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Climate Change summary by Royal Society - Greenhouse Effect
I said I'd go through some of this report in more depth, so let's start at the beginning. (If I'm unclear, feel free to ask me to try and explain better. Better explanations are also welcome in the comments)
Their text is in red, mine is not.
The greenhouse effect
4 The Sun is the primary source of energy for the Earth’s climate. Satellite observations
show that about 30% of the Sun’s energy that reaches the Earth is reflected back to
space by clouds, gases and small particles in the atmosphere, and by the Earth’s
surface. The remainder, about 240 Watts per square metre (Wm-2) when averaged over
the planet, is absorbed by the atmosphere and the surface.
What this basically boils down to is that clouds are reflective. They're white, so this is hardly surprising. If they were black, then they'd absorb far more heat instead. Gases also reflect radiation, as do particles. Anything that interacts with light is going to do its own particular mixture of absorption, reflection and refraction.
Clouds have a big impact on how much radiation reaches the surface. This is a pain for climate modellers as clouds are hard to predict. (ask any weather forecaster)
Radiation that is not reflected by clouds, gasses and particles eventually reaches the surface of the Earth and is an average of 240 Watts/m2 (Yes, some radiation will get absorbed in the atmosphere and re-radiated, we'll come to that later)
How much energy is that? Visualise an old style 250W light bulb every metre of your ceiling. (Daylight is far brighter than indoor light, but we tend not to notice because our eyes adapt so well.
5 To balance the absorption of 240 Wm-2 from the Sun, the Earth’s surface and
atmosphere must emit the same amount of energy into space; they do so as infrared
radiation. On average the surface emits significantly more than 240 Wm-2, but the net
effect of absorption and emission of infrared radiation by atmospheric gases and clouds
is to reduce the amount reaching space until it approximately balances the incoming
energy from the Sun. The surface is thus kept warmer than it otherwise would be
because, in addition to the energy it receives from the Sun, it also receives infrared
energy emitted by the atmosphere. The warming that results from this infrared energy is
known as the greenhouse effect.
Second law of thermodynamics coming into play. Heat moves from hotter bodies to colder bodies.
If the Earth gets hotter, it radiates more heat back into space. Thus, the Earth radiates 240W/m2 (allowing for stuff bouncing back and forth from those dratted clouds and gasses).
Clouds (and atmospheric gasses) are a bit like leaky mirrors over the Earth. They reflect both ways and only reflect about a third of the stuff coming from any given direction.
Radiation coming in from the sun is one mixture of wavelengths and radiation emitted by the Earth is a different mixture of wavelengths.
The easiest way to see this happening is to leave a sheet of black paper in the sun. If you hold your hand over the paper, it will be radiating heat at you. It's absorbed radiation in the visible light spectrum and some of that energy has been re-emitted as infra-red (heat).
Clouds also reflect infra red - that's why cloudy nights aren't as cold as clear nights. The clouds act as a 'blanket'. (Or 'duvet' these days)
6 Measurements from the surface, research aircraft and satellites, together with
laboratory observations and calculations, show that, in addition to clouds, the two
gases making the largest contribution to the greenhouse effect are water vapour
followed by carbon dioxide (CO2). There are smaller contributions from many other
gases including ozone, methane, nitrous oxide and human-made gases such as CFCs
(chlorofluorocarbons).
Clouds are easy to see and account for a large chunk of atmospheric reflection. The next biggest reflectors are water vapour and CO2, followed by a host of less common gasses.
How do we know this? Partly by lab observations (send radiation into something and see what comes back again) and partly field work (measure atmospheric concentrations and see how much radiation is reaching your instruments compared with what you get under different atmospheric conditions. Field data obviously needs to be collected over long periods of time to get meaningful results)
Their text is in red, mine is not.
The greenhouse effect
4 The Sun is the primary source of energy for the Earth’s climate. Satellite observations
show that about 30% of the Sun’s energy that reaches the Earth is reflected back to
space by clouds, gases and small particles in the atmosphere, and by the Earth’s
surface. The remainder, about 240 Watts per square metre (Wm-2) when averaged over
the planet, is absorbed by the atmosphere and the surface.
What this basically boils down to is that clouds are reflective. They're white, so this is hardly surprising. If they were black, then they'd absorb far more heat instead. Gases also reflect radiation, as do particles. Anything that interacts with light is going to do its own particular mixture of absorption, reflection and refraction.
Clouds have a big impact on how much radiation reaches the surface. This is a pain for climate modellers as clouds are hard to predict. (ask any weather forecaster)
Radiation that is not reflected by clouds, gasses and particles eventually reaches the surface of the Earth and is an average of 240 Watts/m2 (Yes, some radiation will get absorbed in the atmosphere and re-radiated, we'll come to that later)
How much energy is that? Visualise an old style 250W light bulb every metre of your ceiling. (Daylight is far brighter than indoor light, but we tend not to notice because our eyes adapt so well.
5 To balance the absorption of 240 Wm-2 from the Sun, the Earth’s surface and
atmosphere must emit the same amount of energy into space; they do so as infrared
radiation. On average the surface emits significantly more than 240 Wm-2, but the net
effect of absorption and emission of infrared radiation by atmospheric gases and clouds
is to reduce the amount reaching space until it approximately balances the incoming
energy from the Sun. The surface is thus kept warmer than it otherwise would be
because, in addition to the energy it receives from the Sun, it also receives infrared
energy emitted by the atmosphere. The warming that results from this infrared energy is
known as the greenhouse effect.
Second law of thermodynamics coming into play. Heat moves from hotter bodies to colder bodies.
If the Earth gets hotter, it radiates more heat back into space. Thus, the Earth radiates 240W/m2 (allowing for stuff bouncing back and forth from those dratted clouds and gasses).
Clouds (and atmospheric gasses) are a bit like leaky mirrors over the Earth. They reflect both ways and only reflect about a third of the stuff coming from any given direction.
Radiation coming in from the sun is one mixture of wavelengths and radiation emitted by the Earth is a different mixture of wavelengths.
The easiest way to see this happening is to leave a sheet of black paper in the sun. If you hold your hand over the paper, it will be radiating heat at you. It's absorbed radiation in the visible light spectrum and some of that energy has been re-emitted as infra-red (heat).
Clouds also reflect infra red - that's why cloudy nights aren't as cold as clear nights. The clouds act as a 'blanket'. (Or 'duvet' these days)
6 Measurements from the surface, research aircraft and satellites, together with
laboratory observations and calculations, show that, in addition to clouds, the two
gases making the largest contribution to the greenhouse effect are water vapour
followed by carbon dioxide (CO2). There are smaller contributions from many other
gases including ozone, methane, nitrous oxide and human-made gases such as CFCs
(chlorofluorocarbons).
Clouds are easy to see and account for a large chunk of atmospheric reflection. The next biggest reflectors are water vapour and CO2, followed by a host of less common gasses.
How do we know this? Partly by lab observations (send radiation into something and see what comes back again) and partly field work (measure atmospheric concentrations and see how much radiation is reaching your instruments compared with what you get under different atmospheric conditions. Field data obviously needs to be collected over long periods of time to get meaningful results)
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Typo. :~)
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Now fixed. Nice to know someone is reading it in detail!
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Try it if you like but you far better notter!
- Flanders and Swann
;-)
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