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how atmospheric
chemistry and physics

effects global warming

 

a briefing document

 

 

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click to see all the indexHow atmospheric chemistry and physics effects global warming is one of a series in a series of briefing documents investigating the indicators, science, analysis and argument surrounding global warming.
One of a grouping of documents on global concerns at abelard.org.
on energy on global warming
On housing and making living systems ecological
sustainable futures briefing documents
Tectonics: tectonic plates - floating on the surface of a cauldron
click to see all the indexIndex
the greenhouse effect
so where are we right now (march 2007)
isotopes and global warming
oxygen in ice and oxygen in water: dating methods
mercury and venus


 

 

 


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the greenhouse effect

Radiation on Earth, with isotope componentsSome people do not like this expression, but it suits me just fine. It is a pretty damn good description. The light from the sun comes in, and the greenhouse gases trap some of the heat radiated from the Earth. It’s an energy balance. The more heat that comes in, or the more heat that is trapped, the higher the energy balance. The bigger the ice fields, that is the higher the albedo, the lower the Earth’s energy balance. For more detail, see radiative forcing components diagram.

But, setting aside any changes in conditions, incoming energy from the Sun minus energy reflected out into space from the Earth is zero. This what is meant by an energy balance. The various ways in which the incoming energy from the Sun interact with the Earth and its atmosphere can be crudely seen in the series of diagrams to the left. The energetic ‘particles’ from the Sun are, by stages, converted to lower energy forms (by particle collisions). Thus, the outgoing energy spectrum will be shifted rightwards (towards the infra-red and beyond) of the incoming energy spectrum, while what remains is perceived by us on Earth primarily as light and warmth.

A fundamental error is being spread that, because a global warming gas (forcing) is transparent to a large amount of the incoming radiation from the sun, it cannot be having much effect in terms of warming the planet. This is an error that ignores a real-world fact that incoming radiation is converted to longer wave-lengths as it impacts the atmosphere and the planetary surface. It is these longer wave-lengths that are then blanketed from escaping back into space. If it were not for this wave-length conversion (mechanism), the greenhouse gases (GHGs) would not be causing the rise in the level of the Earths energy balance (examine the graphs to the left to get some idea of how this works).

The energy balance for the Earth is about 33° Centigrade higher than it would otherwise be if there was no greenhouse effect. The average temperature on Earth is about 16° Centigrade. Thus, it would be more like -17° Centigrade, if there were no greenhouse gases in the atmosphere and, therefore, no greenhouse effect.

This page will give you a rough idea of physical and chemical data related to greenhouse effects. See also planetary heat circulation.

This section is developing in response to the desperate attempts of the fossil fuel industry, various amateurs and conspiracy theorists who wish to rubbish the growing consensus on the anthropogenic global warming effect. Much of this page is in response to the almost surreal claims that carbon dioxide is, somehow, not a major part of the problem. It is my, perhaps vain, hope that by the end of this page, any such nonsense will be taken less seriously. Things are moving very quickly, as vast amounts of data is being collected year by year.

In the next section, pay close attention to the dates of the three items quoted.

 

so where are we right now (march 2007)

“On the one hand, as scientists we are ethically bound to the scientific method, in effect promising to tell the truth, the whole truth, and nothing but - which means that we must include all the doubts, the caveats, the ifs, ands, and buts. On the other hand, we are not just scientists but human beings as well. And like most people we'd like to see the world a better place, which in this context translates into our working to reduce the risk of potentially disastrous climatic change. To do that we need to get some broadbased support, to capture the public's imagination. That, of course, entails getting loads of media coverage. So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have. This 'double ethical bind' we frequently find ourselves in cannot be solved by any formula. Each of us has to decide what the right balance is between being effective and being honest. I hope that means being both.” [Steven Schneider in Discover, pp. 45-48, Oct. 1989]

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“[...] I readily confess a lingering frustration: uncertainties so infuse the issue of climate change that it is still impossible to rule out either mild or catastrophic outcomes, let alone provide confident probabilities for all the claims and counterclaims made about environmental problems.” [Steven Schneider in January 2002 Scientific American]

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“[...] The way the SPM [Summary for Policy Makers] works is that the scientists write a report, and then are put together in a room with representatives of the world’s governments, and between them they agree a text that has full support, the idea being that there is nothing left that can be contested: that the SPM has the full support of all the relevant scientists and their governments. Since the governments in question include the administrations of George W. Bush, King Abdullah, John Howard and Hu Jintao, this is not a straightforward process; in fact there is something heroic about the firm stand the SPM manages to take. The price for this is that the SPM makes no policy recommendations of any kind, a fact which has drawn some negative comment; but the consensus on the basic facts is so remarkable that we can live without the unenforceable policy advice.

