Division Director’s Message

Maurice L. Blackmon

            In last year’s message, I wrote about the need to reduce uncertainties in our understanding of climate change.  I would like to continue this discussion and relate it to some of the accomplishments in Climate and Global Dynamics Division (CGD) over the past year or more.

            CGD scientists and our collaborators have developed an updated, improved version of the Community Climate System Model, CCSM2. This model has a host of new or improved components.  We are currently running a multi-century simulation with this model to understand its mean climate and “natural” climate variability.  Preliminary examination suggests that the model will be improved over the original CSM in many ways, but will continue to produce some of the same flaws in CSM.  Further work is necessary.

            An early experiment with a prototype version of CCSM2 has been an attempt to simulate the climate of the 20^th Century.  Our first attempt to simulate the climate of the 20^th Century using the original CSM was not completely satisfactory.  The simulation with the newer model is better.  What was needed to produce the better result?  There are several factors that had to be included in the simulation.  The first was the growth of greenhouse gases.  The second is the inclusion of sulfate aerosols.  The third is solar variability, i.e., changes in the radiation emitted by the sun. The final ingredient was volcanic eruptions over the past 130 years. 

            Uncertainties are involved in several of these ingredients.  The growth of greenhouse gases is well known.  The output of solar radiation has only been measured to the necessary accuracy in the past twenty years.  Consequently, a reconstruction has been necessary, and this has uncertainty associated with it.  Furthermore, the way this effect was included in the new model has some flaws in it, producing further uncertainties.  The situation is similar for volcanic eruptions.  There are no good records for the effects of volcanoes in the late 19^th or early-to-middle 20^th Century.  Only the eruption of Mt. Pinatubo in the early 1990’s was well monitored.  Consequently, a reconstruction of the earlier eruptions and their effects was necessary, and this resulted in further uncertainty.

            The inclusion of aerosols is also uncertain, but for different reasons than previously believed.  CGD scientists have participated in several field programs over the past few years in which the amount and characteristics of aerosols have been studied.  One of the major results has been that, at least in some areas, the aerosols emitted by human activity have a strong absorbing effect on solar radiation.  Previously, the most common assumption was that the dominant aerosols were sulfate aerosols, which scatter but do not absorb solar radiation, thus cooling the atmosphere.  Absorbing aerosols warm the atmosphere.  This effect was not included in the latest simulation of the climate of the 20^th Century, thus producing some uncertainty in the results.

            The situation is more complicated, however, in that the geographical distribution of the absorbing aerosols is unknown, as is its seasonal variability.  Further field work and observational work is necessary in order to have a better understanding of human-produced climate change. 

            This leads me back to the subject of reducing uncertainty and what to do about it.  Representatives of the federal agencies participating in the U. S. Global Change Research Program have been considering how to make the program more effective and responsive to the national needs.  I believe that specific goals must be identified, and programs developed to meet these goals.  A particularly valuable goal would be to reduce the uncertainty in our estimates of human-induced climate change.  This will require a combination of observational programs related to the needs for developing improved models.  Some of this is already being done.  Better answers for national and international policy makers require that this coordination be done even more effectively.