Hi everyone! Now, you probably didn’t miss that in a few days will begin in Paris the “COP 21 The major international conference about climat change. And I thought this was the opportunity for me to answer to a question I’m frequently asked. Frequently, friends or familly ask me if I -“the science man”- believe in global warming. As you may know, I am originally no climatologist but rather a physicist But today we will actually try to see how you can get an idea on that subject by using the physician’s perspective. SHOULD YOU BELIEVE IN GLOBAL WARMING? To start, lets recall the basic reasoning generally held about global warming. This reasoning holds 3 points: 1: Human activities release CO2 2: CO2 is a greenhouse gas 3: Greenhouse effect warms up the planet Now, let’s analyse each of these 3 points in detail. You maybe know that when you burn fuel which contains carbon (such as wood, petroleum or -as in here – propane), it produces carbon dioxide – the CO2- which is released in the atmosphere. The Earth’s atmosphere is composed by approximately 78% of Nitrogen N2,
21% of Oxygen O2
and 1% of Argon Ar. CO2 there is much less So we don’t measure it by percentage but in PPM. PPM means “parts per million” It’s like a percentage, but instead of 1/100, it is 1/1’000’000. Currently, CO2’s rate in the atmosphere is around 400ppm (or 0.04%) This is today’s value. But what’s interresting is to take a look at how this value has been evoluating for the past centuries. That’s something we can do because we know how to directly measure CO2 in the atmosphere for over 50 years and we are able to go far back in time thanks to the analysis of bubbles contained in ice core samples. Let’s look at the evolution of CO2’s concentration in the atmosphere. Let’s go back to 1000 A.D. You can see that at this time, and for centuries, the concentration stayed around 280 ppm, with some fluctuations… Then, around 1850 A.D, something happened. The industrial revolution happened, and CO2’s concentration in air started to rise… . rise… rise.. So much that I need to change the scale of my graphic. Actually, several times. That’s it! The actual record in 2015 is just over 400ppm. So I don’t think we need to fuss about this graphic. You can see the trend. To be perfectly clear on one point: no one has any doubt on this curve line. No one contests it. Even the most climato-skeptical of climato-skepticals is forced to admit that this is what is currently happening with CO2 values in the atmosphere. That’s for the 1st point, but before treating the 2nd, let’s look at the 3rd. 3. The idea that greenhouse effect warms up the planet You must know that Earth gains almost all its energy from solar radiance. As the surface of the sun is approximatively 5’500°C (9’930° F) the light it sends us is mainly visible light, UV rays, and what is called near-infra-red IR. Earth directly reflects part of those rays (±30%) and absorbs the rest. That’s what provides its energy. As any body warmed up to a certain temperature, earth also emits radiance. But because it’s far from being as hot as the sun, this radiance is not visible or UV but what’s called far-infra-red. You know, the one you can see with night vision binoculars. You must know that if there wasn’t any greenhouse effect all this far-infra-red radiance emited by earth would leak back to space, and ground’s temperatures would be ± -18°C (0° F) No need to do great climatology to find this number. It’s only 2 ligns of calculation if you know thermic ray’s physics. But we see that’s not what’s happening on earth. It’s not ± -18°C (0°F). The reason is that this far-infra-red radiancy emited by the earth is strongly absorbed by earth’s gases and is partly reemited towards earth. That creates an additional flux that warms the planet, which is what’s called “greenhouse effect”. And thanks to this geenhouse effect, earth average temperature is not -18°C (0°F) but 14°C (57°F) And you can find that in 5 or 6 calculation ligns. So you see that greenhouse effect is an undeniable phenomenon. No debate on it’s reality. There’s no need of huge climate model to demonstrate greenhouse effect. It’s just physics. Actually, thank god there is greenhouse effect, because It allows us to have an earth temperature around ±14°C (57°F) Rather than ± -18°C (0°F). Avoiding us to live in a ice desert Another famous illustration of greenhouse effect is that -what seems paradoxal- Venus’s surface is on average hotter than Mercury’s, despite Mercury being nearer to the sun. That is because Venus’s atmosphere is composed by 96% of CO2, which creates a huge greenhouse effect That brings us to our 2nd point: 2: CO2 is a greenhouse gas. Yeah, why CO2 and not oxygen for example? What makes a gas a greenhouse gas? As we saw before, to have greenhouse effect you need far-infra-red radiance to be absorbed by the atmosphere. So, without going into details, for a gas to be able to absorb infra-red, it must be composed by – at least 3 ATOMS in its molecule
– or eventually only 2 ATOMS, on the condition that the 2 atoms are DIFFERENT. CO2=3 atoms so greenhouse gas (YES) Oxygen (O2) and Nitrogen (N2)=NO
Both composed by identical atoms Methane (CH4)=YES Argon (Ar)=1 atom so NO Ozone (O3)=3 atoms so YES Yes, Ozone which layer protects us from UV is also a greenhouse gas. And by the way, all the “CFC” gases which destroy the ozone layer are also greenhouse gases. And… water steam (H2O)! Yes 3 atoms.
