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GG 140: The Atmosphere, the Ocean, and Environmental Change
- Global Warming
The issue of global warming is discussed. Recent climate change over the last half of the 20th century is thought to be driven largely by greenhouse gas emissions, with carbon dioxide playing a large role. The carbon cycle describes the reservoirs of carbon (atmosphere, terrestrial biomass and ocean) and the exchanges that occur between these reservoirs. Inputs of carbon to the atmosphere include burning of fossil fuels and respiration from biomass. Vegetation also removes carbon from the atmosphere through photosynthesis, and a similar uptake of carbon from the atmosphere occurs in the ocean through biological processes. Residence time for carbon in the atmosphere can be computed and is estimated to be a few hundred years. Atmospheric carbon dioxide has been measured directly since the 1950s, and longer records exist over geologic time from ice core data.
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The Atmosphere, the Ocean, and Environmental Change
GG 140 - Lecture 27 - Global Warming
Chapter 1: Global Warming [00:00:00]
Professor Ron Smith: OK, so we are starting this new subject, which is a really important one. And we’ll be covering at least these subtopics, starting with the carbon cycle, then the question of whether we can use the Holocene as a reference period for judging recent climate change. We’ll go over the history and the theory of the warming and cooling we’ve had over the last hundred years or so, talk about the IPCC forecasts for the future, and then go back, in a way, and kind of summarize some of this by talking about this interesting and useful concept of climate sensitivity. So it’s quite a lot of material.
The book is chapter 13. A couple of you have commented that in the earlier editions, there may not even be such a chapter. So please check with your classmates that have the current edition and look at their chapter 13 and see what the deal is. If you don’t have that chapter, you may want to borrow someone else’s textbook for a few hours while you read through that chapter.
But a lot of the material, as I’ll indicate in a minute, is coming out of the IPCC reports. And those are–some are already put online on the class’s server for you, others you can find very easily online. Just Google IPCC. It’ll take you to their big website. There’s lots of different reports, you just have to figure out which ones you’re looking for there. So even if you don’t have a chapter 13, you can get a lot of this from the IPCC reports.
Now I wanted to point out that this subject we’re dealing with now, I think, is one of the four great paradigm shifts in the earth sciences. The four, in my mind, being the theory of evolution, that is, Darwin’s idea of the mutability of species back in the 1860s. The theory of the Ice Ages, which we’ve spoken about in this course. The idea that just 14,000 years ago, we had a thick layer of ice covering over much of what is today open, useful land. The theory of plate tectonics, which helps us to understand the layout of the continents, how that has changed through geologic time. The difference between continental crust and ocean crust. It even helps us to understand the shape of the ocean basins. And then the theory of global warming, which we’ll be dealing with now.
Now, these each have their own characteristics, but on a scientific front, they all were true revolutions. In other words, they changed the very most basic ideas of how we thought about the different subjects of biology and climate and geology, and then, not only climate for global warming, but also the role of humans on the planet. There are books, interesting books, written on the history of each of these. Fascinating reading. How they were resisted, how they were promoted, what kinds of evidence finally tipped it over so that the majority of the scientific community finally accepted these. Fascinating reading in the history of human thought.
Of course, the theory of evolution was resisted strongly by the church at that time because it changed the way we thought about humans and human nature. Still resisted to this day in some areas. The theory of the Ice Ages and plate tectonics I don’t think had any kind of resistance or even that much notoriety in the general public. So they were mostly battles fought within cadre of scientists. So that’s a little different.
But then we get to the theory of global warming, and now we’re back in a subject that is of great interest to the general public as well, and comes straight into politics. And it has a stake, a current stake. Now, all of these things were interesting–those first three were interesting intellectually, but none of them required that we do much in response to whether the theory was accepted or not. Where the theory of global warming is a little different because it has to do with not only the role of humans on the planet and the question of whether there are too many people and our lifestyle is wrong, but also in many respects it brings in the issue whether we should be changing our lifestyle or changing the way we do things.
So here we are. And so you and I are living through this one. Not only is it a scientific debate, but it has these political and economic consequences. It’s really a mess, because it’s all these fears of discussion are engaged in the theory of—in the discussions of the theory of global warming.
