If the latest research is correct, our oceans are heating up much faster now than they have in the past 10,000 years. This is one of the conclusions that is drawn from a recently published paper in Science. The researchers (Yair Rosenthal, Braddock Linsley, and Delia Oppo) cleverly traveled back in time to explore how ocean temperatures have changed. Comparison of those temperatures to today's helped them quantify the impact that human greenhouse gas emissions are having on the planet.
The story begins in a remote location in the western equatorial Pacific. Using sediments along the Makassar Straight and the Flores Sea in Indonesia, the researchers extracted benthic foraminifera, which are small creatures that live near or at the bottom of the ocean waters. The chemical signatures in the shells of these creatures can provide valuable information about the ocean temperature in the recent or long past.
Why these locations? It turns out the ocean is anything but static. While it is true that much of the ocean is stratified (warm, less dense waters floating atop cold, denser waters), at the high latitudes, colder and denser surface water sink to the ocean bottom where they spread out across the globe. The study took place in the equatorial Pacific Ocean where these stream of water traveled from the poles. The sediments allowed the researchers to deduce temperatures in the ocean depths, which could be connected with surface waters.
After analyzing the foraminifera shells, the authors concluded that ocean waters at mid-depths were warmer during the early/middle Holocene (about 10,000 to 8,000 years ago) then they are today. Over the next few thousand years, the oceans cooled. Then, in recent decades, there was a significant change as the oceans began to heat quickly.
Here is what the author himself had to say:
"It is clear that much of the heat that humans have put into the atmosphere through greenhouse gas emissions will be absorbed by the ocean. But the absorption time takes hundreds of years, much longer than the current rate of warming and the planet will keep warming. Our study puts the modern observations into a long-term context. Our reconstruction of Pacific Ocean temperatures suggests that in the last 10,000 years, the Pacific mid-depths have generally been cooling by about 2 degrees centigrade until a minimum about 300 years during the period known as the Little Ice Age.
After that, mid-depth temperatures started warming but at a very slow rate. Then, since about 1950, temperatures from just below the sea surface to ~1000 meter, increased by 0.18 degrees C. 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. But our results also show the temperature of the ocean interior is still much colder than at any time in the past 10,000 years thus, lagging the changes we see at the ocean surface."
As with any new study, there are tough scientific questions to ask. For instance, one of the best metrics for global warming is the rate of sea level rise. As oceans heat, their waters expand causing sea level to rise. In fact, sea levels can be viewed as a thermometer of the Earth. The results of the present paper seem to be at odds with sea-level data which suggest that waters have risen over the past 2,000 years. What are the explanations for this discrepancy?
Another issue has to do with the accuracy of the measurements. It turns out that sediment studies have a hard time providing recent temperature information. Mixing of the sedimentary layers by animals (bioturbation) or uncertainties in the rates at which sediments form cause a great deal of uncertainty in the upper layers. Consequently, there is a real question about how well this study captures recent temperature changes.
What can we take away from this?
Posted by John Abraham on Tuesday, 5 November, 2013
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