Web hub says (4.42): “Last time I checked, orbital changes don’t create a feedback loop, and neither does angle of incidence. Those are known perturbations, with an easily estimated forcing function.” The net annual and global radiative forcing from orbital changes and angle of incidence is indeed close to zero. This fact is used by some people to discount the role of such variations in global climate change. However that argument misses the point, which is that what seems to count is a regional (rather than global) and seasonal (rather than annual) control, namely (as Milankovich suggested) the slow cyclic variation in summer insolation on the earth’s land hemisphere. It is mainly on the NH land areas that the initial positive feedback is felt, with growth and shrinkage of the area covered by snow and ice and attendant albedo changes. So far so good. However the question of how this regional pattern can extend to become a global one is still a matter for further debate. Some people argue that atmospheric CO2 feedback plays an important role; others find that solution unfeasible because of the time lag. At least that’s how I see the situation. There seems to be plenty of scope for further study.
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