Scientists studying 60 years in the life of a California tidal zone report today that populations of sea creatures, including snails, crabs, starfish and anemone, are migrating northward in reaction to rising ocean temperatures.
While the shifts may have been helped along by a number of factors, including the so-called El Nino effect, the population changes may provide intriguing new evidence of the impact of global warming.
"The fact that creatures who prefer warmer water are now thriving in a place where they were once relatively rare came as a big surprise to us," said Charles Baxter, one of two scientists at the Monterey Bay Aquarium Research Institute in central California who headed the research project. "What we see is evidence that the effects of global warming may already be apparent, at least in the Northern Hemisphere."
Mr. Baxter and three colleagues published their findings in a paper appearing in today's issue of the journal Science.
There is broad agreement among scientists that the Earth has gotten warmer by at least one degree during the past 100 years. But there is considerable controversy over the impact climate change is having on the Earth. There is also much disagreement whether man-made gases such as carbon dioxide are fueling the increase in temperature and whether it could lead to disastrous fluctuations in weather patterns.
In the Monterey Bay study, scientists returned to the exact location of a research project done in the early 1930s that took stock of the creatures inhabiting a 35 square-yard patch of rocks and tide pools.
The new study documented dramatic changes in the dispersion of 16 invertebrate species. In an inch-by-inch inventory, the aquarium's research team documented significant increases in the population of eight species more commonly found in Southern California, and a decrease in five "northern" species that the 1930s study found in abundance.
The scientists attribute the population shifts to a slow, steady warming trend.
"The 60-year period of faunal and floral change coincided with . . . warming of ocean and air temperatures along the western North American coast," the researchers wrote in Science.
Over the past 60 years, according to the scientists, the average shoreline temperature has risen a bit more than one degree, while the average maximum water temperature has gone up four degrees.
By moving species northward on warm ocean flows, the El Nino phenomenon of unusually warm currents generated in the central Pacific could be a factor in altering the balance of Monterey Bay species.
However, the scientists point out that El Nino is a temporary force.
"As a periodic event, El Nino might be able to bring animals north, but it wouldn't sustain them over a period of many years because temperatures would eventually go down again," said Rafe Sagarin, one of the researchers who participated in the study.
The change in the population balance may result, in part, from the actions of one species on another and from aquatic predators who feed on invertebrates. But that theory, too, has its limitations, said Mr. Sagarin.