Good morning OCuK!
(sorry for the length of this post...
)
Just before xmas I had to research and prepare a seminar on global warming/greenhouse effect - heres some things that came of this:
1. The natural greenhouse effect is real - simple radiation balance calculations show that the
surface of the earth is warmer that it 'should be'. It should average -6°C . Infact studies show that it is 15°C . This is because of natural greenhouses gasses (GHGs) in the atmosphere which carry heat to higher altitudes where the emission is re-radiated out of the atmosphere. Incidently this occurs at an altitude of ~6km, which corresponds to the temperature of approx -6°C (required for radiation equalibrium). THIS EFFECT IS ACCEPTED GLOBALLY.
2. The anthropogenic or enhanced greenhouse effect describes the human influence on the natural greenhouse effect. This is obviously due to altering the levels of GHGs in the atmosphere. The effect of higher GHG concentrations is that emissions occur at a higher altitude (thus at a cooler temperature) resulting in positive radiative forcing (more radiaition comming into the atmosphere, than leaving). The response is to try and level this out, and the earth's atmosphere warms so that the temperature at the emitting altitude (lets say its now ~8km) is -6°C and balance is restored. This means that surface temp rises in accordance with a linear temperature profile.
3. UNFCCC reckon there's a net increase in global temperatures of 0.6°C over the past 100 years which is attributed solely to enhanced greehouse effect. In the past 40 years, direct CO2 measurements from Mauna Loa show a rise in atmospheric CO2 levels of around 50 parts per million (now at ~365ppm).
However, using records of the Earths geomagnetic index as a proxy for solar activity, M. Lockwood has deduced that Solar Coronal activities have doubled in the last 100 years (1999). This is backed up by sunspot records.
4. Crash course in a possible (one of many possible) link between solar activity and Earth climate:
Solar activity means high ejections of solar plasma which carry with it, a frozen-in magnetic field. This magnetic field extends out to the very edge of the solar system, where galactic comsic rays (high energy protons and ions) meet the influence of the solar magnetic field. During high activity, the magnetic field hinders the cosmic rays traversing the solar system (thus less appear at earth).
Cosmic rays in the atmopshere produce ionisation at low altitudes, acting as nucleation centers for (low level) cloud formation. Low clouds are optically thick, and so sunlight gets reflected away before reaching the earths surface. This is negative radiaitive forcing leading to a cooling effect on earth.
(N. Marsh & H. Svensmark (2000))
Summarise:
Past ~100 years
Solar Mag flux:
up 131%
--> Cosmic Ray Flux:
down 11.2%
--> Low cloud cover:
down 8.6%
Expected value for radiative forcing: +1.4 Watts per square meter
Compare this to the estimated contribution from enhanced GH: +1.5 W/m^2
As you can see these figures are very close considering the simplicity of this argument. As far as I am concerened, there is no doubt in my mind that global warming is occuring, but it can be attributed to BOTH human and natural effects. The real question is which effect dominates the trend in rising temperature? Unfortunatly there is the inclination of scientists and polititions to look for the one answer, when infact it can be the sum of many subtle effects. Above only counts CO2 and the role of cosmic flux due to solar variation. There is evidence to suggest that solar UV emissions that increase during high activity disassociate particles and aerosols in the atmosphere that can also increase cloud production, equally methane from livestock add more GHGs to the atmosphere. Its not until all therories are considered together that a wider picture/model can be deduced for the Earth's fate.
Sorry if the above is a bit disjointed.. I may have over simplified some aspects to make it more readable. Feel free to ask questions or expand on any points.
