(Image source: Sea-level–induced Seismicity and Submarine Landslide Occurrence.)
At the end of the last ice age as the climate warmed, glaciers began to thaw, and sea level began to rise, a troubling spike in the number of undersea landslides and related tsunami events occurred. In total, almost half of all the undersea landslides of the past 125,000 years occurred during this period of deglaciation occurring from 8,000 to 15,000 years ago. A rate many times that seen during either the glacial period or during the Holocene.
This large increase in subsea landslide events had long been observed in the science. But, up until this point, there has been little research to determine why so many landslides occurred. But this year, a team of scientists developed a model to investigate the cause of these continental shelf slope failures and large undersea landslides. The report, published in Geology, is available here: Sea-level–induced Seismicity and Submarine Landslide Occurrence.
The findings of this study and related model run were stark. According to results, rising sea waters spurred by climate change greatly increased the pressure on undersea slope and fault structures. This pressure, rising over 7,000 years to roughly equal that of a human bite over every inch of these undersea fault systems, was found to result in numerous catastrophic failures.
In a slope failure, a strain on an undersea fault in a debris, rock or sediment zone first begins to grow. The region of sediment already rests over a sloping undersea terrain and so the weight of the ocean above constantly pushes down on these structures. Over time, the stress increases due to slow structural change, seismic stresses, the passing of large waves and/or strong storms, and/or loss of slope integrity. If sea levels rise, the added weight of a deeper ocean overhead further increases stress. Eventually, the fault line catastrophically fails causing separation and rapid collapse of slope material toward the ocean bottom.
(Image source: UNCW)
Slope collapse zones can stretch for miles and miles along seabed drop-off zones. A slope failure can release millions or billions of tons of material, displacing an equally high volume of water. Under the right circumstances, such large slope failures can result in very large tsunamis, similar to those caused by major ocean earthquakes.
More ominously, perhaps, is the fact that large slope failures can directly expose previously buried deposits of methane hydrates. If large stores of this substance are rapidly uncovered in a warming sea environment they can swiftly out-gas and greatly contribute to an already ongoing warming.
The most vulnerable regions for slope collapse include large, shallow continental shelves that have multiple or large fault zones and that border an ocean drop-off to deeper water. A particularly sensitive region is the shallow East Siberian Arctic Shelf which contains numerous fault zones and extends out into deeper Arctic waters as well as slope structures near the Gakkel Ridge. The ESAS also contains one of the Arctic’s largest stores of methane, estimated at 500 gigatons. Many structures in these regions are already emitting significant, but not catastrophic, volume of methane from undersea hydrate stores. A slope collapse of the kind mentioned in this report in any of these locations would have severe consequences for ESAS, Arctic and global methane release.
Rate of Sea Level Increase During the End of the Last Ice Age
(Image source: Commons)
During the period of deglaciation at the last ice age’s end, a global temperature rise of about 5 degrees Celsius caused a 395 foot sea level rise over the course of 70 centuries. On average, sea level rise matched pace with temperature increase. Once ice sheet destabilization began, each .1 C temperature rise coincided with about a 6 foot rise in sea level, or a rate of slightly more than 5.5 feet per century.
Current glacier systems hold enough water to increase sea levels by about another 200 feet. Ocean thermal expansion will add its own increase to this potential sea level rise. At a .8 C temperature increase since the 1880s, current temperatures are near or just above the Holocene maximum, large ice sheet destabilization has begun, and rates of sea level rise are continuing to increase. With temperatures expected to rise between 4 and 7 degrees Celsius by the end of this century under business as usual fossil fuel emissions, a temperature increase roughly equivalent to one that took 7000 years to complete at the end of the last ice age, it is highly likely that a 5 foot per century rate of sea level increase will be matched or exceeded. Such a rapid rate of sea level rise would create stresses to sub-sea ocean slope systems that meet or exceed that seen during the end of the last ice age, greatly increasing the risk of catastrophic slope failure and resulting tsunamis and potential methane release scenarios.
(Image source: AVISO)