HR: 10:50h
AN: U42A-03 INVITED     [Abstracts]
TI: Long-Term Extra-tropical Land Temperature Variability and Its Association with Solar and Volcanic Forcing
AU: * Cook, E R
EM: drdendro@ldeo.columbia.edu
AB: It has long been hypothesized that a significant fraction of observed Northern Hemisphere (NH) temperature variability at multi-decadal to centennial time scales can be attributed to variations in solar activity. This argument has been supported by some modeling experiments that used long-term changes in solar irradiance, explosive volcanism, and anthropogenic trace gases to model temperature changes over the past 1,000 years. The modeled temperatures based on these radiative forcings often compare reasonably well to proxy-based NH temperature reconstructions covering the past 1,000 years, with solar forcing being important up to the mid-20th century, after which anthropogenic forcing dominates. Volcanic forcing is also important, but it operates in a more impulsive, episodic fashion. Consequently, its impact on proxy estimates of NH temperatures over the past 1,000 years may episodically distort and even overwhelm the effects of solar forcing on temperatures, thus breaking down what would otherwise be the appearance of a strong solar influence on past temperatures. While this problem can be investigated with models, the level of radiative forcing attributed to both solar and volcanic effects is still poorly constrained. Here, we statistically investigate the combined influences of solar and volcanic forcing in a land-only, extra-tropical NH temperature reconstruction. This record has been shown to have a strong statistical association with instrumental annual temperatures over the same region of the NH. Using estimates of solar irradiance and volcanic forcing, we show evidence for a centennial time-scale influence of solar forcing on past temperatures on the order of 700-1,000 years ago during a period of relatively high solar activity and little explosive volcanism. After that time, the frequency of explosive volcanism increases substantially, during a period of generally reduced solar activity, which appears to overwhelm solar forcing. In an effort to better reveal the underlying solar signal in the temperature reconstruction, we try to factor out the volcanic forcing signal, with limited success. It appears, therefore, that the emergence of solar forcing as a long-term agent of temperature change over extra-tropical NH land areas is highly dependent both on the magnitude of its forcing and on the frequency and timing of explosive volcanism as a competing agent of temperature variability and change.
DE: 8409 Atmospheric effects (0370)
DE: 1600 GLOBAL CHANGE (New category)
DE: 1650 Solar variability
SC: Union [U]
MN: 2004 AGU Fall Meeting