Any comments or questions on this? So, of course, I’m going to try to approach this in the most boring, scientific way that I can and try to keep to a large extent out of the politics and these other discussions. But we can’t avoid dealing with some of these. It used to be that, when you went to someone’s house or a cocktail party, your companion would whisper in your ear, now remember, don’t talk about politics or religion. And now you have to say, don’t talk about politics or religion or global warming, because you get into some terrible battles quite innocently, making the most benign remarks. And suddenly, everybody’s in an uproar. So, particularly hard for someone who works in this field to avoid that kind of awkward party situation.
So as I’ve indicated, for a lot of the discussion, then, that you’ll do, and that I will do, and that everybody else does, your primary reference material are these very valuable IPCC reports. There have been a series of IPCC reports, and the most recent one, called the Fourth Assessment, was finished in 2007. And of course, the Intergovernmental Panel on Climate Change is busily working on the next one. I don’t know when the target date is, but probably within a year or two, there will be the Fifth Assessment of climate change. And some of that material is already being leaked. You can find it on their website. But for the most part, we’re going to be going back and focusing most of our attention on the 2007 report.
But there have been other things more recent, such as an interesting National Academy of Sciences report on, what’s the word they used? Sudden, they used a different word–sudden climate change, they did a particular study on how fast climate change will occur. And I’ve got that posted on the lab course website. A special report, also by the National Academy, on high latitude climate change, and both of those are more recent than 2007, so those will give you some updated material.
And there is other–for example, if you go to this website, RealClimate.com, which has a reputation for being a little bit on the liberal side of the issue, but nevertheless, a lot of scientists post recent results on RealClimate.com. And if you’re going to do that, though, you should be evenhanded and go to some of these other anti websites, like, what are they called, ClimateSkeptic.com and things like that. So be sure you, if you’re going to bookmark some of these things, try to get an even number of bookmarks on the two sides of the issue. You’ll have to decide for yourself, in the end, which material you accept and which ones you reject. I’ll try to help with that a little bit.
Chapter 2: The Carbon Cycle [00:08:45]
So let’s start out by looking at the carbon cycle, which is really one of the core issues in the debate. It has to do with what controls the carbon dioxide level in the atmosphere. We know the carbon dioxide is a powerful greenhouse gas. It has a linear molecular structure, but it can flap and it can vibrate in such a way to absorb and emit infrared radiation. So it’s a powerful greenhouse gas. It’s in part controlled by anthropogenic processes, so it’s at the center of the whole debate. You have to know this. You have to know that when you burn something like coal, the chemical reaction looks like that, carbon plus oxygen. You oxidize the carbon to form CO2, and that’s a gas that goes into the atmosphere. This comes from the atmosphere, and this would be coming from some buried deposit of coal. If you’re burning methane, which is CH4, you combine that with oxygen and you get two by-products, water vapor and carbon dioxide.
The water vapor we pretty much neglect, because it goes into the atmosphere as water vapor, but then instantly, it becomes part of the normal hydrologic cycle. And if you remember early in the course, we computed the average residence time for water vapor. Anybody remember what that was? Nine days, yeah, eight days, nine days. So in other words, it’s cycled back in rain to the earth’s surface or the ocean very, very quickly, and so it really plays no role in the greenhouse effect.
Let me be careful. Water vapor is also a very powerful greenhouse gas. And because there’s so much of it, it’s actually in a sense the strongest of all the greenhouse gases. But it is controlled by the natural system. It doesn’t matter how much we add into it. The level is not going to change for that reason. It’s going to be controlled by the own, the internal processes within the atmosphere. Nothing we can do in adding water vapor is going to have any influence on changing the amount of water vapor in the atmosphere. So it’s important to know that water vapor is one product, but in terms of what that does to climate, that particular piece is negligible in the sense that I have just described.
Propane is another form of gas. It can be burned for heat, and the reaction–the stoichiometry is a little bit different. I’ve tried to balance those reactions. I’m not a chemist, so you may want to check me. Add up the carbons, add up the oxygen, be sure everything adds up properly, and correct if I’ve gone wrong here. But basically water and CO2 again, and the same thing implies. The CO2 stays in the atmosphere, influencing climate. The water vapor quickly gets cycled into the natural hydrologic system.
