In virtually every tropical forest, grassland or warm, humid and wooded area around the world, one can see porous castles, 2-5 m tall and made of soil, water and saliva of their habitants, termites. These small insects, who live in the mounds as colonies of 60,000-200,000, are not just skilled architects and engineers but also a surprising source of methane, a greenhouse gas more potent than carbon dioxide.

Termites are counted among natural sources of methane like wetlands, wild animals, livestock and geological features like volcanoes, and are estimated to account for 1-3 per cent of emissions globally.

In 2008-17, the world emitted 576 Tg (1 Tg or teragram is equal to 10¹² g) of methane per year, of which termites contributed 9 Tg, says the Global Carbon Project, in its “Global Methane Budget” published in 2020. Other estimates put the emissions at 2-15 Tg per year.

However, scientists say actual emissions could be greater or lesser than these estimates. To establish certainty, there is a need to understand the relationship between termite colonies and methane.

Termites are known to wreak havoc on agriculture, forests and construction, due to their affinity for plants and wood. However, only 10-15 per cent of 3,000 termite species identified globally are categorised as pests.

In natural ecosystems, they feed on and recycle the nutrients present in dead and decaying plant and animal matter. It is this cellulose-rich diet that causes their emissions. Methanogenic microorganisms that live in the gut of termites break down the cellulose entering the body and release methane.

In the 2000s, however, it was found that termites undo the damage partially by depositing in the mound a select group of microbes that can capture some emissions.

Final emission depends on the balance between methane-emitting bacteria in the termites’ gut and methane-consuming bacteria in the mounds, Joyeeta Singh Chakraborty, an assistant professor at SRM University, Sonipat, tells Down To Earth (DTE).

Termites’ contribution to global methane emissions was first highlighted in a 1932 paper in The University of Chicago Press by S F Cook, a physiologist from University of California Medical School, Berkeley.

While conducting experiments on termites’ respiration, he identified another gas but did not decode its composition. He speculated termites, like cattle, utilise cellulose in food to produce methane, likely through gut microbes.

In 1982, researchers from the US, Germany and Kenya published a paper in Science saying termites are responsible for 150 Tg of methane emissions per year, or 40 per cent of total methane emissions at the time.

This was an overestimation due to the methodology used, explains Chakraborty. The value of termite emissions has been revised in different studies over the years: in 1993, it was set at 26 Tg per year; in 1996 at 19.7 Tg; in 2013 at 11 Tg; and in 2018 at 20.03 Tg annually.

Several influences

There are multiple reasons for the uncertainty in estimation of termite emissions. First, emissions data on only 5 per cent of the 3,000 known termite species is available.

Second, estimating termite biomass (mass of all termites in all mounds globally), which is essential for calculating emissions, is an invasive process that involves destroying the mound and manually counting the insects. Third, there is limited information on the degree to which mounds capture methane.

Currently, there are two primary ways of estimating termite emissions. The first is to study individual termites in a laboratory setting. This method, however, does not calculate methane capture by mounds.

The second is to check emissions released by individual mounds in forests; but here, emissions by individual termites outside the mound are missed.

Chakraborty conducted such a study as a doctoral student at Forest Research Institute in 2015-16, in the forests of Doon Valley in Uttarakhand. She studied mounds of Odontotermes obesus, the most common termite species in India which dwells in the forest and sometimes near agricultural fields, but is less common in urban areas.

She covered the mounds with transparent plastic sheets that had a hole sealed with a rubber septum and captured the gas released by poking a syringe into the septum. The gas samples showed that, on average, the mounds released 0.38 mg per square metre per hour (mg m⁻² h^-1) of methane. Her study was published in 2021 in the journal Ecosystems.

Chakraborty’s estimates for Odontotermes obesus are lower than those for Thoracotermes termite species in Africa (17.3 mg m⁻² h⁻¹) but higher than those for species in tropical forests of Malaysia (2.3 × 10-14 mg m⁻² h-^-1).

She explains that such differences between species are due to variations in the kind of food consumed, metabolic process and gut microbial content. Studies indicate wood-feeding termites release less methane than soil-eating and fungus-growing species.

This is because wood-eating termites possess microbes that predominantly release acetate. In the other groups, methane-releasing microbes dominate, says a 1993 Chemosphere paper.

Even within species, emissions vary across hierarchy. A 2009 paper in the Academic Journal of Plant Sciences says the queen of a colony, whose primary job along with the king is to reproduce sterile workers and soldiers, is the highest emitter. Next come the soldiers who defend the colony from predators, the workers who build the nests, and finally the larvae.

Then there are external influencing factors like soil temperature and moisture. Chakraborty’s 2021 study says high temperature increases metabolic activity, and greater moisture levels raise vegetation growth and food consumption among termites. Both can increase emissions—for Odontotermes obesus mounds, the highest were in July (0.66 mg m⁻² h⁻¹) and the lowest in November (0.135 mg m⁻² h⁻¹).

Mitigation limit

Researchers have also tried to study methane capture within the mounds. In a 2018 study published in PNAS, scientists in Australia inject methane and argon into 29 termite mounds in the country’s Northern Territory, where three native termite species exist, and then pump out the gases.

They observe that some methane was lost while argon was the same. They repeat the process along with difluoromethane, known to prevent methane-eating bacteria from oxidising the gas.

This time, they find the levels of methane and argon injected and pumped out to be comparable. This indicates that methane-munching microbes mitigate 20-80 per cent of the gas.

To understand why the microbes do not consume all the emissions, in 2020 researchers from multiple nations excavated and studied 17 mounds of three termite species from Northern Australia.

They found that methane-eating bacteria comprised less than 0.5 per cent of the bacterial community in the mounds, suggesting their abundance is low. However, these results do not apply to other parts of the world, the researchers write in a paper published in The ISME Journal.

Warming risks

Knowing about emissions from one termite colony does not necessarily translate to larger scales. There are uncertainties about number of colonies globally, of individual termites per colony, and applying findings to areas that have not been studied, says Thomas Kleinen from Max Planck Institute for Meteorology.

Another emerging unknown is how termites respond to climate change. As temperatures increase and vegetation becomes more productive in warmer areas, termites are expected to move northwards, colonising new regions.

In a 2021 study in Environmental Research Letters, Kleinen and his colleagues use a computer model to simulate how climate change impacts termite emissions. They estimate that in 2000-09, termite emissions were 11.7 Tg per year.

Next, they determine food availability for termites in a specific location and the number of individuals that could live there. Based on this, their simulations show that in future scenarios where average temperatures rise by 3.6°C by 2100, termite emissions could rise to 88 Tg per year in 2200 and 95 Tg per year in the 2280s.

Decreasing biodiversity and decline in predator numbers could also increase termite population, suggests Chakraborty. In her 2021 study, she notes that termites are more active in degraded, species-poor homogeneous patches than in diverse spots. Hence to mitigate emissions, she says, one solution could be to restore ecological balance.

This was first published in the 1-15 May, 2023 print edition of Down To Earth