- Today's lecture is very much a continuation of the last two lectures in my previous series. Specifically, I'm thinking of the lecture on Planetary Universe and also the one on Life in the Universe. And in this previous lecture, I talk quite a bit about what it is that makes a planet hospitable and a really good place to live. Well, let's just remind ourselves of the array of planets and, indeed, one dwarf planet that we don't really want to forget about, we love you Pluto, honestly, which is over here on the right-hand side. So these images are all taken from various different NASA missions. Obviously, we have the Sun on the left, and then Mercury, and then Venus, which I'll be talking about quite a bit today, then Earth, which will not be forgotten during the course of today, then Mars, then Jupiter, the prettiest of all the planets, Saturn, Uranus, Neptune, we're going to be talking about a little bit, and I'm afraid Pluto doesn't get a mention in today's lecture at least. So Venus is often regarded as Earth's twin. I don't think that's a very good analogy at all for reasons that I'll develop during the course of the lecture. But it's certainly one of the neighbors, and we do well to look at the neighbors and compare and contrast their situation to cast perspective on our own situation. So we know for sure that Venus is closer to the Sun than we are because, just occasionally, we see it to transit in front of the Sun, so we know that it lies between the Sun and ourselves. This is only a rare event, and it's not going to happen again in the lifetime of any of us sitting in this lecture theater tonight. But let's go further away to start with, and let's think about Neptune. Neptune is the furthest planet from our Sun, and, as such, it's very cold, but that's not the only thing about it that's different from planet Earth. The spin, rotation on Neptune is a tad faster than on Earth. One day takes about 16 hours. So, if you think we're racing through the days in September, and here I very much agree with you, then you'd find that you rattle through the days rather more quickly on Neptune. Neptune's axis of rotation about which it's doing that spinning every 16 hours is about 28 degrees, with respect to the plane of its orbit around the Sun. And so that's fairly similar to the axial tilt here on Earth, which means that Neptune experiences seasons just like we do here on Earth. However, because the year on Neptune is so long, then each of the seasons, each of the four seasons on Neptune, lasts for over 40 years. That will seem rather longer than the three months of summer that we just went through. And that's because a year on Neptune lasts about 165 Earth years. So Neptune would be very different indeed. It is way outside the habitable zone. It is just too cold, too far to receive enough heat energy from the Sun to be warm enough for creatures like us to inhabit. Although that may sound a tad gloomy, I hope I can cheer you up with some very new images taken by NASA's newest space telescope, the JWST. I don't know if you've already seen this really beautiful image taken in the direction of Neptune. It was taken back in July. I thought it was breathtaking. I still think it's breathtaking. I love the fact that even though Neptune is really quite large compared with Earth, if the Earth were the size of a walnut, then Neptune would be as big as a football. That's the kind of difference in scale that we're talking about. But in this particular image, we see Neptune against a wide field of view, and down there, there's a really beautiful spiral galaxy. It's well worth downloading this image from the NASA website just to admire the many galaxies in this beautiful wide-field view. This image was taken by what's called NIRCam, the Near Infrared Camera. So that's the portion of the infrared energy band between the optical and the full-on, serious, thermal infrared wave band. It's just an absolutely spectacular image. So Neptune is dark, because it's so far away from the Sun, and it is cold. It is 30 times more distant from the Sun than planet Earth is. Neptune is the only planet in our solar system not to be visible at all to the naked eye. Interestingly, Neptune was the very first planet to be predicted on the basis of mathematics. Because of deductions about the behavior of orbits nearby to Neptune, it became clear that something else was going on. Hence, its discovery. It was discovered in 1846. And it was only 11 years ago that Neptune completed an entire year, a Neptune year, since someone on Earth had discovered it. Not only is it too dark and too cold for life to survive, but the atmosphere just doesn't work for human lungs. It's largely composed of hydrogen and helium, very light elements indeed. The weather is pretty dreadful as well. Winds whip around. There's also quite a bit of methane in Neptune's atmosphere, and some of the winds in the top of the atmosphere move across the planet at speeds of about 2,000 kilometers per hour. That's about 1,200 miles per hour. Whichever units you prefer, it's still really fast and really inhospitable. It is utterly hostile to life, yet it is a thing of beauty. So here is just a zoom in of Neptune. And this new image from the JWST showing in very sharp detail the beautiful rings which were discovered by a previous NASA mission, Mariner 2, in the late 1980s. Here are some of the moons