We are already used to hearing jokes about polluting cow farts. But even small underwater shellfish can contribute to global warming.

Indeed, a new study has found that through digestive processes, clams and worms produce 10 percent of greenhouse gases in the Baltic Sea.

But Stefano Bonaglia, the lead author of the study, says that does not mean we should start blaming these organisms for the climate change humans have created.

Bivalves have been releasing greenhouse gases for millions of years without a negative impact on the climate, he points out.

Only after humans began farming them, and once fertilizer runoff has polluted the seas, has impact worsened.

Shellfish are fast-growing and could be a vital source of protein for a rapidly growing global population

Humans, not shellfish, are warming the planet

Clams and worms in the Baltic Sea release as much greenhouse gas as 20,000 dairy cows - which seems like a big number. But it's quite relative.

In Germany alone, there were more than 4 million dairy cows in 2016, according to the dairy division of the Agriculture and Horticulture Development Board - 20,000 would represent not even 1 percent of that.

Methane emissions from underwater animals in the Baltic Sea are part of a natural cycle - as opposed to cattle, which are farmed by humans.

These mollusks are not to blame for global warming - humans are, Bonaglia points out.

"When humans interfere with the natural cycle - for example through intensive farming - we may have negative consequences, such as these unwanted emissions," Bonaglia told DW.

Cattle farming is still - by far - among the most polluting of all human activities

Keep farming, stop polluting

While aquatic clam farming should be controlled to limit greenhouse gas emissions, Bonaglia highlights that fertilizers reaching the seas and the coasts are a much greater threat for the climate.

The Baltic Sea Centre explains that the elements included in fertilizers, such as nitrogen and phosphorus, lead to eutrophication in the Baltic Sea.

Fertilizers increase nutrients, enabling algal blooms and consequent bacterial activity that can potentially increase greenhouse gas emissions.

While clams and worms represent 10 percent of the Baltic Sea greenhouse gas emissions, the other 90 percent comes from bacteria, mainly present in the dead zones of the Baltic Sea - deep waters with low levels of oxygen, suffering an excess of nutrients due to human activities.

A global concern?

The Baltic Sea makes up only about 0.1 percent of the Earth’s oceans - drawing back, the amount of emissions from underwater shellfish around the world could be a concern when the scale of shellfish farming is taken into account.

Bonaglia called again for calm, saying his findings cannot be extrapolated to other seas.

"We don’t know what is going on, for example, in the Adriatic Sea, the Mediterranean Sea or the Black Sea," Bonaglia said.

Each water body has its own characteristics and requires therefore of specific studies, he pointed out. "We need more studies, definitely."

The Atlantic Ocean, for instance, does not generate much methane, since its turbulent waters metabolize it and prevent it from going into the atmosphere.