Science doi:10.1126/science.1146436 (2008)

Credit: SIMON PEARSON

Uneven warming of the North Atlantic Ocean during the last half-century may be caused by changes in the natural climate system, concludes a new analysis.

Susan Lozier of Duke University in North Carolina and colleagues compared heat-content measurements in the North Atlantic region from 1950 to 1970 and from 1980 to 2000. Over the 50-year period, the North Atlantic as a whole heated up moderately, but finer-scale changes were more complex, they found. Tropical and subtropical areas gained up to ten times more heat than the North Atlantic average, but in contrast, the subpolar zone cooled almost as markedly.

Using a modelling approach, the researchers showed that the observed heating and cooling pattern may have been caused primarily by changes in a large-scale climate system known as the North Atlantic Oscillation (NAO). Since the observed variations in regional heat gain and loss are great enough to mask an underlying greenhouse warming trend, the authors warn it is too early to know whether the changes in heat content are partly due to anthropogenic climate change. They say that long-term monitoring is needed to tell whether humans have heated the North Atlantic, perhaps even by affecting the NAO itself.