As an oceanographic contribution to the quincentennial celebrations of Columbus’s voyage of discovery in 1492, a collaborative expedition was carried out in July-August 1992 to make a transatlantic, deep-ocean hydrographic section along Columbus’s route at 24 degrees N. The 24 degrees N section is of interest for studies of climate change because it has been surveyed twice before, during the International Geophysical Year of 1957(1) and during 1981(2) and hence represents one of the best known of all oceanographic sections. Here we use the temperatures from all three surveys to show that the waters between 800 and 2,500 m depth have consistently warmed over the past 35 years and that the warming since 1957 is remarkably uniform across the east-west extent of the North Atlantic. The maximum warming, found at 1,100 m depth, is occurring at a rate of 1 degrees C per century. This trend is broadly consistent with model predictions of climate change due to increases in atmospheric CO2 concentration(3,4), but the observed warming occurs in the interior ocean, in contrast to the surface-warming predicted by the models. The observed patterns of decadal-scale changes in ocean temperature are thus powerful signatures that can help us to understand the nature and causes of climate change.