To: neolib who wrote (1598 ) 6/10/2007 11:46:19 PM From: Nadine Carroll Read Replies (1) | Respond to Predictably we differ there. The problem is that global warming has a very large integral term, which is a problem. W. M. Schaffer [scipio_a@ix.netcom.com]: The Devil being often in the details, it would be useful to non-climate modelers if Professor Dyson could elaborate on the following assertion: "Concerning the climate models, I know enough of the details to be sure that they are unreliable. They are full of fudge factors that are fitted to the existing climate, so the models more or less agree with the observed data. But there is no reason to believe that the same fudge factors would give the right behavior in a world with different chemistry, for example in a world with increased CO2 in the atmosphere."Freeman Dyson: I am not able to provide details of particular climate models and their deficiencies. Typical examples of fudge factors occur in the treatment of clouds. Each cell of the atmosphere in the model is characterized by a set of numbers which specify the temperature, pressure, density, humidity, wind-velocity, cloudiness, etc. in that cell. Since the cell is much larger than a typical cloud, the "cloudiness" number is only a rough measure of the fraction of the cell that is occupied by clouds. An empirical formula then gives the rate of precipitation in the form of rain or snow for a cell with a given humidity and given cloudiness. The empirical formula contains several coefficients which are fitted to the observations to make the model agree with the existing climate. These coefficients are what I call "fudge-factors". They are not based on a detailed understanding of clouds and rainfall but only on fitting a formula to observations. If now the model is run with enhanced CO2, there is no reason to believe that the same fudge factors will still give the right answers. There are many other fudge factors concerned with processes such as snow-melting and vegetation-growth that cannot be modelled in detail. I agree with the opinion expressed by Schaffer in the last sentence of his fourth paragraph.
