Ocean on-pagers: Do we know enough of Oceans?
Oceans and the carbon cycle

The oceans influence the climate by absorbing and storing carbon dioxide. Climate change is caused by the accumulation of man-made carbon dioxide (CO2) and other greenhouse gases in the atmosphere. The rate of accumulation depends on how much CO2 mankind emits and how much of this excess CO2 is absorbed by plants and soil or is transported down into the ocean depths by plankton (microscopic plants and animals). Scientists believe that the oceans currently absorb 30-50% of the CO2 produced by the burning of fossil fuel. If they did not soak up any CO2, atmospheric CO2 levels would be much higher than the current level of 355 parts per million by volume (ppmv) - probably around 500-600 ppmv.

Plankton influence the exchange of gases between the atmosphere and the sea. In any given region, the relative amounts of CO2 contained in the atmosphere and dissolved in the ocean's surface layer determine whether the ocean-water emits or absorbs gas. The amount of gas dissolved in the water is in turn influenced by the amount of phytoplankton (microscopic plants, particularly algae), which consume CO2 during photosynthesis. Phytoplankton activity occurs mostly within the first 50 metres of the surface and, although oceanographers don't fully understand why, varies widely according to the season and location. Some areas of the ocean do not receive enough light or are too cold. Other areas appear to lack the nutrients or trace minerals required for life, or zooplankton (microscopic animals) that feed on phytoplankton so limit the population growth of the latter that not all of the available nutrients are consumed.

Rather like a pump, plankton transport gases and nutrients from the ocean surface to the deep. Their role in the carbon cycle is quite different from that of trees and other land plants, which actually absorb CO2 and serve as a storehouse, or "sink", of carbon. Instead, ocean life absorbs CO2 during photosynthesis and, while most of the gas escapes within about a year, some of it is transported down into the deep ocean via dead plants, body parts, faeces, and other sinking materials. The CO2 is then released into the water as the materials decay, and most of it becomes absorbed in the sea-water by combining chemically with water molecules (H2O). Although a small but possibly significant percentage of the sinking organic material becomes buried in the ocean sediment, most of the dissolved carbon dioxide is eventually returned to the surface via ocean currents - but this can take centuries or millennia.

Measuring the level of plankton activity in the ocean is difficult. The rate at which plankton consume carbon dioxide and convert it into sugars for producing tissue and energy varies enormously. This makes it difficult to sample and estimate their annual consumption of CO2. The enormous expanse and remoteness of the oceans (few oceanographers want to go to Antarctica in the middle of winter) also hampers sampling. Satellite pictures of chlorophyll (cell pigment that converts sunlight into energy) give a general idea of the amount of phytoplankton present, and oceanographers hope that future satellite measurements will further clarify the picture.

Climate change will affect plankton, and vice versa. Warmer temperatures may benefit some species and hurt others. Changes in carbon dioxide levels may not have a direct impact, but related "feedback loops" could be important. For example, because plankton create a chemical substance called dimethylsulfide (DMS) that may promote the formation of clouds over the oceans, changes in plankton populations could lead to changes in cloudiness. At the same time, more clouds would reduce the amount of solar radiation reaching the oceans, which could reduce plankton activity. Another possible feedback could occur near the poles. If global warming causes sea ice to melt, more light would reach and warm the surface waters, either benefiting or damaging certain plankton. (The depletion of the ozone layer by CFCs also increases the amount of ultra-violet light reaching the surface, which could have negative effects on the plankton.)

Most scientists are sceptical about proposals to artificially increase CO2 absorption by "fertilising" key ocean regions. For example, because Antarctic phytoplankton are surprisingly sparse considering the quantity of available nutrients, a few scientists have theorised that fertilising the Southern Ocean with iron would boost populations and thus the amount of CO2 absorbed from the atmosphere. Insufficient iron, however, is only one of many possible reasons for low biological activity in the Southern Ocean, and too much iron could poison some plankton. Computer models also indicate that an increase in plankton off Antarctica may not actually lower atmospheric carbon dioxide levels significantly over the next 100 years. But the real danger, of course, is that manipulating biological systems that are not thoroughly understood could have negative consequences just as easily as positive ones.

For further reading:

Berger, W.H., V.S. Smetack, and G. Wefer, (eds.), "Productivity of the Ocean: Present and Past", Wiley: New York (1989).

Mann, K.H. and J.R.N. Lazier, "Dynamics of Marine Ecosystems: Biological-Physical Interactions in the Oceans", Blackwell Scientific Publications: Boston (1991).

Schlesinger, W.H., "Biogeochemistry: An Analysis of Global Change", Academic Press: San Diego, CA (1991).

Source: Climate Change Factsheets of Information Unit on Climate Change (IUCC)-UNEP, 1993

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