Introduction to the Science of Climate Change

[00:00:00] Peter Hess: Peter Hess: My name is Peter Hess. I'm a professor here in BEE. This course is Perspectives on Climate Change Challenge. I'd like to start with this figure here, and this shows the Earth, the atmosphere and energy from the Sun. The green here shows the aurora borealis. This is a picture also showing energy, lights from the surface of the Earth, reminding us of the activities of man at the surface. You can see the atmosphere here in a very, very thin blue line shown here. Most of the atmosphere is below 10 kilometers, so it's a very, very thin shell over the surface of the Earth. And because it's thin, it's susceptible to change. And in fact, we're changing it quite rapidly with emissions. Okay, so the atmosphere, what does it do for us? It protects us from the harmful rays of the Sun from ultraviolet light. Without the atmosphere, and particularly ozone in the stratosphere, life cannot exist on the surface of the Earth. The atmosphere also mitigates the temperature, so there's a natural greenhouse effect of the atmosphere, and it protects us from the very high temperatures and the very low temperatures that would occur without the atmosphere. So finally, the atmosphere is essentially able to connect various aspects of the Earth together. For example, pollen moves in the atmosphere. Nutrients that plants need move in the atmosphere, like nitrogen, phosphorus, etc. We need the atmosphere to transport these nutrients for plant growth. Finally, the atmosphere, because it acts as a shell and it connects the Earth, the surface of the Earth through pollution, it connects China with the U.S., the U.S. with Europe, etc. So that CO2 emitted in one place, for example, China, impacts us in the U.S. here. So a global atmosphere means, ultimately, we need a global solution to climate change. Now, as I mentioned, the atmosphere is very thin and is susceptible to change, and in fact, the ice core record shows that, in fact, the CO2, as shown here on the left, has changed considerably in ice ages over the last 600,000 years. Okay, so CO2 has ranged up and down, perhaps by about 20%. But you can see very, on the right hand side of this graph, CO2 now is increasing very, very rapidly. So in the past, CO2 has increased on the order, on the timescale of maybe 10,000 years. Now the increase in CO2 is increasing on the order of 10 to maybe a hundred years. So that's part of the problem, that CO2 is increasing very rapidly, and so rapidly, it's difficult to adjust to the CO2 increase and to adapt to it, both for people and for ecosystems. Okay, the last time CO2 was as high as it is now. It may have occurred over two million years ago. At that time, the global temperature was about four degrees warmer than it is now, four degrees centigrade, and the sea level was 25 meters higher than it is now. Okay, so the outline of the talk is, first I'm going to give some evidence for anthropogenic climate change, talk a little bit about the physics of climate change, carbon cycle on Earth, climate models, and a future climate, and adaptation and mitigation to climate change. Okay, so this is a well-worn picture. This is CO2 at Mauna Loa, the CO2, so this is on time scale of 50 years. CO2 has increased in 50 years from 320 parts per million to over 400 now, and one can see the little wiggles in the red here, here due to the uptake of CO2 from vegetation. So first question we might ask is, how do we know the CO2 has is due to anthropogenic activities? How do we know it's been emitted by people? Okay, and there's a number of evidences for this. First of all, if you look at the isotopic signature of the increase in CO2, so that's on the left side of your screen here, it shows the CO2 increase and the isotopic signature, which is the DEL 13 carbon, so the isotopic signature of 13 carbon. Plants have a lower C13 to C12 ratio than the atmosphere. So if CO2 is from fossil fuels, essentially old decayed plants, then we would expect this ratio to decrease, and that's exactly what we see. As CO2 has gone up, the ratio of DEL 13 carbon has decreased, which suggests that this carbon is derived from plants. And moreover, if you look at other isotopes of carbon, and the increase in carbon, it suggests that the source of carbon is very old, plants that lived a long time ago, again pointing to fossil fuels. If you look at the atmospheric carbon dioxide versus oxygen, so carbon dioxide again is from 1970 to about 2005. Carbon dioxide is on the left hand side of the scale, O2 is on the right, so when one burns fossil fuels, what happens? You react the hydrocarbons with oxygen, you release CO2, and the oxygen levels in the atmosphere go down. They're not going down very much, but the decrease in oxygen is measurable, and sure enough, oxygen has been decreasing as CO2 has been increasing, suggesting again this must be the combustion of fossil fuels, and the levels of oxygen decrease are consistent with the level with burning the hydrocarbons. A budget of the carbon cycle also suggests that