Non-specialist Summary:
Along with lead author Yair Rosenthal (Rutgers) and co-author (Delia Oppo) we published a paper in Science discussing the results of our research into reconstructing Pacific Intermediate water temperatures over the last 10,000 years.
To set the stage to discuss our new results we must first discuss the instrumental record of intermediate water temperatures in the ocean that extends back to 1955.
Oceanic intermediate waters refer to waters between approximately 500 and 1000m below the surface, so mid-depths. Previous work based on instrumental thermometer data has shown that these intermediate waters have been warming everywhere (in all ocean basins) since the late 1950s. But we know little about changes in intermediate waters before this time.
In our research we were able to reconstruct Pacific intermediate water temperatures using the Mg content of bottom dwelling fossils call foraminifera over the last 10,000 years. These single celled protozoans live in the mud and it turns out that their Mg chemistry serves as a paleo-thermometer. The warmer the water, the higher the Mg concentration in the fossil shells.
Our new results indicate that the intermediate waters in the Pacific have cooled ~ 2°C over the last 7,000 years. The cooling trend was rather steady until approximately 1,500 years ago when temperatures rose a little during a time known in the northern Hemisphere, as the Medieval Warm Period (a time when there were vineyards in England). Following this relatively warm interval, intermediate water temperatures continued their long-term cooling trend reaching a minimum approximately 400 years ago (~1600AD) during what is known as the little Ice Age. After this the sediment record shows that intermediate water temperatures stopped cooling and the trend reversed.
Our results place the current warming of intermediate waters in temporal perspective and indicate that the present rate of heat gain in intermediate waters is ~15X that of any time in the last 7,000 years, based on the resolution of our sediment-based data.
Our results also demonstrate that these century-scale climate events known as the Medieval Warm Period (a.k.a. Medieval Climate Anomaly) and Little Ice Age were global in scale and actually effected water temperatures between 500 and 1000m down in the western Pacific. Thus the deep ocean is sensitive to century-scale changes in Earth’s surface temperature.
Intermediate waters in the Pacific and Atlantic actually form in mid-latitudes where surface water sinks after equilibrating with atmospheric temperature. Thus our results also demonstrate that the ocean is taking up heat from the atmosphere much more efficiently that previously thought.
The oceans are thus working to buffer the global climate system and the ongoing rise in atmospheric temperatures.Unfortunately we do not know what the down-stream effects of this heating will be and how much time this will buy us. There could be positive effects or negative effects. The heat will eventually come back out. In our view, there is a lot of uncertainty and more work is needed to understand intermediate water formation and heat storage in the oceans.