The Why Files The Why Files --

Fertilizing the ocean: Fix for global warming, or a cure that’s worse than the disease?

Cooking our planet: Could geoengineering lower the thermostat?

The warnings over greenhouse warming just keep steaming ahead. 2°C, once deemed a calamitous heating, is now in the center of warming forecasts. Melting of ice sheets in Greenland and even parts of Antarctica now seem less like sci-fi than a matter of time.

And the fossil fuels continue to burn. With varying degrees of abandon, China, India, and the old economies of Europe and North America continue to feed fuel into furnaces, pumping out more carbon dioxide and trapping more heat in the atmosphere.

The rest is simple physics: When heat can't escape, it warms Earth.

 Indian waters are colored red.  East central African waters are cooler colors like green and blue
When phytoplankton cannot perform photosynthesis due to a shortage of a nutrient like iron, they release the excess energy as fluorescent light. Notice that the area of the West Indian Ocean with the highest fluorescence in the image below has low iron concentrations in the image above. Iron-starved plankton in that area cannot consume as much carbon dioxide as their more ferrous friends off the southernmost coast of India.
High fluorescence is red.  A red elliptical cloud is shown along the central African latitudes The phytoplankton look like clumps of segmented incandescent orange curls in green water
Maps: Mike Behrenfeld, Oregon State University/NASA Phytoplankton: Maria Vernet, Scripps Institution of Oceanography/NASA

One common and destructive response is to crank up the air conditioner, burn more electricity (and more fossil fuel), and hope for the best.

Theoretically humans, being the animal that can see the future, could enact regulations to slow and then reverse the flood of greenhouse gases. But since 2000, carbon dioxide pollution has been 33 percent above the rate of previous decades, and the rate of warming is exceeding earlier forecasts.

With greenhouse pollution soaring, we are forced to consider some last-ditch planetary defenses that go by the menacing moniker of "geoengineering."

A solution in the oceans?

One geoengineering proposal, spraying sulfate aerosols high into the air to reflect sunlight back to space, emulates the cooling effect of volcanoes. A more down-to-Earth proposal involves fertilizing the ocean to grow more microscopic floating plants, which could transfer carbon dioxide from the atmosphere into long-term storage in the deep ocean.

The oceans cover about two-thirds of the planet and absorb about one-third of the carbon dioxide that we dump in the atmosphere. Ocean storage has slowed the pace of warming, but the carbon dioxide is acidifying the ocean, endangering many shell-forming creatures like coral, krill and clams. The ocean-fertilization geoengineering proposal could control acidification as well as warming, proponents say.

The root of the idea is this: floating plants, or phytoplankton, are scarce in large reaches of the vast Southern Ocean, mainly due to a deficiency in the micronutrient iron, which is needed in the formation of chlorophyll, the compound that helps convert carbon dioxide and other raw materials into sugars. Phytoplankton are the predominant biomass in the ocean.

On land and water, green plants convert carbon dioxide into carbohydrates through a solar-powered process of photosynthesis that removes the major greenhouse gas from the atmosphere.

The putative link among iron levels, ocean plants and climate was raised in the late 1980s, when oceanographer John Martin suggested that iron carried to the ocean in dust from the continents had fertilized phytoplankton, removing carbon dioxide from the air and dampening the past greenhouse effect. Martin speculated that the process had withdrawn enough carbon dioxide to help trigger periodic ice ages.

A photo of a ship with a crane-suspended barrel and workers is placed within the rainbow bloom map
To confirm John Maritn's iron hypothesis, in 1993 Iron Enrichment Experiment (IRONEX) researchers dumped iron into a 64-square-kilometer area and measured the resulting phytoplankton bloom (yellow and red at bottom center). The added iron was equivalent to one Geritol tablet per hundred tons of seawater. If this biomass sinks deep enough into the ocean, it could slow global warming.

A fertile mind

The beauty of ocean-fertilization is its leverage: By stimulating plant growth, one ton of iron can theoretically remove up to 100,000 tons of carbon from the air. This multiplicative impact caused Martin to coin the quintessential quotable quip: "Give me a half a tanker of iron and I'll give you the next ice age."

To a planet that's entering a steam age (carbon dioxide levels in the air may be the highest in 20 million years, according to the Global Carbon Project), an ice age may sound pretty good...

This graphic shows digested phytoplankton need to fall below surface circulation zones to remove CO2
Graphic: LBL
Carbon dioxide from burning fossil fuels enters the atmosphere, and dissolves in ocean water. Stimulating the biological pump (photosynthetic carbon uptake by phytoplankton and downward transport of carbon in sinking organic matter) with iron could slow greenhouse warming.

Many people find the idea of manipulating the planet, even for beneficial ends, to be nettlesome. Are we smart enough to do it right? What about the environmental damage caused by introducing exotic species, building roads, destroying wetlands, obliterating forests and hunting animals to extinction? Good ideas can boomerang. All in all, it would smarter, safer and faster to cut greenhouse pollution.

But the trend in greenhouse gases is moving the wrong way, and that returns us to geoengineering, and particularly to ocean fertilization.

After 12 experiments in the ocean, however, a crucial question remains -- how long will the atmospheric carbon be tied up in the ocean? When plants decay, their carbon may quickly return to the atmosphere. In the ocean, some portion of the atmospheric carbon trapped by plankton will sink to the deep ocean and be isolated from the surface for centuries.

But unless this is a big portion of the carbon in those fertilized plants, ocean fertilization does nothing to stop global warming.

So solving the storage question is critical to deciding whether ocean fertilization can cool our overheating planet.


Terry Devitt, editor; Nathan Hebert, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2017, University of Wisconsin, Board of Regents.