the carbon is from people, okay? We have a pretty good estimate of how much fossil fuel is burned, okay? Both in the US, we know how much gas is used, we know how much oil is used by industries, how much natural gas and coal are used by power generators, we know pretty much around the world how much fossil fuel is burned, and how much carbon is released per year. Okay? And one can see that this estimate of emissions from people shown in red, and this atmospheric increase in CO2 shown above here, that the amount of CO2 burned more by people more than compensates for the atmospheric increase in CO2, okay? And it more than compensates because a substantial amount of CO2 has been taken up by the oceans and the land. Okay, so if you start with the atmosphere here in 1870, you add the burning of fossil fuels here, you add the land use, and if CO2 hadn't been uptaked, if we didn't lose CO2 to the land, to the soils, to the trees, and to the oceans, then atmospheric CO2 now would be 550 parts per million. So luckily, some of the CO2 that we're emitting to the atmosphere is removed into the trees and into the ocean, otherwise the CO2 content of the atmosphere would be much larger. So the sinks of CO2, one thing to keep in mind is that the sinks of CO2 are climate dependent, and most of the forecasts suggest that in the future, as the climate warms, the sinks of CO2 will not be as effective as removing CO2 as they are today. So less of the CO2 we release will go into the atmosphere, and less of it will go into the oceans in the future, as projected. Okay, so we have a pretty good idea that CO2 is from fossil fuel burning. We know it's increasing. What's the relation between CO2 and temperature? Okay, and this shows a global temperature anomaly, and it shows also the various discussions of this temperature anomaly from the IPCC. The IPCC is the Intergovernmental Panel on Climate Change. It consists of a number of volumes reviewing climate change, reviewing the literature on climate change over five, over the previous years. And one of these panels comes out, one of these reports comes out every five or six years. If you want to know anything about climate change, this is really a great place to go. You can look it up on the web, and it's and it's free. Okay, you can download the pds. Okay, so this curve shows what the IPCC said about the temperature anomalies. Starting in 1990, the unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more. Okay, and then as years go by, the temperature keeps going up. It ends with, in 2013, it's extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. Okay, so the climate, as the temperature increases, we can be more and more confident that this decrease is not natural, but it's likely due to the increase in CO2 in the atmosphere. So how do we relate? So now we have two correlations. One, we have CO2's increasing, we have temperature increasing. How do we know one causes the other? Okay, and there's various, there's a number of evidences for this. One is what we call climate fingerprints, and that is that the warming due to CO2 has a unique fingerprint and how temperature changes in the atmosphere. Okay, and one of the fingerprints is that the troposphere, the lower level of the atmosphere increases, the temperature there increases with time, and the temperature in the stratosphere decreases with time. And that's exactly what we see, and in this plot here we see a warming in the mid to upper troposphere and a cooling in the lower stratosphere. So as we add in CO2 to the atmosphere, it acts as a blanket, essentially traps heat in the lower levels and lets the upper levels cool. So that's one evidence that in fact, the change in temperature is due to CO2. The other evidence, or one of the other evidences, there's lots of evidence, is knowing the physics of the atmosphere. And there's really not, this isn't rocket science, the physics of the atmosphere and of the transfer of radiation through the atmosphere is well understood. And in fact, in 1903, Arrhenius looked at what was the effect of adding CO2 to the atmosphere. And the price sought by Arrhenius was the solution to the riddle of the ice ages. He focused on a decrease in CO2 as a possible cause of cooling and found that cutting the level in half could indeed bring an ice age. But he also took the trouble to estimate what might happen if the amount of gas in the atmosphere at some distant time in the past or future, so that would be, say, in 100 years after he lived, he didn't know this, but was double his present value. He computed that would bring roughly 5 or 6 degrees centigrade of global warming. And he says, "I should certainly not have undertaken these tedious calculations," Arrhenius wrote, "if an extraordinary interest had not been connected with them." And the estimated, and the numerical calculations cost Arrhenius month after month of laborer's work. And since he understood, he undertook some of this work due to personal reasons: a divorce from his wife and child custody. So this is sort of a little bit of history