A temperature record built from the shells of tiny sea creatures suggests the middle depths of the Pacific ocean have warmed 15 times faster in the last 60 years than at any time over the last 10,000 years.
By analysing the chemical composition of fossilised sea creatures called foraminifera, the authors of a new study in the journal Science have reconstructed a record of Pacific ocean temperatures stretching back over the past 10,000 years.
Scientists can glean information about what the climate was like at the time the sea creatures were alive. The warmer the water, the more magnesium the shells contain relative to the amount of calcium.
The reconstructed temperature record suggests that for most of the past 10,000 years, water 450 to 1000 metres deep in the Pacific ocean was cooling.
As the graph below shows, there were some ups and downs – the reconstruction suggests that this bit of the ocean was comparatively warmer during the Medieval Warm Period, about a thousand years ago. It also suggests that the ocean cooled faster during a period referred to as the Little Ice Age, which followed a few hundred years later.
The change in Pacific Ocean temperatures over the last 10,000 years, relative to the 1850-1880 average. “Age ky B.P.” means the number of years before present, measured in thousands of years. Source: Rosenthal et al. (2013)
About 300 years ago the water reached its coolest point. From about 1950 onwards the water warmed very rapidly. Over the last 60 years, these middle depths of the Pacific warmed by 0.18 degrees Celsius – a rate of warming 15 times faster than anything seen over the last 10 millenia.
Lead author of the study, Yair Rosenthal, told us this rapid rate of change is another piece of evidence showing that current climate change is unusual:
“This seemingly small increase occurred an order of magnitude faster than suggested by the gradual change during the last 10,000 years thereby providing another indication for global warming.”
The recent warming adds to the idea that oceans are taking up much of the extra heat trapped as a result of human activities. In its major review of climate science last month, the IPCC concluded that the oceans had absorbed more than 90 per cent of the heat trapped by human activities since the 1970s.
Taking the ocean’s temperature
Scientists have been measuring how much heat is stored in the world’s oceans for decades using basic instruments onboard ships. More recently, a network of underwater buoys has also been relaying temperature measurements to labs via satellites.
The use of climate proxies allows scientists to look back further. In this case, scientists used the shells of sea creatures found in sediments from the sea floor.
Hyalinea balthica – the foraminera scientists used to piece together their temperature record. Source: University of Southampton
But shells trapped in sediment cores only give information about what the temperature was like one location, not worldwide. Although in this study samples were only collected from a few locations, the authors say their results give a good indication of temperatures across the entire Pacific, because a number of ocean circulations meet at this one location, mixing water from across different regions of the ocean.
“The intermediate water in the western Pacific consists of water that once was near the surface in the northern and southern Pacific.”
The authors also suggest their record gives some indication of what the climate was like elsewhere in the world too – since it picks up temperature changes around the time of major climate events like the Little Ice Age and the Medieval Warm Period.
Storing up heat
The study shows that Pacific waters were warmer in the past. This tells scientists the ocean can store a lot more heat than it currently does. Rosenthal explained:
“We may have underestimated the efficiency of the oceans as a storehouse for heat and energy.”
So does that mean the ocean will simply absorb all the extra heat trapped in the atmosphere by greenhouse gases? Probably not, according to Rosenthal – ocean heat absorption is “not going to stop climate change”, he says.
Co-author Braddock Linsley agreed the ocean’s buffering capacity is no ‘get out of jail free’ card, describing modern day warming as an experiment in which humans put heat into the ocean “without quite knowing how it’s going to come back out and affect climate.”
Rosenthal et al. (2013) Pacific Ocean Heat Content During the Past 10,000 Years. Science. DOI: 10.1126/science.1240837