Another thing that’s going on, we’ve talked about calcium carbonate in this course a couple of times because that excess calcium that we found in the Quinnipiac River that wasn’t there in ocean water, we argued goes into making calcium carbonate in the oceans, shells, limestones, and so on. There’s a lot of that around in the earth’s climate system. If we take a little bit of that and heat it, we can separate it into the material that’s used to make cement, but that releases CO2 as well. So there’s another source of CO2. It’s not quite as powerful as these. I’m going to show you some numbers in a minute. But that is another significant input of CO2, is the making of cement from natural calcium carbonate that was precipitated in the oceans by either living or non-living processes.
Stop me if you have any questions on this.
So when you burn fossil fuels then, you’re putting carbon dioxide in the atmosphere. And that comes from carbon that’s been stored in the earth, typically for 60 or 100 million years. Remember in the Cretaceous Period, way back when the dinosaurs were alive, that’s when a lot of forests were being–the earth was very warm during that period of time. You had tropical forests almost everywhere on the continents. Those trees would eventually fall. And the productivity was so high, instead of their rotting and putting that CO2 back in the atmosphere, they would be covered over by another and then another and another. So you ended up sequestering all of that carbon down in the crust of the earth, where it’s been sitting there for 60 or 100 million years. And now we’re suddenly pulling it back out and burning it and putting it back into the atmosphere.
And in the oceans, too, you had algae growing, phytoplankton, that would sink to the bottom of the ocean, and instead of rotting and returning to the ocean system, they would get covered over. And today you have all of these oil deposits beneath the ocean bottom. And then we dig those up and we burn those, too. So both on the land and in the oceans, we are removing ancient, very ancient fossil carbon and burning it to put it back in the atmosphere.
Now the natural, the modern biosphere is active in this as well. For example, trees remove a lot of carbon dioxide during the summer season when they’re growing and use that to build their woody biomass. Remember, the trunk of a tree is about 1/5 or one quarter carbon. Well, where did that carbon come from? It didn’t come from the soil. From the soil, the tree is getting water and it’s getting nutrients, like nitrogens and phosphorus, but almost all of the carbon to build a tree trunk is coming from the atmosphere.
I mentioned in another context that I did a project down on a small island in the Caribbean last spring. And we were flying an aircraft around the island. One of the sensors we had on the airplane was a carbon dioxide sensor so we could measure how much carbon dioxide was in the atmosphere upstream of the island, over the island, and downstream of the island. The island was forested. It’s the island of Dominica, one of the few Caribbean islands that still has its original forest. And when we flew downstream of the island, across the wake of the island, we found a slight deficit in carbon dioxide. In other words, the air that had come from upstream and passed over the island’s forests had lost a little bit of carbon dioxide to those trees, and we found that deficit downwind. Now, this has been well known. You can measure this in lots of other ways. But this is yet another way to convince yourself that the forests are actively removing carbon dioxide from the atmosphere all the time that they’re growing.
But then, they recycle some of that. For example, the material in leaves that falls to earth at this time of year then rots and respires over the winter season to put that little bit of carbon back in. Now it still has its woody biomass, which is most of it, but a little bit goes back into the atmosphere, which gives that little bit of wiggle in the Keeling curve that you were studying in your Time Series Lab. So I want to emphasize that the biosphere is a very active player in this carbon cycle. Certainly on the long term, but then on the modern time scales as well.
Any questions there? Yeah.
Student: Does the carbon that’s sequestered in the wood, when, like, the wood from the tree rots, is that also, is that rereleased in the atmosphere?
Professor Ron Smith: That’s right. So if a tree, if a modern tree today does fall and then you watch it over some years, it rots and kind of disappears into the Earth. That carbon ends up back in the atmosphere. A little bit may be left in the soil as organic carbon, but then over a longer period of time, maybe ten or twenty years, that too will go back to the atmosphere. So there are few places on earth, however, where you’re getting this sequestration, where you’re getting things falling on top of it faster than it can rot. But I would say, for example, in most of these New England forests, most of the ancient tree material is going back into the atmosphere.