of someone who really did some fundamental calculations over 100 years ago. So now these calculations are much easier, we understand this better, it's no longer quite as tedious. In fact, one can solve the radiative transfer in a simple model in five minutes, okay, instead of the months Arrhenius took. So the atmosphere, we can consider the Earth in space, and the Earth really is composed of mass that essentially doesn't escape Earth. The carbon cycles between the land and the atmosphere and the ocean, but has a very important exchange of energy. And so in fact, the Earth absorbs energy from the Sun, as we should know. And what keeps, so it keeps absorbing energy, what keeps the Earth from heating up? Why doesn't the Earth's temperature increase and increase and increase? And the answer is that the Earth also emits energy, except the energy we see it emit is in the infrared, okay? We cannot see it, it emits its thermal energy. And the Earth reflects quite a bit of energy. So to the left is a view of the, what we see is a view of the Earth. This is reflected solar radiation, watts per meter squared. There's joules per second per meter squared. This is the amount of energy reflected from the Earth. We can see that the Earth reflects energy where there are clouds, so clouds are very important reflectors of energy, and where there are deserts. Okay, the Earth emits radiation, as shown on the right. This is thermal radiation that, that we don't see, and this is emitted from the Earth where there are no clouds, and also from desert, from desert regions where it's very, very hot. So the Earth energy comes in from the Sun, and then the Earth reflects some of the energy to space, and emits energy as thermal, as thermal radiation. So all objects emit radiation, and at room temperature, most of the radiation is emitted in the infrared. So this shows a boy holding a, holding a ball, and he's emitting radiation from his surface, okay? Normally you can't see it unless you have some sort of infrared camera. So the amount of radiation that emits, that an object emits, increases with temperature. So say all of a sudden the Earth is getting more energy than it's emitting, okay? If it's getting more energy than it's emitting, the Earth would act to heat up. If the Earth heats up, the temperature of the Earth increases, the outgoing radiation increases, and at some point the incoming radiation and the outcoming radiation are balanced, okay? So there's a balance between energy in and energy out. And in fact this is what we expect from the Earth, and over long periods of time, that the energy coming in and the energy coming out are about the same. Now consider the Earth with no atmosphere. The Earth absorbs about 240 joules per second per meter squared, or watts per meter squared of energy. It emits that amount of energy to keep in balance, and this would suggest that the surface of the Earth would be 255 degrees K. that's below freezing, okay? So there's no atmosphere. Now what happens if we add an atmosphere? Again, the same amount of energy comes in from the Sun. The Earth emits some of this energy as infrared light. It reflects some of the energy. Some of the energy is trapped by the atmosphere. Some of it escapes through the atmosphere to space, but some of this energy is emitted from the Earth's atmosphere back to the surface of the Earth. So now the surface of the Earth not only absorbs its 240 watts per meter squared from the Sun, but it's absorbing additional energy that's radiated back from the atmosphere, and this energy is radiated to space. So the net result is we have a greenhouse effect where the Earth is now warmer than it would be without an atmosphere. Okay? The problem is that we are adding CO2 to the atmosphere so that the Earth is heating up. The atmosphere absorbs more and more energy that wants to escape the surface of the Earth. So what I've talked about now is essentially we've talked about the increase in CO2. We've attributed the increase to the burning of fossil fuels. We showed that the increase in CO2 can cause an increase in temperature due to well-known physics and to other climate fingerprints. Now we're going to go on from there and talk about what some of the consequences are. First, I wanted to show this figure here, and this is in terms of radiative forcing. If you haven't heard of radiative forcing, this is a very famous plot that almost all climate scientists look at. And this shows a change in energy at the surface of the Earth. The increase in the amount of energy absorbed by impinging on the surface of the Earth since the pre-industrial time. And what species, what atmospheric species, is causing that change in energy at the surface. So we can see that the radiative forcing of CO2 dominates the increased energy impinging on the surface of the Earth since pre-industrial times. But we also can see that long-lived greenhouse gases, methane, N2O, halocarbons, tropospheric ozone, shown here, also increase the amount of energy that's absorbed at the surface of the Earth. But very important, but