around Neptune, with Triton showing just up there, and here is a beautiful zoom in. It's a little saturated out on the screen here, but, in fact, there are some bright patches in the southern hemisphere of Neptune, that's in the bottom half of the main disc, and these correspond to high-altitude methane ice clouds. For the first time, the JWST was able to image some of the meteorological conditions on Neptune, and so it's extremely helpful for atmospheric physicists to be able to study the atmosphere of planet Neptune in this detail. So this is the kind of amazing image that you can get from a $10 billion space telescope. I'm now going to show you an image taken with my phone outside of my kitchen door during lockdown of another planet in the solar system. That's Venus right at the top and our moon right at the bottom. This is literally handheld with my phone outside of our kitchen door. The clarity of the sky was something that seemed to characterize the first few weeks of that first lockdown, which is when I took this photograph. Venus is often known as the Evening Star or the Morning Star because of that business I talked about earlier, that it's really close in. It's on a really relatively smaller orbit closer to the Sun, so it's never very far from the Sun. So, when the Sun is setting, it's not far behind. When the Sun is rising, it's not far behind. Whether we see it as the Morning Star or the Evening Star depends very much on what part of the Venus year we are in. Venus is shrouded in cloud. We can't see all the way to the surface because of all the clouds in its atmosphere. More of those a little later on. But Venus appears so bright in the night sky because those clouds have what we call a very high albedo. What do I mean by albedo? The albedo measures how reflective a surface is. So snow and ice and white clouds, they have a very high albedo. They reflect a lot of sunlight, so we see the object that's doing the reflecting to be very bright, which is why you can take a photograph of Venus even with a phone. So snow and ice have a high albedo, sunlight is reflected, and, in fact, that then cools the ice and the snow underneath a bit more because it's reflected the heat energy away. In contrast, seawater is not as reflective as ice and, therefore, it absorbs more heat from the Sun because it doesn't reflect as much, and, therefore, it gets hotter. In Venus, it's those clouds that are responsible for its high albedo, its high reflectivity, and that's why we can see it to be so very bright in this image. The albedo of Venus is something like 0.7. Albedo is measured between zero and one. Black velvet would have an albedo very close to zero, but perfect ice would have, perfect snow or a perfect mirror would have an albedo close to one. The albedo of Venus is about 0.7, so that means it reflects 70% of the light from the Sun that strikes it. When the Moon is close to being a full moon in Earth's sky, it can look a lot brighter than Venus, even though the Moon's albedo is only about 0.1. The Moon is basically a lump of coal, a sphere of coal, but with really good PR. But because it's much closer to us, it appears to be much, much brighter. So Venus looks really quite tranquil and really quite beautiful, doesn't it? Let's just learn a little more about Venus and see if we can understand what's different about Venus compared with planet Earth. One thing to note first of all is that Venus takes a really long time to spin, a really long time to rotate. So one day on Venus takes, forget the 16 hours of Neptune, one day on Venus takes 243 Earth days. The other interesting thing about Venus is it rotates in the opposite direction compared with Earth. So, on Venus, the Sun rises in the west and it sets in the east, the opposite to Earth and, indeed, opposite to all the other planets in the solar system. But because the day on Venus is so long, the Sun rises really slowly and it sets really slowly. The Venus year, in other words, the time taken for Venus to do a complete orbit around the Sun, is 225 Earth days. So a day on Venus lasts longer than a year on Venus. So I put it to you, it's a really good thing that there is no life on Venus because birthdays would be so very confusing and poorly defined. But why doesn't life exist on Venus? And why is Venus said to be hostile to life? Well, it's the atmosphere. The atmosphere on Venus is mostly CO2. It's mostly carbon dioxide. Here on Earth we have approximately 80% of our atmosphere is nitrogen, N2, which is a very neutral sort of gas, and then 20% approximately, I'm rounding this up, is oxygen, O2, and that's what human lungs need in order to breathe. But the atmosphere on Venus is about 96% carbon dioxide. Is this unusual in planets? Well, let's ask that question. What's the best way of finding out what the constituents of other planets are? Well, when you've discovered them, and I described in my last but one lecture, the one on planetary universes. At the end of that lecture, I said that 5,035 planets outside of our solar system had been discovered, but that's now been updated. We now know about 5,157 exoplanets, planets outside of our solar system. Many of these planets, including the one that I'm about to talk about, were discovered by a project called SuperWASP. WASP stands for the Wide Angle Search for Planets. And I was able to visit this in South Africa a few years