Greenhouse gases from an unseen world

The list of ways we humans produce greenhouse gases is long and varied, starting with the combustion of coal, oil, and natural gas, which releases prodigious amounts of carbon dioxide. That most important greenhouse gas is also emitted by deforestation, the making of cement, the cultivation and harvesting of crops, and the raising of cattle, pigs, and chickens. Agriculture accounts for an even larger fraction (40 to 50 percent) of two other greenhouse gases, methane and nitrous oxide, released by anthropogenic sources. The list goes on.

Yet we aren’t the biggest emitters of greenhouse gases. Depending on the gas and the study, the biggest emitters are often the smallest organisms on the planet: bacteria, fungi, and a few other inhabitants of an unseen world.

Let’s start with carbon dioxide. The gas is measured in petagrams, or Pg, which is one million billion grams or a billion metric tons. Today, there is over 870 Pg of carbon in the atmosphere. The roughly 10 Pg released each year by fossil fuel use is small compared to what comes from nature, such as the roughly 50 Pg of carbon dioxide emitted by bacteria and fungi in soils each year. Bacteria in the ocean release over 145 Pg per year, although most of that carbon dioxide never gets to the atmosphere. A huge amount of carbon dioxide is discharged every year by natural processes, many of which are mediated by microbes.

More methane and nitrous oxide also come from microbes than from fossil fuels. Microbes in wetlands and other natural habitats produce about three times more methane than what is leaked from natural gas pipes and coal mines. For nitrous oxide, the difference is even greater—about a factor of 10.

Yet even though microbes release a lot of greenhouse gases, humans are the reason why greenhouse gases have been building up in the atmosphere since the early 1800s. We are the cause of the climate change now threatening the planet. Before the Industrial Revolution in the 19th century and agriculture’s Green Revolution in the 20th, levels of greenhouse gases in the atmosphere were roughly constant for eons. The carbon dioxide released by soil microbes was taken up by trees and other plants, aided by symbiotic fungi. The carbon dioxide from marine bacteria didn’t get to the atmosphere because it was used by microscopic phytoplankton during photosynthesis. Methane and nitrous oxide from microbes were degraded by other microbes before the gases could escape into air. The cycles of carbon and nitrogen were in balance.

Humans have disrupted that balance. Unlike the carbon dioxide released by soil microbes or marine bacteria, the carbon dioxide emitted by a gas-guzzling SUV or a coal power plant is not counteracted by any organism or process that soaks up the greenhouse gas. Methane has been increasing in the atmosphere not only because of natural gas use and coal operations (methane seeps out of coal mines and waste piles), but also because anthropogenic global warming has stimulated methane production by microbes in wetlands faster than microbes can degrade the gas. Global warming may lead to even higher emissions of methane and carbon dioxide as bacteria and fungi decompose organic matter previously frozen in Arctic soils and permafrost. The huge rise in nitrous oxide in the atmosphere started in the 20th century when fertilizer nitrogen became abundant and cheap after the invention of the Haber-Bosch process. The widespread application of fertilizer nitrogen is a big reason why the Green Revolution was so green. The problem is: as much as 75% of the nitrogen spread on a field isn’t used by crop plants; too much is transformed by microbes into a potent greenhouse gas, nitrous oxide. The Haber-Bosch process has doubled the supply of useable nitrogen to the biosphere, greatly upsetting the nitrogen cycle.

A big challenge facing climate-change scientists is figuring out what happens next: how will the biosphere respond to disruptions in the carbon and nitrogen cycles, global warming, and other manifestations of climate change? The answer largely depends on microbes. Understanding the most serious environmental problem facing us today depends on the smallest organisms—the microbes.

_{Featured image by querbeet via iStock.}