An eight-century-long record from the Caribbean reveals warmer sea surface temperatures in medieval times than today.

Credit: © NASA

Past climate records provide a long-term perspective on recent temperature changes. A new annually resolved sediment record from the Cariaco Basin highlights past variability in tropical Atlantic sea surface temperatures for the last eight centuries and suggests that the twentieth century is not the warmest hundred year period on record.

Sampling a core from the Cariaco Basin off the coast of Venezuela at 1 mm intervals, David Black of the State University of New York at Stonybrook, USA, and colleagues1 were able to analyse annual layers in the core. The team then used magnesium–calcium ratios and stable oxygen isotope ratios of fossil planktonic foraminifera to reconstruct sea surface temperatures in the Caribbean Sea from ∼AD 1250 to the present. Temperatures rose by ∼1 °C between AD 1425 and 1500, followed by an abrupt decrease of 1.5 °C during the Little Ice Age (AD 1500 to 1640).

Although the recent temperature rise represents the largest and most rapid increase of any 100-year period in the record, sea surface temperatures have generally been cooler in the twentieth century than during the Medieval Warm Period. However, temperatures in the last two decades are rapidly approaching the warmest in the record.