The NAO - natural variability vs forcing factors

Since we know that the NAO exerts a big influence on the wintertime climate over the Euro-Atlantic part of the hemisphere, one might think that winter warming in recent times is due to this natural mode of variability and that we’ve overestimated the effects of anthropogenic greenhouse gasses (GHGs) in the atmosphere. In my last post I talked about the pronounced positive trend in the NAO index and questioned whether this is part of its natural variability or if the NAO might be influenced by external forcing.

I’ve read a paper by Stephenson et al. (2006) that is trying to answer this question by looking at the response of wintertime NAO to increasing concentrations of atmospheric carbon dioxide. In the study they’ve used 18 global coupled general circulation models (GCMs) with a 1% per year increase in concentrations of CO₂. 15 of the models were able to simulate the NAO pressure dipole, which is one of its main features, but none of the models were able to reproduce a decadal trend as strong as that observed in its later time series. 14 out of those 15 models simulated an increasing trend in the NAO index with increasing CO₂ concentrations, but the magnitude of the response was generally small and highly model-dependent. Despite their different NAO responses, all the models showed a similar increasing NAO-like pattern in temperature and precipitation trends with increasing CO₂ concentrations, like warming and increasing precipitation over Northern Europe (which is fitting with my memories of “bad” Norwegian winters).

                   Norway; what I would call a "good" winter....................and a "bad" winter

Although the models in the study do suggest that the NAO show a weak positive response to increasing amounts of CO₂, the authors note that with the large amounts of model uncertainty as they found in this study “one has to be exceedingly careful about making inferences concerning future climate change”. Since the models simulate differences in NAO response, but similar responses in temperature and precipitation over Europe, the authors suggest that NAO is not the key determining factor for such changes.

So it seems like we cannot blame warming over Europe solely on the NAO's natural variability. There are so many factors interacting in the climate system and it is hard to know what is responsible for what. But it looks like there are external forcing mechanisms, including anthropogenic greenhouse gasses, contributing to the extreme winter climate over recent decades.

Various studies have investigated NAO trends and ended up with similar results. I would like to quote one of them; “[…]most authors agree that greenhouse gases are likely to be at least partly responsible for the long-term trend in the boreal winter NAO index”, Gillett et al. 2003. It is clear though, that a lot more research needs to be done on this subject to understand to what extent anthropogenic forcing from greenhouse gasses influence wintertime climate over Europe.