Well, in fact, that’s one of the problems with this idea of solving the global warming problem by planting lots of trees. Because what that will do in the short run, indeed, it will draw extra CO2 from the atmosphere and store it in the biomass of the tree. But a typical tree of the species you’re familiar with, both conifers and deciduous, typically have lifetimes of 60 to 80 to 100 years. And then they will fall and they’ll rot and the material will go back. So it is a temporary fix, perhaps, to reducing carbon dioxide buildup in the atmosphere, but by no means is it permanent because of the short turnover time of trees in the forest.
So this diagram will summarize some of this. And there are two types of numbers on here. There are, for example, storage. These numbers are in boxes. And those units would be in gigatons of carbon. And then the arrows, which are the fluxes, going between these reservoirs, and those units are in gigatons per year of carbon. So for example, in the atmosphere typically–and this is a calculation you could do–but we have about 760 gigatons of carbon. Remember, that’s the C in the CO2. If you want to know what the mass of the carbon dioxide is, be sure to include the mass of the two oxygens that are on the carbon dioxide. This is just the carbon in the carbon dioxide.
For example, let’s look at how it interacts with terrestrial production, the one I’ve just been talking about, of trees and other biomass on the continents. So about 60 gigatons per year is put in and about 62 is taken out every year. So it’s a very dynamic reservoir. The difference, however, is rather small. There’s an excess going from the atmosphere into the biosphere of about two or three gigatons per year. It’s this big exchange, the 60 and the 60 that’s responsible for that wiggle in the Keeling curve. And it’s the annual difference, that two or three units, that’s responsible for that trend in the Keeling curve. I’ll show you that Keeling curve in a moment. You’ve seen it before, but that’s a central part of our argument.
The oceans are somewhat similar, so there’s biological activity taking place in the oceans. We’ve talked about that. We’ve seen that it’s confined to certain parts of the ocean where you can get nutrients coming up into the euphotic zone. The numbers are even bigger than for the terrestrial biomass, but they too have a similar characteristic. Large instantaneous cycling, but over a year, only about two gigatons, and going in the same direction, going from the atmosphere into the ocean.
There’s the cement production, about 6.5. And there’s some other things here. This is one to look at. The amount of fossil fuel reserves still in the crust of the earth is 4,000 units of gigatons. So you can imagine if you burn the rest of that, what that would do to that number. It would be 4,760, unless, you probably should take into account this removal as well. But it would be a huge number if one were to burn all of the fossil fuel reserves and put that carbon dioxide in the atmosphere.
Questions on this busy diagram here? I’m going to use these numbers to do something we did early in the course. I want to compute residence times, because sometimes that’s helpful to get a physical feeling for the way these reservoirs work. So I’m going to be using the atmosphere as the reservoir, and then either these large monthly–month by month fluxes or the smaller annual total fluxes to compute the residence time. So residence time is for CO2.
Chapter 3: Residence Time of Carbon Dioxide in the Atmosphere [00:23:48]
On the month by month, on the seasonal basis, I’ll take that 760 gigatons and divide it–here I’m just using the atmospheric number. I probably should have summed up the atmosphere and the ocean number, but I’m just using the atmospheric number. And when I do that, I get a pretty short time, 13 years. I get even a shorter time if I use, if I put the ocean numbers in there as well. So there’s this rapid cycling that’s taking place. But in a way it’s kind of irrelevant because it cancels itself out at the end of the year. So I’m not going to put a great deal of stock in this number. It is a residence time. It is computed in the proper way, but the fluxes we’re using are balanced over the course of a year. And so, the numbers is not of great use.
On the longer term, I’ll take 760 and divide it by one of those imbalances. Now again, I’ve used the imbalance for just the terrestrial biomass. Maybe I could have changed that to four by adding up two here and two there. But you can see what that will do to the numbers pretty easily. I’ve just used two gigatons per year here. And I got 380 years. Now this is a more useful number because this is actually how long a carbon dioxide molecule is likely to stay in the atmosphere before it’s returned into one of the biosphere sinks.
So when we’re trying to predict the future of carbon dioxide concentrations in the atmosphere, we have to realize that it stays there for quite a long period of time in spite of these fluxes back and forth between the atmosphere and the biosphere. Questions there?
