Let's take stock of what we've done so far. We have seen presented evidence that temperature anomalies have definitely been rising in the post-industrial era. That is one piece of information. We have established correlation between the increase in temperature anomaly and the increase in CO_2 concentration. So far, we have not presented a model linking the two, but we have certainly seen that the two things are in sync. We've also explored what has happened to the climate in the past. Why is this important? Because we want to put the present climate change. We have established that the climate is changing and is natural to ask, is this change exceptional? One with a tentative conclusions we have reached so far, this change is not exceptional on a geological scale, but is definitely exceptional during a historical timeframe of human life. There are still lots of pieces of a puzzle that we have to put together. One important piece of the puzzle is what we're beginning to look at with this session. It is trying to build a case on whether there can be other reasons for these increasing the temperature anomaly other than human activity. Here, I have chosen a picture which is a winter landscape in Northern Europe, circa 1600. What we're seeing is country dwellers, peasants, or burgers, happily skating on a river in Northern Europe, something that you would be ill-advised to try and do today. In the years of Shakespeare, in winter, if you had been so inclined, you could have been skating on the Thames and other activity that say, don't recommend to attempt today in winter in London. Since in 1600 or thereabouts, humans were not emitting a lot of CO_2 because of burning of fossil fuels. One could reasonably ask, perhaps these changes outgrew. Perhaps yes, the temperature is changing, but has nothing to do with human activity. Can we build a case as to why for current change in temperature anomaly is due to human activities? What else could be at play? What else could be contributing to the increase in temperature? If you think about, the first thing is a change in how much energy the sun emits. This is not constant,. It changes over time. Perhaps we are experiencing a warmer period now simply because the irradiance, the amount of energy emitted by the sun is now increasing. Also, we shall see in some detail in the rest of the session, there are periodic changes in the orbit of the Earth. What matters? It is not just the energy emitted by the sun, but it's also the energy received by the Earth due to it's inclination of its orbit, etc. We must not forget the possibility of non-human emissions such as volcanic emissions. In this session, we're trying to answer the question, can these non-anthropogenic mechanisms convincingly account for the current observed temperature change? If the answer is no and we have data, and mechanisms for an anthropogenic forcing, then we can be confident that human emissions of CO_2 are at the root of a current increase in temperature. This is how we're building our case and it is how this is linking to our previous sessions. Let's begin from most obvious suspect. The sun is our fundamental source of energy. The amount of energy emitted by the sun is not constant. If this energy changes over time, it will have an obvious impact. Also, we have to take into account how much energy is received by the Earth and how much energy is retained. For instance, it is known that a decrease in solar activity, coupled with an increase in volcanic activity, is what we think has triggered the Little Ice Age that lasted approximately between 1650 and 1850. Let's look at these three possibilities in some detail. Let's start from irradiance. Can the current changes in climate perhaps be reconciled with differences in irradiance? Here, I have a picture, that shows with a red line, the changing temperature from 1880 up to very recent dates. The yellow line depicts the solar irradiance. If you look carefully you see a thin red line, which is the annual variation, and the thick line, both yellow and green, depict the moving averages, but it gives a bit more of a stable picture. You can see that roughly up to the 1950s, we have a pretty good correlation between the two trends. But after the 1950s, they have decoupled rather radically. If anything, the solar irradiance has been going down exactly when the temperature anomaly has begin to climb, most decisively. Therefore, at first blush, it is very difficult to impute to explain the increase in temperature with a change in solar irradiance. So much for the energy received from the sun. What about changes in the Earth's orbit? The Earth's orbit changes in three ways. From our high school textbook, we know that the orbit of the Earth around the sun, is an ellipsis. But in the high school textbook, they show the ellipsis as an extremely eccentric one, extremely elongated one. In reality, the orbit is pretty close to being circular, with a tiny bit of eccentricity. Eccentricity of zero, means a circular orbit. Now, this eccentricity, obviously, has an impact on how much energy is received, and varies over time. That is the first change. The second one, is the change in the tilt of the axis of rotation of the Earth. This is, as we know, is what gives rise to the season and seasonal changes. Furthermore, if we think of a rotation of the Earth around this rotation axis, this axis wobbles over time. All these things, with different periodicities, give changes to the amount of energy received by the Earth. This orbital and axial variations, produce long-term natural climate cycles of ice ages and warm periods, glacial and interglacial periods. This picture here, shows the three sources of variations of the eccentricity, of the obliquity, and of the precession. Calculating these things with