you notice here that there are some, there are some constituents, some species emitted to the atmosphere, that are causing a cooling and are essentially counteracting this increase in greenhouse gases. And those are atmospheric aerosols. So this is a figure of atmospheric aerosols from above. So atmospheric aerosols are essentially particles in the atmosphere. They could, they're suspended, suspended particulates. They could be mixed with water. And for the most part, though not totally, they are white. So, and this figure here is what might look like from the window of an airplane. If you're landing in LA, you're looking down near the Earth's surface, and it looks very white because of these aerosols. This figure here shows some satellite, and these aerosols here are over India. They're a little bit browner, but they're mostly lighter than the surface of the Earth. And so what do aerosols do? They reflect light from, from space, and they cool the surface of the Earth. The energy that's coming in never makes it to the surface of the Earth because their, their energy is reflected by the aerosols back into, into space. Now the other thing to know about atmospheric aerosols is they're really dangerous to breathe. Okay, so if you've heard about air pollution in India, in China, lots and lots of people are sickened by atmospheric aerosols. In 1950, 10,000 people died in London because of atmospheric aerosols. In the meantime, Europe and the US has largely cleaned their atmosphere because these aerosols, these fine particulates will kill you. Okay, they'll cause lung disease, respiratory disease, etc. And so countries that are industrialized and have enough money have pretty much cleaned, cleaned the air. The countries that are rapidly industrializing like China and India have not cleaned the air as of yet. Though there's, there's progress particularly in China to, to clean up the air because so many people are becoming ill because of atmospheric aerosols. So this is a connection between pollution and climate. And what most countries are doing is weighing in on the side of pollution. They're saying atmospheric aerosols are bad. We're going to clean up our air because it's making people sick now, even if it warms the climate. So if you look, if you look back here, the atmospheric aerosols have a cooling effect. They're cleaned up, the climate will warm that much more because these aerosols are counteracting the effect of greenhouse, of greenhouse gases. Okay, so what we've discussed so far is warming of the climate system is unequivocable. And um, the greenhouse gas emissions since pre-industrial era driven largely by economic and population growth are now higher than ever. This has led to atmospheric concentrations of carbon dioxide, methane, and nitrous oxide. They're unprecedented in at least the last 800,000 years, most likely the last few million years. The effects together with those of other anthropogenic drivers have been detected throughout this climate system are extremely likely to have been the dominant cause of the observed warming since the mid-20th century. So there's very little doubt that climate warming is due to people and is connected with the emissions of greenhouse gases. Now I think it's instructive to look a little bit at the carbon cycle because most of the pollute, most of greenhouse gas warming is due to CO2. And in fact CO2 is going to be the dominant driver of greenhouse gas warming in the future. So fossil fuel is emitted from industry, um, and cars, et cetera, from fossil fuels in the ground is emitted to the atmosphere and then the atmosphere exchanges CO2 with the ocean and with land. And you notice here the exchanges are large both ways, but more of the CO2 is coming from the atmosphere to the ocean and from the atmosphere to land than it's leaving it. So as we said in the beginning, the ocean and land act as CO2 sinks. But this also means there's large exchanges between the atmosphere, the land and the oceans means it's very hard to get CO2 out of the system. You put CO2 in the oceans, it doesn't stay there, it goes back to the atmosphere. You put CO2 on land, it doesn't stay there very long, it goes back to the atmosphere. So to get CO2 out of the system so it's not exchanged with the atmosphere takes very many years, 10,000, 100,000, et cetera. So what happens to a pulse of CO2 added to the atmosphere? So this shows essentially a simulation, you're adding a large pulse of CO2 to the atmosphere and in the first hundred years about 60% is taken up by the oceans and by the land. So that's a hundred years, you lose 60% of the pulse. And the next, so this timescale here is 2,000 years. In the next 2,000 years, the ocean uptakes more and more as the ocean overturns and as the ocean chemistry is allowed to take up more CO2. But even after 2,000 years, you still have 60% of the CO2 left, 40% of the CO2 left in the atmosphere. And then slowly, on a timescale of 10,000 to a million years, you finally lose the CO2 from the system. So once you put the carbon into the atmosphere, it