In this session, we're going to try to find the answer to some really important questions. For instance, do we know for sure that the climate is changing and what could be the causes for this changing climate as we shall see? The key message of these and the following sessions are going to be the following. We are virtually certain that the temperature is changing, past changes in temperature have been correlated strongly correlated in the past. And again, I'm using the word correlated carefully with changes in concentration of Co2. It is virtually certain that the changes in Co2 concentration today are anthropogenic and it is very likely that the changes in climate and anthropogenic notice the difference changes in concentration of Co2 changes in climate. Also in this session, we're going to define what I call with temperature anomalies and explain what they are essential to understand temperature changes. And we are going to discuss some global trends for temperature anomalies. This is how we can tell whether, for whatever reason climate is changing. And then we're going to examine some skeptical arguments before getting started. A question that is often asked is climate science proper science. How do scientists reach conclusions? That hypothesis is worth taking seriously? Well to begin with, the theory must be consistent with data and when the data are noisy, the theory must be consistent with data coming from different sources. This gives us confidence that we are. What we're measuring is actually when it is difficult to measure is actually robust. Also, it is not just a matter of collecting lots of data point. There must be a theory to explain the existing data. A correlation is not enough and this theory must do more than explain things that we have already observed. The theory ideally should be able to predict new phenomena or to explain observation that it was not built to explain in the first place. So how does climate science score on this criteria. Well, you'd be surprised to know that data about temperature and Co2 concentration now and in the distant past from very different sources are broadly consistent. This goes to the first point when my data is noisy, I want to make sure that different sources give me roughly the same picture. We do have a solid model to link Co2 concentration to the temperature of the planet. And when I say over planet, I don't just mean the earth in the future session, we will look at a planetary climate model which will explain the temperature of the Earth of Venus, of mass of Mercury etcetera. So we're not just relying on concentration and you will also be surprised to know that global warming was predicted more than a century ago. The first important person who wrote something in this area was for here in 1820 for Tyndall in 1859 show that some gases trap infrared radiation. And in 1896 Arenas produced the first simple climate model in which Co2 played an important role. So climate science takes all the main boxes or solid science. There is far more to climate science than drawing pretty hockey sticks a few more things to keep in mind. Climate change. Climate change has become a political and ideological minefields. Being skeptical is always good. Being a denier is always bad. Being source aware is essential to be very careful. If you look fixed up to look things up on Google or Wikipedia, it can it can be dangerous sources. Looking over sources is key. Okay, so let's go into the heart of the matter to get started. Let's define what we mean by climate and how this differs from weather. And then we're going to focus on one particular aspect of climate, which is temperature. There is a lot more to climate than just temperature. However, temperature is clearly defined and we can summarize in one number. It is relatively and I stress relatively easy to measure. We have direct and indirect record of past temperature. And if we have a handle of temperature, we can explain many other things because the effect of changes in temperature can bring about the most severe consequences for humankind. And these go beyond the increase in temperature itself, but they can affect sea levels, human forced migration, spread of diseases, etcetera, etcetera. So our main focus is going to be on temperature. So what is climate? I have here a good definition from a book by Note House, which is called Climate. Climate, casino and climate is the statistical average and variability of temperature over a period of time ranging from months to thousands of years. Climate change is a change in the statistical properties over long periods of time. So climate is distinguished from weather, which is a realization of a climatic process for a short period of time. Okay, so let's focus on temperature changes Measuring temperature changes over the last 100 years. Doesn't tell us very much, we need to know how this temperature changes compare to what we have observed in the past and when I say the past, we have to distinguish historical past, biological past and geological past. Only from this perspective can we gauge whether recent temperature changes are exceptions, how the planet coped with past temperature changes and whether recent temperature changes are likely to be anthropogenic? Remember we only have a decent chance of altering the course of temperature changes if these are anthropogenic, if we are responsible for it is bad news and good news, bad news for obvious reasons, but good news because we can try to do something about it. If a change in temperature we're due for instance, to a change in irradiate from the sun, it would be extremely more difficult to do something about it. So let's start with temperature anomalies. Scientists always talk about temperature anomalies never speak about temperature itself. Climate scientists, and what is the temperature anomaly? And what is so good about it? The temperature anomaly is the difference between the temperature at a given location at a given point in time and the temperature at the same location at a different point in time. So it is the difference in temperature which at different points between the temperature measured at a reference time and a later time. So why do we want to look at temperature anomalies instead of temperatures directly? Well, temperature at the same time can vary a lot just by moving over relatively little distances. If I am in a city and I take my car and I drive out of the city, the temperature can easily go down by two centigrade. Anomalies are constant over much longer distances, so it might be true that the countryside around London in winter might be 2 Centigrade cooler than London itself. However, if over a long period of time, London becomes half a degree warmer. The increase in temperature in the surrounding countryside is going to be very close to half a degree as well. So, despite baseline differences, despite the fact that London and the countryside around it have different temperature at the same point in time, the temperature in the countryside will also increase or decrease roughly by the same amount as the temperature in London. So the key point is that a given temperature anomaly tends to extend over hundreds and sometimes thousands of kilometers. So these are the scales were interested in. And as a consequence of this constancy of temperature anomalies over wide areas, we only need fewer than 1,000 records of temperature and anomalies to get a sufficiently accurate picture for the whole Earth. So let's start from the easy questions. What can we say about short term changes in temperature anomalies? We can produce lots of charts showing changes in temperature anomalies. A different locations, et cetera. However, we can extract a simple message that I'm going to show you in pictures one and two. And the simple message is that over the last 130 years temperature has certainly been rising. So this is my first light that shows this point. And there is the mean land ocean temperature anomaly. So look on the Y axis you have 0.5. There is a temperature anomaly over the last 130 years. The reference is 1951, and clearly the red line is a smooth curve that goes through the annual mean, which are the black dots. And the gray band gives you the uncertainty around the measurement. And this is another very interesting graph that shows you average seasonal variation over the last 130 years and the different colors show the seasonal variation of temperature anomaly recorded from the 1880s all the way to 2019. And clearly the biggest effect is they want you to a seasonal variation in July and August. We are in the Northern Hemisphere are warmer than January and February, but you can see that there is a steady increase in colors and the colors are linked to the dates as shown in this slide here from 1880 to 2019, so yeah, there seemed to be little doubt that the temperature is increasing. This hasn't stopped some, I'll call deniers to say the planet has stopped warming over the last 15 years. Comma data shows this is from Fox News, careful about making statements about carefully chosen periods of 10 or 20 years. It is very easy to show from the data that in any segment of 10, 20 years the temperature has not risen at all. Or perhaps it has even gone down. Andrew Dressler is on a very interesting exercise and it shows that you can even choose seven consecutive segments from 1970 to 2015. Such that the best fit in every single segment shows a decline in temperature. And this is what the result of what Dressler did is in every segment. The temperature goes down a tiny little bit. And clearly those segments have been tendentiously chosen in order to make this point. But look at the same data which is the green curve. And this is now a best fit linear interpolation to the whole data. So this is how realist sees temperature change. This is how a skeptic. I shouldn't use skeptic because skepticism as I said, is good. You should always be skeptic. But that is, I would say a denier can present temperature change. So unless we subscribe to the Fox News will view of the world end of statistics. It is pretty uncontroversial that global temperature as Ghana over the last 130 years before we get too excited. The big question is how exceptional at the last 130 years been? Have we been there already? Well, now things become a lot more interesting.
