Tim the Plumber wrote:
The basic idea of AGW is that;
1 Human activity produces CO2 (absolutley)
You need to go further than that the evidence is clear that human activity has increased the the level of CO2 in the atmosphere by 40% since the late 1700 and we are responsible for CO2 in the atmosphere continuing to rise by about 2 PPM per year.
2 CO2 increases greenhouse warming of the atmosphere (Beyond me, let's assume so). The important question here is to what degree (sensitivity).
The reason co2 increases temperature at the surface is simple co2 is opaque to the heat waves that the earth gives off when it tries to cool especially at night. From experiments we know that any doubling of the amount of CO2 in the atmosphere will directly increase the temperature by about 1 deg C + or - 0.2 deg C. We also know that any warming of the atmosphere will lead to knock on effects particularly an increase in the amount of water vapour in the air which is even more opaque to heat trying to escape from the surface. The best estimate is that a doubling of CO2 in the atmosphere will lead to surface warming of about 3 deg C.
3 That increased temperature results in ice melting at the poles which increases sea level. Again how sensitive is this to temperature change being the key question.
The increased sea level is caused by:-
1 Thermal expansion of the ocean, the average depth of the ocean is 3.8 kilometre. A thermal expansion of just 0.01% will lead to a sea level rise of 38 cm or a bit over a foot.
2 The ice on land melting of which the Greenland , the Antarctic and glaciers at lower altitudes, are the major contributors.
If the pre-industrial activity of humans resulted in increased sea levels as a result of increased temperature (3-4 degrees c), as has been suggested,and the increase in sea level is demonstrated by the e3xistance of submerged villages off the coast of lots of places, then the sea level must have been of the order of 4m. That requires a melting of 1.4 million cubic killometers of ice.
I don't accept the 3-4 C figure as been correct although human activity may have been responsible for a small amount of the warming that took place prior to the industrial revolution.
It is however worth noting that for each doubling of the level of CO2 the temperature is expected to rise by the same amount.
Where was the big ice sheet which has disapeared in the last 7,000 years?
Or perhaps the land has a tendancy to move up and down. This could explain the drouned cities.
From Roman times to about 1870 the amount of sea level rise has not been significant but since that time, the ocean levels rose on average at about 1.7 mm per year up to 1993 based on tide gauge data, after that time we have satellite data which indicates that current rate is about 3.4 mm per year. Note that where the two types of data overlap the results are consistent. This shows that the rate of sea level rise is accelerating.
If humans have changed the climate by pre-industrial CO2 release then what we are doing now should have extreemly obvious results, the sort of thing which is quoted by the more insane alarmist types who speak of boiling seas etc.
The answer is yes the result are obvious. The temperature gradient of the atmosphere with height has changed dramatically and in line with that expected from greenhouse gases.
The temperature in the high latitudes has also increased by up 4 deg C which is also expected from increased levels of GHGs, but not from any other possible cause that we are aware of.