So YES water steam is a greenhouse gas It’s even one of the strongests. So now you maybe wonder: “Why p***ing us off with CO2 and never talk about water steam?” Really, if I boil some water to make myself some tea, do I contribute to greenhouse gases and global warming? Well, in fact no. There is a reason for that which is important to understand. It’s impossible to durably modify the quantity of water steam in the atmosphere. The reason for that is that water steam is in balance with oceans water. If we artificially try to raise the quantity of water steam in the atmosphere (for example we all make tea and take long hot showers all together) In a few days, oceans would have absorbed all the surplus and the concentration of water steam in the atmosphere would go back to normal. So yes, water steam is one of the most important greenhouse gases but we cannot durably change its concentration in the atmosphere contrary to CO2 that, as we saw, dangerously accumulates. So if we go back to our 3 points we see there’s no doubt on the mechanism of global warming. It’s not even climatology, just physics. And not even super difficult physics. So I hope I convinced you that the reasoning is incontestable. Except… Except that on such serious questions we cannot just be contempted by a qualitative reasoning. Ok, the mechanism is proven, but what does it make in numbers? SHOULD YOU BELIEVE IN GLOBAL WARMING? We saw before that CO2’s concentration in the atmosphere was 280ppm before the industrial revolution Since then, it rose by nearly 50%. This additional CO2 in the atmosphere since the industrial revolution provokes a bigger greenhouse effect than it should naturally be. To quantify this, we’ll translate this as a surplus of radiance due to this additionnal greenhouse effect. First, let’s look at what the sun sends us. If you went just above the atmosphere and faced the sun, face-to-face, you’d get an approximate 1366 W/m2 flux That’s the maximal value, but you should aknowledge 2 things. 1st: No spot on Earth is permanently facing the sun (orientation changes and half of the time is night). So what every m2 of earth averagely receives is only 1/4 of the maximal value 2nd: Earth directly reflects 30% of the radiance. That’s what’s called albedo. In total, if we calculate, earth receives an average sun radiance of 240W/m2 The effect of accumulated CO2 rejected in the atmosphere since the industrial revolution is equivalent to a surplus of radiation of approximately 2W/m2 If we add the other greenhouse gases, the total is approximately 3W/m2. This surplus of radiation is called the “radiative forcing”. A radiative forcing of 3W/m2 is like the sun was radiating us by 243W/m2 instead of 240. Or like if the sun was shining 1% stronger. That doesn’t look like a lot but that is what’s threatening of changing everything. * SMALL DIGRESSION: A classical climato-skeptial argument is that climate change we observe now is not due to human’s activity but to the natural variations of the sun’s power. The power that the sun emits does indeed vary naturally according to cycles. And we can even measure it. So you see the curve here. Behind the average number of 1366W/m2 I gave you before are variations of more or less 0.5W/m2 according to a cycle which lasts 11 years. This is the maximal value, once you divide by 4 and take the albedo of 30%, that makes a difference of more or less 0.1W/m2 on the ground. So the radiative forcing due to sun’s natural cycle is of 0.1W/m2. And we saw that radiative forcing due to greenhouse gases caused by human’s activity since the industrial revolution is 3W/m2. So yeah, there’s no comparison.* SHOULD YOU BELIEVE IN CLIMATE CHANGE? So we saw that greenhouse effect by gases emited by humans since the industrial revolution is equivalent of a radiative forcing of 3W/m2. The question now is: “what impact will it have on the average temperature on earth’s surface?” If you add these 3W/m2 in our simple model, we find that this additionnal greenhouse gas effect provoques a rise of the average temperature on earth’s surface by 1°C (2°F) Doesn’t look like a lot, 1°C (2°F). Except that this is for the actual concentration of greenhouse gases. If we continue to reject them as if nothing happens it will of course rise. And… There’s another factor to aknowledge: it’s water steam. So yes I told you that water steam is the most important greenhouse gas but that we cannot change it’s concentration in the atmosphere Except that there’s one thing that can change its concentration: if you raise the temperature. As I told you, the stability of water steam in the atmosphere is linked to its interaction with oceans. But you maybe know that hot air can contain more humidity than cold air so if the planet warms up, there would be more water steam in the atmosphere. If temperature rises because of greenhouse effect due to CO2 ->Atmosphere will stock more water steam ->which will create more greenhouse effect. ->which will rise global warming. We have here what’s called a “positive feedback” We call it “positive” feedback because the