The North Atlantic Oscillation and ecology: links between historical time-series, and lessons regarding future climate warming

Indices of the North Atlantic Oscillation (NAO) have been very useful for explaining interannual variability in many ecological time series. We suggest that this is based on a combination of 3 factors: (1) the strong relationship between NAO and meteorological conditions in winter; (2) qualitative changes in environmental conditions in response to winter conditions, especially temperatures; and (3) the central importance of those conditions for the distribution and population dynamics of species in temperate and boreal regions. The increase in winter temperatures associated with a shift of NAO towards its positive phase in recent years has resulted in a relief from winter stress for many species and populations. This has reduced mortality rates during winter, thereby influencing local population dynamics and allowing, for example, the northward expansion of many species. In contrast to winter warming, the recent increase in summer temperature has had fewer ecological consequences, as it has not been large enough to cause an increase in heat stress to critical levels during summer. The difference in the ecological consequences of winter and summer warming also explains why reductions in the ranges of species have been observed less often than expansions during the past few decades. However, with further warming, summer heat stress might become an increasingly important determinant of the response of species to climate warming. This suggests that studies analysing the effects of the winter NAO on species dynamics and distributions will give us only a limited perspective on the further consequences of climate warming.