There is an interesting and important comment posted by Tas at December 2, 2006 at 02:31 AM on Prometheus with respect to the nonlinearity of the climate system in that small perturbations can result in critical threshold shifts in climate with resultant important environmental and social impacts.
The comment states in part,
“…we have strong indications that we live in a non-linear world. The climate system almost certainly has thresholds which when crossed cause an essentially irreversible mode-switch. For example, once the Greenland ice sheet collapse passes a certain point, it is unlikely to regrow in the current regime.
So the argument that incremental changes don’t matter because they are “insignificant”, fails at some point – an unknown point, a priori – because once crossed such thresholds cannot be “uncrossed”. This is compounded enormously by the inherent lags in the system. It is the potential for committing now, to a non-linear change that will only be realized sometime later, that really underlies the need for caution. To dissect changes to their minutest components and argue that any or all are not significant is a convenient rhetorical device, but, to paraphrase Richard Feynman, nature will not be fooled. Whether it is an air conditioner in Melbourne, or an oil furnace in Madison, the final mole of CO2 may push us to a point where the world is no longer the same.”
This comment is correct that the natural climate system is nonlinear. We present evidence of this in our paper
Rial, J., R.A. Pielke Sr., M. Beniston, M. Claussen, J. Canadell, P. Cox, H. Held, N. de Noblet-Ducoudre, R. Prinn, J. Reynolds, and J.D. Salas, 2004: Nonlinearities, feedbacks and critical thresholds within the Earth’s climate system. Climatic Change, 65, 11-38.
Climate Science agrees that the human disturbance of the climate system certainly could also result in threshold changes (as well as, inadvertantly move us away from a natural climate change threshold).
However, what is missed in the discussion is that it is not only the radiative effect of CO2 that is being perturbed by human activity, as has been extensively documented on Climate Science (see the weblogs in the Category “Climate Change Forcings and Feedbacks”), and in the 2005 National Research Council Report “Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties“.
Thus if we accept that small perturbations can result in significant changes in the climate system through nonlinear interactions, then all of the human- and natural climate forcings need to be assessed in this context. Moreover, since the radiative forcing of CO2 is more dispersed than a number of other climate forcings, as we have documented for aerosols (see) and land cover/land use change (see), it reasonable to propose that they pose a greater threat of causing threshold changes in the climate system since they are more concentrated in their spatial forcing. [as a simple example of this, we find a greater impact of sunlight on a piece of paper when we focus it with a lens!).
The more heterogeneous climate forcing of land cover/land use change is why NASA reported, based on our peer reviewed research, that “Landcover changes may rival greenhouse gases as cause of climate change“. The comment on Prometheus inadvertently provides further reasoning on why the IPCC needs to move beyond its focus on the radiative effect of the well-mixed greenhouse gases, and its treatment of the multi-decadal behavior of the climate system as a quasi-linear system.
The subject of the importance of the nonlinearity of the climate was also discussed in the Climate Science weblog of December 24 2005.