ago. It's a very simple project but brilliantly executed. It comprises a bunch of cameras and a bunch of expensive Canon lenses, or expensive by the standards of photographers, but it's a relatively cheap way of building a telescope to have a set of Canon lenses operating there together. And this is one of a number of projects that's been responsible for the discovery of that 5,157 planets. One of them in particular is known as WASP-39b, and that's a planet that was discovered just over 10 years ago, which people had been looking at to try and identify what the constituents of its atmosphere might be. So there was a space telescope put up some years ago, again, by NASA. This particular space telescope is called Spitzer after Lyman Spitzer. And this is an infrared space telescope, and it was a very, very successful mission. And it can take spectra a bit like the ones that I described in my lecture a couple of years ago now

Unravelling Rainbows, where I talked about spectroscopy and spectra as being rainbows, split-up light from stars. And this Spitzer satellite can sort of take spectra a bit like the rainbows that I described in that lecture. By a rainbow or a spectrum, I mean that, as a function of color or wavelength, we've got differences in intensity that tell us about different constituents in the atmosphere of whatever planet we're looking at or the star that we're looking at. So, don't judge. This is not a great spectrum that I'm showing you here, but you can see at different wavelengths in the infrared band, there's some evidence from the Spitzer satellite for slight differences as a function of wavelength or as a function of color. So, on the basis of these hints, WASP-39b was then a really good target to look at with the wonderful JWST, which I'm sure many of you remember. We had the exciting launch of this on Christmas morning, late on Christmas morning in the Christmas just gone. It was just before lunchtime here in Europe. This was the triumphant success of a lot of hard work by engineers and scientists in NASA, in ESA, and in the CSA, the Canadian Space Agency and the European Space Agency as well as the North American Space Agency, irrespectively. It was launched from an Ariane 5 rocket from the Guiana Space Centre in South America, and it was a fantastically successful launch, and the deployment, the commissioning and the deployment of that telescope has been every bit as successful as the first launch. That's why we got that amazing image of Neptune and its rings that I showed you at the start of this lecture. It is an absolute triumph of the expertise and the painstaking hard work of international cooperation by engineers, especially, and of scientists. It's gone beautifully according to plan every step of the way since launch. So, if we now look at the spectrum, the previous spectrum prior to the launch of the JWST, which is this one from the Spitzer satellite, and what the JWST brings. That's the spectrum in these blue dots. And I'm very grateful to my former graduate student, David Grant, for preparing this spectrum in such a clear way. That big spike in blue dots there tells you about the presence of CO2 at a wavelength of about 4.4 microns, 4.4 millionths of a meter. So CO2 in planetary atmospheres is a thing. Practically the first exoplanet to be looked at showed CO2. We had a slight hint that that was the case from a preexisting spectrum, but we now know for sure, very clearly, that there is CO2. This spectrum is actually representing CO2 in absorption. It doesn't matter too much about those details, but it's there. It's in the atmosphere of that exoplanet, WASP-39b. Well, now let's go back to Venus. So, Venus. This image is from NASA's Mariner 2 spacecraft. It's got all the glamor of a slightly mucky ping pong ball, but it's vastly more deadly and hostile and dangerous because of all that CO2, because of the immense density of CO2. It's 96% of CO2 in its atmosphere. So, where does CO2 come from? And how come Venus has got so much CO2 in its atmosphere relative to, for example, what we have here on Earth? CO2 on Venus, CO2 on Earth, CO2 on other planets is very frequently released by volcanic activity. Venus has way more volcanoes on it than Earth does. Plate tectonic activity, when tectonic plates shift on a planet, can give rise to lots and lots of CO2 being released. We no longer believe that Venus is seismically active these days, but we believe it was in the past. So CO2, when released from volcanoes or tectonic activity, will make it into the atmosphere unless there's something to sequester it, unless there's something to store it. What might be able to store CO2? Well, a nice big ocean. We have wonderfully deep oceans here on Earth. You can also, in contrast with Venus, by the way, Venus we don't believe ever had very deep oceans. It has no oceans these days. It's quite possible that they boiled off as the consequence of a runaway greenhouse effect. More of that later on. But plants and trees and animals can also sequester CO2. CO2 can combine with water in the presence of sufficient energy and sufficient enzymes or catalysts in order to make the hydrocarbons that make up plants and trees and animals. But Venus doesn't have any oceans to absorb CO2. Earth's does, thankfully, and, at present, life is very much, for the most part, thriving here on Earth. Let's talk a little more about