Multi-decadal oscillations of surface temperatures and the impact on temperature increases

The last IPCC assessment report indicated that natural climate variability could temporarily amplify or obscure anthropogenic climate change on decadal time scales. Here we analyse global mean surface temperatures in terms of such long-period variations. We find two main oscillations, a strong oscillation with a period of about 70 years and an amplitude of about 0.09 K and a quasi-bidecadal oscillation with an amplitude of about 0.06 K. The strong oscillation shows large hemispheric differences. In the Northern hemisphere the period is longer and the amplitude is larger (about 82 years and 0.18 K) compared to the Southern hemisphere (about 47 years and 0.065 K). No obvious hemispheric differences are observed for the quasi-bidecadal oscillation. Such long-period oscillations can strengthen or weaken the temperature increase if the oscillation positively or negatively adds to the underlying long-term trend.