One of the stickiest problems that climatologists face when confronting climate model predictions on controversial issues such as C02 or other trace gas increases and their effects on climate is how to verify the models. Since direct validation is only possible by 'performing the experiment' on our planet -with all living things along for the ridemost rational attempts to validate climate model sensitivity are based on finding surrogate forcing and response analogs such as the seasonal cycle. The ability of a model to produce the 15 ~ Northern Hemisphere or 5 *C Southern Hemisphere seasonal cycle of surface temperature in response to seasonal solar forcing, is indeed a powerful test of model validity, but the seasonal test certainly is not sufficient to validate a model's sensitivity to all conceivable radiative forcings, including greenhouse gas increases. Clearly, other independent surrogate verification tests are needed. The best place to turn to find other such tests is to paleoclimatology, since ancient climates have been well documented to have been very different from today's conditions. For example, one of the most successful simulations to date was performed by Kutzbach and Guetter (1984), in which they attempted to explain the warmest period in recent climatic history, the so-called 'climatic optimum' that occurred between about 5 and 9 thousand years ago. It was a time when summertime northern continental temperatures were probably several degrees warmer than at present and monsoon rainfall was more intense throughout Africa and Asia. Kutzbach and colleagues found that the 'optimum' could be explained simply by the fact that the tilt of the earth's axis (its obliquity) was slightly greater than now and, at the same time, the orbit was such that the earth was closer to the sun (i.e., perihelion) in June rather than in January as it is now. These variations, do not make substantial changes in the annual amount of solar radiation received on earth, but do change substantially the difference between winter and summer heating periods. About 5-10% more solar heat in the Northern Helnisphere summer and a comparable amount less in winter occurred 9 000 yrs ago compared to today. This was sufficient in Kutzbach's simulations to substantially alter mid-continental warming in summer months, which led to the enhanced monsoon rainfall and river runoff in the model. Eurasian warming on the order of 2.5 ~ was obtained throughout most of central Eurasia and part of west central North America in Kutzbach's simulation. His results match quite well with considerable amounts of paleoclimatic evidence and help to explain one of the important mysteries in the paleoclimatic record.
Another example of possible surrogate verification of climate models is the so-called Younger Dryas period. When the earth emerged from the grip of the last Ice Age about 12 000 yrs ago, conditions around the globe were rapidly turning toward the warmer interglacial values that we have been experiencing for the last 10 000 yrs. Although the great glaciers of northeast Canada and part of the Scandinavian ice sheet had not fully melted