via two mechanisms—nucleation impacts and changes in the fair weather electric circuit of the
atmosphere. A central question is then, Where does the role of energetic particles fit in the effort to
understand the influence of the Sun on climate? The discussion among several workshop participants
indicated that although TSI is the main driver of response, there is the question of whether smaller (in
terms of energy) perturbations can have a tangible effect on climate and by what mechanisms, and
whether that impact might show a significant location dependence.
The panel discussion concluded with a general discussion of whether the workshop had addressed
the issues raised by NASA and NSF—what is known about the role of the Sun in determining climate,
and what are the future research directions? During the discussion period, several workshop participants
stated that many of the major issues had been aired in the workshop but that significant and important
work remained. In addition, participants discussed what sorts of investments agencies could make now
with available resources in, for example, improving existing data sets, versus continuing measurements or
starting new measurements or new observations, or making new investments in modeling efforts to
address these open issues. Suggestions included looking at model studies in a systematic way; examining
the paleoclimate record to see whether there were natural oscillations in the system that could result in the
system transitioning from one mode to another; developing an understanding of the inherent timescales in
the system and the feedbacks that might amplify effects; determining through model or data studies if
there are certain regions that are more susceptible to solar influence on climate; evaluating the quality of
the past record of environmental response to establish a better chronology; or developing a better
understanding of the issues associated with some of the proxies used in studying the Sun-climate linkage.
At the end of the panel discussion Gerald North summarized the issues that were developed during
the workshop. He made three principal points:
1. NASA has led the way in providing a model for ready access to data from many sources—the
challenge is to provide better access to paleoclimate data while recognizing the effort it takes to acquire
and archive those data in a form accessible to the community.
2. Coupled models, with their inherent complexity, are the future and need to be used more
widely for well-designed studies. It is fortunate that climate modeling has advanced to the point that such
projects can be undertaken with some confidence.
3. The directly measured record is limited and not without its issues, and so the challenge is to
make sure that a means is developed to infer the time history of TSI variability and the limitations on the
ability to specify that past behavior.
North also summarized other issues that he felt had been addressed during the workshop and that
were particularly noteworthy. Those issues included, among others, the need to be careful in making
inferences from the isotope record, which may reflect influences of atmospheric circulation; the need to
understand the role galactic cosmic rays may play in cloud nucleation; and the influence of variations in
geomagnetic field on the paleo-climate record. North noted that Peter Foukal's discussion of the Sun was
particularly interesting because of the unresolved issues with understanding variability and the sources of
variability in TSI arising from the details of the quiet network. Also, a better understanding is needed of
how solar brightness, TSI, and the spectral and spatial distribution of energy are affected by the faculae
and the dynamics of the Sun.