Discussions about ocean and global warming tend to focus on the threat of rising sea levels or the possibility that hotter tropical waters might spawn more frequent typhoons. But one also needs to remember that, in a fundamental sense, the oceans are important allies in the struggle against troubling climatic change. Of all the heat-trapping carbon dioxide that is released into the atmosphere every year from tailpipes and smokestacks, about a third goes into the sea, which scientists therefore recognize as an important “sink” for this gas.
The carbon dioxide dissolves in the shallow layers of the ocean. where, thankfully, it cannot contribute to warming the atmosphere. Much of the carbon transferred in this way is used by phytoplankton, the ubiquitous microscopic plants that grow near the surface of the water. After these short-lived organisms die, some of the carbon in their tissues sinks to great depth, Climatologists call this process the “biological pump” because it draws carbon out of the atmosphere and stores it deep in the sea. Naturally enough, some people have pondered whether this phenomenon could be artificially enhanced. This tactic would be the marine equivalent of planting more trees to isolate carbon in a form that does not contribute to greenhouse warming.
One researcher closely associated with this concept is the late John H. Martin of Moss Landing Marine Laboratories in California. Martin and his colleagues were aware that large oceanic regions contain high levels of nitrate (a normally scarce nutrient) but show low concentrations of the photosynthetic pigment chlorophyll. That combination was curious: with abundant nitrate to fertilize their growth, tiny marine plants should multiply rapidly, greening the sea with chlorophyll. Yet vast high-nitrate, low-chlorophyll areas can be found in the equatorial and northern Pacific and over large stretches of the southern oceans.
Martin and his co-workers knew that the growth of phytoplankton in these places was not limited by any of the major nutrients—nitrate, silicate or phosphate. They believed that the deficiency of a trace element, iron, was curbing the growth of phytoplankton, because experiments with cultures had shown that adding a dash of iron to water taken from these areas boosts its ability to support the growth of common types of phytoplankton.
They reasoned that this connection between iron and plant growth, if it indeed operated the same way in the ocean, would have profound consequences. For example, it could explain why carbon dioxide levels in the atmosphere were much lower during the last ice age; iron carried in dust blown off the cold, dry continents of the time would have fostered the growth of marine phytoplankton, which then acted to pump carbon from the atmosphere to the seafloor. When the continents became warmer and wetter at the end of the Pleistocene (roughly 10,000 years ago), the land gave off less dust to ocean-bound winds, robbing some marine phytoplankton of the iron needed for growth.
Although this argument was compelling, many other theories could also explain past changes in atmospheric carbon dioxide levels. To impress on some of his skeptical colleagues the importance of iron as a plant nutrient, Martin jokingly proclaimed in a lecture in 1988 that adding even modest amounts of iron in the right places could spur the growth of enough phytoplankton to draw much of the heat-absorbing carbon dioxide from the atmosphere. His often quoted jest “Give me a half a tanker of iron, and I’ll give you an ice age” foreshadowed more serious considerations of actually using this approach to help cool the planet.
54- The passage is primarily concerned with which of the following?