takes a long, long time to go away. So what are the consequences of this? The largest fraction of CO2 recovery will take place on timescales of centuries as CO2 invades the ocean. A significant fraction of the fossil fuel, ranging in published models in the literature from 20 to 60%, remains airborne for 1,000 years or longer. Ultimate recovery takes place on the scale of hundreds of thousands of years, a geologic longevity typically associated in public perceptions with nuclear waste. The notion is pervasive in the climate science community and in public at large that the climate impacts of fossil fuel CO2 release will persist only a few centuries. That's not true unless we figure out a way to take carbon dioxide out of the air in mass quantities. So these curves are a little bit complicated, but what they show here is adding CO2 to the atmosphere. So this time scale here is up to 3,000 years after you add CO2. You add CO2 in this model at the percent of 2% per year, and then you cut it off at a certain year, a certain concentration. So for example, in this curve here you cut it off at 150 parts per million. Here you cut it off at 1,200 parts per million. So you increase CO2 and then you stop the emissions completely. You can see that the atmosphere, as we discussed in the previous slide, retains the CO2 for over a thousand years. So the CO2 does not disappear. The surface temperature here increases and it remains high throughout. So you increase CO2, now you cut the emissions, the emissions are zero. You're not releasing any more CO2. The temperature stays high for another 1,000 years. And the sea level, the thermal expansion of the ocean, continues to get worse. And this is a thermal expansion of the ocean. This does not account for ice melt from Greenland or Antarctica. And so this also persists, the increase in sea level for a thousand years. So once you put CO2 in, it stays in for a long time and it doesn't disappear very quickly. So one of the ways that we look at the impact of climate, and we want to know what is in 100 years or 200 years. So now we're not talking about thousand year time scales. We're talking about 100 year time scales, generally, is the use of climate model. Okay. So climate models include the essential elements of the climate system. And we use these extensively for projecting future climate change. The elements of the climate, so these are essentially models, they're a set of numerical equations that show how things evolve once we add CO2 to an atmosphere. Okay. It consists of the atmosphere. We look at the changing carbon dioxide in the atmosphere, the changing water vapor as the earth warms. We look at the impact of aerosols, clouds, heat transports, and radiation as it's transferred between the sun and the earth, and the earth and space. They include an ocean, which also absorbs CO2, which absorbs heat. So it's been projected that most of the temperature change, the extra heat that's coming into the earth, because of climate warming is actually going into the ocean. And includes a land component too. Okay. So it models trees, the CO2 absorption in soils, etc. And ice, because ice is a large modifier of climate. It reflects a lot when it's present. It reflects a lot of light back to space. So scientists use these very large, complicated climate models. And this shows the, essentially, this is the NSF computer facility. It's located in Cheyenne, Wyoming. It's a big building. It's solely designed for running these computers that people use climate models on. If you go inside, it has many, many ducts and pipes that are used for cooling the computer. Rows and rows of computers here, computer banks, etc. It's these computers that people run these climate models on for hundreds of years, are massive. And ironically, they produce, they need a lot of energy. So the reason why these computers are located in Wyoming is coal energy is cheap there. And so NSF, National Science Foundation, got a real deal to build this computer modeling facility in Wyoming. Okay. So one test, a necessary test of climate models. You want to make a model. You add all these components. You want to know, is the model any good? So one necessary test, necessary but not sufficient, is to be able to model the climate over the last hundred years. So if you take a climate model, so the red shows the observations of temperature. This is from 1850 to 2000. And you add natural forcing to the climate system. So what we mean forcing and change is in the output of energy from the sun, the impact of volcanoes, etc. We can see that the temperature record and the impact of these sort of natural changes in the model are completely different. So we cannot simulate the observed temperature record with just natural forcings. Now we add the human influences alone. We don't add the natural forcing. We just add the increase of CO2 due to people. We add the greenhouse gases and we add the aerosols. Okay. Again, we can see that now the temperature is increasing here in the model, in the gray here. And it produces a better fit to