Could ocean acidification lead to further global warming? That’s the theory floated in a new paper in the journal Nature Climate Change. In the long run – eventually, rather than immediately – this new feedback could warm the earth an extra quarter or half a degree Celsius, the study suggests.

The paper is interesting because it points to a new climate feedback – one current predictions don’t include. But research on the issue is at an early stage, which means it’s subject to the kinds of uncertainties you get with new findings and theories.

Sulphur-seeded clouds

The theory all boils down to tiny sea creatures like phytoplankton, which produce a chemical containing sulphur. When released into the atmosphere, this sulphur compound encourages clouds to form, helping to keep the earth cool by reflecting incoming sunlight back out to space. The sulphur also directly scatters incoming sunlight. So surprisingly, perhaps, there is a link between phytoplankton and cloud formation.

Normally these marine creatures produce a ready supply of this sulphur compound. But that could change in the future, say the authors of the new study. Seawater is becoming more acidic as it absorbs some of the increased levels of carbon dioxide in the atmosphere.

If that acidification makes the seas less hospitable for creatures like phytoplankton, the evidence from the limited studies available suggests that emissions of dimethyl sulphide – the sulphur compound in question – could decline.

Jean-Pierre Gattuso, a senior research scientist who has studied ocean acidification in great detail tells us:

“On balance and based on today’s knowledge, dimethyl sulphide emissions are projected to decrease as a result of ocean acidification.”

And if lower sulphur emissions lead to less reflective clouds, that would mean more sunlight reaching earth’s surface, and more warming.

Some serious uncertainties

How much extra warming depends on how future greenhouse gas emissions pan out, and how sensitive marine species are to increasingly acidic oceans.

If greenhouse gas emission peak by mid-century and marine species are not too sensitive to changing ocean conditions, the feedback could result in between a quarter and a half of a degree Celsius warming, the researchers suggest.

That’s the temperature rise expected once the entire climate system adjusts to the extra heat – a measure scientists call the equilibrium temperature response. Essentially, it’s the amount of warming we’re committed to, even if we don’t feel if for hundreds or even thousands of years.

If marine species end up being largely insensitive to the acidity of the ocean, the warming could be much less. But if they are highly sensitive to ocean acidification, the temperature rise could be as much as three quarters of a degree.

For a previously unknown feedback, these levels of temperature rise are pretty big. That makes it important to remember the scientific context. This is the first study to look at the links between ocean acidification and emissions of this sulphur compound on a global scale, and so caution should be applied when looking at the conclusions.

Jean-Pierre Gattuso told us:

“This is a potentially very important result, but this is a “what if” experiment. It remains to be seen whether this conclusion will stand the test of time.”

The result is based on a complex model of the earth, and based on trends seen in laboratory experiments and in a small number of studies in controlled bodies of ocean water. And not all research agrees that clouds are so sensitive to emissions of dimethyl sulphide.

Other factors

In the open ocean worldwide, there will be a number of other factors at play which could affect how much dimethyl sulphide is produced.And other effects of climate change may further complicate the picture.

Gattuso tells us that future changes in temperature, the availability of nutrients, and the makeup of plankton communities could interact with ocean acidification to alter how much dimethyl sulphide is produced.

But despite the many factors at play, this study flags a new and potentially important feedback – one that’s currently not included in climate model predictions, as the lead author of the paper says:

“The main point of our study is that there’s a mechanism that introduces an additional radiative forcing on top of the CO2 forcing.”

Six et al. (2013) Global warming amplified by reduced sulphur fluxes as a result of ocean acidification. DOI: 10.1038/nclimate1981

Updated 27/8/13 to reflect that reduced sulphur emissions could lead to less reflective clouds. Previously the article said less clouds.