“The first crucial component of the scientific consensus concerns a figure called the ‘climate sensitivity’. This is the amount by which the climate will grow warmer if the amount of CO2 in the atmosphere doubles. It is not a straightforward figure to calculate because many of the values change as the temperature changes; water vapour, for instance, is an important greenhouse gas, and as the oceans warm, water vapour in the atmosphere increases both in amount and in its greenhouse properties. Arrhenius thought that it would take three thousand years for our activities to double the level of CO2, which in 1750, before the Industrial Revolution, was about 280 parts per million (ppm). By now the level is 379ppm and rising sharply. As the Chinese and Indian economies take off and global levels of CO2 begin to rise even more quickly, it seems a racing certainty that we will achieve that level of doubled emissions some time this century; at which point the ‘climate sensitivity’ will become the most important number in the world. So the fact that according to the IPCC ‘an assessed likely range’ for climate sensitivity can now be given ‘for the first time’ is of more than academic interest. That figure is likely - between 66 and 90 per cent probable - to be between 2 and 4.5ºC. The best estimate is for climate sensitivity to be 3ºC. ‘Values substantially higher than 4.5ºC cannot be excluded.” [John Lanchester, from lrb.co.uk, March 2007]

 

isotopes and global warming

All this global warming stuff gets complicated. You may hear all the amazing claims and be inclined to ask, “How do they know that?”

A lot of the advances and discoveries are made by a careful analysis of isotopes. Isotopes are versions of chemical elements which have slightly different atomic weights, from having different numbers of neutrons. The isotopes that vary from the basic element are, in general, more rare than the basic isotope.

Most of the carbon in the world has six neutrons, and six protons, which gives carbon an atomic weight of 12.

But there are also carbon atoms out there with seven or with eight neutrons, which are known as carbon 13 (13C) and carbon 14 ( 14C) .

Now, the ratio for the different quantities of these various carbon isotopes in the air are known and can be measured with great accuracy. The ratios vary according to circumstances.

Plants prefer to use 12C, rather than 13C, for photosynthesis. They take in both 12C and 13C version of carbon dioxide (CO2) from the inhaled air. During respiration, plants exhale the unused carbon dioxide. Because the plant has used 12C carbon dioxide preferentially to 13C carbon dioxide for photosynthesis, the percentage of carbon dioxide that is 13C carbon dioxide is higher in the exhaled air than in the inhaled air. Thus, scientists talk of the exhaled air being 12C depleted.

Also, with carbon, the rare isotope 14C is not found in the carbon-rich fossil fuel deposits of coal and oil. 14C is radioactive, with a half-life of 5,730 years. It has had millions of years during its sojourn underground to decay into the more common forms.

Thus from ratios like this (the ratio of 12C carbon dioxide to 14C carbon dioxide) the amount of carbon in the air from anthropogenic sources can be estimated. And the scientists are not just guessing, believe it or not. The table below shows some isotopes, together with some of the uses to which they are put.

As you will see, there are several naturally occuring isotopes of both carbon and oxygen. Thus, you will immediately guess that there are various possibilities for the composition of carbon dioxide (CO2). There are six relevant permutations for this discussion (3 carbon isotopes times 2 oxygen isotopes). The other isotopes of these elements are not relevant to this discussion.

oxygen in ice and oxygen in water: dating methods

"Much of what we know about Cenozoic ice ages comes from the offshore record, where continuous sequences of sediment are preserved. Scientists in the early 1970s discovered that the deep-sea sedimentary record, recoverable by drilling ships, could be used to reconstruct glacial/interglacial climates. In order to determine the pattern of climate change, a technique known as oxygen isotope analysis was developed. This technique involves determining the ratio between the two varieties of oxygen, the light isotope 16O and the heavy isotope 18O. As ocean water evaporates, preferentially more 16O is released, but in non-glacial times is returned almost immediately to the ocean as runoff from the land. In glacial times this excess 16O is stored in ice masses, leading to enrichment of 18O in the oceans. Marine sediment containing microfossils called foraminifer, reflects the composition of seawater. As the sediment accumulates on the sea bed, a continuous record of oxygen isotopic variations is produced, with the highest values of 18O occuring during interglacial periods.” [2]

Oxygen take-up, that is oxygen trapped in ice or incorporated in shells, is used for detecting two different things - an assessment of the amount of water tied up in the ice sheets, and the current ambient temperature. This take-up is recorded in the shell formation of foraminifer microfossils in oceanic sediments. However, these two causes can be confused. My reading suggests that both will act in the same direction and the causes may be separated, but as yet I am unconvinced that this is so.