the red, to the observed temperature change. Okay. And now we add both. We add the change in the sun output. We add the natural force in volcanoes. And we add the human influence. And we can see that these models then do a pretty good job in matching the temperature record over the last hundred years. So this gives us some confidence that these models are doing the right thing. Okay. It's a necessary condition that you can model what has occurred. So what do we do in these models? Then what we want to do is add the change in projections and the change in CO2 in the next hundred years and see what happens. Okay. So we don't know what's going to happen in the next hundred years. We could do business as usual. That would be the red curves here. Or we could really cut emissions as shown in these blue curves here. And the emissions could decrease. Okay. So we take these current models. We add CO2. Each year we add the emissions. And we see what happens. So each of these lines here represents the socioeconomic and emissions scenario used in climate models. Provide a plausible description of how the future may evolve with respect to a range of variables. Including socioeconomic change, technology change, energy and land use, emissions of greenhouse gases, and air pollutants. Okay. And they're used as inputs for climate models. Now one thing to note here is that this blue curve, most of these blue curve here, and this is a temperature increase of 0.9 to about 2 degrees. Almost all of these curves involve a scenario where we have negative emissions. So that means there's a way, or the scenario assumes there's a way of sucking out CO2 from the atmosphere at a large scale. Okay. And this technology has not been invented yet. Okay. There's some hope that maybe something like this is plausible, but right now no one's quite sure how to do this. So these blue curves with negative emissions rely on technology that doesn't exist. This black curve here, with the temperature rise of about 3 degrees, 3.7 degrees. This curve here is consistent with the agreements from Paris, with the Paris Protocol a few years ago, which the U.S. recently withdrew from. So this Paris Protocol here is we're not following this business as usual scenario, but there's still quite a bit of climate warming projected from it. And you can see in this black curve here where we are in 2016 emissions. So we're a little bit below this deep red line, this business as usual, but that's close to where we are at present time. Projected warming by 2100. Okay. So now we've talked about putting these scenarios in these climate models. And what do we, what do we get? Okay. If we, if this low climate scenario where we assume that we can decrease emissions, we can suck carbon out of the atmosphere, we get a warming up about one degrees C. And this high climate, in this high scenario where we, as business as usual, we get about four degrees C. That's about 10 degrees Fahrenheit. Okay. We get very enhanced warming on the poles here, warming over the land masses. And on these two things, on these two scenarios, essentially the climate models as indicated by stippling, no matter what climate model you use, it gives the same answer. What is not known so well, although we know globally, the climate models seem to give the same answer, and they give whether you increase or decrease, whether temperature increases is the same. What we don't know is very well, is regionally, how much the temperature will increase. So we know global increases, we have a pre, all the models agree fairly well in the global increases. Increases in a place like Ithaca or any other locale, we don't know how much that will be. We don't know the regional impact of climate warming without, with any certainty. Precipitation is even less well known. Okay. So again, on the left shows the precipitation with the low climate scenario. This is an increase by one degree C, and this is increased change in precipitation at four degrees C. Okay. The stippling shows regions where the projected change is large compared to natural internal variability, and where 90% of models agree on the sign of the change. So I don't know if you can see the stippling in the back of the room, but very few regions are stippled. So in terms of precipitation, we do not have a very good handle on how it will go with climate change. And again, especially, we don't know how it will go regionally except in a few places. [00:37:59] Speaker ?: Okay. [00:38:00] Peter Hess: Globally average rainfall is increased in general. In general rural places are wet get wetter, places that dry get drier, and extreme precipitation events will very likely become more intense and frequent. Okay. But other than that, if you want to know what's going to happen to precipitation in Ithaca, and you can see why we might want to know it, to ensure adequate drinking water, to ensure adequate water for agriculture, etc., we don't really have enough information to plan ahead. Okay. And one curve here showing sea level rise, projected sea level rise, and you can