What is the physics supporting global warming?

I don’t know how technical an answer you want. For the benefit of other users I’ll keep it relatively simple but do ask if you want anything explaining in more detail.

For all intents and purposes the Sun is the sole source of heat we have on this planet, it accounts for 99.97% of the heat we have. Heat energy is radiated outward from the Sun in all directions, some of it reaches us here on Earth.

With a surface temperature of a little over 6,000°C, the transmitted energy is coming from a very hot source and therefore has a correspondingly short wavelength.

The amount of solar radiation that reaches Earth is some 1363 Watts per square metre per year on a plane perpendicular to the Earth’s surface at the upper edge of the atmosphere. Some of this incoming solar radiation is reflected back into space by clouds and the various surfaces on Earth. The whiter surfaces such as freshly fallen snow have a reflectance of about 80%, old snow is about 50%; darker surfaces such as vegetation have a reflectance of about 15% and the deep oceans about 8%.

The solar radiation that penetrates our atmosphere and is absorbed by the Earth is about 342 Watts per square metre per year. If all the sunlight reaching us could be converted into electrical energy it would provide us with 174 petawatts per year.

As the surfaces upon the Earth absorb the solar radiation they begin to warm up and they’ll keep absorbing more of this heat energy until the ambient temperature drops (heat travels from hot to cold). At such time the absorbed energy will be radiated back outward again, a classic example of this are the warm road surfaces acting like radiators on a cool night.

Because the heat source of this re-radiated energy is much cooler than that of the Sun, the wavelength is therefore much longer; it’s also a different form of radiation – thermal radiation (infrared).

This longer wavelength corresponds to the vibrational frequencies of the greenhouse gases. Different gases vibrate at different frequencies and the same gas can have a number of frequencies at which they absorb thermal radiation.

The key point to remember is that the incoming heat energy has a wavelength too short to be of consequence to the greenhouse gases, it’s the outgoing thermal energy that can be absorbed.

Once absorbed by the greenhouse gas molecules, the heat is subsequently retransmitted. Some of this goes upward and is lost into space but some of it returns back down to Earth. It’s this trapping and returning of heat energy that we refer to as the greenhouse effect.

Global warming / cooling occurs when the amount of greenhouse gases in the atmosphere increases / decreases. This is an over-simplification as there are many other factor that contribute to warming / cooling such that temps can, for example, be falling at a time when greenhouse gas concentrations are rising.

Other essential points to consider are the numerous influences consequent to changes in orbital variations, changes to total solar irradiance and the physical properties of the greenhouse gases; namely their concentrations, global warming potentials and atmospheric residence periods.

To put matters into context: For millions of years the amount of greenhouse gas in the atmosphere fluctuated between 180 and 290 parts per million by volume, since the onset of industrialisation levels have risen so fast that they’re now at 394ppmv (or 595 part per million by mass) and are rising by 2ppmv per year. With equilibrium climate sensitivity calculated to be around 3°C it means that by the time atmospheric concentrations reach about 600ppmv the average global temperature will have risen by some 3°C.

Given that GHG concentrations have increased by 41% so far ,we would expect to have seen a temperature rise of about 1.2°C. In reality the rise is 0.9°C but due to the fact that peak warming occurs many years after the release of the GHG’s this is pretty much bang on what we would have expected to see.

Indeed, the very earliest climate models (the JASON’s 1979 climate model of the world), made just such a prediction. And 32 years later those predictions are now coming to fruition.

For further info:

http://www.randombio.com//co2.html

http://www.wag.caltech.edu/home/jang/genchem/infra...

the basic properties of greenhouse gases, specifically CO2 has been known for over 100 years. The physics includes but is not limited to thermodynamics, isotope analysis, chemical analysis.

That is a long question to answer. I cannot do it in this small box. Please do some background research on the subject elsewhere.