Although carbon dioxide is always mentioned first in climate-damaging gases, methane is the much more potent greenhouse gas. The colourless and odourless, combustible gas with the formula CH4 is about 25 times more harmful to the climate than CO2, which is why, despite its low concentration in the air, it contributes around 20 percent to the man-made greenhouse effect.

The Intergovernmental Panel on Climate Change (IPCC) has calculated that methane emissions in 2010 will have to be reduced by 35 percent over the next 30 years if the target of 1.5-degree Celsius is to remain achievable. A small glimmer of light, however, is that methane remains “only” between nine and 15 years compared to CO2, which takes a century to decompose in the atmosphere.

At present, however, there is no improvement in the methane situation in sight, on the contrary. Without human intervention, the content of the gas in the atmosphere was around 700 ppb (parts per billion) by the end of the 18th century. In 2017, on the other hand, the share was 150 percent higher at about 1860 ppb. This means that there is more methane in our planet’s gas shell today than at any time in the past 800,000 years.

Problematic natural methane

While anthropogenic methane emissions could be reduced by appropriate measures, against the methane that comes from natural sources they are largely powerless. Microorganisms in swamps and waters as well as volcanic activity play the most important roles. In addition, there are the greenhouse gas emissions that arise from the thawing of the Arctic permafrost. These are due to global warming and thus indirectly also to human activity, but are at least at present just as difficult to limit. Scientists fear that these emissions could multiply in the coming decades.

Now, an international team of scientists has demonstrated, based on recent research, that natural ecosystems such as lakes and rivers release more methane from global warming than previously thought. The study, published in the journal Nature Climate Change, attributes this to changes in the balance of microbial communities in the various ecosystems that regulate methane emissions.

Producers and degraders

The emission and reduction of methane from ecosystems is essentially regulated by two types of microorganisms: methanogens that naturally produce methane, and methanotrophes, which remove methane by converting it into carbon dioxide. Previous studies have shown that these two natural processes have different temperature sensitivities and could therefore be influenced differently by global warming.

Scientists, lead by Mark Trimmer of Queen Mary University of London, analyzed the effects of global warming on freshwater microorganisms and methane emissions by observing the effect of warming artificial ponds over an 11-year period. They found that rising temperatures led to a disproportionate increase in methane production compared to the reduction of methane. The bottom line was a significant increase in methane emissions.

Microbial communities in imbalance

“Our observations show that the increase in methane emissions is far higher than previously expected due to a purely physiological response to the temperature increase,” says Trimmer. Long-term warming is therefore also changing the balance in the microbial community of freshwater ecosystems. This produces more methane, while proportionally less methane is oxidized to carbon dioxide. “Because methane is a much stronger greenhouse gas than carbon dioxide, these effects increase the global warming potential of the carbon gases released,” trimmer said

These observations were complemented by a meta-analysis of available data on global methane emissions from wetlands, forests and grasslands. These underpin the experimental studies and show that the warming of natural ecosystems produces disproportionately a lot of methane.

Relevant for future predictions

“Our results are in keeping with what we have been able to demonstrate in the real world with a greater diversity of ecosystems. Taken together, these data suggest that natural ecosystems, with rising Earth temperatures due to global warming, continuously release more methane into the atmosphere,” Trimmer said. The scientists stress that future predictions must take into account how ecosystems and the microbial communities present there will change as they are exposed to further warming.