This column originally appeared in the June 24, 2007 issue of the Poughkeepsie Journal.

By burning fossil fuels, humans have added carbon dioxide to the atmosphere, causing the Earth's climate to change. Carbon sequestration refers to natural and human-enhanced ways of removing carbon dioxide from the atmosphere. In an attempt to rein in climate change, there has been a scramble to understand how to maximize carbon sequestration. The last installment of Ecofocus explained how forests store carbon. Here we investigate the oceans.

Places that retain carbon dioxide for long periods of time are called carbon sinks. Oceans are the Earth's largest carbon sinks, storing more than one-third of the carbon dioxide we emit each year through fossil fuel combustion. Some carbon dioxide is removed through ocean mixing - a physical process - and some is taken up by marine algae - a biological process.

Carbon dioxide from the atmosphere dissolves into the surface of the ocean. Ocean mixing, which is driven by temperature and currents, causes carbon-rich surface waters to sink into the deep ocean, where it is locked away from the atmosphere. This process removes more than two billion metric tons of carbon dioxide annually. Without the ocean's physical sink, atmospheric carbon would be increasing even more rapidly.

Scientists are working to increase the ocean's ability to store carbon dioxide. Much of this research is funded by The Department of Energy, where projects focus on capturing the carbon dioxide produced by coal- or oil-fired plants, concentrating it, and converting it to an inert liquid called bicarbonate salt brine. This brine would be injected deep into the ocean, where it would be kept away from the atmosphere for about a thousand years. This technology is feasible, but extremely costly.

Capturing carbon dioxide from power plants can only reduce current emissions. A more ambitious plan would be to use carbon dioxide collectors to remove the excess carbon already in our atmosphere, like a global air filter. If successful, this technology would have the potential to lower atmospheric carbon dioxide levels. Unfortunately, this approach is also expensive and would require a great deal of energy.

Microscopic phytoplankton drive the ocean's biological sink. Like their terrestrial counterparts, aquatic plants use carbon dioxide during photosynthesis. Unlike trees, these tiny organisms don't store much biomass. During ocean mixing, a small percentage of phytoplankton sink to great ocean depths, bringing carbon stores with them. While their storage capacity is small - less than one-tenth of the physical sink - it is still being investigated as a future means of carbon sequestration.

Just as you fertilize your garden to increase the growth of your tomatoes, adding the right fertilizer to the ocean can stimulate phytoplankton growth. There are vast areas of the ocean where iron is in short supply for plankton growth. By adding iron to the Antarctic Ocean, researchers stimulated the growth of diatoms. This plankton group has a hard silica shell, which is beneficial for sinking carbon to the deep ocean.

By fertilizing areas of the ocean with iron, we could offset a small percentage of the carbon dioxide we emit by burning fossil fuels (about 2 percent of current emissions). However, this might alter the entire ocean ecosystem, something of great concern to many oceanographers.

While carbon sequestration has a place in dialogue about climate change, the reality is that it will not solve the atmospheric carbon dioxide problem. In addition to being extremely costly, many sinks come with unknown side effects. Present efforts should focus on reducing emissions and exploring alternatives to fossil fuels.

Dr. Jonathan J. Cole is a biogeochemist at the Institute of Ecosystem Studies; he has spent more than 25 years researching carbon in aquatic systems.