some of the details of why Venus is so hostile to life. That atmosphere that I mentioned that's 96% CO2. It's not just the chemical composition of the atmosphere that would be deadly to life. It's also that Venus has extreme pressure. The atmospheric pressure that we have here on Earth is pretty good at sea level. It's about right. It's a bit strenuous to operate in on some of Earth's highest mountains, on Mauna Kea on Hawaii, for example, or Mauna Loa, or some of the high mountains in Chile. But the pressure on the surface of Venus is 90 times higher than the atmospheric pressure on Earth. It's equivalent to the pressure that you would get if you were about one kilometer deep in our ocean. You'd be crushed. You would not survive. Another way in which Venus is way too hostile for life to survive is its extreme temperatures. All of that CO2, all of the clouds that I mentioned keep heat in. They trap heat in. More of that a bit later on. But the temperature at the surface of Venus is about 460 degrees Celsius. That's way hotter than my oven, and I have a pretty fancy oven. 460 degrees Celsius is hotter than lead can survive in. 460 degrees Celsius is above the melting point of lead. No way would life or humans survive. And amid these extreme temperatures, there is no relief because days last so long. There's no cooling off at night. There's very little annual variation because the tilt of Venus' axis, about which it spins jolly slowly, is none of this 28 degrees that you have on Neptune or 23 degrees that you have on Earth. It's three degrees, so you don't even get seasons and a cool winter season in which to cool off. It's been suggested that the Venus atmosphere has been so hot in the past that hydrogen cannot remain in its atmosphere. And the reason for this is that we know the ratio of H2O, gaseous water, and D2O, which is like water, but where, instead of hydrogen, you've got what's called heavy hydrogen, deuterium, which is chemically the same as hydrogen but much more massive. And while you have something like 100 times the D2O to H2O ratio on Venus compared with what you have on planet Earth, even though we think Venus and Earth formed out of the same bit of gas cloud that gave rise to our solar system, we think that, because the atmosphere of Venus was so hot, the hydrogen was able to escape in a phenomenon known in planetary atmospheres as Jeans escape. So Venus, I hope I've convinced you, is absolutely deadly to humans and absolutely deadly to everything else. Let's now start to make some comparisons with planet Earth. How much CO2 is in Earth's atmosphere? I'm sure we're all relieved to know that it's much less than in planet Venus, but let's just consider how much less there is, how much less CO2 there is. The units we normally use for CO2 are parts per million, parts of... You ratio the number of CO2 molecules relative to other particles in the atmosphere, whether they're nitrogen or oxygen or whatever else it might be. The parts per million of CO2, carbon dioxide, in Earth's atmosphere has been rising steadily since the start of the Industrial Revolution and has climbed really quite dramatically in the last several decades. The parts per million of CO2, now well over 400, there are about 420 parts per million, have risen by about a third since Queen Elizabeth II ascended to the throne and by about 50% since the start of the Industrial Revolution. This plot here shows the parts per million of CO2 at pretty much today's date. This was up to date earlier this summer. And you can see the black line is the sort of steady fit averaged over full 12-month periods, over complete years, and the red zigzags tells you about the annual variation that you get, that has been known for some time. It rises and it falls each year. It falls each year because of the growing season in the Northern Hemisphere. The Northern Hemisphere of Earth has much more land, and the Southern Hemisphere has much more sea. Sea doesn't do that much growing, but the growing period on the Northern Hemisphere is responsible for attenuating that fall, but, of course, it's superimposed on an increasing rise. The reason is that plants use CO2 from the atmosphere along with sunlight and water to make food and to make hydrocarbons and substances that they need to grow. This is the chemical reaction that's undergoing. CO2 and water and, sorry, that should say energy. I apologize. It's not very visible. Give rise to the hydrocarbons, the complex molecules involving carbon and hydrogen and oxygen. And oxygen is usually released in that process because of the different ratios, the stoichiometry of the molecules that get made. So plants sequester CO2, and then CO2 is stored as hydrocarbons in plants and trees. So let's ask the question now. Is 420 parts per million of CO2 in Earth's atmosphere significant, even allowing for our relief at the fact that the CO2 quantity in our atmosphere is not as deadly as the percentage of 96% that there is in Venus? Anyone who's tempted to think that 400 parts per million of CO2 in our atmosphere is not significant should be confronted with the following. If you think 400 parts of anything amid a million parts of other stuff is not significant, probably isn't a coffee drinker. Why do I say that? Well, 400 parts per million of caffeine is very significant to those of us who are very definitely in need of a cup of coffee