Newton dynamics, is relatively easy to do, but we are not talking about quantum physics or general relativity. Classical mechanics is perfectly fine. Here there are calculations of changes over time. On the x-axis, I have thousands of years, before the present, going from zero to 2,000 and 5,000 of years. I have eccentricity as the top, obliquity in the middle, and precession at the bottom. Eccentricity is when the number is zero, it means that the orbit is almost exactly circular. As the number increases it's more elongated. The obliquity, it is the inclination of the axis, so 22, 23, 24 degrees. They are expressed in degrees. The procession, which is this, is a very small angle that you can see there and is expressed as a sine of Omega, sine over precession angle. Since Omega is so small, you could just say Omega just is equal sine of x is almost equal to x, when x is very small. These are the calculations. These are the observed changes. I draw your attention over the same scale, changes in temperature. I draw your attention to the bottom part of the graph. At first blush, you see, yes, there are some periodicities, but actually it looks a lot more complicated. The reason why it is more complicated, because here we have three super-imposed phenomena, with very different frequencies. What scientists do, is what is called a Fourier decomposition, which is sorting out with different characteristic frequencies of this signal here. What we have at the bottom is the change in oxygen 18, we have seen how oxygen 18 conveys information about the temperature. Higher values means warmer, lower values means cooler. Milankovitch cycles tie the climate of the Earth to the slow and regular variations of the Earth's orbit. The scientific community was expecting that past climate change must also been slow and gradual, but as we have seen, sometimes we have had very abrupt instances, or rather instances of very abrupt climate change. Could this be explained to the Milankovitch cycles? Scientists have tried to reproduce these abrupt changes in climate using different combination of Milankovitch cycles. But they could not get the models to simulate observed past changes in climate unless they also added big changes in carbon dioxide levels. In the past, when the climate warmed, the change was accompanied by an increase in greenhouse gases. I'm very careful when saying accompanied and not putting an explicit cause of link because it works both ways. If the temperature increases, this gives rise to mechanisms that we have seen, for example, a permafrost that increase the concentration of CO_2 that in turns increases the temperature. When we look at past records, we find that sometimes changes in temperature can slightly had earlier, and then changes in CO_2. Sometimes we see the opposite. It's a complex phenomenon. In any case, ice core measurements reveal the carbon dioxides are now at much higher levels. It's at anytime, roughly in the last 800,000 years. If we could find a mechanism to link CO_2 concentration to temperature, there will be a very strong reason to believe that this high concentration of carbon dioxide are causing temperature increase. Since we can establish a link, obvious link, between carbon in the atmosphere and the burning of fossil fuels in the post-industrial age, then we have a solid link that creates an anthropogenic explanation for global warming. Have CO_2 concentration really increase in recent times? Well, let's look at some pictures. This picture here goes back 800,000 years and shows the concentration of CO_2. You can see at the far right, the spike, which is totally unprecedented up until 1950. We could argue, yeah, we are within oscillation that we've seen in the past, but then it's just spiking up beyond anything that we observed almost in the last million years. Then I can blow up, and I can go with the magnifying lens, this picture is split in two, the last roughly 1,000 years and then post the Industrial Revolution. It is the famous hockey stick picture. You see that there is a very sharp rise. At the onset of the Industrial Revolution. There is the well-known hockey stick graph. But as we shall see, there is a lot more to establishing these links and drawing pretty hockey stick graphs, but the fact that it is there is obviously important. From 1769 -2006, World Annual co-production has increased almost by a factor of 1,000. Production of coal and other fossil fuels is still increasing. The burning of fossil fuels is the principal reason why CO_2 concentration has gone up. Not everybody agrees, and it is difficult to disagree when we look at the synchronicity of the increase in emission of CO_2 because of coal and other fossil fuels burning during the Industrial Revolution, which is shown by this graph here. Bringing over things together, we have established a very strong correlation between past and current episodes of climate change and changes in CO_2 concentration. We've looked at the Milankovitch cycles and we have seen that on very long periods, ice ages and the like, they have a strong explanatory power, but they do not account for the abrupt climate changes that we have observed in the past. In order to explain this abrupt climate changes, we have to invoke changes in CO_2 concentration. We have seen convincing graph that correlate changes in CO_2 concentrations with the burning of fossil fuel that came with the onset of the Industrial Revolution. What we have at the moment is a strong indication that the increase in CO_2 concentration is anthropogenic. Therefore that the increasing temperature we are observing at the moment is also anthropogenic. What we still need to put together is a model that links the CO_2 in the atmosphere to the increase in temperature. It is what we're going to do next.
