A snail's take on the climate change.

The snails know. So do the limpets, barnacles, anemones, chitons, sea stars, and crabs living along the shore of Monterey Bay, Calif. While humans continue to debate how global warming will alter societies and ecosystems, this intertidal community is already showing changes linked to rising temperatures, reports a team of California scientists in the Feb. 3 Science.

James P. Barry of the Monterey Bay Aquarium Research Institute and his colleagues tracked shifts in the intertidal ecosystem by taking a census of the animals living in a 100-meter-long stretch of beach belonging to Stanford University's Hopkins Marine Station. They compared their counts with those compiled by Stanford researcher W.G. Hewatt, who studied the same patch of coastline in the early 1930s. Brass bolts sunk into the seaside granite by Hewatt enabled Barry and his colleagues to locate the original study area.

In the 61 years between surveys, the population of intertidal denizens shifted in favor of species that prefer warmer water. The researchers noted statistically significant increases in eight of nine southern species living in Monterey Bay. Conversely, five of eight northern species suffered declines.

A gradually warming climate could explain the southern takeover, the scientists say. The annual average water temperature at the shoreline in this area increased by 0.75oC during the past 60 years, while summer maximum water temperatures have climbed 2.2oC since the 1920s.

The scientists also note that El Nino warmings may have played some role. Such episodes are known to cause short-term changes in marine ecosystems. But the researchers argue against a major El Nino influence, because both Hewatt's study and the present one followed decades with similar numbers of El Ninos.