature have been very closely coupled at least over this record and we have no reason to think that that didn't continue further back in time which you know is a large part of the evidence of the link between uh carbon dioxide and temperature but um it's not everything um so there's also direct evidence that recent changes in the greenhouse gases in the atmosphere are affecting the radiation budget so this is a uh a figure that shows the difference in outgoing long wave radiation um during two periods in 1970 and 1997 as a place in the mid-pacific so this is wavelength on the x-axis and brightness temperature up here if you look at the difference there's a difference in part because the planet has warmed over those 30 years but if you take that away then you can look at the change in the outgoing long wave radiation just due to the composition of the atmosphere and that's what's shown here and you can see that in general the um the brightness temperature is lower and that that's because the radiation is emitted from a level higher up in the atmosphere remember where the temperature is lower and consequently the radiation is smaller the outgoing long wave radiation is smaller so lower by about two degrees two degrees up that temperature curve in the wavelengths that are affected by carbon dioxide you can also see methane on here ozone some of the cfc's so direct evidence that things we've put in the atmosphere um have influenced the outgoing long wave radiation so it's not all about um a paleo story of carbon dioxide okay so given that that that evidence um why do we still need to explain the um the evidence for human influence on climates well because of um people like this um so scott pruett is head of the u.s environmental protection agency and when asked the question do you believe it's been proven that co2 is the prime we can told not for climate um replies i would not agree that it's a primary contributor to the global warming that we see and so people in immense positions of power and responsibility that still don't um understand the the problem and so um yeah so i'm gonna lay out following miles's lead in his lectures um i'm gonna lay out a very simple um test i guess of the kind of no hypothesis that's that's implicit in this question the idea that the recent warming has nothing at all to do with the human emissions of carbon dioxide i'm going to test that hypothesis and see if we can reject it this is something that miles amongst others uh have been doing for um decades and continue to do um regularly evidently not very successful so um everyone agrees the world's warm since 1900 uh this is the first time maybe not everyone but at [00:36:21] Speaker 2: least scottberg doesn't most people right no i know but you'll find us on the internet but i mean okay [00:36:28] Speaker 1: so there is uncertainty in how much uh you know these numbers are not certain everyone sorry everyone everyone's serious agrees even people are moderately yeah yeah so there is i mean joking aside there is uncertainty in how much the world has warmed and in the details of this graph but it's not um yeah not not enough to change the story significantly um we also um know i've just shown you some evidence for that that uh human activities have been disturbing the global energy balance this balance between incoming and outgoing radiation um at the top of the atmosphere um you can see that here this is the radiative forcing if you like the top of the atmosphere imbalance in orange the human um contribution to that we also know that natural factors have also disturbed that balance uh over the last century so there's long and short-term solar variability more and less radiation coming from the sun in the first place shown in the in the green lines here and then um volcanic activity now it may be confusing to look at this and to see that um volcanoes actually um look like from this like they cool the climate they have the opposite uh we've just talked about volcanoes emitting carbon dioxide okay and volcanoes impact climate in two ways they do emit carbon dioxide largely balanced by that in subduction zones as we've seen but also they emit sulfate aerosols which you know up in the stratosphere influence clouds and radiation and the dominant effect is a short-term one that involves a negative radiative forcing okay so so we know the planet's warming and we know that um human and natural activities are modifying that top of the atmosphere balance um we also know that the climate system takes a while to respond to things it conserves energy and so things like these short volcanic events um are expected response to them is a little longer term um uh yeah what we don't know is the magnitude of the response to these various forcings i explained earlier that various feedbacks in the system mean that it's it's not clear exactly how um how the surface temperature responds to each of these different forcings okay so it's just the shape of the curves here you should be focused on at this stage we can dial them up or down because because we don't know what the constant if you like multiplying each of these forcings is okay and so one way forward is to try and estimate them objectively from the data okay so to try and work out um so for example um we could multiply the solar forcings by 10 um and take it up there um so to see if we allow you know we don't understand the mechanisms by which this would happen but if we allow the natural um climate drivers to have a big response on the surface