[MUSIC] I would like to start this session with a beautiful picture. This is a picture of a surface of Mars and as we know today, Mars is an extremely dry planet. But if you look at this picture carefully, we can see clear indications that they used to be flows of liquid rivers on Mars. And this raises a question, well, if there were rivers on Mars, what caused the changes in the climate of Mars? And the scary question is could similar changes happen on planet earth if global warming goes out of hand. So this is just to give a bit of a interesting motivation to this session. In this session, we're going to ask questions this. How do we know what the past climate was like? And what do we know about past climate? What does it matter to understand what past climate was like? Can the changing temperature on earth can have s catastrophic consequences on earth as perhaps it has happened on other planets? Could the earth become like Mars as we shall see the key messages are that we have been there already, but we as human beings, we're not around. And piece of good news know, we do not expect fossil fuel driven climate change to turn the earth into another mass. So why bother? Why should we be interested in past climate change? Paleoclimate data yield our best indications of what is called the climate sensitivity. Which is how much with temperature changes in response to a specified climate forcing. Forcing is a concept we're going to meet over and over again in the future sessions. Forcing is the difference between energy in and energy out, so a climate forcing is an imposed change of the earth energy balance. Forcing can be due to a variety of causes. It can be due to a change in the sun's brightness. It can be due a change in the earth orbital cycles. It could be even be due to a change in plate tectonics, and it could be due to changes induced by human activity. And for convenience, scientists often consider a standard forcing, which is called double atmospheric CO2 forcing, which is the level forcing. They would double the concentration of atmospheric CO2 with respect to the pre-industrial levels. We have to be careful forcing its energy in minus energy out and is directly thermo dynamically linked to temperature. However, forcing can be caused by factors other than increased CO2 concentration. So the standard forcing of doubling CO2 concentration focuses on the CO2 concentration to forcing. And assumes that we know with confidence with links between forcing CO2 concentrations and temperature. As we shall see, we know something about these links, but nowhere as precisely as we wished we could. We still have to make the case that CO2 is the main cause of forcing and that the increasing CO2 is anthropogenic.. But let's go in step. So past climate is important to put the current climate into perspective and to predict future climate. But how do we measure passed climate? The problem is that temperatures have only been recorded since we made 1700s. And therefore scientists have to rely on what are called climate or temperature proxies. Water proxies process of quantities which are related to temperature. They're not exactly temperature, but they can be used to infer the temperature and how it has changed over time. I'm going to discuss in detail one particular technique to infer past climate. It is not the only technique, it is the one with the highest resolution and the reason why I'm going to go into some detail in explaining how it works. Because I want to give you a clear idea of a degree of ingenuity, difficulty and how delicate these measurements are. Whenever you read about past climate and past measurements of anything. So I want to give you a flavor of the difficulties inherent to climate measurement. So there are several proxies. Tree rings in dry, warm, wet cold ears, trees grows to a different extent and the width of tree rings changes, ice core, fossil poland ocean sediments, corals. These are all possible process. They all offer different degree of certainty. They go back in the past two different extent and they all have different degrees of resolution. Ice cores of as one of the highest resolutions every going to look at that in details. So these are some pictures of ice core distraction. You go to the pole, you drill a hole and you have the drilling machine. They're showing this picture and this whole is incredibly deep and you extract the core. And you segment the core and hopefully you put on some labels. Otherwise you got lots of pieces of ice and you don't know where that came from is a total mess. So you you label them carefully and this is how they are stored in Denver in a very cold place for obvious reasons. And from these we can reconstruct two things at the same time past temperature and passed C02 concentration. How do we do that? We have to measure, first of all the gas composition of the little air bubbles in the ice core. So the key thing to understand is the ice core. The depth in this ice core can be correlated to snowfalls that happened so many years ago. So from the depth in this on the ice core I can first reduce the temperature step one. Then I look at one particular layer in the ice core and I find little bubbles of air trapped in this record of past snowfalls. In this little bubble there are gases that were present in the atmosphere at the time of that snowfall many thousands of years ago. Using a mass spectrometer, I can analyze the CO2 concentration present in those bubbles. So that is, believe it or not this is the easy part of the exercise. Estimating past temperature is more difficult because it cannot be measured directly. If you want I have a tiny little time capsule in those bubbles. And that time capsule is really the air of 10 or 12 or 15,000 years ago when the ice fall happened. We can't do anything like this for the temperature. Past temperature must be obtained from the isotopic composition. I'll explain what this means of the water