Global warming may get a nudge from its "evil twin," ocean acidification.
That possibility is raised in a new study that suggests that the activities of a tiny plankton – affected by the growing acidity of the world's oceans – could raise average global temperatures by as much as 1 degree Fahrenheit above current estimates.
The study, published Sunday in Nature Climate Change, represents a first cut at the issue and so faces a range of uncertainties. Trends in greenhouse-gas emissions, for example, could impact the findings.
Still, the work is "important" and "unique," says Richard Feely, a senior scientist at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle.
"I don't think any of the previous modeling includes the effect of acidification on biological feedbacks" to the atmosphere, says Dr. Feely, who was not a member of the research team.
This "biological feedback" involves a compound produced by the plankton, called (rather unmercifully) dimethylsulfoniopropionate. Mercifully, scientists have reduced the name to the acronym: DMSP.
DMSP breaks down into forms that, once they reach the atmosphere, are readily converted into tiny aerosol particles. These aerosols, in turn, serve as the seeds for thick, low-level clouds over the ocean – clouds that are effective at reflecting sunlight back into space.
Lab and field experiments have shown that when seawater becomes more acidic, planktons' output of DMSP drops. The potential effect on cloud formation globally could be significant. By the end of the century, reduced cloud cover from this feedback could raise projected global average temperatures from 0.23 to 0.48 degrees Celsius (roughly 0.4 to 0.9 degrees Fahrenheit), according to the research team led by Katharina Six, with the Max Planck Institute for Meteorology in Hamburg.
Sulfur-based gases are the primary raw materials for aerosols that stimulate cloud formation, Feely says. Bacteria in the ocean break down DMSP into two byproducts, one of which is dimethyl sulfide (DMS) – said to be the atmosphere's single largest source of sulfur. DMS also is responsible for the smell of the sea as you approach the beach.
"It's really quite important to have a good sense of that," says Feely of the processes and effects of DMS on global climate.
Acidification occurs as the oceans take up a significant portion of the rising levels of carbon dioxide that human activities emit. No one expects the seas to mimic battery acid. But while the changes are undetectable to anyone wading in the surf from one year to the next, they are daunting to marine organisms that have adapted to a very narrow range of pH levels, a measure of relative acidity, in seawater.
The study published Sunday grew out of earlier field studies of ocean acidification in the Arctic, explains Stephen Archer, a senior scientist at the Bigelow Laboratory for Ocean Sciences in East Boothbay, Maine. His work focuses on how the ocean and atmosphere exchange gases and is a member of the team reporting these latest results.
Acidification is happening at the fastest pace in the cold seas at high latitudes. The relationship between plankton and DMSP production under acidic conditions could be important to melt-season cloud formation and climate change in the region. Dr. Archer and colleagues conducted the field studies in a fjord on the coast of Spitsbergen island to address the question as a part of the European Project on Ocean Acidification.
The results showed a significant decline in DMSP/DMS production, which ran counter to what models were producing. This raised the question of the potential effect globally, leading to the latest study, which uses the Spitsbergen data.
Archer acknowledges that this study represents a test-of-concept exercise for the feedback, based on a region where acidification is most pronounced. And at this stage, it's difficult to take into account any adaptation the organisms might undergo to survive in more acidic waters.
He and colleagues are preparing to look at similar DMSP processes in tropical and sub-tropical waters to get a more representative geographic sample and to get a better understanding of the mechanisms that lead to the feedback.
"If we could really understand the mechanisms behind that response ... we could more robustly predict what's going to happen" as ocean pH changes, he says.
"We've got the funding to do that; hopefully we can improve on what we've done," he says.