when booting up this morning. If you don't believe me, then tomorrow morning, when you're in need of a cup of coffee, try drinking decaf. A normal cup of coffee contains about 400 parts per million of CO2. Sorry, let's try that again. A normal cup of coffee contains about 400 parts per million of caffeine molecules amongst all the water molecules and milk, if you take it, and all that sort of thing. So I put it too you, there's a big difference between caffeinated coffee, which has about 400 parts per million of caffeine molecules, and decaf. I should, of course, acknowledge that this is a culturally dependent statement because Italian coffee has a much higher concentration of caffeine molecules and in certain places in the US coffee contains rather less caffeine. So I want to think now about whether, if we've got a tiny amount of CO2 in the atmosphere on planet Earth, can it be significant? Can it give rise to feedback processes which thereby alter or have the potential to alter the nature of the atmosphere on planet Earth? I want to talk about two distinctive phenomena, two distinctive feedback processes, which are well known in, for example, system engineering. The first of these is something we call positive feedback. Now, you might be tempted to think that positive feedback is a good thing. I'd like to pour cold water on that straight away. Positive feedback is not a good thing because it reinforces the thing that you'd rather have less of. Let me give you an example from everyday life. Supposing you can't sleep because you're anxious about something, and then you become anxious about the fact you're not sleeping, and so you become more awake, and you can't sleep, and that increases the anxiety levels. I'm sure many of us have been there. That's positive feedback, where you reinforce a vicious cycle. The same thing happens in the context of atmospheric physics. If something is causing the planet to warm up a bit, then you are at risk of melting the polar ice caps. Remember I said earlier that ice has a really high albedo. It reflects a lot of sunlight back into space. If you lose that and it becomes sea water, there's a load of bad stuff that happens because the sea levels increase, and that encroaches on the land where humans are living, particularly in very low lying places like Bangladesh. But, in addition, Earth's albedo goes down. Less sunlight is reflected, and so the planet heats up. That happens because less solar energy, less sunlight is reflected, and so the globe warms, and so more ice on the ice caps can melt. This is called positive feedback because it reinforces the bad thing you really don't want to be happening. Is positive feedback a good thing? It's really not, and it's a process that is very much at play in all sorts of different ways in Earth's atmosphere, in all planetary atmospheres. Almost certainly, if there ever were oceans in Venus, this kind of spiral led to the deadly situation, the hellish situation that we now have on Venus. So that's positive feedback, and we need to remember positive feedback is really bad. So anything that promotes the heating of Earth's atmosphere risks pushing you into this vicious cycle. Too much CO2 could contribute to that. More of that later. Now, let's think about negative feedback. Is negative feedback a good thing? Normally negative things are things we want to avoid. Is negative feedback a good thing? It really is. Negative feedback is about regulating. So a simple example of this is the thermostat in a room. When a room gets cool, if you've got a working thermostat, the thermostat will then turn the heating on, so the room will warm up. Then, when it gets to the temperature it's been set to, the heating will turn off, so gradually the room will start to cool again, and then, after a while, the thermostat will turn the heating on for a bit, the room will get hot, and then, after a while, the thermostat will turn off and the room will cool again. That's negative feedback, when you regulate the important property, in this case, temperature. Unfortunately, there's no thermostat that we know of in planetary atmospheres. Positive and negative feedback, positive feedback processes and negative feedback processes, are a thing in biology, in system engineering, in chemistry, in economics. Hmm.(audience laughs) Let's stay focused on Earth's atmosphere, shall we? It's highly relevant to atmospheric physics and to the evolution of planetary atmospheres. Now, a specific type of positive feedback is that of the greenhouse effect. What is the greenhouse effect? And surely greenhouse is a really positive thing. Why yes, but context is everything. The greenhouse effect is a wonderful thing in the context of growing tomatoes. This is my dad's greenhouse, and he grows the best tomatoes in the universe. So, why am I associating the greenhouse effect with positive feedback, which we've just said is something bad? Context is everything. The glass of a greenhouse can contain and trap heat energy. When sunlight comes in during the day, it predominantly comes in in the form of UV light and optical light, which heats the surroundings, heats the sand the tomatoes are growing in, heats the tomatoes. They grow bigger and more delicious. And then, that heat energy wants to reradiate as infrared photons. But glass isn't transparent to infrared photons. They get trapped, and they