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At the beginning of the 21st century, terms such as the ‘greenhouse effect’, ‘greenhouse gases’ and ‘greenhouse warming’ are printed or spoken thousands of times a week in the context of climate change caused by human activities. This module is designed to consolidate your understanding of the basic science behind these terms, and then to review what is known about the human impact on the composition of the atmosphere since the dawn of the industrial age, commonly put (in this context) at around AD 1750.
We start with a couple of fundamental questions about global climate. What determines the Earth's global mean surface temperature (GMST)? And how does the composition of the atmosphere come into that equation?
Electromagnetic radiation is the only form of energy transfer that travels through the vacuum of space, propagating as a wave.
By convention, the full spectrum of electromagnetic radiation is carved up into regions, each characterised by a particular range of wavelengths. The wavelength (symbol λ ) is just the distance between successive crests of a wave.
Wavelength is given in micrometres, μm: 1 μm = 10-6m. Note that the wavelength changes by a factor of 10 for each division along the top scale, so this is a logarithmic scale.
Our eyes are sensitive to visible radiation, which corresponds to the wavelength range from about 0.4 μm (violet light) to 0.7 μm (red light). When all wavelengths in this range are present, we perceive this as ‘white light’. To either side of the visible band lie the ranges known as ultraviolet (uv) radiation (with wavelengths below that of violet light) and infrared (ir) radiation (with wavelengths above that of red light).
As with any propagating waves, the shorter the wavelength, the higher the frequency (f ) (i.e. the higher the number of waves passing a point in a given time). For electromagnetic radiation, the two multiplied together give the speed of light (c): c =fλ .
The Sun is the ultimate source of energy for the Earth's climate. A planet such as the Earth will have a stable temperature as long as there is a balance between the rate at which energy comes in from the Sun and the rate at which it is returned to space by the planet. If the two rates fail to match, the planet will either warm up or cool down until a balance is restored.
Thus, it is appropriate to begin with a review of this global balancing act. The heart of the matter is that the energy flows to and from space are in the form of radiation - or to be more precise, electromagnetic radiation (see previous pages).
The Sun emits electromagnetic radiation with a range of wavelengths, but its peak emission is in the visible band - the sunlight that allows us to see. The wavelength of radiation has important climatic implications, as we shall see shortly. For now, we are mainly interested in the overall rate at which energy in the form of solar radiation reaches the Earth.
The Earth intercepts an amount of solar radiation equivalent to that falling on a disc with the same radius (R ) as the Earth, facing the Sun: this comes to (1368 × πR2 ) W, where πR2 is the area of the disc (in m2).
However, the Earth is spherical , so the area presented to the incoming solar radiation by the rotating Earth (over a period of 24 hours or more) is 4πR2 (i.e. four times as great). Thus, the solar input per unit area averaged over the surface area of the whole Earth is a quarter of the solar constant:
1368 Wm-2 /4 = 342 Wm -2
Not all of the incoming solar radiation is available to heat the Earth: some of it is reflected directly back to space. The proportion of incident solar radiation that is reflected by a given surface is called the albedo. The photo shows an image of the Earth from space formed from reflected sunlight (solar radiation at visible wavelengths).
Clouds and the ice-covered mass of Antarctica (at the bottom of the image) appear bright because they reflect strongly (i.e. they have a high albedo - up to 90% in the case of fresh snow and sea-ice). By contrast, the oceans have a low albedo (typically less than 5%) and appear dark in this image. In general, most land surfaces have moderate albedo, with values ranging from 10-20% for forests to around 35% for grasslands and deserts.
Suppose now that the Earth's atmosphere is stripped away, but the planetary albedo is unchanged. (This may strike you as a curious proposition, but it will help to expose just how important the atmosphere really is.) The steady-state balance between incoming and reflected solar radiation (orange arrows) and outgoing terrestrial radiation (reddish arrow) for an Earth-like planet without an atmosphere.
100 units represent the globally averaged rate per unit area at which solar radiation reaches the planet; i.e. 342 W m-2. To the left of the figure, a nominal 100 units of solar radiation reach the planet; 31 units are reflected away and all of the remaining 69 units are absorbed by the surface.
By itself, the effect of this continual input of solar energy would be to warm the surface up - it would get progressively hotter. Fortunately, there is a compensating cooling effect. Like the Sun, all objects emit electromagnetic radiation. Further, they do so at a rate that depends on the temperature of the object: the hotter an object becomes, the higher its radiative power - the rate at which it emits radiation.
For our planet, a steady or equilibrium temperature is maintained by a dynamic balance: the rate at which solar energy is absorbed (the 69 units to the left in the diagram) must be balanced by the rate at which the planet loses energy to space as emitted radiation (the 69 units to the right in the diagram). Note that this emitted radiation originates with the ‘jostling about’ of atoms within the surface; it is not the same thing as the reflected solar radiation, which merely ‘bounces off’ the surface. To emphasise the distinction, we shall refer to the radiation emitted by the planet as terrestrial radiation.
Expressed in quantitative terms, the relationship between temperature and radiative power is the basis for a well-established law of physics. The appropriate calculations tell us that, for an Earth-like planet to emit radiation to space at a steady rate of 236 W m-2 (the 69 units depicted in the diagram), it should have an equilibrium temperature of -19 °C.
This equilibrium temperature is known as the effective radiating temperature and, were it not for the atmosphere, this would also be the Earth's global mean surface temperature (GMST). Conditions on Earth at this temperature would not allow life as we know it. But the atmosphere perform the vital trick of keeping the GMST at a more temperate 15 °C.
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