Separating the impact of human activity from natural variability in the global climate record is difficult because of the paucity of long-term climate data and the relative magnitude of natural variability. Wigley et al. (1998) use lagged correlation and cross-correlation of hemispheric mean observed and model-simulated temperatures to conclude that both anthropogenic and solar forcing significantly have influenced global climate. They use model results to evaluate the roles of enhanced greenhouse gases alone and greenhouse gases with sulfate aerosol (anthropogenic) and solar irradiance changes (natural). They reject volcanic eruptions as a factor in long-term influences and show that the El Nino/Southern Oscillation has only a marginal effect.
Reference
Wigley, T. M. L., R. L. Smith, and B. D. Santer, 1998: Anthropogenic influence on the autocorrelation structure of hemispheric-mean temperatures. Science 282, 1676-1679.