Carbon fixation

“The total amount of data provided by ocean research is equivalent to that collected daily by meteorologists.” That paraphrased comment, made some years ago, is fast getting out of date. As was evident at a conference¹ held last month, the upshot of merged research agendas from biology, chemistry and physical oceanography over the past decade is that the balance is being redressed. This is especially so where oceanic carbon is concerned — the central issue for research is of course how the oceans will respond to, and maybe accommodate, the human passion for CO ₂ emission and the climate change it is likely to cause.

The greatest uncertainties seem to lie more with the biology than with the physics and chemistry of ocean processes. Maybe this is not surprising. Global-scale oceanography has its roots largely in the physical sciences, so we have a better picture of how physical and chemical parameters — pH, temperature, circulation and so on — can regulate carbon exchange between the ocean and the atmosphere. But it is marine organisms that can really tap into physicochemical carbon cycling and lock up carbon on geological timescales.

Equally, it is the biological response to climate change that seems to be the most difficult to predict. For example, the formation of calcium carbonate shells by certain algae — a process that counterintuitively releases CO₂ — appears to be reduced under conditions of higher levels of atmospheric CO₂ (I. Zondervan, Alfred Wegener Inst., Bremerhaven). So if the balance between carbonate chemistry and photosynthesis shifts in the future, these organisms might switch from being a carbon source to being a carbon sink.

As to the requirements for phytoplankton growth, limiting nutrients such as iron and silicon are known to govern biological productivity in certain oceanic regions. Nutrient supply, whether from the atmosphere or the deep oceans, is bound to change with climate perturbation. But contrary to the more rigid biological assumptions made so far, it seems that algae can modify their nutrient take-up capacity as nutrient availability changes (B. Quéguiner, CNRS, Marseille), with knock-on effects on primary productivity. Such observations call for a more dynamic biology to be incorporated into multi-element biogeochemical models.

Ecological responses further down the food chain also have to be considered. Sinking carbon escapes from surface waters primarily when blooms of larger algae, such as diatoms, outstrip the rate at which the larger grazers can crop the excess. But at the meeting it was clear that we have no answers as to how climate change will affect species assemblages, and hence operation of food chains and carbon export to the ocean depths. Can anything be learned from work on terrestrial systems? Most importantly, experience shows that early conclusions as to the CO₂ response of vegetation often provide little indication of the long-term response because they underestimate plants' ability to acclimatize. Long time-series of data (and the patience to acquire them) are required.

In the meantime there are plenty of other gaps to be filled in, especially in the ‘middle ground’. Such areas include the mid-ocean depths, the ‘twilight zone’, where sinking carbon is processed (R. Armstrong, SUNY Stony Brook) and physical features at intermediate spatial scales (S. Doney, NCAR, Boulder). These features are on the 10–100-km scale, and are too large to be investigated with shipboard measurements but too fine to be resolved with global approaches such as satellite observation or modelling.

Ocean biogeochemistry models are likely to be in for a shake-up when all these considerations are taken into account. This is no academic matter — the next generation of models will produce the predictions that shape future policy on carbon usage. There are also data to come from the whole-Earth approach, linking up ocean, land and atmosphere, to help understand and quantify the carbon cycle, and from impending further experiments (with nutrient fertilization, for instance). Finally, the new Earth-observing satellites, such as Terra, can even provide information on the physiology of marine plankton as well as its abundance.

The message from the meeting is that the future course of carbon-fuelled research is set fair. But should governments and grant-giving bodies ask where it is heading? Is there still the hope that scientists will show that the Earth may be able to save itself in some Gaiaesque feat of self-sustainability? As the error bars come down on predictions of how the carbon cycle will react to climate change, perhaps policy-makers and industry can move on and face the reality of a different world. Then again, with the prospect of carbon becoming a tradeable commodity, maybe the governmental push to track its every movement is just a sign of a very thorough market-research campaign.