maintain the heat, and that's tremendous for growing tomatoes. But if you've got something called a runaway greenhouse effect, and that's not a cutesy cartoon thing, by the way. That's when the greenhouse effect has turned into a vicious cycle. That's when you've got problems. And this term, the greenhouse effect, is used in the context of planetary atmospheres. It's been hypothesized in the case of Venus. A runaway greenhouse effect occurs when a planet absorbs more energy from the Sun than it can then reradiate back into space. Sunlight can warm the ground, Earth's surface during the day, and then, at night, the energy should radiate back into space in the form of infrared rays, and it should cool down. But what could trap the heat in the context of Earth's atmosphere? There's no glass there like there is in a greenhouse. What is it that could be trapping the heat? Well, if your atmosphere consisted entirely of homonuclear molecules, that is a molecule which comprises more than one atom, obviously, 'cause it's a molecule, but the same sort of atom, that actually wouldn't contribute to trapping the heat. So, if our atmosphere consisted solely of nitrogen in the form of N2 and oxygen in the form of O2, we wouldn't be having a greenhouse effect. And that's because in a homonuclear molecule, where you've got an oxygen atom here and an oxygen atom here, this is a completely symmetric situation between the two atoms. The nucleus of the oxygen here and the nucleus of the oxygen here will be equally attractive to the cloud of electrons associated with that molecule and orbiting around the two oxygens. So you don't have any separation of charge. A separation of charge is something that we refer to as a dipole moment. Don't worry if you're not familiar with that term. It just means we've got a net separation of charge, a net electric field. So you don't have that in the case of homonuclear molecules like N2 and O2. In such a case, the atmosphere is approximately transparent. Light does not couple to homonuclear molecules. Light gets in and out. It does not trap the infrared photons, and it's perfectly permeable, perfectly transparent to optical ultraviolet photons as well as infrared photons. But what about heteronuclear molecules? Molecules like CO2, where you've got different species within one molecule. Well, let's remind ourselves what a light ray is, what a photon is. A light ray is where you have varying electric and magnetic fields. This is something that I discussed in my very first Gresham lecture in a lecture entitled Faster than Light? So, why might light in the form of these electric and magnetic packets of energy, why might they couple to heteronuclear molecules? Why might sunlight couple to heteronuclear molecules? Well, let's just imagine this little symbol here represents the heteronuclear molecule of H2O. The charge on the nucleus of the different atoms in this particular molecule is going to be different for the different types of nucleus. The hydrogen nucleus only contains one proton. It has a positive charge of plus one. Whereas, the oxygen nucleus has a charge of plus eight because it's got eight protons. So the electrons associated originally with the oxygen and the hydrogen, while they're going to be whizzing around within that molecule, they're going to be preferentially more drawn to the more positively charged oxygen. Hence, you have a little bit of an electric field between the O and the two Hs. Entirely similarly with CO2, the carbon nucleus has a positive charge of plus six and the oxygen charges are plus eight, on the nuclei, I mean, so the electrons are going to be preferentially drawn towards the more positive charge. So there's going to be a slight dipole moment, a slight, because there's a separation of charge, there's going to be a slight electric field, and that's why light rays, photons, those waves of electric field and magnetic field, can couple to them and impart energy to those molecules so that you can then store energy within the bonds between the C and the O and the H and the O. This is another reason why we can't even see the surface of Venus. It's because it's full of CO2, and light is not transparent in situations where it couples to the molecules, which is all the time if you've got heteronuclear molecules. As I said, the atmosphere of Venus is 96% CO2, and that's why we can't see the surface of Venus from Earth. The only way we can see the surface is if we actually land an explorer on the surface of Venus, which has been done at some times in the past, which is how we know about its chemical constituents, its pressure and its temperature. So Venus' atmosphere is not transparent to light because that light is absorbed by the CO2 and imparts energy to the CO2, heating it up, contributing to that vicious cycle that I mentioned. Incidentally, the converse of this is why we can easily see the surface of the Moon. Doesn't have an atmosphere at all. No heteronuclear molecules, no homonuclear molecules. We can always see the surface of the Moon as long as there are no clouds of H2O, which is water droplets in the case of Earth. So what I've been describing, the greenhouse effect, has its origins in ideas first put forward by the French physicist and mathematician Fourier. Fourier figured out the relevant physics of atmospheres that we now know in terms of this positive feedback effect called the greenhouse effect. Fourier is a really good guy. Not only did he figure out that important part of atmospheric physics and other important aspects of thermodynamics, he also developed the mathematical tools which are widely used today in CT scans and MRI scans, and, indeed, for important parts of astronomy, such as interferometry, which means that we get beautiful images of the radio sky, as I described in my lecture a year and a half ago,

