Arquivo de etiquetas: balanço energético

The Basics of Climate Change Science I

The Earth’s Energy Balance

We will be talking about the basics of climate science. We need to build on these basics in order to understand our choices about deep decarbonisation and other actions related to other greenhouse gases in order to understand why, how, when, at what pace we should be reducing emissions of greenhouse gases in order to stabilize the concentration of greenhouse gases at safe levels. What’s safe? What’s the relationship between the greenhouse gases and climate?

That’s the purpose of this lecture: to give us an introduction to this very rich, very sophisticated a hundred ninety years of science, and in the first chapter I’m going to talk about the Earth’s energy balance.

This goes back as we’ve already noted about a hundred ninety years to Joseph Fourier who first realized that the Earth’s average temperature would be determined by a kind of balance or equilibrium between the incoming energy of solar radiation and the outgoing energy that the earth radiates back to space.211

That’s when the Earth’s temperature is at a level such that the incoming radiation and the outgoing radiation are in balance. That we have an equilibrium, a place of stationary temperature for the Earth and understanding how greenhouse gases affect that balance has been the core of climate science since Fourier’s very creative understanding of this process since the 1820. This diagram that you’re looking at shows in very simple, schematic terms this global energy balance.

At the center of the graph is the center of the whole issue and that is that the Earth receives radiation from the sun, that the radiation from the sun warms the planet. If we look at the amount of radiation at the top of the Earth’s atmosphere that’s determined by the distance of Earth from the Sun and from the sun’s energy in radiative flux and we can measure that just as we do with our lightbulbs in watts: that’s a unit of power and watts per area or watts per meter squared is the standard unit that scientists use to measure the incoming solar radiation and what you can see from this diagram is that on average given the solar constant output we have an average amount of about 341 watts per meter squared at the top up the atmosphere.

Now as this incoming sunlight comes in, that’s mainly how we experience this electromagnetic radiation is sunlight, that is radiation at a certain frequency much of which is within the visible range, part of that is immediately reflected, the part that you see towards the left hand side bouncing off clouds back up into space Another part of the incoming solar radiation is reflected by the surface of the Earth.

When the sunshine comes in and hits an ice sheet, say the Greenland ice sheet, or hits sea ice floating in the North Atlantic and and the sunshine just radiates, is reflected in re-radiates back out into space that is reflected on the left hand side. But of course a certain amount of the radiation doesn’t bounce off the clouds back into space and doesn’t bounce off of the earth’s surface but is absorbed by Earth and warms the planet. And the basic idea is that any body, including the body of planet Earth, when it has a certain temperature itself radiates energy.212

This is a basic fact of physics This is a basic fact of physics and the basic study of it is called the study of blackbody radiation and in fact the Earth absorbs radiation from the sun. You see that from the center to the left of the diagram but then it radiates energy back into space, and one of the most interesting and basic facts of all of this is that the incoming radiation is  in the form of visible light or ultraviolent radiation, UV radiation, and that is relatively short wavelength, high-frequency radiation, and the radiation that the earth itself causes by it being a warm body is a bit longer wavelength called infrared radiation. So the incoming arrows are visible sunshine for example and the outgoing radiation on the righthand side of the diagram is infrared radiation.

That’s gonna play a very very key role in our understanding of climate and the greenhouse gas affect because the basic idea is that the greenhouse gases (carbon dioxide, methane, nitrous oxide, some industrial chemicals) are basically transparent to the incoming solar radiation, they allow it to come in, but they are not quite so transparent to the outgoing infrared radiation from Earth itself, the longer wavelength. In fact, they absorb that infrared radiation in part, and it’s that absorption of the Earth’s own infrared radiation that traps energy that otherwise would go out to space. It traps energy and creates a kind a blanket or the greenhouse effect if you will that makes Earth warmer than it otherwise would be.213

You see that in this simple diagram by the fact that we have surface radiation aiming back towards space and then you see that some of that circles back after it hits the greenhouse gases in terms of what’s in this diagram call the back radiation towards the plane. Now in balance or in equilibrium the Earth’s temperature is determined such that the arrows coming in equal the arrows going out, and if we were to have no greenhouse gas in the atmosphere if we were a a planet without an atmosphere, if we were like the moon, then there would be only radiation going back out in space, none of that back radiation going from the greenhouse gases back to earth, and the balance would be reached at a relatively low temperature of the planet.

Indeed, the temperature would be roughly 33 degrees Celsius lower than it actually is on the planet. The actual temperature of Earth on average is about 14 degrees Celsius. If we didn’t have the greenhouse gas cover we would be roughly 18 degrees minus, negative 18 degrees Celsius, instead of the actual 14 degrees Celsius that we have, and that is the difference of having a greenhouse effect that traps some of the outgoing infrared radiation and not having an atmosphere with that greenhouse effect that would just allow the radiation to go back into space directly. Now this diagram’s filled with all sorts of complications  and this is why be underlying science of the greenhouse effect has many challenges.214

How much of the incoming radiation actually reflects back to space? That depends on cloud cover, that depends on the surface of the earth, how much is ice for example, how reflective is the Earth’s surface, what’s called the albedo of the Earth. If ice melts then what used to be reflected back into space of the solar radiation now gets absorbed and you get a kind of feedback effect where a warming up the planet melts the ice, reduces the reflectance of the incoming radiation, increases the absorption of the incoming radiation, and further warms the planet. And many other dynamic effects are present here meaning that your simple simplest calculations can’t quite do the job telling us precisely how an added level love greenhouse gas is going to change the radiative  balance and thereby change the equilibrium temperature but this simple illustration is very very helpful in explaining the basic greenhouse effect.

