EVIDENCE FOR SHORT-TERM FORCING OF LAND-SURFACE AIR TEMPERATURE BY ANTHROPOGENIC CO2 EMISSIONS

We compare records of atmospheric CO₂ concentration (1), land surface-air temperature (2), and anthropogenic CO₂ emissions (3) during the period 1968-1993. Time-series data were de-trended by linear regression. Seasonal and ENSO-related interannual variations of temperature and CO₂ were removed by spectral filtering (4). The de-trended, filtered residuals of temperature and CO₂ vary synchronously; maximum correlation of 40% between the two residual signals occurs when the curves are shifted 122 days in the direction of a CO₂ lead, temperature lag relationship. A 'greenhouse' cause-effect link is one plausible explanation for the observed correlation. The magnitude of this hypothesized effect is indicated by the correlation to be 5.0 °C for a CO₂ doubling, which is marginally consistent with model predictions (5). A similar analysis also demonstrates that changes in anthropogenic CO₂ emissions track together with atmospheric CO₂ variations; maximum correlation of 45% between the two signals occurs when the curves are shifted 338 days in the direction of an emissions lead, atmospheric CO₂ lag relationship. The magnitude of the effect indicated by the correlation is 1.8 Pg C/ppmV CO₂, compared to mass balance estimates of 1.0 Pg C/ppmV CO₂ (6). By the naïve approach of transitive and direct comparison, anthropogenic CO₂ emissions statistically account for 20% of observed land-surface air temperature residual variability within the period of time described by these data. This result does not rely on atmospheric model predictions.

(1) Conway, T.J., and Tans, P.P., NOAA Cooperative Flask Sampling Network, 1967-1993. (2) Easterling, D.R., Karl, S.A., et al., US Global Historical Climatological Network. (3) Marland, G., Boden, T., and Andres, B., Carbon Dioxide Information Analysis Center. (4) Hughes, G.B., Giegengack, R., and Kritikos, H.N., 1999, International Journal of Climatology, v. 19, pp. 813-819. (5) Boer, G.J., Arpe, K., et al., 1992, Journal of Geophysical Research, v. 97, p. 12,771. (6) Keeling, R.F., Piper S.C., and Heimann, M., 1996, Nature, v. 381, n. 6579, pp. 218-221.