Watching the Radio. So this is Fourier's original paper, where he takes the reader through all the different key bits of relevant physics. The greenhouse effect does appear to be manifesting itself in the atmosphere of planet Earth and causing global warming and a changing climate, whose behavior gives rise to more and more extreme events. This is how the temperatures have changed in the last 150 or so years. This is data supplied by NASA, same people who did so much to explore space and make careful measurements of space, and the brilliant engineers who devised this measuring equipment, who devised the actuators that work so brilliantly on the JWST today. These measurements of the global temperatures are not by cranks or people who are out to mislead, but the same skilled and expert engineers and scientists who can launch wonderful telescopes at the same time. There's no doubt that the temperature on this planet is increasing. But could it be that the reason why the temperature on Earth is increasing is just the Sun's fault? I mean, perish the thought that it's our fault. Could it be that an active Sun is more responsible for our warming climate? In principle, it could be, but that isn't the case. It's just not. The reason why the Earth's atmosphere is getting hotter and hence Earth's surface is getting hotter is because heat is trapped inside. If it were the Sun's fault that we were getting hotter, you would expect the temperature in the upper atmosphere of Earth to be getting hotter and then for that heat to transmit into the lower down layers of the atmosphere and then of Earth's surface. But that's not what's been observed and measured. What has been observed and measured is that the upper atmosphere is cooling, and it's the closer in layers that are getting hot. Heat is being trapped inside the atmosphere by greenhouse gases. In much the same way that we trap heat close to our bodies when we put on a warm winter coat, heat doesn't escape as easily. Greenhouse gases, heteronuclear molecules, not just CO2, but also H2O and methane and a few other things, these are responsible for trapping heat energy, originally from the Sun, close to the Earth and heating it up. And even though the levels are only the same as the relative number of caffeine molecules that you have in a cup of coffee, it's enough to contribute to that positive feedback effect. Terribly sorry if I'm coming across as a bit gloomy, but I would like to quote Carl Sagan, who pointed out,"It is far better to grasp the Universe as it really is"than to persist in delusion,"however, satisfying and reassuring." This is Carl Sagan, for those who don't know, is the American, the late American astronomer and cosmologist and writer. I wonder what future generations will say of us, perhaps by the time King George VII is on the throne. I think they might very reasonably ask the question, what on Earth did you think you were doing using up fossil fuels, hydrocarbons buried in the ground, to heat up buildings? What on Earth did you think you were doing using fossil fuels to heat up water? That reaction in reverse now. Hydrocarbons and oxygen, when burnt to heat water, to heat buildings, gives rise to CO2, which is really bad, even if it's an exothermic reaction, and we get energy out of it and a nice warm building. The problem is the following, that hydrocarbons, such as coal and oil, are the buried products of solar energy, photosynthesis, and geological pressure that formed over a few hundred million years. Fossil fuels, like coal and oil, are the sequestration of CO2, the storage of CO2, keeping it out of the atmosphere in a way that certainly no longer happens on Venus. But these same fossil fuels have fueled our industries and our transport since the beginning of the Industrial Revolution. On a cheerier note, energy from the Sun is enormous. It's something like 100,000 terawatts of energy that the Sun provides. The energy from one hour of sunlight is equivalent to all the energy that humankind currently uses in a year. And it is fusion energy, by the way, but harnessing it at a distance of 93 million miles. Each square meter on average of the illuminated Earth receives about a kilowatt of energy. But I wouldn't want you to think that I think solar energy is the only way that we should be powering Earth going forwards in the future. I reckon that we need all the tools in the toolkit. I don't think any one method for providing the energy that we need here on Earth is perfect, but it's true that better is the enemy of good. We really do need all the tools in the toolkit. We mustn't be put off by the cacophony of soundbites from those who are in denial. I want to really recommend to you another course of lectures that's starting at Gresham this term by my Oxford colleague, Myles Allen, now my Gresham colleague, recently appointed as Gresham Professor for the Environment. He's going to talk about the what, the