temperature and initially assume that um human carbon dioxide emissions has um no influence on the temperature can we explain this temperature increase so without recourse to the orange line initially can we fit those data just um using a kind of least squares fit if you try and do that um allowing for any scaling of the volcanoes and the solar forcing this is what you end up with as a kind of best fit explanation of the observed warming um you can see that it's not terrible for the first part of the record in fact if you drew a line in 1980 maybe you might say there's something in that maybe uh maybe it is solar variation but you can clearly see that you know since 1980 the sun seems to have gone into a bit of a decline and the um yet the surface temperatures have continued to rise and so that so there's a lot of variability now that's not explained by this best fit line what really makes it um clear that uh we really do need to include some human induced uh warming into the system is the the resist the residual so the plotted in black here is the residual that's not so the difference between the black curve and the red curve okay and it's plotted alongside what we expect from um the human induced uh warming and you can see that the residual fit here looks very suspiciously like the um human forcing okay so it sort of takes off in the 1970s uh when uh co2 emissions really take off um so what we're going to do now is shown by this little blue diamond here so we're going to gradually dial up the amount of human influence warming we allow in this fit and see if we can do a better job of explaining that observed temperature trend okay so we start with about point i'm not going to explain the units miles to us here um but uh pruitt is 0.1 degrees yeah i don't think anybody else uses this unit good um i don't think we should dignify the name okay so if we start with about 0.1 degrees of human induced warming um since the pre-industrial era then we do slightly better and you can see that as we increase up in 0.1 increments you can see that not only do we fit a bit better but the residual comes down um we keep going by the time we get up to um one degree of human induced warming you can see we've clearly gone too far there's now a negative residual it's unexplained and we can oh sorry we keep going until we find the best fit which um with no unexplained residual that suspiciously resembles human induced warming um lies about 0.8 degrees c of um co2 induced warming to date okay so the key the key take home message from that is you know the best explanation of the observed global mean surface temperature record is that carbon dioxide emissions from human activity have contributed about 80 percent of the observed warming since 1870 now we can try and explain the observed warming with natural factors alone but even allowing for any amount of amplification of the response to solar and volcanic activity leaves an unexplained residual that's very suspiciously well correlated with the expected response to human activity as i said you know this so this hypothesis null hypothesis that humans have nothing to do with the rising temperature can be comfortably rejected was decades ago still is in eci they still do this regularly so you can go to the globalwarmingindex.org web page and get an updated version of this kind of fit and attribution you can see that you know updated as of september 2017 we've had about um about one degree of human induced change [00:43:50] Speaker 2: just where the extra point two is other greenhouse gases sort of native other stuff so so the 80 co2 and then if you include all the methane and all the other greenhouse gases that's what takes it up to watch [00:44:02] Speaker 1: yeah and then i thought just um just to make it clear that we don't just do this with statistical fitting which hasn't really involved any climate models and that it's not only relevant for global mean temperature should include this figure from the ipcc which basically shows a similar kind of thing so the black lines here are observed trends in the white ones are sea ice extents the blue ones are operation heat content and the yellow ones are land surface temperature in various regions and and the blue bands in each one are climate models run with only natural forcings and the pink bands the climate models run with natural and anthropogenic forcings and you can see that almost without exception it's only when you include the anthropogenic forcing alongside the natural forcings that you can explain the trends in in various features of regional climate as well as this global mean temperature okay so moving on to the future what about future change and what does science say we should do about it okay so sort of sort of aka can we meet the goals of the paris climate agreement so i thought i put this in just because i'm sure it's something you've all seen but i thought we should have some projection um in here um so this is surface temperature projections from the ipcc report which of course is your go-to resource for consensus view on um all of these things um so this is the global average surface temperature changes there's a slightly different baseline of where zero is but you can see that for a business as usual scenario that's shown in like a pink here um the temperature change between now and the end of the um century you know it's it's up um you know four degrees here you can see in most likely four degrees but quite a range but even the lower parts of that range are comfortably