Scientists are seeking new ways to reverse the build-up of carbon dioxide in the atmosphere, which many believe is the cause of global warming.  VOA's Brian Wagner reports one U.S. project aimed at stimulating plankton growth in the oceans is raising some debate about its possible effectiveness.

Environmental groups have long advocated planting trees as one way to reverse the build-up of carbon dioxide in the atmosphere.  Trees absorb small amounts of carbon dioxide to survive. Marine scientists say that growing tiny organisms in the ocean known as plankton also can help cut carbon dioxide levels in the atmosphere. 

The U.S. firm Planktos is planning to revive plankton in several regions of the Pacific Ocean as an effective way to absorb atmospheric CO2 and counter global warming. Planktos climate expert Melodie Grubbs says iron is the key nutrient needed to stimulate plankton growth. 

"Once we put that in, iron acts as a catalyst for photosynthesis, which creates plankton," she said.  "It's a missing nutrient in the ocean, and from that sparks a natural plankton bloom."

Researchers say plankton blooms are down, because less iron dust is being blown onto the ocean's surface by nutrient-rich winds. Planktos says dust supply has fallen 30 percent over the past 30 years, resulting in a 10 percent decline in plankton populations. The Planktos team hopes to reverse the trend by seeding the waters off the Galapagos Islands and several other spots in the Pacific Ocean.

Researchers have been studying the effects of iron seeding on carbon dioxide levels in the atmosphere for more than two decades. Frank Millero, professor of marine chemistry at the University of Miami, helped pioneer some of the techniques in the 1990s. 

"Even at that time people were thinking about this as a solution," he said.  "We were more interested in showing that [lack of] iron was limiting to plant growth."

The Planktos team says it is building on the research of Millero and other scientists. But unlike academics, the California firm hopes to get financing from major companies that release carbon dioxide into the atmosphere. The companies would compensate Planktos for reducing greenhouse gas emissions globally, without altering their own manufacturing process.   

Planktos says it hopes to sell these so-called carbon credits or offsets.  Profits from the sales will create new opportunities for environmental restoration, says Planktos project manager Michael Bailey.

"We are part of the international carbon credit community, part of that marketing plan, basically trying to find a way to make money by restoring the oceans and the earth's atmosphere. So this is a beautiful business model," he said.

Experts, however, disagree on how much carbon dioxide is absorbed by plankton blooms, which tend to last no more than two or three weeks. Professor Millero says a key concern is whether plankton seeded with iron can have a long-term effect.

"The problem, if there is one, is if the carbon dioxide in the plant material does not sink to the deep waters and become oxidized there, then the net effect is not as large as you might hope for," he noted.

Scientists say plankton has a positive impact on global warming only when carbon-based material in it sinks to deep waters, where it can remain for centuries. Often, some of the material remains near the water's surface, where it is broken down and released back into the atmosphere as carbon dioxide.

Another concern is whether iron seeding can produce more plankton blooms than those that occur naturally. 

David Archer is a professor of geophysical science at the University of Chicago.

"Fertilizing the ocean, it is not at all clear there is any effect on CO2 in the atmosphere," he said.  "In models of the oceans, if you fertilize the ocean here, and reduce the fertility of the ocean somewhere else, the net effect globally will be very small."

Archer also notes that it is nearly impossible for Planktos to measure how much carbon dioxide is pulled from the atmosphere, if any, and as a result, to calculate the carbon credits that Planktos and other firms want to charge to fund environmental restoration projects. Archer says carbon credits would be better used on more reliable methods of reducing greenhouse gases. 

"You can buy carbon offsets that are used to pay for construction of windmills, or other carbon-free sources of energy to displace coal energy," he added.  "And that has a very clear, easy to demonstrate effect on CO2 in the atmosphere."

Experts may not agree on the best method to fight greenhouse gas emissions, but they do agree that lowering emissions from fossil fuels alone is not enough. Many say the world needs to find techniques to reverse decades of emissions and pull carbon dioxide from the atmosphere.