see at the worst projection, sea level rise increases at 2100 by 8 feet. Okay. The sea level rise now has increased a little bit since the pre-industrial. If you pay any attention to the hurricane impacts, etc., to flooding in southern Florida, you know the small sea level rise now has caused a lot of problems, especially under cases of storms. Okay. Okay. So, finally, or almost finally, we get to this plot here, which is really very, very important conceptually. And this shows the temperature anomaly, and this shows the accumulated total CO2 emissions from pre-industrial. Okay. So, you add all the CO2 emissions together, year in and year out, and you can see that the temperature increases almost linearly as a function of the accumulated CO2 emissions. So, what this implies is there is no sustainable CO2 emission rate compatible with climate stabilization. We increase in emissions are, might be small, but if they're still positive, temperatures can still increase. The temperature at the peak of warming is the temperature the climate remains stuck in for millennium following cessation of emissions. And notice also, too, there's a large temperature spread, and this is global temperature anomaly. Okay. So, we don't know, for example, if emissions increase from where they are now, right here, to about 500 cumulative CO2 emissions in giga-cons-carbon to, say, 2,000. Oops. To 2,000, there's a large temperature spread of over 4 degrees. Okay. So, there's, again, a large uncertainty in how bad it will get. The other thing this plot tells us is, say, we want to limit the, we're here now, we're about 400 gigatons carbon, and we want to limit climate change to 2 degrees. Okay. At 2 degrees, we must have accumulated emissions of about 800 gigatons carbon. So, at 400 now, we can release no more than in our 400 if we want to stay low. Okay. So, this tells us how to, so, we have 400 gigatons carbon to distribute. If we want to limit climate change to 2 degrees, how are we going to distribute it between different countries? [00:41:36] Speaker ?: Okay. [00:41:37] Peter Hess: And this is how it works for the Paris Peace Accord. Okay. So, this is CO2 emissions. If we want to limit the emissions, we want to limit the emissions to under this 400 gigatons carbon. This would be a plausible path of emissions. Okay. With year. If you take the U.S. pledge to, in the COP, in the Paris Peace Accord, the pledge from Europe, the pledge from India, the pledge from China, we can see that the rest of the world has no emissions. Okay. So, how are we going to distribute this? If we want to stay under two, how are we going to distribute these emissions equitably? Okay. So, I'll just mention two solutions. We can mitigate human intervention to reduce sources or enhance the sinks of greenhouse gases. We can also adapt, and we're going to have to do both. We're going to have to both mitigate, but we know that climate is going to change. We believe it's probably going to change more than two degrees, so we're going to have to also adapt to a warmer climate. Okay. So, the adaption process for adjustment to actual expected climate change and its effects. And we're going to have to mitigate and adapt in the face of large uncertainty. We don't know how much the climate is going to change, especially locally. We don't know how much it's going to cost us, etc. There are multiple factors in global environmental change. The Anthropocene is a geologic area of significant impact of Earth's east ecosystems. There's pollution, climate change, ozonal, acid rain, deforestation, etc. And so, these are the environmental impacts. Climate change is only one of them. There are many other environmental impacts that we should be concerned of. And, of course, these environmental impacts are one of the problems we face. We face national poverty, poverty within the U.S. We face global poverty. There's a huge disparity between how people live in different countries. And connected with that, we face global and national inequality. Okay. So, all these things and climate change are all connected together. Okay. So, with that, I will end. We'll take a few minutes for questions. [00:44:10] Speaker 2: Can you talk about the relative importance of adaptation versus mitigation to meeting sustainability goals? [00:44:18] Peter Hess: Yes, I will try. I mean, they're both important. The more you mitigate, the less you have to adapt. So, it really depends where you want to end up. And the ideal between mitigation and adaptation might come down to, you know, how much you'll cost. Okay. So, I don't know if there's an ideal balance. One has to do both. If one doesn't mitigate, you're going to have to adapt to huge temperature changes. And when we talk about adaptation, we're often referring to the adaptation of people. Right? People have to move inland from the shore because of sea level rise. You have to adapt to changes in agriculture, et cetera. Ecosystems, coral reefs, natural world is going to have a much harder time adapting to climate change than we do. Well, that's what we do.