On the Effect of Historical SST Patterns on Radiative Feedback
It is now recognized that climate feedbacks vary in time, and depend upon the forcing applied to the system. The pattern of warming in recent decades is suspected to have more stabilizing feedbacks than is expected for long-term warming. The dependence of radiative feedback on the pattern of SST change has been termed the “pattern effect.” In this study, the radiative feedback is quantified over the historical period is estimated in 14 AGCMs forced with observed SST and sea-ice from 1870 to near-present. This feedback is compared to coupled experiments with quadrupled CO2. There is good correspondence between historical and long-term feedbacks generally, but it is found that feedbacks since about 1980 are significantly different from the long-term feedbacks.
This study strengthens previous findings by showing that the pattern effect, defined here as the difference in feedback strength between historical and long-term climate change, is robust across a large number of models. Additional experiments using an alternative SST dataset show that the details of the SST reconstruction matter for the quantification of feedbacks and the pattern effect, but the qualitative interpretation is unchanged. Over the period 1870-1980, the Earth warmed with a relatively uniform pattern and feedbacks is largely consistent with long-term experiments driven by increased CO2. In contrast, since 1980 a pattern with a strong tropical SST gradient emerged. This pattern drove large negative feedbacks and pattern effects in these models. These more recent feedbacks are not correlated with long-term feedbacks.
This study raises questions about whether climate models can be used to constrain climate sensitivity based on recently observed decadal warming. Coupled climate models generally do not capture the observed pattern of warming in recent decades, so their recent feedbacks may be more correlated to long-term feedbacks than observed feedbacks are expected to be. Models that do capture the observed global warming may do so with too small a pattern effect and too much ocean heat uptake. This study highlights the need to better understand the processes involved with recent warming as well as how the pattern is expected to evolve. This understanding will require testing various hypotheses that have been suggested, including that the recent warming is a mode of unforced variability, that aerosol forcing has contributed to the pattern, that the eastern Pacific is warming more slowly because of upwelling unperturbed water, or teleconnections from other regions have influenced the tropical Pacific in ways that are not understood.