The major method of finding the age of sea sediment cores is magnetism. The Earth’s magnetic field switches direction from time to time, and therefore the magnetic alignment of different times represented is in the core and indicates when that part of the core was laid down. Magnetism is also used for distant core samples to locate their original position on the globe, as the angle between position and the pole varies with latitude. Obviously, these two usages can be confused.

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element isotopes
carbon 12C 13C 14C
uses of isotopes

coal, diamonds, graphite

(98.9%)

absorbs radio waves, used in nuclear magnetic resonance spectrometry to study organic compounds (1.1%) radioactive, half-life: 5730 years,
used radiocarbon dating objects [1]
(trace)
oxygen 16O, 17O, 18O  
uses of isotopes

stable components of atmospheric oxygen:
16O: 99.76%, 17O: 0.038%, 18O: 0.21%

 
hydrogen 1H - protium
(standard hydrogen atom)
2H - deuterium
(‘heavy’ hydrogen)
3H - tritium
  99.985% of naturally ocurring hydrogen 0.0115% of naturally ocurring hydrogen traces only
half-life: 12.32 years

The changing ratio of 14C to 12C, as we burn fossil fuels, allows a calculation of how much anthropogenic (fossil) carbon is in the air.

As 16O is preferentially evaporated relative to 18O, an assessment can be made from ice core and microfossil analysis as to the amount of water tied up in ice at any one time.

There are several other isotopes of both oxygen and of carbon, but they are only usually available through laboratory fabrication.

Hydrogen isotopes are used in ice core analysis, but I don’t know what for yet!

mercury and venus

The average temperature of Venus is much higher than that of Mercury, even though Venus is nearly twice as far from the sun as Mercury!

“[...] a young American physicist called James Hansen, whose 1967 PhD thesis studied Venus and came to the conclusion that it was the greenhouse effect which made the planet so warm – 400ºC on the surface, hot enough to melt lead. A probe later the same year showed that the atmosphere of Venus was in fact 96 per cent carbon dioxide [...] ” [Quoted from lrb.co.uk]

planet name average distance to sun (AU) solar constant (W/m2) real average temperature (oC) average temperature without atmosphere, and with zero albedo (oC) average temperature without atmosphere (oC)
Mercury 0.387 9147 167 173 167
Venus 0.723 2620 464 55 unknown
Earth 1 1370 16 5 -17
Mars 1.524 590 -63 -47 -58

An Astronomical Unit (AU) is defined as the average distance from the Sun to the Earth. It is approximately 150 million kilometres, or a bit over 8 light minutes.

The Solar Constant is different for each point in space. It is the amount of energy from the Sun that reaches that particular place. It can be calculated very easily from this simple formula:

S.C.x = S.C.earth/r2

Where S.C.x is the solar constant at point x, and r is the distance from the Sun to x (in AU). This formula is an example of an inverse square law - the closer you get to the Sun, the more of a difference going a little bit closer will make. Thus, while the difference in the distance from the Sun to Mercury and from the Sun to Venus is approximately the same as the difference in the distance from Sun to Venus and from the Sun to the Earth, the amount of energy drops to less than a third between Mercury and Venus, but only drops to about half between Venus and the Earth.

The average temperatures without atmosphere and albedo assume that each planet is black. A black planet has an albedo of 0, which means that it absorbs everything and reflects nothing. However, planets are not black! The Earth, for example, has an average albedo of around 0.3. This means that 30% of the incoming energy from the sun is reflected straight back. Thus, a more realistic figure for the average temperature of the Earth without an atmosphere would be -17oC, rather than +5oC.

Nobody knows the albedo of Venus, because the atmosphere is so thick that nobody can see the surface. In all likelihood, however, Venus’ surface is not black, so the average temperature of Venus without an atmosphere would be significantly lower than 55oC.

 

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end notes

  1. Thus, after ten half-lives, less than one-thousandth of the carbon 14 remains. Carbon 14 is regarded as at its extreme limit of carbon dating at about 60,000 years ago. Carbon 14 is created in the upper atmosphere by cosmic rays. There are a large number of other radiometric dating methods using isotopes of other elements. (Carbon 14 decays to nitrogen 14.)

  2. As you go further back in time, tectonic plate movement has redistributed the surface of the planet.



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