This science has improved greatly since the 2001 ICPP report, and predictions are therefore more reliable. It is a very complicated problem [8]. There are standard physics equations that tell you how air and water move and how heat is transferred from one medium to another, but weather varies with location and changes by the hour. To predict climate, one has to divide the space into a finite number of cells, few enough for computers to handle. These cells are about 200 km laterally and 1 km vertically (in the atmosphere), decreasing vertically to maybe 100 m near the ground. To divide up time, 30-min averages are taken for climate, and shorter time steps for weather forecasting. The computer program then takes the average condi­tions in one cell and predicts what the conditions will be in the next time step. The conditions include, for instance, temperature, wind speed, water vapor, snow cover, and all the effects mentioned earlier in this chapter. We did not mention the history of CO2. About 45% of the CO2 that man generates goes into the atmosphere, 30% into the oceans, and the rest into plants. The CO2 absorbed by oceans diffuses downward over many years. The CO2 in the atmosphere has a mix of different life­times; roughly speaking, half goes away in 30 years and half stays for centuries. All such effects have to be accounted for in the models.

The key word is “average.” How does one find the average conditions in a 100 x 100 x 1 km cell 1 km above Paris, for instance? Clouds are forming and mov­ing all the time. Modelers have developed parametrization, a technique for averaging over small-scale and short-time conditions. Clearly, it takes many decades of expe­rience to get parameters that give the right averages, and different workers will arrive at different parameters. This does not inspire great confidence, and most skeptics of climate change distrust modeling and correctly point out that this is the weak point in forecasts of impending disaster. Fortunately, there is a way to check. Starting a couple of centuries ago, accurate data on temperature, CO2 content, and so forth became available. A modeler can take those data and predict what hap­pened later using his or her parameters. Then he/she can check with what actually happened and adjust his parameters to give the correct result. The only uncertainly is then whether or not the parameters of a century ago are the same today. We will show that different workers have varying success in their predictions, but all show that the current global warming is man-made.