above two degrees above pre-industrial um yeah and i don't want to talk much about regional change because i know that's where you're going in the next few lectures um into impacts but i thought i would put these up so this is in the 2.6 scenario so this kind of you know drastic action scenario and the um business as usual scenario and it shows you that the world really is warming everywhere you know global means hide a lot of structure but the world really is warming everywhere and that climate change will mean different things to different people um uh yeah and that four degrees whilst undeniably important for everybody will be more important for some than others um so the situation before the paris meeting um was that we uh were committed to uh limit the increase in global average temperature to two degrees c um that's a target that was banded around so it's interesting to think a bit about where that target comes from and as far as i can tell it was a um an economist actually at yale um called willham north house in 1977 first suggested that we might want to keep um temperature the temperature of global mean temperature within a range um not too far out of what's the normal range of variation uh within uh the climate system and he can sort of define normal as um you know not beyond something that we've experienced in the last few hundred thousand years okay so suggested that if you know one one degree is about where fluctuations sit if you get up to two degrees they're more comfortably out of anything that humans have experienced um yeah for a very long time okay so that was the situation in terms of temperature targets before paris and um key um strategy for getting there was this idea of contraction and convergence so the idea that whilst um greenhouse gas emissions need to be scaled back there should be um a delay in the requirement for the developing world to do that and that we should converge on a common amount of carbon emissions per person um to make this an equitable process but we shouldn't do that too soon in 2050 now there's a huge problem with this strategy um and that's that um with contraction and convergence the contraction is fine so long as we converge on zero um there's no uh there's no way we can stop temperature increasing if we don't stop emitting net carbon dioxide into the atmosphere if we're not squirrelling away as much as we're emitting ultimately temperatures will continue to go up and that's because of this graph which shows that co2 induced warming depends linearly on cumulative emissions okay so here's our total cumulative emissions since the pre-industrial era and here's the temperature anomaly relative to that period okay and this is due um so we saw before that a doubling of co2 leads to a roughly the same change in radiative forcing okay but there's a compensating effect which miles knows way more about than i do um about the airborne fraction the amount of carbon that stays in the atmosphere versus being transferred into the ocean um and that they completely um coincidentally compensate for each other and give us this linear trend so what's plotted on here from a whole range of climate model um predictions projections with climate a range of complexities of climate models um temperature change against cumulative emissions and you can see whatever rate you make those emissions at however long they go on for we still seem to track up this same linear curve but this is kind of an estimate of uncertainty so this has um oh okay so um before i move on so we've emitted about 560 or 70 gigatons of carbon already you can see that if we emit another um another 500 or so that takes us to a trillion tons of carbon uh and that's consistent with a warming of about two degrees above pre-industrial okay and so if we want to limit um temperature increase to less than two degrees then we need to emit less than a trillion tons is that straightforward basically and those are numbers you probably have heard before okay a few other points um a few other points on here um if we postpone um how long we get started on reducing emissions it doesn't necessarily mean we're um doomed to exceed two degrees but it does mean that we i'll have to work much harder to get there once we do get started okay and we really do have to get down to zero if we want to stop temperatures increasing so the aim um aim the limit um pre-paris was a two degree i don't like the word target two degree threshold um what happened in paris so that changed quite quickly um with the suggestion that 1.5 degrees c might be um better and um and lots of people got on board with that so these are these are two key parts um of the paris climate agreement uh article two this agreement aims to strengthen the global response to the threat of climate change by holding the increase in global average temperature to well below two degrees c and to pursue efforts to limit the temperature increase to 1.5 degrees c so this realization that perhaps when we get to two degrees c actually especially for some parts of the world things are already looking pretty bad we might not want to let it get that far um and then article four which arguably more important might not look very important um on a first read so to achieve the long-term temperature goal parties aim to undertake rapid reductions so as to achieve a balance between anthropogenic sources and removals of greenhouse gases in the second half of this century there's two things buried in here one is this undertake rapid reductions