What's the problem with Water?

Previously, I pointed out that Carbon Dioxide and water vapour are greenhouse gases. So lets look at water vapours roll in the climate equation.

First of all lets consider water in general. The Water molecule consists of two hydrogen atoms and one oxygen (H2O). It can change between solid, liquid and gas within a narrow range of temperatures, the sort of conditions found on Earth. So we find H2O exists in the form of gas (water vapour), liquid (water, clouds, mist) and solid (ice, snow). The state that H2O appears in our eco-system depends on the amount of thermal energy that is present in the system.

So if there isn't a lot of thermal energy, water will become a solid because the water molecules aren't excited (vibrating and moving around). If you add more thermal energy, then the water molecules in the solid (ice) get excited and turn to liquid. If you add even more thermal energy then the liquid water becomes a gas (water vapour) as the molecules fly about individually. Of course the opposite happens when you take the thermal energy away, which is why you get condensation on windows in the morning. The gas (water vapour) in the air condenses on the cold glass of the window, producing liquid (water droplets).

Simples.

Previously I showed that water vapour is the most abundant green house gas (GHG) in the Earths atmosphere with Carbon Dioxide coming second. So how do the two interact and create the problem of Climate Change?

Lets look at a few scenarios:

1. The amount of Carbon Dioxide (CO2) in the atmosphere remains stable. 
In this scenario, unless some other input source of energy - such as the Suns output changes or there is an alteration to the tilt of the earths axis - changes the amount of thermal energy present in the Earths eco-system, then the proportion of water vapour in the atmosphere is unlikely to change much over periods of decades or centuries, because there isn't a suitable increase of thermal energy to generate more vapour/gas. The amount of water vapour will of course fluctuate regionally and on a daily basis as the Earths rotation and the Suns energy output cause the day/night cycle and weather patterns. Seasonal cycles will also occur as the Earth orbits the Sun and the Earths tilt induces the seasons (Summer, Winter etc.).
In this scenario the climate trajectory remains relatively flat.

2. The amount of CO2 in the atmosphere reduces.
Under this condition, the amount of thermal energy retained by the earth would drop. Although water vapours roll as a green house gas would continue, as CO2 in the atmosphere depletes over decades or centuries, some water vapour would return to the liquid state and some liquid water would become solid (ice). This in turn would reduce energy levels, because there would be less water vapour in the atmosphere acting as a greenhouse gas and more ice would reflect sunlight back into space (the polar albedo's will increase). Since water vapour has an impact on thermal energy levels in the system and in this case would help to reduce the levels. It creates a feedback mechanism whereby the cooling reduces the amount of water vapour and produces more ice, which in turn causes more cooling etc. until a new cooler steady state is achieved on Earth.
With all this going on, there would also be weather, day/night cycles and seasonal cycles as in scenario. However the trajectory of the climate would be one of cooling.

3. The amount of CO2 in the atmosphere increases.
This is effectively the exact opposite of scenario 2. Under this condition CO2 causes some warming, which in turn causes more water vapour to be generated and the amount of ice cover is reduced, this in turn generates more heat in the system. Less ice means less sunlight is reflected back into space and instead sunlight warms the ground and oceans. So here we have a positive feedback mechanism, the opposite to scenario 2 where negative feedback was engaging in the system.
Again with this scenario there would also be changing weather, day/night cycles and seasonal cycles as in scenario. However in this case the trajectory of the climate would be a warming one.

So from this we have learnt that water is potentially a powerful feedback mechanism that enhances the impact of other green house gases that we generate, principally CO2, but also methane and other more complex gases used in industrial processes such as electronics.

But what about clouds?
Yes indeed, in the middle of this process are clouds, which aren't generally gas or solid yet can influence climate. Clouds are made of droplets of water, although obviously some contain ice as well.
Clouds have different influences depending on the height they are in the atmosphere, as well as shape or type. At some heights they reflect some light away from the earth, at other heights they help keep heat energy in (a cloudy evening is usually warmer than an evening with a clear sky).

Some useful links to info about clouds and climate:
http://www.giss.nasa.gov/research/briefs/delgenio_03/
http://www.metoffice.gov.uk/climatechange/news/latest/clouds.html
http://www.ucar.edu/news/features/clouds/
http://public.web.cern.ch/public/en/research/CLOUD-en.html

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