Annals of Global Warming: Bering Strait, choke point and butterfly effect

January 17, 2010

From the National Science Foundation, we get more stuff that students in high school ought to learn, stuff which would give the conservatives on the Texas State Board of Education full-fledged conniptions, stuff that just doesn’t fit into Texas’s Teach-To-The-Test™ education standards.

Reading the press release brings home two points to me.  First, for the sake of Texas social studies standards, this story tells the physical effects of “choke points.”  The Bering Strait limited human migration to populate the America’s, meeting the definition of choke point favored by Texas Essential Knowledge and Skills (TEKS), and this story sheds light on that issue.  But it also points out that choke points affect more than just humans.  This was a key finding of the President’s Commission on Americans Outdoors that led to a recommendation that we create green corridors favorable for animal migration.  In the Bering Strait, it’s migration of ice and cold and warm water that get choked off — and that, too, affects human history.  Second, this may enlighten students to the butterfly effect noted in the science we call chaos, where a small physical perturbation in one area can have enormous consequences later, and far away, in the dynamic systems that keep our planet alive.  (Yeah, the Bering Strait is bigger than a butterfly.  I know.)

Press Release 10-003

Global Ice Age Climate Patterns Influenced by Bering Strait

Small geographic feature has large impacts on climate

Ice pouring through the Bering Strait, from the Arctic to Pacific Ocean - NASA photo via NSF

Ice is shown choking the Bering Strait in recent times; the ice moves from the strait south to the Bering Sea. Credit: NASA

January 10, 2010

In a vivid example of how a small geographic feature may have far-reaching impacts on climate, new research shows that water levels in the Bering Strait helped drive global climate patterns during ice age episodes dating back more than 100,000 years.

The international study, led by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., found that the repeated opening and closing of the narrow strait due to fluctuating sea levels affected currents that transported heat and salinity in the Atlantic and Pacific Oceans.

As a result, summer temperatures in parts of North America and Greenland oscillated between comparatively warm and cold phases, causing ice sheets to alternate between expansion and retreat and affecting sea levels worldwide.

While the findings do not directly bear on current global warming, according to Steve Nelson, National Science Foundation (NSF) program director for NCAR, they highlight the complexity of Earth’s climate system and the fact that seemingly insignificant changes can lead to dramatic tipping points for climate patterns, especially in and around the Arctic.

“The global climate is sensitive to impacts that may seem minor,” says NCAR scientist Aixue Hu, the project’s lead scientist. “Even small processes, if they are in the right location, can amplify changes in climate around the world.”

The research results are published this week in the journal Nature Geoscience.

Funded by NSF and the U.S. Department of Energy, the scientists used the latest generation of supercomputers to study past climate at a level of detail that would have been impossible just a few years ago.

Hu and his colleagues set out to solve a key mystery of the last glacial period: Why, starting about 116,000 years ago, did northern ice sheets repeatedly advance and retreat for about the next 70,000 years? The enormous ice sheets held so much water that sea levels rose and dropped by as much as about 100 feet (30 meters) during these intervals.

In other cases, scientists have associated such major oscillations in climate with fluctuations in Earth’s orbit around the Sun. But in the time period the research team looked at, the orbital pattern did not correspond with the geologic movement of the ice sheets and associated sea level changes.

The researchers considered an alternative possibility: that changes in the Bering Strait, the main gateway in the Northern Hemisphere between the Atlantic and Pacific Oceans, might have affected ocean currents across much of the globe.

Although small–the strait is currently about 50 miles (80 kilometers) wide between Russia and the westernmost islands of Alaska–it allows water to circulate from the relatively fresh north Pacific to the saltier north Atlantic via the Arctic Ocean. This flow is instrumental to regulating the strength of a current known as the meridional overturning circulation, a key driver of heat from the tropics to the poles.

Using the NCAR-based Community Climate System Model, a powerful computer tool for studying worldwide climate, the researchers compared the responses of ice age climate to conditions in the Bering Strait.