okay so to achieve the long-term temperature goal we're going to reduce carbon dioxide emissions we're not going to try and change the incoming solar radiation with mirrors in space or aerosols injected into the upper atmosphere we're not going to do um any short-term uh expensive fixes we're going to try and reduce our um condo oxide emissions and then and then the implicit in this bottom part this idea that we need to get to a net balance between sources and sinks of greenhouse gases if we're actually going to make this make any um stable temperature work and you know and a suggestion of when we might need to do that bar so if you look at the um uh carbon budget if you like for 1.5 degrees as estimated in the ipcc fifth assessment report so 2013 reports again this is the same uh graph you can see that to um limit global uh temperature change to 1.5 degrees then we need to be emitting no more than about you know 600 and something 650 something like that um gigatons of carbon um given we've already emitted 560 something that doesn't leave as much our current emission rate is about 10 gigatons of carbon per year okay so it doesn't leave us many years however this there's some um uncertainty in this figure and you can see this this large range so where we are in here you're in here somewhere now there's a large range in the temperature change since the pre-industrial area and actually we know much better than that what the temperature change has been since the pre-industrial area so era so we can re-plot this figure relative to the to this decade the 2010s okay and we we say okay we know we've had about 0.91 degree of warming so if we want to stay within 1.5 degrees of pre-industrial then we can't have more than about 0.5 or 0.6 above where we are now okay so if we reinterpret it in that framework then we we can see that we can have um so to get to about 0.6 above where we are now we can have about 200 gigatons of carbon okay so now we're talking about 20 years worth of current emissions at the current level okay so can we do it um from the science side yes okay we can do it there's 20 years of current emissions it doesn't have to take it we if we started reducing them now and we reduce them linearly um right down to zero then we have 40 years to get to zero okay 40 years if we start now and reduce them by the same amount each year um we'll get there another way of thinking about this um we're already about a degree above pre-industrial we're warming about 0.22 degrees per decade we have about 22 years before we cross 1.5 okay 0.5 divided by 0.22 gives us 2.2 decades and so the current rate um which again gives us 22 years at current emission rates or 44 years to get our emissions to zero if we start reducing them immediately in a straight line down to zero so yes we can do it but we really need to get started i think is the that's the upshot um so just to sum up then so most likely warming under a business as usual scenario is about four degrees c by 2100 with more warming after that um if we can get the emissions to fall to zero by about 2070 you can see that um we do have the possibility of um equilibrating temperatures about 1.5 degrees above pre-industrial we're not it's not actually too late in any sense you know people will tell you that um we're committed to lots of future warming even if we stop emitting greenhouse gases today but actually we're not committed to much um and i'm have to say this is physics i don't really understand some compensation between the carbon cycle right miles and and the ocean uptake of heat but if we were to stop emitting um greenhouse gases tomorrow we wouldn't actually see much further temperature rise so it's not it's not too late um and then i'm not going to stray into policy but just one last slide to say that the the um the current pledges from the paris agreement are not sufficient here they are relative so this is the um co2 emissions um gigatons of carbon dioxide now is about a factor of four difference between gigatons of carbon dioxide and gigatons of carbon and this is what's required to get us down well below two degrees c c falling to zero by you know 2070 2080 and this is the paris pledges so it's not enough and it's only limited to 2030 it doesn't say what will happen happen beyond then so more as a primer for what comes later in the course really um yeah and then some key points really just to make sure the take-home messages are all clear so that the the first one here you know greenhouse gases they warm the planet by raising the level from which long wave radiation escapes to space and hence reducing the amount that's escaping okay and causing an imbalance at the top of the atmosphere the amount of imbalance the radiative forcing is relatively well pinned down but the surface temperature response is not and that's due to this myriad of complex feedbacks that goes on and atmospheric levels of carbon dioxide at the moment are high and they're changing incredibly fast in geological terms and then as we've seen the best explanation by far of the observed global mean surface temperature record is that carbon dioxide emissions from human activity have together with other human related activities have the root of the of the warming since 1870 this linear link between co2 induced warming and cumulative emissions is crucial in how we think about mitigation strategies and and it is possible to meet the paris 1.5 degree target if we can ramp emissions down to zero over the next few decades and i'll leave it there