|Single-celled foraminifera provided data for each study. Photo: David Field.|
Two recent studies from the Scripps Institution of Oceanography at the University of California, San Diego (UCSD) illustrate the effects of warming on the ocean, ocean organisms and climate.
In the first, Flávia Nunes and Richard Norris probed a four- to seven-degree warming period that occurred some 55 million years ago during the closing stages of the Paleocene and the beginning of the Eocene eras by investigating the chemical makeup of the shells of single-celled foraminifera retrieved from ocean cores at various locations around the world.
The unique data set they constructed revealed a monumental reversal in the circulation of deep-ocean patterns around the world. The researchers conclude that it was triggered by the global warming the world experienced at the time. The research, published in the January 5 edition of the journal Nature, is one of the few historical analogs for large-scale sea circulation changes tied to global warming.
The earth is a system that can change very rapidly. Fifty-five million years ago, when the earth was in a period of global warmth, ocean currents rapidly changed direction and this change did not reverse to original conditions for about 20,000 years. What this tells us is that the changes that we make to the earth today (such as anthropogenically induced global warming) could lead to dramatic changes to our planet.—Flávia Nunes
The global warming of 55 million years ago, known as the Paleocene/Eocene Thermal Maximum (PETM), emerged in less than 5,000 years. The researchers indicate that 5,000 years can be considered an upper limit and they believe the warming could have unfolded much more quickly than geological records can show them.
The PETM set in motion a host of important changes around the globe, including a mass extinction of deep-sea bottom-dwelling marine life. Fossil records indicate key migrations of terrestrial mammal species during this time—including evidence of the first horses and primates in North America and Europe—likely allowed by warm conditions that opened travel routes not possible under previously colder climates.
In the Nature study, the scientists analyzed foraminifera called Nuttalides truempyi from 14 sites around the world in deep-sea sediment cores maintained by the Integrated Ocean Drilling Program. Analysis of carbon isotopes was used to reconstruct changes in deep-ocean circulation through the PETM period. Nutrient levels tell the researchers how long a sample has been near or isolated from the sea surface, thus giving them a way to track the age and flow path of deep sea water.
The results revealed that deep-ocean circulation abruptly switched from overturning—a conveyor belt-like process in which cold and salty water exchanges with warm surface water—in the Southern Hemisphere, where it virtually shut down, and became active in the Northern Hemisphere. The researchers believe this shift drove unusually warm water to the deep sea, likely releasing stores of methane gas that led to further global warming and a massive die off in deep sea marine life.
Overturning is a fundamental component of the global climate conditions that we know today.
In the paper, the authors note that modern carbon dioxide input from fossil fuel sources to the earth’s surface is approaching the same levels that estimated for the PETM period, which raises concerns about future climate and changes in ocean circulation. Thus they say the Paleocene/Eocene example suggests that human-produced changes may have lasting effects not only in global climate, but in deep ocean circulation as well.
The research was supported by the National Science Foundation (NSF) and the U.S. Science Support Program. The Integrated Ocean Drilling Program is sponsored by the NSF and participating countries under management of Joint Oceanographic Institutions.
The second study works on a much shorter timeline. In it, sediment cores collected from the seafloor off Southern California reveal that plankton populations in the Northeastern Pacific changed significantly in response to a general warming trend that started in the early 1900s.
As ocean temperatures increased, foraminifera became more abundant. Foraminifera that live in cooler waters decreased, especially after the mid-1970s. These changes are unlike anything seen during the previous 1,400 years.
Oceanographer David Field discovered these changes during his Ph.D. work at Scripps Institution of Oceanography at the University of California, San Diego. Field and his co-authors describe their findings in the current issue of Science magazine.
Field studied fossilized forams in one- to three-meter-long sediment cores collected at the bottom of the Santa Barbara Basin, off Southern California. In this area, dead plankton and sediments settle onto the seafloor to form distinct annual layers similar to growth rings in a tree. At 600 meters beneath the ocean surface, seawater in the Santa Barbara Basin contains very little oxygen, so few bottom-dwelling animals disturb the sediments and the annual layers remain relatively intact.
Field and his coauthors examined yearly sediment layers that were formed up to 1,400 years ago. Counting the different species of foraminifera in each layer, they discovered that many species of tropical and subtropical forams became more abundant after about 1925. Although previous studies have shown an ocean warming trend beginning at about this time, scientists have debated how much of this warming trend was due to natural variability.
However, Field’s data set extends far enough back in time to demonstrate that the 20th century warming trend surpassed the range of natural variability.
Many previous studies have shown that a rapid warming and a dramatic change in eastern North Pacific ecosystems occurred in the mid-1970s. At this time, species of plankton, kelp, fish and seabirds that prefer warmer waters increased and species favoring colder conditions decreased. Most scientists agree that part of the warming of the global oceans and atmosphere since the mid-1970s has been caused by human emissions of greenhouse gases.
However, it has been unclear whether the ecosystem changes at this time were associated with anthropogenic warming. Field’s sediment cores show that tropical and subtropical species of forams became even more abundant during this period while forams that prefer cooler waters decreased. The resulting foram community was unlike anything seen during the last 1,400 years. These long-term data indicate that the ecosystem changes since the mid-1970s are best explained by anthropogenic warming.
These data show that ocean warming has been affecting foram populations prior to the late twentieth century. However, changes since the 1970s have been particularly unusual, and show that ocean ecosystems in the northeastern Pacific have passed some threshold of natural variability.—David Field
The study was conducted as part of the National Science Foundation’s Long-Term Ecological Research (LTER) program.
“Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period”; Flávia Nunes and Richard D. Norris; Nature 439, 60-63 (5 January 2006);doi:10.1038/nature04386
“Planktonic Foraminifera of the California Current Reflect 20th-Century Warming”; David B. Field, Timothy R. Baumgartner, Christopher D. Charles, Vicente Ferreira-Bartrina, Mark D. Ohman; Science 6 January 2006: Vol. 311. no. 5757, pp. 63 - 66; DOI:10.1126/science.1116220