July 17, 2009
Glaciers Not Melting As Fast As Once Thought
The world's glaciers are indeed melting, but a monumental study released Thursday in the journal Science discovered that it might not happen as quickly as once thought.
The study notes that there are more gradual shifts in the current systems in the North Atlantic Ocean, instead of a rapid meltdown that could cause swift climate changes to Europe and other places.
The research, based on a "general circulation model" that replicated the Earth's climate 21,000 years at the height of the last Ice Age, indicates that although huge changes in the ocean current systems are probable, they might take place more gradually than originally thought.
The current pace of glacial melt will "give ecosystems more time to adjust to new conditions," stated study coauthor Peter Clark, a professor of geosciences at Oregon State University, to AFP.
"Our data still show that current is slowing, and may decline by 30 percent by the end of this century," Clark said. "That's very significant, and it could cause substantial climate change. But it's not as abrupt as some concerns that it could shut down within a few decades."
"For those who have been concerned about extremely abrupt changes in these ocean current patterns, that's good news."
Clark and his colleagues built a huge computer model that simulated the atmospheric and oceanic circumstances of the last ice age and the shifts that resulted in Earth's last global warming 14,500 years ago.
"Being able to successfully simulate thousands of years of past climate for the first time with a comprehensive climate model is a major scientific achievement," said Bette Otto-Bliesner, an atmospheric scientist and climate modeler at National Center for Atmospheric Research (NCAR) and co-author of the Science report.
"This is an important step toward better understanding how the world's climate could change abruptly over the coming centuries with increasing melting of the ice caps."
The power of computation on the experiment was so huge that it took more than a year to run, Clark said. It was the longest experiment of its kind that ever reviewed past climate with such specifications.
Beginning 21,000 years ago, the research team replicated the conditions in the Bølling-Allerød warming, which happened 14,500 years ago. The replication matched with temperature, sea level and glacial coverage records collected from this geologic period.
The simulations helped identify the three environmental factors as leading causes of the Bølling-Allerød warming: an surge in atmospheric carbon dioxide, the re-animation of delayed heat-moving ocean currents and a bulky buildup of subsurface heat in the ocean while those currents were not moving.
Huge amounts of water then surged into the North Atlantic and reduced the ocean's salt levels, which are important, as they power the convection current that moves warm tropical water north and cooler water south.
The fresh water caused the current to halt, chilling the North Atlantic area and sending heat to southern waters. The cooler temperatures slowed down the melting of the glaciers and gradually the ocean current began moving again.
The replication found that as a result, ocean circulation came to a stop. Lacking warm water moving to the north, the North Atlantic chilled and heat was stored in southern waters. As a result, glacial melt also halted.
"All that stored heat is released like a volcano, and poured out over decades," Liu said. "That warmed up Greenland and melted (arctic) sea ice."
While the model could not figure out the speed of ice melting in the period, it determined that both an unhurried decrease and a quick end to melt run-off amounted in the same result: a 15-degree warming.
"That happened in the past," Liu said. "The question is, in the future, if you have a global warming and Greenland melts, will it happen again?"
The experiment does not alter worries about global warming. Temperatures are still anticipated to rise from four to 11 degrees by the end of the century, and the study agrees that many of the world's climate models are correct.
"The findings from this study, which also match other data we have on recorded climate change, are an important validation of the global climate models," Clark said. "They seem to be accurately reflecting both the type and speed of changes that have taken place in the past, and that increases our ability to trust their predictions of the future."
"We want to know what will happen in the future, especially if the climate will change abruptly," said Liu.
The researchers will continue to replicate the environment, which has simulated part of the last 21,000 years. With 4 million more processor hours to go, the replications will eventually meet the present and 200 years into the future.
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