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GRANTS PASS, Ore. – Trees in old growth forests across the West are dying at a small, but increasing rate that scientists conclude is probably caused by longer and hotter summers from a changing climate.

While not noticeable to someone walking through the forests, the death rate is doubling every 17 to 29 years, according to a 52-year study published in the Friday edition of the journal Science. The trend was apparent in trees of all ages, species, and locations.

"If current trends continue, forests will become sparser over time," said lead author Phillip J. van Mantgem of the U.S. Geological Survey's Western Ecological Research Center.

"Eventually this will lead to decreasing tree size," he said. "This is important because it indicates future forests might store less carbon than present."

Old growth forests, particularly those in the Northwest, store large amounts of carbon, making them a resource in combatting global warming, said Jerry Franklin, a professor of forest ecology at the University of Washington. But as trees die, they decompose and give off carbon dioxide, contributing to the amount of greenhouse gases. Young forests store very little carbon, and it takes hundreds of years to replace old growth, he said.

The researchers considered several other possible causes for the higher death rate — air pollution, overcrowding of young trees, the effects of logging, large trees falling on small ones, and a lack of forest fires, which keep forests healthy. But the data showed the trend affected trees young and old, in polluted and clean air, in crowded and sparse stands and at different elevations.

The likely cause, they concluded, was warmer average temperature across the West, about 1 degree over the study period, said co-author Nathan L. Stephenson, also of the USGS Western Ecological Research Center. That results in greater stress on the trees from lack of water, leaving them vulnerable to disease and insects.

Stephenson said the rising death rate could also produce a cascading decline in forests that leads to less habitat for fish and wildlife, an increased risk of wildfires, and a vulnerability to sudden forest die-offs.

"If it's a gradual process, we may be fine," said Mark E. Harmon, professor of forest ecology at Oregon State University. "If it is a real sudden process, it could be problematical."

Barbara Bond, a professor of forest physiology at Oregon State who was not involved in the study, said it would be wrong to definitively conclude that the rising tree mortality was caused by warmer temperatures.

"An enormous amount of additional work would have be done before any rational scientist would draw some cause and effect," she said.

The geological survey paid for the study, which examined data between 1955 and 2007 in 76 research plots in British Columbia, Washington, Oregon, California, Idaho, Colorado and Arizona. The average age of the forests examined was about 450 years, with some as old as 1,000 years. Of the 59,736 trees counted, 11,095 died over the study period.

The death rate increase varied, with the highest in California's Sierras, from about 0.9 percent in 1980 and rising to about 1.3 percent.

SYDNEY – Scientists said Sunday they had uncovered new marine animals in their search of previously unexplored Australian waters, along with a bizarre carnivorous sea squirt and ocean-dwelling spiders.

A joint US-Australian team spent a month in deep waters off the coast of the southern island of Tasmania to "search for life deeper than any previous voyage in Australian waters," lead researcher Ron Thresher said.

What they found were not only species new to science -- including previously undescribed soft corals -- but fresh indications of global warming's threat to the country's unique marine life.

"Our sampling documented the deepest known Australian fauna, including a bizarre carnivorous sea squirt, sea spiders and giant sponges, and previously unknown marine communities dominated by gooseneck barnacles and millions of round, purple-spotted sea anemones," Thresher said.

Using a submersible car-sized robot named Jason, the team explored a rift in the earth's crust known as the Tasman Fracture Zone, a sheer two kilometre (1.24 mile) drop to 4,000 metres (13,200 feet) below the ocean's surface.

Blogging on board the ship, researcher Adam Subhas said the team witnessed some "cool biology" as they descended the fracture, including the sea squirt, which he described as "basically an underwater Venus fly trap, but much bigger."

The sea squirt, also known as an ascidian, stands 50 centimetres tall on the sea floor at a depth of just over 4,000 metres. It traps prey in its funnel-like front section if they touch it when they swim past.

"The geology was fascinating too -- the sediment was incredibly fine and lightly packed; it made me think of powder snow," Subhas wrote.

Fossil coral fields were found, dating back more than 10,000 years. Thresher said samples taken would provide ancient climate data for use in global warming projections.

"Modern-day deep-water coral reefs were also found, however, there is strong evidence that this reef system is dying, with most reef-forming coral deeper than 1,300 metres newly dead," he said.

Though close analysis of samples was still required, Thresher said modelling suggested ocean acidification could be responsible.

"If our analysis identifies this phenomenon as the cause of the reef system's demise, then the impact we are seeing now below 1,300 metres might extend to the shallower portions of the deep-reefs over the next 50 years, threatening this entire community," he said.

Rising sea temperatures are blamed on global warming caused by the build-up in the atmosphere of greenhouse gases such as carbon dioxide -- which is also blamed for higher acidity in sea water.

A UN report warned in 2007 that Australia's Great Barrier Reef, described as the world's largest living organism, could be killed by climate change within decades.

The World Heritage site and major tourist attraction, stretching over more than 345,000 square kilometres (133,000 square miles) off Australia's east coast, could become "functionally extinct", the report said.