This recent report, from the session on 'Tipping Points' at the important Dec.07 American Geophysical Union meeting in San Francisco, illustrates the complexity of current technical discussions about the validity of the increasingly disruptive climate change scenarios being projected by various Climate Change computer models. The bottom line is that our models may be seriously underestimating the rapidly of the coming changes, as indicated in the previously posted article re the melting of arctic sea ice. ~rj

---

...In Hansen's talk, he did try to clarify what he meant by a tipping point. His notion of this has less to do with what mathematicians understand as "bifurcations," and more to do with a kind of inertia in the climate system. He means things like having passed a threshold of CO2 which, given warming in the pipeline and the lifetime of CO2, commits a certain discrete event — e.g. loss of perennial sea ice or the Amazon rainforest– to occurring even if we were to later reduce emissions to zero. He tried to distinguish between reversible and irreversible tipping points...

...where things get interesting is where you try to explain a magnitude of signal this big in terms of basic physics. This is important because there is a perception that GCM's vastly underestimate the amplitude of the response to total solar luminosity, leading to a perception that there is some "missing physics" (whether it be exotic amplification of a stratospheric response, or something like clouds and cosmic rays)...

But — the take-home point is that at this point the study of solar cycle response very strongly supports the notion that there is no need to invoke any mysterious or exotic missing physics (like cosmic ray modulation of clouds) in order to represent the response of climate to solar variability. If some models underestimate the response, this is likely to have more to do with errors in the vertical mixing of heat than any missing fundamental physics. ...   more »