• Climate variability;
  • Decadal ocean variability;
  • global warming

[1] A basin-wide warming in the North Atlantic Ocean has occurred since the mid-1990s; however, the cause of this basin-wide warming is controversial. Some studies argued that the warming is due to global warming in association with the secular increase of the atmospheric greenhouse gas of carbon dioxide (CO2), while others suggested that it is caused by the Atlantic multidecadal oscillation (AMO) – an oscillatory mode occurring in North Atlantic sea surface temperature. Here we show that both global warming and AMO variability make a contribution to the recent basin-wide warming in the North Atlantic and their relative contribution is approximately equal. It is further shown that after removing a linear trend and the seasonal cycle, atmospheric CO2 measured from 1958–2008 varies approximately with the AMO. On the assumption that a linear trend can be removed from the CO2 time series, then there are suggestive similarities between CO2 and AMO temperature anomalies. That is, atmospheric CO2 increases (decreases) when the AMO is in the warm (cold) phase. This would suggest that the recent basin-wide warming of the North Atlantic might contribute to global ocean warming via its associated increase of atmospheric CO2.