phenomenon reinforces itself, it auto-aggravates. Of course that is not positive for us at all. Other positive feedback like that exist. For example, banquise melting causes to reflect less solar rays ->so absorbs more, ->which aggravates global warming. These positive feedbacks could maybe lead to a “runaway phenomenon”, where global warming feeds itself and earths warms faster. We think that’s what happened on Venus. Thankfully, specialists estimate that it is not very probable to happen to us. What makes us think that is that 100 million years ago, the atmosphere on earth contained 5 to 10 times more CO2 than today and temperatures were probably superior of 5°C (9°F). And the planet coped with it. That means that with positive feedback also exist “negative feedback” (mechanisms that counterbalance the greenhouse effect) For example: clouds. With more water steam could come more clouds ->Which contributes to reject part of solar rays->contributes to moderate global warming. But estimating the exact contribution of these negative feedback is quite hard and the best way to do it is with very detailed models which take into account atmosphere, currents, oceans etc and simulate it. That’s what’s called a GENERAL CIRCULATION MODEL. So yes, with these models are uncertainties, due mainly to the fact that each model gives different credit to each positive and negative feedbacks. So when you simulate a situation with all models, they don’t give the same augmentations of temperature. This here for example is one scenario of emissions and each blue lign is the prediction of a model There are 42 models. All make a different prediction, so there is an uncertainty about the rise of temperatures. That comes from the GIEC, so contrary to what we sometimes hear, their results are totally transparent about having uncertainties of modelisation. But frankly, do you think it changes the message? So we have a phenomenon which is indiscutable in physics. 1. Humans emit CO2
2. CO2 is a greenhouse gas
3 Greenhouse effect warms up the planet The only blurred facts are if, with luck, negative feedback will do a little bit better than what we imagined. Let’s take an analogy: you want to jump from a window at the 6th floor Basic physics is rather clear on what will happen; you are going to fall and crash on the ground. But… But yes you always have uncertainties, because there could be a balcony on the fifth which you would land on… Or you fall on a huge pile of trash which would slow down your fall and you’ll survive. So there are some uncertainties But it’s not safe. So, do you jump? SHOULD YOU BELIEVE IN CLIMATE CHANGE? I hope my message is clear Yes, it is true there are uncertainties on the exact rise of temperature due to all the greenhouse gases we reject in the atmosphere. What you see here are GIEC’s simulations for the scenario in which we do nothing to change it. You see the average curve and the uncertainties linked to the model. So there’s a doubt to know if in 2100 we will have +3°C (5°F) because negative feedback would have been favorable or +5°C (9°F) We’ll probably be in between, but it’s not because there are still uncertainties that we should not do anything. (It was the sense of my analogy with jumping out the window) You know, we often talk about saving the planet but that’s not true, the planet doesn’t give sh*t. The planet will be ok. I told you there already has been periods of Earth where you had 5x more CO2 and +5°C (9°F). And Earth was perfectly fine. By the way, these were rather nice periods as it was in the jurassic (you know, the era of dinosaurs among others) No, Earth is going to be fine. But if climate change we talked about really happens, there is on species that will specifically suffer: Us. That’s it. So, I’m not going to lecture you, I think you know it. We all have to make efforts to try to use less energies, particullary fossil fuels. But it’s not certain that’s sufficient. I think science and technology will also have their role to play in it. and as a science poplarizer that is what I wanted to end on. I think that societies in which we live also have to make collective efforts to encourage scientific reseach that could permit to produce technologies to consume less energies. or produce cleaner energy or, I don’t know, cheaper and more efficients photovoltaic pannels or something around hydrogen or nuclear fusion… But what is sure is that will not be invented by itself. In brief, take the bike, take public transportation But also don’t forget to write to your deputy to tell him to support research in science and technology because it’s also a key to the problem. THANK YOU FOR WATCHING THIS VIDEO As always, don’t hesitate to share, morevover if you have friends who don’t really believe in climate change. As always, you can find me on social networks (facebook, twitter, the best places to get some info about the channel) Those who want can support me on tipeee.com. An infinite thanks for the tipers who support me And you can also find me on my blog called “science étonnante” Thank you. A bientôt.