On the horizon

News from the frontiers of science.

Fewer sharks = fewer scallops

Tourists may have been unhappy with "Jaws" roiling the waters, but scallops didn't mind. Researchers have found that, at least off North Carolina, overfishing of the largest sharks has led to the inadvertent collapse of scallop populations. Fish that the big sharks once ate are thriving, feasting on scallops and other shellfish.

The results highlight the ecological – and economic – damage that comes when people skim off an ecosystem's top predators, the researchers say.

The team, led by scientists from the United States and Canada, found that from 1970 to 2005, fishing had decimated some key shark species, essentially knocking them off their perch at the top of the food chain in North Carolina's coastal ecosystems. On-site observations that began in the early '80s showed that at the time, migrating cownose rays had little effect on the long-term scallop population. As sharks dwindled, cownose rays surged. By 2004, the cownose had virtually cleared the sampling areas of scallops – except for control sites protected by pens. This "collateral" ecological damage from commercial shark fishing points to the need to manage fisheries as whole ecosystems, not by species, according to the researchers. Their results appear in the current issue of the journal Science.

A long look back at C02's impact

Just how sensitive is the climate to changes in carbon dioxide? It's a key question for scientists trying to gauge the impact of burning fossil fuels on today's climate. Now a team has looked at patterns over the past 420 million years and found that when CO2 concentrations double, the global average temperature rises by at least 1.5 degrees Celsius (2.7 degrees Fah­renheit). The best estimate: 2.8 degrees C.

February's report from the Intergovernmental Panel on Climate Change (IPCC) gave a range of 1.5 to 4.5 degrees C, with a best estimate of 3 degrees C. Others have suggested 1.5 to 6.2 degrees C. But these estimates are based on studies that cover the past several hundred thousand years of ice-core and other data – periods when temperatures were similar to or cooler than now.

The latest results, compiled by a team led by Dana Royer of Wesleyan University in Connecticut, drew on computer models and measurements using thermometer stand-ins to look at the 420-million-year record of CO2 and global temperatures. Because their timespan covers periods when the planet was far warmer, the results suggest that the IPCC's sensitivity figure is pretty robust. The results appear in the current issue of the journal Nature.

Double stars host planets, too

Talk about spectacular sunsets.... New results from the infrared Spitzer Space Telescope hint that planets may be as common in double-star systems as they are in systems like ours, with a single sun.

Planets have been detected in at least one double system – but the stars are 1,000 times farther apart than Earth is from the sun, or 1,000 astronomical units (AU). Astronomers wondered if planets could form where twin suns were closer together.

The University of Arizona's David Trilling led a team that examined binary (two-star) systems with stars separated by 500 AU or less. Out of 69 binary systems it imaged, the team found dusty debris disks circling 40 percent of them – a slightly higher percentage than single stars boast. Surprisingly, perhaps, the disks appeared most often around the tightest binaries.

Debris disks don't prove that the stars host planets, but they are the kind of features one would expect to see if planet-building had begun. The results are in the April 1 issue of The Astrophysical Journal.

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