November 18, 2008

Study Examines Effects of Global Warming on Oceans

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No, the Gulf Steam is not coming to a stop, but major changes are taking place in the world’s oceans, according to the author of a recent Cornell study. Prof. Charles Greene, earth and atmospheric sciences, was the leading author of an oceanography study recently published in the journal Ecology.
The study examined trends in global climate and the major oceans surrounding North America.
Polar ice is part of a positive feedback loop that exacerbates climate changes. As temperature increases, ice melts. Since ice is white, Arctic melting decreases the reflectivity of the Earth. As a result, more solar radiation gets absorbed, further increasing temperatures and perpetuating the cycle. The opposite is true for temperature decreases.
According to the study, a new trend in Arctic melting is affecting the Atlantic Ocean. The primary change has been an abnormally large influx of water originating from the Pacific and traveling through the Arctic.
Water in the Pacific Ocean is less salty than water in the Atlantic. Approximately once every decade, some water from the Pacific travels through the Arctic and spills into the Atlantic.
However, in 1989, the freshwater infusion was more powerful than expected. Proposing that greenhouse gases were the main cause of this anomaly, Greene said, “Greenhouse gases are contributing to more fresh water in the Arctic Ocean [since global warming can lead to] greater precipitation, runoff, melting tundra and melting ice.”
The study identified two major symptoms of this change: a dramatic movement in the oceanic biosphere and a “regime shift” in the composition of Atlantic water.
The 1999 freshwater infusion had some peculiar characteristics. In May of that year, specimen of Neodenticula seminae, a species of boreal plankton, were detected in the Atlantic Ocean. N. seminae had been absent from the northern Atlantic fossil record for 800,000 years. However, since its discovery, the species has proliferated along the Eastern seaboard.
Greene pointed out that this southward migration of cold-adapted species has run contrary to some public ideas concerning climate change.
He said, “When most people think about global warming, they think of more southern species extending to higher latitudes and northern species retreating further.”
In oceans, a process called stratification causes less dense water to float above denser water, forming a distinct layer. In the Atlantic Ocean, warm surface water carried by the Gulf Stream approaches the eastern coast and transfers its heat to the surrounding land. Once the water cools, it becomes dense and sinks deep into the ocean, forming North Atlantic Deep Water. NADW is the first part of the global “conveyor belt” which redistributes heat from equatorial regions to higher latitudes.
However, fresh water is less dense than salt water. “If you throw in a lot of fresh water … you add buoyancy, so what is coming in as salty water in the Gulf Stream, is diluted and no matter how cold you make it, it won’t become dense enough to sink and form NADW,” Greene said.
Since the water remains stratified for longer, the Atlantic growing season has been extended. The study observed a recent surge in copepods, one of the main predators of phytoplankton.
However, not all species have benefited from this transition. The study hypothesized that the infusion of fresh Arctic water has played a role in the dramatic fall in cod stocks in the northwest Atlantic.
The study also discussed the effects of the regime shift on the global oceanic conveyor belt. Approximately 10 years ago, scientists seriously discussed the possibility of an extreme disruption in NADW possibly halting global heat redistribution. However, the study reported, “results from more recent climate change models have downplayed the likelihood of this scenario, at least in the near future … NADW formation and the global ocean’s [conveyor belt] are likely safe from abrupt change during the 21st century.”
However, Greene is cautiously optimistic. “A great reduction or even a complete shutting down of NADW formation [are] real possibilities in the coming centuries … change would likely be quite abrupt.”
In spite of the powerful implications of the study, Greene warned strongly against taking the study for more than it was worth. He criticized several publications for distorting or sensationalizing the point of the paper.
Nicole Ceci, a Master’s Degree candidate in environmental processes, collaborated with Greene on the study as a research assistant. After taking a course in satellite sensing in 2006, Ceci worked during the summer, developing programs to analyze satellite temperature data.
Ceci admired the interdisciplinary nature of the study, saying that it “explores some of the ties between climate change and ecosystem change.”
The study was funded in part by the National Science Foundation.