Reconciling anthropogenic climate change with observed temperature 1998-2008

Abstract

Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations. As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well known warming and cooling effects.

Fig. 1.

Radiative forcing of anthropogenic sulfur emissions (purple line), net anthropogenic forcing (blue line), linear estimate of net anthropogenic forcing (blue dash), total radiative forcing (red line), radiative forcing of solar insolation (orange line), and observed temperature (black). The SOI (divided by 10) is given in green. SOI data are presented as annual mean sea level pressure anomalies at Tahiti and Darwin. Post-1998 period of interest (highlighted gray).

Fig. 2.

Observed temperature (black line), the out-of-sample simulation generated by the model estimated with a sample period 1864–1998 (orange line), the out-of-sample simulation generated by the model estimated with a sample period 1920–1998 (blue line), and the out-of-sample simulation generated by the model estimated with a sample period 1960–1998 (green line). Error bars represent 95% confidence intervals (see SI Appendix).

Fig. 3.

Observed temperature (black line), the out-of-sample forecast for global surface temperature driven by anthropogenic changes in radiative forcing (red line) and the out-of-sample forecast for global surface temperature driven by natural variables (solar insolation, SOI, and volcanic sulfates) (green line). Error bars represent 95% confidence intervals (see SI Appendix).