Atmospheric carbon dioxide and land-surface air temperatures in geologic and modern-instrument records: Identifying patterns and formulating alternative hypotheses through spectral analysis
Over geologic time scales, atmospheric CO2 concentration and surface air temperature display a remarkable synchronicity. This tandem variation has been attributed to atmospheric CO2 forcing of surface-air temperature via the ‘greenhouse effect’. But since controlled experimentation at regional or global scales is not feasible, obtaining conclusive evidence of ‘greenhouse’ forcing is problematic. Conclusions must be drawn from empirical observations of the dynamic climate system. Uninterrupted, daily atmospheric CO2 records are available from Pacific and North American stations from 1968 through 1993. The Fourier spectrum of CO 2 shows significant power at one year and its harmonics, and at the difference frequency of the atmospheric pole tide and the annual signal. The spectrum suggests that sub-annual, annual and inter-annual atmospheric CO 2 variability is not complex. Daily records of land-surface air temperatures are also available for the same time period, and a comparison is made between the two histories. Time-series data were de-trended and de-seasonalized, and residuals were compared. Significant correlation was found when the temperature signal lags the CO2 signal by 122 days. This relationship indicates that a cause-effect connection is a likely explanation for the observed correlation over the period described by the data. Similar analysis indicates anthropogenic emissions may affect atmospheric CO2 levels. Therefore, fluctuating emissions appear to drive changes in surface temperature. Since the modern instrument record is short, no conclusions can be made about inter-decadal to millennial scale relationships. Long time span, high-resolution records of temperature and CO2 are required. Geologic archives can provide this information, but methods are needed for extracting high-resolution data. Two possible sources are sclerosponges, and annual rhythmites (varves). An electron microprobe technique is demonstrated for extracting a 2,000-year, monthly-resolution record of El Niño/Southern Oscillation cycles from sclerosponges. A combination of electron microprobe analysis and optical scanning is demonstrated for investigating solar variability in Archean varves. Both of these archives show promise for addressing issues relating to geologic relationships between CO2 and temperature.