Methane (CH₄) is an important trace gas in the atmosphere, contributing significantly to longwave absorption, and to the chemistries of both the troposphere and the stratosphere. In the troposphere, methane acts as a sink for hydroxide (OH) and as a source for carbon monoxide (CO). In the stratosphere, methane is a sink for chlorine (Cl) molecules and a source of water vapor, which is the dominate greenhouse gas. Analysis has shown that atmospheric concentrations of methane have increased by about 30% over the last 40 years. Such increases may greatly affect future levels of stratospheric ozone and, therefore, the climate of the earth. Recent estimates of the total annual source strength of CH₄ vary from 400 to 1200 Tg. Activities such as rice and cattle production, the mining and use of fossil fuels, and biomass burning are believed to be the cause of increasing methane levels in the atmosphere. Added to this list of sources is the termite, which produces measurable quantities of CH₄, with estimates ranging from 2 to 150 Tg per year. However, data indicate that while there are large variations in the amount of CH₄ produced by different species, the total methane source due to termites is probably less than 15 Tg per year, thus making a contribution of less than 5% to global CH₄ emissions.

Methane production by termites was first reported by Cook (1932) who observed the evolution of a gas from a species of termite. Studies during the following years indicated that methane is produced in a termite's digestive track during the breakdown of cellulose by symbiotic micro-organisms. Later studies showed large variations in the amount of CH₄ produced by different species. More recent research by Zimmerman et al. (1982) found average CH₄ production rates of 0.425 ug CH₄/termite/day for the lower termite species and 0.397 ug CH₄/termite/day for the higher termite families.

Environmental conditions such as light levels, humidity, temperature, and CO₂ and O2 concentrations play a part in methane production. Termites prefer the absence of solar radiation; an immobile atmosphere; saturated or near saturated relative humidities; high, stable temperatures; and even elevated levels of CO₂. Although termite populations are active in the middle latitude environments, the vast concentrations of mounds and nests are found in the lower latitude tropical forests, grasslands, and savannahs of Africa, Asia, Australia, and South America. It is estimated that these regions contribute approximately 80% of global termite emissions.

Fraser et al. (1986) performed an experiment using 6 different species of termites from the United States and Australia. Termite mounds under glass enclosures were studied in a laboratory setting, with diet and temperature allowed to vary while all other variables were controlled. It was found that the capacity of termites to produce CH₄ varied from species to species, within groups from different mounds or nests of a particular species, and also with temperature. The 6 different species studied produced methane at rates that ranged over more than two orders of magnitude. Raising the temperature by 5 degs C within each species' preferred temperature range caused a 30-110% increase in measured CH₄ emissions. Prior laboratory and field research seems to show that termites prefer temperatures in excess of 10 deg C above the ambient air temperatures determined by their geographical locations. A positive correlation between amounts of biomass consumed and CH₄ emitted was observed, with the average being 3.2 mg CH₄ per gm of wood.

Seiler et al. (1983) performed a field research project near Pretoria, South Africa, to study termite methane production. His team placed aluminum framed boxes covered by plastic over termite mounds with the goal of separating the mounds from the ambient conditions while keeping the termite colonies in their natural environments. CH₄, CO₂, and temperature levels were monitored inside the mounds and flux rates of the carbon compounds were measured within the boxes by extracting air samples by means of syringes. Also monitored was the exchange of CH₄ and CO₂ at the soil surface within the vicinity of the nests.

The calculated flux rates from termite mounds into the atmosphere showed significant variations which were related to the size of the mounds, the population density of the termites, termite activity, and termite species. It was found that the flux rates exhibited diurnal variations, with maximum values during the late afternoon and minimum values during the early morning. The CH₄ flux rates from individual mounds were directly proportional to the corresponding CO₂ rates, with methane increasing linearly with increasing carbon dioxide. It was also shown that the ratios of CH₄ and CO₂ flux rates measured at different days, different mounds, and different weather conditions were relatively constant for each species, but differed considerably from species to species.

Most interesting were measurements performed on the soil surface at distances of 1 to 20 m from the center of the termite nests, which generally showed a decrease in CH₄, indicating that CH₄ is decomposing in the soil. This observed destruction of atmospheric methane in the termite-free soil areas has led some researchers to suggest that such adjacent areas are a sink for CH₄. But it is agreed that much further tests and measurements are needed to more fully understand the effects of termites on atmospheric levels of methane.

References

Fraser, P.J., R. Rasmussen, J.W. Creffield, J.R. French and M.A.K. Khalil, 1986: Termites and global methane: another assessment, J. Atmos. Chem., 4, 295-310.

Seiler, W., R. Conrad, and D. Scharffe, 1984: Field studies of methane emission from termite nests into the atmosphere and measurement of methane uptake by tropical soils, J. Atmos. Chem., 1, 171-186.