They ran the model on new supercomputers at NCAR and the Department of Energy’s Oak Ridge National Laboratory, enabling them to focus on smaller-scale geographic features that, until recently, could not be captured in long-term simulations of global climate.

The simulations accounted for the changes in sea level, revealing a recurring pattern–each time playing out over several thousand years–in which the reopening and closing of the strait had a far-reaching impact on ocean currents and ice sheets.

As the climate cooled because of changes in Earth’s orbit, northern ice sheets expanded. This caused sea levels to drop worldwide, forming a land bridge from Asia to North America and nearly closing the Bering Strait.

With the flow of comparatively fresh water from the Pacific to the Atlantic choked off, the Atlantic grew more saline. The saltier and heavier water led to an intensification of the Atlantic’s meridional overturning circulation, a current of rising and sinking water that, like a conveyor belt, pumps warmer water northward from the tropics.

This circulation warmed Greenland and parts of North America by about 3 degrees Fahrenheit (1.5 degrees Celsius)–enough to reverse the advance of ice sheets in those regions and reduce their height by almost 400 feet (112 meters) every thousand years. Although the Pacific cooled by an equivalent amount, it did not have vast ice sheets that could be affected by the change in climate.

Over thousands of years, the Greenland and North American ice sheets melted enough to raise sea levels and reopen the Bering Strait.

The new inflow of fresher water from the Pacific weakened the meridional overturning circulation, allowing North America and Greenland to cool over time. The ice sheets resumed their advance, sea levels dropped, the Bering Strait again mostly closed, and the entire cycle was repeated.

The combination of the ocean circulation and the size of the ice sheets–which exerted a cooling effect by reflecting sunlight back into space–affected climate throughout the world.

The computer simulations showed that North America and Eurasia warmed significantly during the times when the Bering Strait was open, with the tropical and subtropical Indian and Pacific Oceans, as well as Antarctica, warming slightly.

The pattern was finally broken about 34,000 years ago, the point in Earth’s 95,000-year orbital cycle at which the planet was so far from the Sun at certain times of year that the ice sheets continued to grow even when the Bering Strait closed.

When the orbital cycle brought Earth closer to the Sun in the northern winter, the ice sheets retreated sufficiently about 10,000 years ago to reopen the strait. This helped lead to a relatively stable climate, nurturing the rise of civilization.

“This kind of study is critical for teasing out the nuances of our climate system,” says NCAR scientist Gerald Meehl, a co-author of the paper. “If we can improve our understanding of the forces that affected climate in the past, we can better anticipate how our climate may change in the future.”

In addition to NCAR, the study team included researchers from the National Center for Scientific Research (CNRS) in France, University of Colorado in Boulder, Catholic University of Louvain in Belgium, Australian National University and Harvard University.

Download the high-resolution JPG version of the image [of ice flowing through the Bering Strait, above]. (1.4 MB)

Bering Strait, NASA photo via National Science Foundation

The Bering Strait separates the U.S. and Russia by only 90 kilometers. Credit: NASA

Download the high-resolution JPG version of the image. (570 KB)

Currents move warm and cold water across the Arctic Ocean - UCAR map

Altered currents once produced ocean warming (right, dark red) that melted ice sheets. Credit: UCAR

Download the high-resolution JPG version of the image. (680 KB)

Media Contacts
Cheryl Dybas, NSF (703) 292-7734
David Hosansky, NCAR/UCAR (303) 497-8611

The National Science Foundation (NSF) is an independent federal agency that supports fundamental research and education across all fields of science and engineering. In fiscal year (FY) 2009, its budget is $9.5 billion, which includes $3.0 billion provided through the American Recovery and Reinvestment Act. NSF funds reach all 50 states through grants to over 1,900 universities and institutions. Each year, NSF receives about 44,400 competitive requests for funding, and makes over 11,500 new funding awards. NSF also awards over $400 million in professional and service contracts yearly.

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