So from economic data, we can get a pretty good handle on the carbon dioxide emissions that have taken place in past years. And here you see various curves. The black is the total, where all the others have been summed up. The two largest independent sources have been coal, in the green, and petroleum, in the dark blue. Each of those is about six billion metric tons of carbon per year. So this is not accumulated. This is what’s emitted each year. And of course, that number has been growing since the beginning of the Industrial Revolution, where we began to use fossil fuels for steam engines and then for electricity generation and transportation and so on. Natural gas is growing rapidly now. Cement production is lower but still growing. And a small amount is the gas flaring that you see in some of these oils fields and gas fields, just burning off excess gas.
So it’s rising very rapidly. No sign at all of any decrease in this. There have been some temporary–for example, in the 1970s there was a kind of an oil crisis, and you see a tiny little bit of a dip there. But generally it’s just a curve that’s rising very, very steeply. And this is the rate at which we are putting CO2 into the atmosphere. This is not the accumulated amount. Questions there?
Chapter 4: Carbon Dioxide Concentration Trends [00:27:31]
So we’ve been monitoring carbon dioxide concentration. The first measurement started on the Mauna Loa Observatory, up on the Big Island of Hawaii, up on top of the high volcanic peak there. There’s a picture of the laboratory. And I know you’ve seen this before, and now I know that you’re familiar with it because you’re working with it in your lab. This is exactly the same data set that I gave you to work on in Lab Number Four. And the features here, of course, plotting–the units are in parts per million by volume, not by mass. So it’s by molecule. It’s a ratio, for every 320–for every million molecules of air, there are 320 molecules of carbon dioxide. That’s the way you would read that number.
There’s an annual oscillation that has to do with the–mostly the Northern Hemisphere biomass drawing carbon dioxide in in the summer and releasing it in the winter. And then on top of that, you’ve got a trend, which you can see from the smoothed line here. And that slope has been increasing. And of course, the increase in that slope is because of this, the rate at which we’ve been putting CO2 into the atmosphere has been increasing. So that rate of change of carbon dioxide concentration has been increasing as well.
So any questions on the–it’s called the Keeling curve because the scientist that got this started back in the ’50s was Charles Keeling. He passed away a couple of years ago, but he’s one of the great heroes in climate science. To have the forethought to get an accurate measurement of this type going early enough, so by the time we got to the year 2000, we had a good record and the beginning of a clear understanding of what’s causing this rise.
Now here’s another diagram from the IPCC report that’s a really important one. You can read through the legend, but I put these little labels up here as well. This is the annual change in carbon dioxide concentration in parts per million by volume. And the bars are the actual annual change. And if that curve looks familiar, if those bar graph looks familiar, it’s because you computed that very same quantity in your lab exercise. So you’re familiar with how to do this. And the increase started out at about 0.5 parts per million change per year and is now about three times that amount of increase per year.
Up above is the expected change from emissions if all the CO2 we’re putting in, namely that, remained in the atmosphere, and it’s about twice what we’re actually finding as the increase every year in the atmosphere. So here’s what we’ve learned from this. The biomass is taking in about half of that extra that we are emitting into the atmosphere. That’s rather remarkable. The reason seems to be that carbon dioxide is in many places around the earth and the ocean a limiting quantity for photosynthesis. So that when you increase the concentration of CO2, you increase the rate at which plants grow. It’s called CO2 fertilization. So as CO2 has increased, the plants have tried to take some of that excess back out. They take about half of it out. There’s some speculation about whether they will continue to be able to take out half, because they need other nutrients as well. They need phosphorus and nitrogen, they need water, if they’re plants growing on land. And it might be that as these other ingredients become limiting, the biosphere will no longer be able to do this for us, and these numbers could creep up. As this one increases, these might actually creep up eventually to become more equal to the amount that we’re putting in every year. Questions on that?
So it’s interesting to look at this from a historical perspective because what we’re doing now is so unusual and so rapid. So if we go back 10,000 years, this roughly covers the Holocene. Here is carbon dioxide concentration in parts per million by volume. And of course, the Keeling curve is the little red part here, it’s just that little part. Before that time, we get that data from the ice cores, the little vesicles, the little bubbles in the ice cores give us the data prior to that time. But it was pretty flat through the Holocene, with a value of about 270 or 280 or 290 parts per million. And then in the Industrial Revolution, on this time scale, it shoots up almost vertically. If you blow that up, blow that time scale up, and show it here going back to 1800, it was slow at first and then has become more and more rapid, as we discussed in the previous couple of diagrams because the rate of carbon dioxide emissions has increased so strongly.
