Q&A On Two Climate Science Questions – A Contribution By Mike MacCracken

As part of my weblog posts, I am inviting internationally well-respected climate science colleagues to answer two questions that I have posted to them.  Today’s scientist is Mike MacCracken. As written at the website of the Climate Institute.

Michael MacCracken has been Chief Scientist for Climate Change Programs  with the Climate Institute in Washington DC since 2002; he was also  elected to its Board of Directors in 2006. Both of these positions are  held on a volunteer basis.

The Q&A with Mike follow:

1.  Is global warming (and cooling) a subset of climate change or does it dominate climate change?

Mike’s Answer

I would say that human-induced climate change is a subset of overall changes in climate over Earth’s history. There are a number of human-induced influences, some of which exert warming and some of which exert cooling influences–and these can have different time scales. Over the next 100 years, the emissions resulting mainly from fossil-fuel based combustion (so CO2, SO2, BC, CH4, and other emissions contributing to tropospheric ozone) are virtually certain to exert a dominant warming influence on sub-continental to global scales (and this is frequently referred to as global warming, although the climatic changes that are induced go far beyond just warming on the global scale). In some local to sub-continental regions land cover change–some due directly to humans and some due to changes in climate induced by other factors, especially due to ongoing use of fossil fuels–may lead to comparable warming or even to cooling on time scales of decades, and so cannot be ignored.

2. What evidence exists that the multi-decadal global climate models can skillfully predict i) the real-world observed behavior of large-scale atmospheric-ocean circulation features such as ENSO, the NAO, the PDO, ect. and ii) CHANGES in the statistics (patterning and in time) of these circulation features?

Mike’s Answer

With or without models, we do not have “predictive skill” of Earth system oscillations on time scales of multiple decades–or even multiple years– and it is not clear, given the nonlinearities of the system, that predictive skill can be achieved. It is also not clear that lacking this skill has anything to do with “projections” of how the climate will be changed by human activities over periods of many decades to many centuries–that is, to the time scales related to the changes in climate being induced by the increasing concentrations of CO2 and other greenhouse gases. The inherent variability of the natural oscillations does make extracting the signal due to changes in land cover considerably more difficult.

I defer to the IPCC analyses (AR4 and pending AR5) on the growing, but limited capabilities of the global models to project (they do not “predict” as my comment on your note in Climatic Change attempted to clarify several years ago–so you should know better than to be saying “predict”) the present and future changes in the statistics of various Earth system oscillations (and what, in some cases, our limited observational record suggests, perhaps incorrectly, may be oscillations). I would only note that I don’t consider this limitation to accurately simulate the current statistics of Earth system oscillations to mean that the models are not reasonably projecting how the mean state of the global climate is likely to change over periods of several decades and more due to combustion of fossil fuels, but do think this limitation makes it more difficult to project the combined changes of fossil fuel combustion and land cover change on regional scales.

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