A study by a National Taiwan University (NTU) researcher overturned a hypothesis that fertilizing oceans with iron to increase the growth of greenhouse gas-absorbing algae might help slow global warming.
Proposed in the 1990s, iron fertilization — seeding oceans with iron to boost the growth of algae to capture carbon dioxide via photosynthesis — has been conducted a dozen times around the world as a way to reduce atmospheric carbon.
However, an international team joined by NTU assistant professor of geology Ren Hao-jia (任昊佳) found that while iron fertilization can stimulate algae bloom in one region, it could inversely prohibit the growth of algae in other regions.
“The increased number of algae exhaust nutrients necessary to the growth of the plant, such as nitrite and phosphate, and the seawater, now containing little nutrients, can no longer nourish algae when it flows to other areas even with iron fertilization,” Ren said.
However, the team arrived at its conclusion with a twist.
Past studies, including one of Ren’s, have supported the idea that iron fertilization could curb temperature increases associated with global warming, as researches showed that iron-laden dust blown from the land into the sea significantly boosted the growth of algae in many regions and helped decrease temperatures.
An analysis of marine sediments in sub-Antarctic waters conducted by Ren and her colleagues in 2014 indicated that in the last ice age, iron levels were relatively higher, while carbon dioxide concentrations were lower; while in between ice ages — when temperatures were warmer — iron levels were lower, while carbon dioxide concentrations were higher.
The findings supported the iron-fertilization hypothesis and were published in the journal Science in 2014.
However, similar research by Ren and her colleagues last year analyzing marine sediments in the equatorial Pacific Ocean found no indications of iron fertilization during the last ice age.
Iron levels in the equatorial Pacific Ocean were high, but nutrients had remained at constant levels, suggesting there was no large-scale algae growth, despite the presence of iron in the region during the last ice age, which contradicted the iron-fertilization hypothesis.
The team said that, while iron fertilizaton can lead to an algae bloom in regions where nutrients are abundant but iron is lacking, it could inhibit the growth of algae in other regions as seawater with depleted nutrients moves there.
The findings were published on the journal Nature in January.
“We also understand some other side effects of iron fertilization. The algae boom it stimulates might cause the ocean to run out of oxygen and lead to massive death of marine animals. It is not to say iron fertilization is useless, but we have to be cautious,” Ren said.
“It was generally believed that iron fertilization was a practical approach to capture greenhouse gases, but Ren dared to challenge a well-received theory and succeeded,” NTU vice president Chen Liang-gee (陳良基) said.
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