Now to move one step more deeply, it’s important to understand this specificity of what makes a greenhouse gas and that is shown by what’s called a radiation spectrum of both the incoming radiation and some of the absorption of that spectrum. Light comes to Earth from the Sun or electromagnetic radiation comes to Earth across different wavelengths and so we go from very very short wavelengths on the left handside of the spectrum to very long wavelengths on the righthand side from ultraviolet towards the left hand side of the electromagnetic spectrum towards infrared and long wavelengths on the righthand side of the spectrum, and the amount of energy that is in the solar radiation is shown essentially by this spectrum of the sunlight and it’s that dark line which shows the radiation spectrum.

How much of irradiance, how many watts per meter squared is coming at each wavelength of solar radiation. You can see that the peak of that radiation is in what’s called the visible range of the electromagnetic spectrum. Visible because that’s what we see that’s the light we see. We don’t perceive ultraviolet or infrared, that’s outside of the visible range for human beings, not for some animals but for us, and so most of the energy, most of the watts per meter squared of the incoming solar radiation is in the visible range.215

Not so for the outgoing radiation from the planet, and this is a part of physics that comes from that theory of blackbody radiation called the Stefan-Boltzmann equation. It basically says that a very hot object like the sun will have more irradiance at the high frequency or low-wavelenth end of the spectrum whereas a cooler body like the Earth will have more radiation at the long-wavelength or infared part the spectrum.

So since Earth is a lot cooler than the Sun, we radiate at the righthand side of the spectrum. Now why does that matter? It matters because certain compounds, these are the greenhouse gases, absorb infrared radiation. That’s part of their chemistry, part of their quantum physics. The compounds that absorb radiation all have more than two atoms so O2 or N2, oxygen as it is in the atmosphere or nitrogen, dinitrogen, as it is in the atmosphere, is not a greenhouse gas.

To be a greenhouse gas you need to be 3 atoms or more. That allows the atoms to jiggle in particular ways and to absorb the infrared radiation So CO2 was three atoms, 1 carbon, 2 oxygen atoms. Nitrous oxide, N2O, methane which is 5 atoms, carbon and 4 hydrogen atoms all have configurations in their bonding that allows them to or makes them absorb infrared radiation and by absorbing the infrared radiation, they absorb the energy that otherwise would radiate to space.

They warm the planet and so one can see shown in this diagram, the so-called absorption bands of carbon dioxide and water. They’re to the right hand side of this figure. What does that mean?

They absorb longer wavelength electromagnetic radiation, the kind that Earth radiates, they don’t absorb the kind of radiation coming. The long and the short of it, they are transparent to the visible sunlight that we see when we go out on a sunny day but they absorb the infrared that we don’t see that the Earth is re-radiating as a warm body at an average of 18 degrees Centigrade and it is precisely the absorption of the infrared radiation that keeps the planet at the average of about fourteen degrees Celsius rather than the minus 18 degrees Celsius that would prevail if we didn’t have the greenhouse effect. And what we know is what Arrhenius told us back in 1896 and that is that as we increase the concentration of carbon dioxide or methane or nitrous oxide or other greenhouse gases in the atmosphere we’re going to get a warming.

We can measure the temperature, you have to do it very carefully in weather stations all over the world, my colleagues at the Goddard Institute of Space Studies, NASA’s leading scientific outfit for measuring the Earth’s mean temperature and one of the major enterprises in the whole world for this has produced very very careful data on changes of Earth’s temperature that’s illustrated by this graph.216

Now in this particular graph, the 0 line is the Earth’s average temperature for the years 1951 to 1980 and what you can see is that by our period by the years after 2010 or so, we’re at about .6 of 1 degree Celsius or about one degree Fahrenheit warmer than the average of 1951 to 1980, and you can see from this upward slope that the Earth is warming warming. It’s not warming every year, there’s a lot of variability. In fact there are even episodes, look at graph from around 1940 to around 1980, where there wasn’t a lot of warming and that raises a lot of questions CO2 and other greenhouse gases were rising but the temperature wasn’t rising all that much so that poses a question of what else is happening but the general direction is unmistakable and it instead of taking an average of 1951 to 1980, we took the average temperature before the whole industrial revolution started we would see that we’re close to a one degree Celsius increase of temperature now, about .9 of one degree Centigrade and this is of course the upward slope that is so frightening because it’s already disrupting the planet and it will cause a lot more disruption. Now let’s turn in more detail to the specific greenhouse gases that are responsible for this human-induced change. That will be the topic of the next lecture.