Modern climate observers can be divided into groups conveniently called worriers and skeptics.

Most climate scientists reside in the former, warning the build-up of carbon dioxide in the atmosphere - already at higher levels than in at least the past 30,000 years - is bound to produce harmful effects on the planet's ecosystems.

On the other hand, some skeptics have claimed increased carbon dioxide is a good thing.

"Carbon-dioxide emissions from fossil-fuel combustion are beneficial to life on Earth," the Greening Earth Society, for instance, said on its Web site.

Such a statement takes a rather simplistic view of ecological relationships, however.

It is true some plant growth increases with higher CO2 concentrations, but most scientific studies show this benefit is limited; it is constrained by other factors. Plants need nutrients as well as CO2 for healthy growth, but many of those nutrients remain in short supply.

Some animals will benefit from warming temperatures, some will not. The skeptical network CCNet, published in the United Kingdom, recently trumpeted findings that an Australian fruit fly adapted to rising temperatures within two decades.

In a classic example of selective argument, CCNet called this "more evidence that species adapt to climate change." Fruit flies have short generational life spans and do, in fact, adapt to change quickly.

This is why they are the preferred species for studying evolutionary detail in the laboratory - their genes respond quickly. If evolutionary biologists used, say, elephants, the lab specimens might outlive the experimenters and the evolutionary knowledge gleaned from them would be of little immediate use.

The fact is animals do evolve to deal with changing environmental conditions, some better than others. Fruit flies, ants and humans probably will emerge relatively unscathed - as species - unless an unforeseen cataclysm sinks them.

Other species, however, almost certain will suffer. Few say with any confidence who the winners and losers will be.

Yes, there are tantalizing anecdotes: A grizzly bear has been seen in habitat usually frequented only by polar bears, while red foxes are expanding northward in Canada at the expense of Arctic foxes.

In the Antarctic, Adelie penguins, for whom the presence of sea ice is essential for survival, are declining. Other penguin species with similar but not identical habits, such as chinstrap and gentoo penguins, seem to be increasing and replacing Adelie in some habitats.

These complex ecological interrelationships cannot be glibly summarized. Most of Earth is covered with oceans. What effect does warming have on the phytoplankton?

They compose both the base of the ocean food chain and an important photosynthesizer, producing about half of Earth's plant-generated oxygen, as well as absorbing CO2.

In a study published last March in Geophysical Research Letters, Watson Gregg, an oceanographer at NASA's Goddard Space Flight Center, and colleagues found phytoplankton amounts increased by more than 4 percent between 1998 and 2003. The increases occurred mainly along the coasts and decreased in mid-ocean areas.

"The study suggests there may be changes occurring in the biology of the oceans, especially in the coast regions," Gregg told UPI's Blue Planet.

Even here, an increase in the volume of phytoplankton does not tell the whole story. Timing of the phytoplankton blooms is just as important.

"That has major implications for entire ecosystems," Gregg said. "The phasing has to be within a reasonable amount of time for the entire ecosystem to take advantage of it. If they bloom too fast, the population moving up the food chain doesn't get any benefit."

Phytoplankton take up CO2, die and sink to the bottom, which removes greenhouse gases from the atmosphere - but it also makes the food unavailable for consumption by many ocean animals.

Another National Aeronautics and Space Administration study, released last week in the journal Science, found a connection between snow melting in the Himalayas under the heat of global warming and increased plankton concentrations in the western Arabian Sea - more than a 350 percent increase over the past seven years.

Even though the Himalayas stand thousands of miles away, the NASA team found that when the winter and spring snow cover declines in Eurasia, Earth absorbs more solar energy - instead of reflecting it back into space -- thereby "creating a larger temperature difference between the water of the Arabian Sea and the Indian subcontinent landmass," the study said.

That difference causes the winds to blow across the Arabian Sea, resulting in monsoons on the Indian subcontinent and an upwelling of cooler, nutrient-rich water - ideal conditions for plankton blooms.

"The associated shifts in winds, and increased phytoplankton levels in the Arabian Sea, could have far reaching consequences for the ecosystem of the region," said Joaquim Goes, senior researcher at the Bigelow Laboratory for Ocean Sciences in Maine and lead author of the study.

Goes said very large plankton blooms can harm ecosystems by depleting oxygen in the water.

Perhaps, but there is a hazard involving climate data that comes from speaking glibly, given the complexity of the interactions involved.

"The immense area and the modest extent of our knowledge of the open ocean hamper predictions of how ocean systems will respond to climate change," said the Pew Center of Global Climate Change in a 2002 report on coastal and marine ecosystems.

"Nevertheless, it is clear that increased temperature or freshwater input to the upper layers of the ocean results in increased density stratification, which affects ocean productivity."

In other words, there are a lot of forces at work that can affect the health or overpopulation of plankton, which in turn affect every other marine creature - and, eventually, humans.

The Pew report went on to predict human-induced increases in atmospheric greenhouse gases are expected to cause much more rapid changes in Earth's climate than have been experienced for millennia.

"If this happens," the report said, "such high rates of change will probably result in local if not total extinction of some species, the alteration of species distributions in ways that may lead to major changes in their interactions with other species, and modifications in the flow of energy and cycling of materials within ecosystems."

In the end, whether global warming benefits life on Earth probably depends on whose life you are concerned about. As with any ecosystem upheaval, it may be good for some, and not so good for others.

The warming probably will cause a decline in overall biodiversity -- a situation which, in past large extinctions, required between 2 million and 5 million years of recovery.

Climate is a weekly series examining the potential impact of global climate change, by veteran environmental reporter Dan Whipple. E-mail: [email protected]

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