Ocean-fertilization advocates suffered another setback last week as 191 nations agreed to a moratorium on large-scale commercial schemes to mitigate climate change.

The agreement, adopted on 30 May at a meeting of the United Nations Convention on Biological Diversity in Bonn, Germany, calls for a ban on major ocean fertilization projects until scientists better understand the potential risks and benefits of manipulating the oceanic food chain. It took 12 days of diplomatic effort to win the support of Australia, Brazil and China, which had opposed the moratorium.

Fertilization projects typically involve seeding the ocean with some form of iron to stimulate algal growth. The algae absorb carbon dioxide from the atmosphere during photosynthesis. A number of ocean-fertilization companies have been formed, anticipating that they could sell credits for carbon-dioxide reductions into carbon markets, but many scientists question the effectiveness and potential side effects of fertilization.

The new language endorses and broadens a similar warning given last November by 35 countries party to the London Convention and Protocol, which governs ocean pollution. Those nations agreed to study the issue and to establish rules this year.

Dan Whaley, who heads Climos, a San Francisco-based company promoting ocean fertilization, declined to speculate on the impact of last week's decision. But he endorsed the call for more science and said he fully supports efforts to address the issue under the London Convention.

Advocates claim that algal blooms will briefly flourish, then die and drop to the bottom of the ocean, taking atmospheric carbon with them. But there are questions about how much of the algae might be consumed by other organisms or be broken down before sinking, reducing the amount of carbon sequestered. And altering the ocean's ecology may have unwanted effects, such as increased acidity or decreased oxygen levels.

Biological oceanographer Mike Behrenfeld at Oregon State University in Corvallis says the moratorium is justified. "We have no idea how long-term, sustained iron fertilization is going to influence the species composition." But the only way to find out is through the large-scale experiments that commercial interests are best poised to carry out, says Ken Johnson, a senior scientist at the Monterey Bay Aquarium Research Institute in California. Johnson is against allowing companies to market carbon credits yet, but he thinks that ocean fertilization is the most viable geo-engineering option for addressing a runaway climate. "This isn't something to rush into, but it's the only solution we've got if climate gets out of control."