Of the absorbed light, more than 90% of the warmth goes into the oceans. The remaining 10% of sunlight warms the land, melts ice, and warms the atmosphere.
It is calculated that doubling the CO2 content of the atmosphere will enhance the greenhouse properties of the atmosphere and directly warm the atmosphere by a little more than 1 degree celsius, however, that warmer atmosphere can carry more water vapor (humidity) and that extra water vapor is also a very powerful greenhouse gas, so, after all is done, the doubling of CO2 will result in more than 3 degrees celsius of warming. (Water vapor acts as a positive feedback which enhances the warming that is started by the CO2.)
The IPCC is a POLITICAL body, NOT a scientific body. They want to have an excuse to tax the world and they have chosen the Global Warming SCAM as their vehicle to do that. The IPCC hasn't made a correct prediction yet and this SCAM they are pushing has been going on for more than 30 years.
In short... they LIE, that's how they come up with the 2 degrees. Actually they have predicted as high as 5 degrees.
It's all a SCAM, watch these and see what real scientists have to say:
The Great Global Warming Swindle
The quick answer is that they program climate models with everything they think they know about climate science, pick a few different input values for what they don't know, and then run a large number of simulations to project possible temperatures in the future. Note that they don't come up with only 2C, they have a whole range of values with a range of probabilities as well.
Here is a good page where they explain all of this: http://www.ipcc.ch/publications_and_data...
Also note that not everybody puts the same credibility into those projections. There is actually quite a wide range of opinion on whether or not those models overestimate future warming.
They pick it out of their derriere and broadcast it to scare you.
Quote by Noel Brown, UN official: "Entire nations could be wiped off the face of the Earth by rising sea levels if the global warming trend is not reversed by the year 2000. Coastal flooding and crop failures would create an exodus of "eco-refugees," threatening political chaos."
I would add to what Mike said and suggest that they first decide what may be considered catastrophic and then adjust the models to make sure they comply. It is a political organization masquerading as a scientific one. In order to use AGW for their political agenda, models which exaggerate any warming are magnified.
They attempt to come up with a number that will keep the feed trough full of taxpayer welfare (grant) money.
A dartboard. And then when someone questions how scientific a dartboard is the alarmists call them a denier.
Simple answer is that they guess, nobody knows for sure.
I am currently working on a paper on the construction of a prominent number in global governance. I chose the IPCC's prediction that the earth will raise to a temperature of 2 degrees Celsius by 2100.
My question is do you have any perspective on this? If not, can you point me towards sources that might help me uncover the IPCC's methodology for constructing this number?
