Decadal variability of anomalies in global averaged atmospheric CO2 concentration (in parts per million by volume (ppm)) and in global surface temperature (in °C) compared with decadal variations in sunspot numbers, modified from Keeling et al. (1989, Figure 66). Upper curves: Approximately 10-year oscillations in atmospheric CO2 concentration (dashed curve) and global temperature (solid curve), calculated as differences between splines of different stiffness. For CO2 the stiffer spline has a standard error (s of 0.245 ppm with respect to monthly averaged data (1958-1989); the less stiff spline has an error of 0.200 ppm. For temperature over the same time period the stiffer spline has a s of 0.088°C. Lower curve: Sunspot numbers (National Geophysical Data Center, 1989), shown as line segments connecting monthly averages. The CO2 record in 1968 and 1969 was modified from that reported by Keeling et al. (1989) as a result of a reassessment of the original calibrating data.