why, and the how of net zero. And I really encourage you to learn more about it. There is no doubt that the situation that we find ourselves here on planet Earth is a bit precarious. It's a bit fragile. But I hope I've given you a sense in comparison with Venus that we have a lot to be thankful for and a lot that is worth saving. At present, we have it pretty good here on Earth. Many situations across the planet are wonderful and are hospitable. But we do need to engage our thinking brains if we're going to preserve that and if we're going to avert catastrophic climate change. The end of this planetary atmosphere, the end of the atmosphere on planet Earth will be a thing if we don't change tack. Let us resolve, in the words that the school that plays host to one of my telescopes in India says, let us resolve to go green before green goes. And that is the end of today's lecture. Thank you.(audience applauding)- Whatever happened about the reports two years ago of finding phosphine on Venus? Is that a possible sign of life?- Thank you very much for that question. So the measurements of phosphine, which is quite an interesting molecule, a little more complex than the ones I've been talking about this evening. There were reports by very reputable scientists, by the way, that this had been discovered in the atmosphere of Venus. Now, this was a startling result because the levels at which it was detected, or at least the levels at which it was reported, were so high that you could only have explained it from a biological origin. In other words, if the detection turned out to be for real, it would've been a very clear indication that life had once been present on Venus. But it's fair to say, as is often the case in science, that although there's reasonably good evidence for a detection of phosphine, it doesn't seem to be borne out that it's at the same high levels of the original report. So phosphine does seem to be present in Venus, but, at present, the concordance of opinion seems to be that it's at the level that you could explain by non-biological processes, processes relating to, for example, original volcanic activity or something like that. So we're nowhere near being able to say that Venus ever played host to life, I don't think.- The ground level pressure of Venus is 90 times that of Earth's. That is hostile to humans, but you said all life forms. Some life forms thrive near thermal vents in the deep ocean, so is it possible that they could exist?- I think it probably is. And I think I should have been a little bit more discriminating. When I said that, to all life, I was thinking of puppies and kittens and things like that. I think everyone would agree that the surface pressure on Venus would be completely hostile to that sort of multicellular life. But to the kind of things that do survive in these very extreme environments in the oceans, for sure, in principle that could survive on the surface of Venus. And that's why, if the very high levels of the phosphine detection were to be borne out, and they don't seem to be at present, but it would be a bit of an indicator that it was that kind of life form.- [Audience Member] You showed an interesting plot of the increase in temperature with the annual variations due to vegetation. One of the things the popular press is always on about in terms of global warming is planting more forests, but, from your graph, the effect seems to be rather small and cyclic, so is there any true reason why it would be beneficial to plant more forests?- I think it would be a really good idea to plant more forests and more plants and so on and so forth. That cyclic behavior that you refer to is just the normal growing season in the Northern Hemisphere which predominates this. It's not in any sense related to a sort of very strategic forest-wide campaign. I think there aren't downsides to doing that, except in very arid places, where there wouldn't be enough water for these things to grow. I think it would be a very worthwhile thing to do. I don't think it would be sufficient. I think we need to stop putting more CO2 into our atmosphere as well as what you suggest. We need all the tools in the toolkit. There's no single silver bullet solution, sadly.- [Audience Member] My question is, the heat which you're talking about, the energy, which is enabling all the planets to survive, have a time limit because of the Sun, which will use up its energy. So, when that happens, will all the planets, the entire solar system disappear, you know, get absorbed in the Sun? The other question which was connected to that is that, the international situation being so bad, suppose, for instance, that there was nuclear bombs all over the place. Would that cause global warming among other things?(audience laughs)- So, to take your second question first, I think there would be so many bad things if the international situation were to lead to nuclear attack in certain locations. Would it lead to a nuclear winter? It's really not clear. It depends on the specificity of the attack. To go back to your first question, the expiry of the Sun and the implications for the planets across the solar system, that is entirely the subject of my next Gresham lecture in November.(audience laughs)- And with that, that is all the time we've got for questions. Thank you, everybody, and thank you again, Katherine.(audience applauding)