Antonis Christofides has graciously sent me an e-mail and short presentation in response to our article
Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell, W. Rossow, J. Schaake, J. Smith, S. Sorooshian, and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union
and my post
He wrote in his e-mail
“Some time ago you had asked the readers of your blog to comment on the completeness of your three hypotheses from “The need to consider human forcings besides greenhouse gases”, and propose alternatives. I’m sorry I was too overloaded to respond at that time, although I did have an alternative hypothesis to propose, based on the research by D. Koutsoyiannis.
So I thought you might like to know about the short presentation I made yesterday in the European Geosciences Union conference in Vienna, where I try to explain that it might simply be meaningless to talk about causes. You can find the presentation at http://itia.ntua.gr/1130.
The title article is
Christofides, A., and D. Koutsoyiannis, Causality in climate and hydrology, European Geosciences Union General Assembly 2011, Geophysical Research Abstracts, Vol. 13, Vienna, European Geosciences Union, 2011.
Their abstact reads
“We often see statements such as “90% of climate change is caused by X” and debates on whether the dominant cause of climate change is human activity, or the sun, or something else. However, in chaotic systems, it can be difficult to defend the meaning of such assertions, because if the “effect” occurs sufficiently later than the supposed “cause”, the relationship between the two is effectively lost because of the sensitivity of the “effect” to the initial conditions. In fact, although “A causes B” initially seems clear, closer examination of what it actually means reveals problems that have tortured philosophers for centuries. We review the meaning of causation in the context of hydroclimatology as well as its possible reformulation in probabilistic terms.”
The entire article by the two very well respected climate scientists is worth reading. Their conclusion reads
“The conclusion is that we should be careful when we talk about causes, and that trends and shifts do not necessarily imply non-stationarity or a change in forcings: they can just happen.”
I certainly agree with this as we have written, for example, 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.
where we stated that
“The Earth’s climate system is highly nonlinear: inputs and outputs are not proportional, change is often episodic and abrupt, rather than slow and gradual, and multiple equilibria are the norm.”
However, our conclusion that only one of the three hypotheses that we presented in our EOS paper can be correct remain. The view that is articulated in the Christofides and Koutsoyiannis 2011 article is of internal feedbacks resulting in long term trends that are then misintrepeted. This is certainly a valid view that I share. However, the human role clearly is also important as we conclude in our EOS contribution, where we report that the only ono-rejected hypothesis is that
“Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and involve a diverse range of first-order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2). Most, if not all, of these human influences on regional and global climate will continue to be of concern during the coming decades.”
Christofides and Koutsoyiannis add valuable insight into the role of nonlinear dynamics associated with internal climate system dynamics in producing long term changes in climate metrics.