We can go back even further in geologic time. And you’ve seen this kind of thing before. This is an ice core showing carbon dioxide concentrations. Here we are in the Holocene, right about here, with preindustrial levels around 270 or 280. But just prior to that, 15,000, 20,000 years ago at the end of last Ice Age, the values were down about 200 hundred, and then oscillated around those lower levels for at least 400,000 years. It’s hard to put this recent data even on the same scale because it plots as almost a vertical line. So the rate at which we’re putting CO2 in and changing atmospheric composition is far higher than anything one could diagnose from previous geologic eras.
Chapter 5: Carbon Isotopes [00:34:40]
There’s another piece to this argument that I find kind of interesting. It has to do with the isotopes of carbon. Carbon, like hydrogen and oxygen that we spoke about earlier in the week, come as a primary element and then various heavier isotopes. And most carbon is Carbon 12, but there’s a Carbon 13 as well. It’s a stable isotope of carbon. And the buried fossil fuel happens to be depleted in Carbon 13 relative to the normal Carbon 12. So when we dig up this fossil fuel and burn it, we’re now putting a lot of the–dominantly we’re putting in the Carbon 12 into the atmosphere, which takes whatever fraction we started with, Carbon 13 to Carbon 12 in the atmosphere, and making the overall carbon in the atmosphere lighter, because we’re mixing in more of the light carbon from the fossil fuel burning. And indeed, if you plot Delta Carbon 13, using the same kind of Delta notation we used before, from 1977 to 2002, it’s a decreasing number. This is data from the South Pole. Here’s some data from Mauna Loa. And it shows also a decreasing trend.
So some would argue, how do we know that that new carbon dioxide in the atmosphere is coming from the burning of fossil fuels? Well, one argument would have been, well we’re putting twice that much in the atmosphere. It makes sense that at least half of that would stick around. It’s a pretty powerful argument. But maybe this is even a more powerful argument, because that carbon we’re putting it has a certain isotopic signature. And now we see that signature showing up in the atmospheric carbon. So it’s another argument for the case that the burning of fossil fuels is what’s leading to that increase in carbon dioxide in the atmosphere.
Questions on that. Yes?
Student: Do we know why it’s depleted in the heavier isotope?
Professor Ron Smith: Yeah, it has to do with the plants that originally took in that carbon. So when a plant grows, as I mentioned, it takes carbon in from the atmosphere to build its biomass. It actually prefers the light isotope over the heavy isotope. So when those plants, way back, 100 million years ago, were growing their biomass, they were doing it preferentially out of Carbon 12. And then when they got buried, that left behind a lot of the Carbon 13 in the atmosphere. They buried a lot of the Carbon 12. Now we’re reversing that, putting the Carbon 12 back in the atmosphere. So it has to do it how plants draw in CO2 into their leaves and then use that to build their biomass. Good question.
So let’s conclude, then, our brief discussion of the carbon cycle. Fossil fuel burning is quickly returning to the atmosphere carbon stored for millions of years in crustal reservoirs. About half of the emitted CO2 stays in the atmosphere, and we estimate it’ll stay there for at least a few hundred years, given the cycling rate between the biosphere and the atmosphere. The CO2 emission rate has increased with time since the beginning of the Industrial Revolution. And the current CO2 concentration, which is approximately 395 parts per million by volume, is the highest in several million years. Let me be careful of what I say here. If I go back to this diagram, which shows the last 400,000 years, clearly, now that we’re up at 390 or 395, that’s way higher than anything we’ve had during the Ice Ages. Yet if you go back way earlier, say 50, 60, 100 million years ago, you’ll find periods in the earth’s history where the carbon dioxide concentration in the atmosphere was actually much higher than it is even today. So I don’t want to make a blanket statement saying that it’s as high as it’s ever been. So instead I have said, it is the highest it’s been in several million years. But again, this is the kind of an argument that arises. Someone will say, well, yes, we’re putting a lot of CO2 in the atmosphere, but it hasn’t by any means reached an unprecedented level. There have been periods in the ancient earth history where carbon dioxide concentrations have been much higher than this. So it all depends on what you want to use for your reference timeframe as to what kind of a statement you make in this regard. So be very careful about that not to overstate the uniqueness of the current carbon dioxide concentration. It’s probably safe to say there’s never been a time in Earth’s history where the CO2 is increasing as rapidly as it is today, where the rate has been as large. But in terms of the absolute amount, the record we’re setting now is only the record for the last few million years, not for–certainly not for all of Earth’s history.
