ANCIENT REEF: The creation of this water park exposed reefs that grew roughly 121,000 years ago, allowing scientists to study sea level rise during that warm period.
Image: COURTESY OF PAUL BLANCHON
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With Greenland's glaciers melting and on the move while vast ice sheets in Antarctica continue to shatter, the proportion of water in the seas continues to grow. And with the climate at the poles expected to continue to warm rapidly in coming decades, many researchers are trying to determine how much and how quickly sea levels might rise. Now newly excavated reefs in Mexico may have provided an answer: high and fast.
Geoscientist Paul Blanchon of the National Autonomous University of Mexicoand his colleagues examined the record provided by ancient reefs uncovered during the excavation for Xcaret, a new theme park on the Yucatan Peninsula. By measuring the decay of thorium in the reefs, the researchers estimated their age at roughly 121,000 years old—from a period in the Pleistocene epoch known as the Eemian interglacial, which saw average temperatures that were roughly 3.6 degrees Fahrenheit (2 degrees Celsius) warmer, higher sea levels, and less ice than today.
The buried reefs revealed that sea level rises of as much as two inches (five centimeters) per year resulted in at least a 6.6 foot (two meter) jump in as little as 50 years, based on a series of reefs retreating closer to a receding shore over time. An older reef's tip crested at roughly 10 feet (three meters) above present sea level but a second reef crest farther inland grew 10 feet higher than that, indicating that sea level had risen by as much as 10 feet by the time the latter formed because corals grow nearly to sea level, according to the findings published today in Nature.
"Twenty centimeters (eight inches) of reef accreted or grew in a little over 50 years," Blanchon says. "We found that the first corals that grew in the new reef were up to 1.5 meters [five feet] tall, indicating that sea level had to be at least two meters [6.6 feet] higher than the older reef which grew up to three meters [10 feet] above present sea level."
Other evidence has shown that 14,000 years ago, at the beginning of the current epoch (the Holocene), ice sheet melting led to sea level rises of as much as 49 feet (15 meters) in 300 years. But this find indicates that sea level can rise even faster, most likely from collapsing ice sheets, Blanchon says.
The dating of the reefs by the decay of thorium as well as comparison with similarly aged reefs from the Bahamas remains in question, however, because Blanchon and his colleagues failed to confidently date the first reef.
"Their accuracy is suspect," the researchers admitted in the paper. Yet, other studies have shown that sea levels rose by as much as 5.2 feet (1.6 meters) per century during the Eemian.
This finding "is the first indication that ice sheet collapse caused a sea level jump during the last interglacial," Blanchon says. "If we can find back-stepping reefs during the last interglacial in [Western Australia and other areas], I think we will have a rock-solid case for ice sheet collapse and catastrophic sea level rise." Given the ongoing meltdown in Greenland and Antarctica, that may be a grim presentiment of our own predicament.
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9 Comments
Add CommentJust to clarify, the rates of sea level rise we have identified from the last interglacial are the same as the rise rates identified during the last deglacial 14 thousand years ago (not higher)
Reply | Report Abuse | Link to thisPaul Blanchon
Just to clarify, the rates of sea level rise we have identified from fossil reefs during the last interglacial are identical to the rates of rise measured from reefs during the post glacial sea level rise 14 thousand years ago (...not greater). This rate is a clear indication of sustained rapid ice loss.
Reply | Report Abuse | Link to thisit's nice to know that corals DO grow in warmer waters, belying the argument lately that corals don't grow in warmer waters, i.e. 'bleaching'...they just grow in different places.
Reply | Report Abuse | Link to thisI think we can all agree that sudden sea level rises reflect sudden ice melting/collapse that reflects sudden warming. If this has happened at least twice when man could not have influenced it, does this not mean that the "sudden" warming we are now seeing is precedented, not unprecedented, and that man may well not be the cause?
Reply | Report Abuse | Link to thisGood question jjrs5. And if climate change and sea level change are part of the natural course of things on earth, why is a rise in sea level, fast or slow, any more "catastrophic" than the disappearance of heat and light every day at sundown?
Reply | Report Abuse | Link to this"If this has happened at least twice when man could not have influenced it, does this not mean that the "sudden" warming we are now seeing is precedented, not unprecedented, and that man may well not be the cause?"
Reply | Report Abuse | Link to thisNo, that is a logical fallacy. Just because something has happened in the past does not mean that it excludes new cause for present warming. This is like arguing that since people have died of natural causes in the past that the body laying in the street with five bullet wounds must be natural too.
"....why is a rise in sea level, fast or slow, any more "catastrophic" than the disappearance of heat and light every day at sundown?"
Reply | Report Abuse | Link to thisYour right. All we have to is point to the New York City of 15,000 BCE and see how it well weathered climate change then and see that.... Oh, wait.
As summer circumpolar temperatures rise, glacial meltwaters sink underground forming important underground rivers, which generally resurge below sea-level, and hence go unseen. This lubricates glacier bases, accelerating ice-flows which open fissures which warm air can then enter. A viscious circle! You have only to enter limestone caves to see the effect of air currents on temperatures...
Reply | Report Abuse | Link to this(QUOTE)No, that is a logical fallacy. Just because something has happened in the past does not mean that it excludes new cause for present warming. This is like arguing that since people have died of natural causes in the past that the body laying in the street with five bullet wounds must be natural too.(END QUOTE)
Reply | Report Abuse | Link to thisSo this reoccuring cycle is just pure coincidence huh?
Warm_periods of Past 5000 years Temperatures
http://www.greenworldtrust.org.uk/Science/Images/Main/Warm_periods.jpg
Is it the case that the rate and magnitude of warming between 1979 and 1998 (the late 20thcentury phase of global warming) were not unusual as compared with warmings that haveoccurred earlier in the Earth's history (Fig. 2a, 2b)?
If the warming was not unusual, why is it perceived to have been caused by human carbon dioxideemissions; and, in any event, why is warming a problem if the Earth has experienced similarwarmings in the past?