ice molecules contained in the ice cores. Now, what is the idea here? Water molecules are always made up of two atoms of hydrogen and one atom of oxygen, H2O. However, there are different isotopes of oxygen. What is an isotope? Every atom is made out of the electrons outside and the nucleus, the nuclear is made up of protons and neutrons. The protons, which are positively charged, are always in the same number as the electrons on the outside. So as a whole, the element is neutral. However, the neutrons can be in there roughly the same number as the protons. But they can be a few more than the protons without changing the chemical properties over substance. So nuclei, which have different numbers of neutrons are called isotopes. Different isotopes do not have different chemical properties because the number of protons and electrons is the same. However, they do have different weight. So there is a picture when I show the two most common oxygen isotopes, oxygen 16. Where there are as many neutrons as protons and oxygen 18 as there are two more neutrons than protons. There is also oxygen 17. But for reason we don't have to go into. It's much, much rarer. So what does this matter? Well, the details can get very complicated, but the intuition is very simple and I like to make these little picture in my mind. I suppose that I have a big tray. I'm holding a tray here and there are some rails on the side of the trade. And on this trade I have lots of balls and I have ping pong balls and I have tennis balls. Perhaps I have some billiget balls, etc. And I began shaking the tray. Now you don't expect the different balls all to have the same velocity. I expect the ping pong balls to rattle a lot more than the billget balls. However, from thermodynamics, I know that it's equilibrium. All the balls will have not the same velocity by the same kinetic energy. Which is one half mass of the ball, v squared big mass, smaller velocity, and vice versa. So the lighter balls with ping pong balls with small mass will be moving faster than the heavier balls with large mass. Why am I bringing up the ping pong in the 10 years and a billion balls? But clearly, the analogy here is that the heavier and lighter balls are the different isotopes of oxygen. So, a thermal equilibrium. The lighter balls, the lighter isotopes will be rattling faster than the heavier balls behavior isotopes. What does it matter? Because in order to evaporate, an atom must break free, must have enough velocity to break free of liquid. Therefore, the lighter isotopes will evaporate a bit more easily than the heavier one. Now, we are halfway through to understanding what is called fractionation, which is creating vapor of a certain substance with different isotopic weights. Let's see how it works. So, back to the oxygen, oxygen camps in three isotopes, oxygen 16, oxygen 17, and oxygen 18. Oxygen 16 is the communist and 17 oxygen 16 by far the rarest. So we're not going to look at oxygen 17 at all. Oxygen 18 is two extra nutrients, and oxygen 16 and therefore it is a bit heavier. As a consequence, at a given temperature, less water with oxygen 18. Will evaporate, then water with oxygen 16. As oxygen 16, I'm going to say oxygen 16 is a shorthand for water with oxygen 16. So as oxygen 16 evaporates more easily. The first water vapor to evaporate as relatively more oxygen 16 than oxygen 18. And the residual liquid is enriched in oxygen 18. And we just have to keep one more thing in mind and we are almost done to understand fantastically interesting thing. It also works in reverse when the vapor condenses into liquid. The heavier water with oxygen 18 condenses a bit more easily and the lighter oxygen 16 water preferentially remains in the vapor. Given this, let's try to explain why they find the water molecules in ice cores always have independent of temperature changes. A lower ratio of oxygen 18 than water at warmer latitudes. And let's start from ocean water around the equator. And I have some pictures here. They will help us understand what is happening. So let's start from the close to the equator. I have evaporation of water vapor and as we have seen before, the water vapor will contain more oxygen 16 than oxygen 18. Now this water vapor moves towards the pole and there is precipitation. And as we have seen, there is more oxygen 18 falling as water as I go towards the pole. So the water vapor becomes richer and richer in oxygen 16. And it continues and it continues. Finally, I arrive at the north pole and there is a final precipitation and it's a final precipitation of water vapor. That contains more oxygen 16 than the water from which it evaporated every beginning at the equator. This is there is no temperature changes in any of this. However, let's assume now that the earth is a bit warmer. If the earth is a bit warmer at the equator, there was a bit more oxygen 18 in the water vapor that evaporated. And during the different rainfalls a bit less oxygen 18 fell as water than when the temperature was called. Therefore, when the temperature increases, we will find that there's no precipitation over north pole will contain more oxygen 18 than with the temperature was lower. And the effect is very delicate. I am looking at a difference in concentration of oxygen 18 over oxygen 16. And I'm looking at a difference indifference because I'm looking at the temperature effect of this phenomenon. luckily it is strong enough this effect for us to be able to draw a beautiful graphs such as this one. It shows a concentration of oxygen 18 as a function of the temperature. So that is on the y axis. I have changes in oxygen 18. And as you can see, there is an almost linear relationship between the temperature and the concentration of oxygen 18. So this is the relation that can be used to calibrate what is called the isotope ratio thermometer. So we can begin to ask the question to answer the question. What does the temperature pattern look like? [MUSIC]
