Over recent decades the boreal winter index of the North Atlantic Oscillation (NAO) has exhibited an upward trend, corresponding to lowered surface pressure over the Arctic and increased surface pressure over the subtropical North Atlantic. This trend has been associated with over half the winter surface warming in Eurasia over the past thirty years, as well as strong regional trends in precipitation over Western Europe. Several studies have shown this trend to be inconsistent with simulated natural variability. Most climate models simulate some increase in the winter NAO index in response to increasing concentrations of greenhouse gases, though the modeled changes are generally smaller than those seen in the real atmosphere. The two other principal anthropogenic forcings, sulphate aerosol and stratospheric ozone depletion, are generally found to have little significant effect on the NAO. Natural forcings may have also had an impact on the atmospheric circulation: volcanic aerosols induce the westerly (positive index) phase of the NAO in the 1–2 years following major eruptions, and multi-decadal changes in the NAO have also in part been attributed to changes in solar irradiance. These natural forcings, however, are unlikely to account for a substantial component of the recently observed positive NAO index trend: it is most likely to be the result of increasing greenhouse gas concentrations. Experiments using climate models forced only with changes in tropical sea surface temperatures suggest that at least part of this trend may be due to remote forcing from the tropics. Some authors have argued that greenhouse gas-induced changes in the meridional temperature gradient in the lower stratosphere may be responsible for the upward NAO index trend, but overall the mechanism of response to greenhouse gases remains open to debate.