Even small increases in temperature from global warming are causing climatology shifts harmful to ocean life, a new study in the Proceedings of the National Academies of Science shows. Modest changes in temperature have significantly altered trade wind intensity in the southern Caribbean, undercutting the supply of key phytoplankton food sources and causing the collapse of some fisheries there.
"Global warming isn't occurring uniformly over the Earth's surface -- it's been much greater at the high latitudes in the Northern Hemisphere than it has been for the low latitudes," said co-author Robert Thunell, a University of South Carolina researcher. "Because of that, some people have said, 'Well, we're probably not going to see much biotic change at low latitudes,' but we show nicely in this paper that even when the climatological changes are relatively modest, they can have a big impact on the marine ecosystem."
The paper is the product of nearly 15 years of observations in a highly collaborative NSF-funded effort between researchers at USC, Stony Brook University, the University of South Florida and several Venezuelan institutions. Since late 1995, monthly observations of a range of variables, from nutrient and chlorophyll concentrations to meteorological readings, have been collected at a single location off the coast of Venezuela to establish a long-range record.
The sea surface temperature was found to have increased somewhat, about 1 degree Celsius, over the decade-and-a-half. But the effect on the sea life was much more pronounced: beginning in 2006, the population of microscopic diatoms, dinoflagellates and coccolithophorids plummeted, along with the local harvest of sardines.
The team thinks the drastic change in ecology results from climatology shifts that go beyond a small temperature increase.
"The intertropical convergence zone is where the northeast and southeast trade winds converge," said Thunell, of the Department of Earth and Ocean Sciences in USC's College of Arts and Sciences. "It's basically the thermal equator, and it doesn't sit right on the geographic equator because it's a little bit warmer in the north. That's because there's more land mass in the northern hemisphere.
"And because of that extra land mass, the northern hemisphere is actually heating more than the southern hemisphere through global warming. We've proposed that this thermal equator, over the last 15 years, has moved a little bit farther northward. So the strength of the trade winds now over our region of study has decreased."
That diminished wind is cutting the food supply for some fish, the team concludes. The upwelling of ocean water, which is a source of nutrients for phytoplankton, depends on the mixing that winds provide. Less wind thus means less mixing, fewer nutrients for phytoplankton, and fewer phytoplankton to sustain the fish population.
"That's a big deal. The plankton near the surface of the ocean are the base of the food chain," Thunell said. "This climatological change is driving a change in the food web structure, which we're now seeing affect the fisheries."
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