On March 1 2010, we posted
Chick, as indicated at the bottom of the above post, has agreed to continue this constructive dialog between us. I will respond in a follow on post tomorrow. Thank you Chick for this interaction.
Following is his second thoughtful contribution.
Guest Post By Charles F. Keller
Perhaps the best way to characterize Roger’s EOS piece is to recognize that, with the publication of IPCC’s 4AR ,we reached the “end of the beginning” (with respects to Winston Churchill’s great speech). That is, we had looked at the major global forcings and come to the firm conclusion that humans were causing warming of the planet mainly by their emissions of so-called greenhouse gases with CO2 as the major one. This is what I will call the “first order” conclusion and it includes understanding of major positive feedbacks such as additional water vapor as well as complications from emission of a variety of aerosols which both cool and warm the atmosphere. Data and computer models were now good enough to allow us to predict the response of large areas of the planet to these forcings in the next half century. Having written this, the IPCC’s authors now will need to turn their attention to the likely much more difficult question of how to respond to this apparent problem of a warming planet with resulting decreased soil moisture the consequences, which could be disastrous for large areas of the planet. But society’s response has been that it now needs predictions of behavior on a decadal time scale and at regional spatial scales. To do this scientists must now include what I will call second order forcings–those that act over shorter time and spatial scales. Among these are the effects of aerosols and their effects on clouds. Other such forcings of course will include the effects of land use changes (not only in albedo, but in moisture content, plant loading, etc.)
Roger’s EOS article considers three alternatives: 1, 2a and 2b. He discards the first and moves to 2a as a more accurate statement of our conclusions than 2b. This is a different way of describing the situation I describe above, for conclusion 2b results from first order, global, long-term forcings while 2a asks us to include the above second order, short term, regional forcings.
Roger says IPCC concentrated too much on the 2b version. I agree but I don’t criticize because it was very important to settle the long-term, global question first. In doing this 4AR did not ignore second order forcings and its working group II has dealt with many of the problems Roger mentions. And so I see Roger’s position not as criticism of what’s been done. Rather it is a call, which many of us recognize, to move on to these next difficult problems. Indeed to make decadal predictions on regional scales will require understanding of second order forcings.
Roger makes a good case for the need to understand the role of diverse human-generated aerosols, both in their forcing of the earth’s radiation budget and their effects on cloudiness and precipitation(see his references to Rosenfeld et al, 2008, Flanner et al, 2007, and others). Indeed much of regional weather will be strongly affected by these.
However, Roger has not dealt sufficiently with what I consider a really large problem, that of understanding and simulating the many pathways of energy within the climate system. Especially Roger ignores the oceans with their great heat capacity and their ability to strongly modify atmospheric behavior. One of the great challenges of the next decade will be to understand oscillations of heat within the oceans. We have already experienced dramatic heat and cold waves across Europe that resulted largely from heat anomalies in the Indian Ocean. ENSO dominates much of the Earth’s weather especially when it affects the Walker Cell. But what drives ENSO or the other oscillations will be a subject for much future research. Indeed these oscillations may be far more important to decadal weather than aerosols. Along these lines it might be well to mention a seminal paper on energy balance by Kevin Trenberth et al. http://www.cgd.ucar.edu/cas/Trenberth/trenberth.papers/EnergyDiagnostics09final2.pdf
Another possible problem for decadal climate is the increasing evidence that solar activity changes cause second order, indirect forcings in addition to changes in insolation. If Judith Lean and David Rind are correct that the 10-11 year solar activity cycle causes global temperature to vary by as much as 0.1°C (insolation changes can only cause about half of that), then some other indirect solar forcings must be at play. Several have been suggested–among them: stratospheric responses to relatively large UV changes, enhancement of ENSO activity, etc. Here it is of interest to note that the prolonged and abnormally low levels of solar activity in the past few years may allow us to tease out which, if any, of these indirect effect are at work. Whether or not Lean and Rind’s work is correct, the recent level temperature record may also allow better understanding of decadal and multi-decadal ocean oscillations such as PDO, AMO, NAO, and others.
On the policy side of his writing Roger calls for a broader consideration of “complementary policies ” that would reduce other unwanted human-generated forcings. Here he echoes recommendations by Jim Hansen and others who call for reduction of other gases/particulates instead of just concentrating on CO2. However, since CO2 is the major contributor to global warming, I would have to support strong measures to curb its emission, while also working to reduce these other lesser forcings. And I think most scientists would agree with Roger’s call for a broader approach. (A note here–Roger mentions in passing that climate is subject to abrupt changes. While this is demonstrably true during deeply cold glacial times, it has been apparently less so during the warmer Holocene suggesting that abrupt climate change is associated with large changes in ice amounts not present in the past few thousand years. A possible exception to this is potential for large ice melt in Antarctica.)
Roger’s call for substituting risk assessment methods for improving our understanding of climate change is beyond my expertise and so I must rely on others to comment. But I do agree, that lacking adequate ability to do regional decadal forecasting, society will be forced to make decisions based on such methodologies.
In conclusion, Roger’s EOS Forum is not too different from the general understanding of the climate community. We have seen the “end of the beginning”. Now we must take on the even-more-difficult tasks of pushing towards the beginning of the end of our studies.