Chapter 6: The Holocene as a Climatic Reference Time Period [00:40:41]
So we’ve got a couple minutes to begin the next section, which is the Holocene as a reference time period. And I wanted you to be aware of these particular often discussed dates. The retreat of the last large ice sheets was occurring around 11,000 years before present. That’s right when the ice was in the process of melting back. There was a warm period that followed that called the Holocene Optimum about seven to eight thousand years ago. That comes into the argument when one is trying to find out whether the current warm climate is unique even in the Holocene. In other words, is our current climate cooler or warmer than this so-called Holocene Optimum.
Much more recently, the Medieval Warm Period, which was roughly 1000 A.D., is another important reference point. This, by the way, is the period when the Vikings colonized Greenland. There was a warm period when they could grow crops in the coastal regions of Greenland. Then came the Little Ice Age from 1400 to 1800 A.D. I’ll say a bit more about that. And then we get into this period, as you’re seeing in your data analysis in the lab, of a more rapid climate change over the last hundred years or so.
So at the very minimum, be aware of these. And I’ll point these out to you as we look at various historical reconstructions of temperature. For example, this one goes back 12,000 years, so that’s covering most of the Holocene. And there’s lots of different data sets represented. The black curve is an average from a number of different proxy data sets. A proxy data set is where you haven’t measured temperature directly with a thermometer, but you’re keeping track of something that you think depends on temperature. Maybe it’s lake levels. Maybe it’s isotopes. Maybe it’s–there’s a lot of different things you can use. They’re all called climate proxies, and they show a lot of variability when you average them together they show some trend. You see a rapid warming coming out of the last Ice Age. Then you see this climatic optimum, what I called the Holocene Optimum, right here. And then it cooled off a little bit. And then there’s some wiggles near the end that we’ll talk about in just a minute. And again these are plotted as temperature anomalies. They’ve chosen some period of time as a reference to compute the temperature anomaly.
This shows the last thousand years or so. So we’re looking at a more recent period of time. And this is from the IPCC report, I think. Now I’ve forgotten which is which. Anyway, these are two different authors’ graphs describing the Northern Hemisphere temperature reconstructions for the last thousand years. And they look a lot alike, but I wanted to point out some differences, because it’s on these differences that many arguments will be built. For example, this author has shown a lot of the different proxies. And if you average those together, you’d probably get something that rose a little bit in this period of time, then sank a little bit more. Here’s the Little Ice Age, a little cooler in this period of time, and then you get the rapid rise and rise again.
This author has suppressed all the different proxies and instead just shown the standard deviation in the data. I think it’s like the two sigma and the one sigma. And while he does show a little bit of a rise, the way he’s plotted it gives you the idea that this is more of a straight line, maybe slightly decreasing, and then rising rapidly at the end. By the way, this is the famous hockey stick diagram. Because that shape looks a bit like a hockey stick. It used to be, twenty years ago, when you Google hockey stick, all you would get would be websites about hockey. Now when you Google hockey stick, you get hockey and you get climate change because of all the controversy over this.
The author this original diagram, Mike Mann, was a PhD from our department. In fact, I sat on his committee. This wasn’t in his thesis, it was in following work. But I’ve tried to follow the arguments. The poor guy has been badgered by the right wing anti-global warming crowd incessantly. Because the implication of this diagram is that what’s happened in the past hundred years is unique in regards to what’s happened over the last thousand years. But it seems to me you’d get that impression from this curve as well, even though they’ve given you more of a warming period in here, in Medieval Warm Period. Still, what’s happening today in steepness and in height seems to be unique. So all the arguments and the dispute over the details of how this diagram were constructed seem to be kind of moot now because we see such a rapid rise the last 20 or 30 years.
So, we’re out of time today, but we’ll be continuing this on Monday.
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