Issues in Climate Science Underlying Sun/Climate Research
Isaac Held, National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory
In this talk I will discuss some aspects of climate research that provide a wider context for
analyses of the climatic effects of solar variability. I divide this discussion into three parts: bottom-up
effects related to the energy balance of the troposphere; top-down effects related to effects on the
troposphere of the state of the stratosphere; and the possibility of solar-modulated cosmic ray-induced
nucleation on cloud cover.
I will first discuss the proposition that the troposphere acts as a strongly coupled system as it
responds to changes in its energy balance, insensitive to the details of the spatial distribution of the
forcing—especially in the vertical, and to a more limited extent, horizontally. This coupling underlies the
utility of the concept of “radiative forcing.” I'll discuss the standard conversion factor (doubling CO 2 Ù
2 percent change in insolation) that is a simple consequence of CO 2 radiative forcing estimates, and the
classic experiments at the dawn of research on climate change with GCMs that showed very similar
tropospheric responses to changes in the total solar irradiance and to changes in CO 2 . The importance of
the frequency-dependence of climate responses in relating greenhouse gas, volcanic, and solar cycle
responses will be mentioned. Hydrological responses provide the most likely route through which to
break away from a universal relationship between radiative forcing and response, and some
geoengineering literature will be introduced in this context.
Stratospheric influences on the troposphere have been discussed in the context of research on the
response to volcanoes, solar cycle UV forcing, the QBO, and, especially, the ozone hole—as well as in
the long-range weather prediction context. I will describe an emerging picture of the mechanisms
responsible for this coupling, especially in regard to coupling to the tropospheric annular modes and the
central role played by meridional temperature gradients at the tropopause/lower stratosphere. I will argue
that the ozone hole plus observed trends in Southern Hemisphere winds provides a useful quantitative
check on models of this coupling, indicating in particular how large the perturbation to lower
stratospheric temperature gradients needs to be to generate an observable effect.
Finally, I will briefly mention research on the indirect aerosol effects through which particles
affect climate not by direct alternation of solar fluxes but by modifying cloud condensation nuclei.
Observational and modeling studies of the susceptibility of cloud radiative properties to changes in cloud
condensation nuclei are especially relevant as background for any discussions of conceivable solar
activity-modulated cosmic ray ionization effects on cloud condensation nuclei and climate.
Indirect Climate Effects of the Sun Through Modulation of the Mean Circulation Structure
Caspar Ammann, National Center for Atmospheric Research
Solar irradiance changes have now been monitored from space for several decades. Based on the
collected data, there are no clear indications that the total irradiance changes might have varied
substantially more than the range directly observed. In fact, climate reconstructions would suggest that
larger irradiance changes are not necessary to explain the mean temperature fluctuations of past centuries
and millennia. There is good confidence in this interpretation because the global/hemispheric mean
temperature of the globe is tightly linked to the radiative balance of the planet, actually remarkably so,
despite the often stated low level of understanding of solar radiative forcing. Therefore, direct irradiance
changes most likely have left relatively small, albeit discernible, imprints in large-scale mean
However, two other issues can be raised that, in combination, might offer a suggestion for an
indirect pathway for the Sun to affect the climate system.
Most recently, the observations by the SORCE spectral irradiance monitor have highlighted large
variability in the higher-frequency component of the solar spectrum, even higher than previously
acknowledged. Independent of how exactly the variations are distributed across the wavelength, a strong