November 15, 2012
Rising Temperatures Could Lead To Sea Levels Rising Three Feet Or More
April Flowers for redOrbit.com - Your Universe Online
Global ice-volume variability has been one of the main feedback mechanisms in climate change during the last few million years. This is because of the strong reflective properties of large ice sheets. Scientists reconstruct ice volume changes in ancient times using sea-level records, however, a detailed assessment of the role of ice volume in climate change has been hindered by a lack of consistent sea-level records and/or their timescales.
For the first time, researchers are able to accurately date continuous sea-level records, allowing detailed comparisons of the ice-volume variability with independently dated ice-core records from Greenland and Antarctica. The findings of this new study were published in the latest issue of the journal Nature.
Scientists have previously relied on the "Red Sea relative sea-level (RSL) record" for reconstruction of highly resolved and continuous records of sea-level variability over the past 500,000 years. The RSL method has a drawback, however. It has no independent age control, inhibiting detailed comparison with other well-dated climate parameters, such as temperature or CO2 records from the ice cores.
Eelco Rohling, Professor of Ocean and Climate Change at the University of Southampton, led an international team of scientists to develop a new method of dating the RSL record by using Mediterranean data from radiometric (Uranium-series) dating of cave deposits, thus providing a much improved timescale for the RSL record. The team then applied the new dating method throughout the entire last glacial cycle — approximately 150,000 years — resulting in an unprecedented continuous sea-level record with an independent age control.
Rohling and his colleagues compared the ice-volume fluctuations with polar temperature reconstructions from the Greenland and Antarctic ice cores, finding that changes in temperature and ice volume/sea level are closely coupled with a response time lag of only a few centuries. Scientists did not previously know this timing relationship, and it reveals a very fast response between global temperature and ice volume/sea level. The team also found that periods of extensive ice-volume reduction/sea level rise were always characterized by changes. These changes are on the order of 3 to 6 feet per century sea-level rise.
"This is the first time that these rates could be measured for any other period than the end-of-ice age 'terminations/deglaciations'. Although it is always hard to step from paleo reconstructions to future projections, it suggests that when significant ice-volume adjustments happen, they are rarely slow," says Professor Rohling, who is currently based at the National Oceanography Centre Southampton but will join the Australian National University next year.
"Ice sheet responses to a change in climate forcing are like the responses of heavy freight trains to firing up the locomotive. They are hard to set in motion (slow to 'spin up'), but once they are reacting, they will be equally slow to 'spin down'. So a lag of a few centuries is worrisome, because we have been warming up the climate for 150-160 years now. If the natural relationship (when changes in climate were slower than today) also holds for the very fast changes in climate today, then we are coming into that 'window' of time where we may expect to start seeing some unprecedented responses in the large ice sheets. This then may tie in with observations of the past decade or so of large ice-shelf collapses around Antarctica and Greenland, the major melt-area expansion over Greenland, changes in the flow speed of major ice streams (both Antarctica and Greenland), and increasing ice-mass loss over West Antarctica/the Antarctic Peninsula and Greenland," Rohling explains.
"We cannot say whether this proves the case, but at least the time delay of the modern ice-sheet responses relative to climate change would seem to agree with the response timescales we have now found in the palaeo-record," he added.
Christopher Bronk Ramsey from Oxford University helped the team with radiocarbon dating and developed some of the age-modeling techniques used. He says, "What is new here is that our dating of the marine sediment is much more direct. This gives us far more accurate age profiles for the sedimentary sequences, which in turn provides valuable insight into the way the earth system has worked in the past."
The results also hint that temperatures over Greenland changed simultaneously with ice volume. Conversely, temperatures over Antarctica were leading ice volume change by up to 700 years.
"Ice volume is related to ice area, and ice is very reflective. So a large ice volume/area reflects a lot of incoming solar radiation. When the ice volume/area reduces, less incoming radiation is reflected, which leads to regional warming. Atmospheric temperature has little inertia, so it will respond very quickly to ice-volume changes," explains Professor Rohling.
"Our observations suggest that the Greenland temperature changes may largely be regional responses to changes in ice volume (at that time over North America and also Eurasia). In contrast, the Antarctic relationship suggests that temperature changes as recorded in Antarctic ice cores may have been driving the ice-volume changes. Possibly, the Antarctic temperature changes reflect the fundamental underlying global temperature changes that drove the ice-volume changes, while the Greenland temperature record primarily represents a regional temperature response to the changing ice volumes. These are intriguing hints at fundamental aspects of the Earth System's response to climate change, which merit further investigation."