Is Photosynthesis the Best Defense Against Climate Change?

What's to come?
July 18 2013 7:29 AM

Reach for the Sun

Photosynthesis, in the form of biochar, may be one of our best defenses against climate change.

Visitors admire fully bloomed lotus flowers at the Kodaihasu-no-sato, the ancient lotus garden, at Gyoda city in Saitama prefecture, north of Tokyo on July 14, 2013.
Visitors admire fully bloomed lotus flowers at the Kodaihasu-no-sato, an ancient lotus garden north of Tokyo, on July 14, 2013. Photosynthesis-based methods of carbon extraction can be a lot prettier than, say, clean coal.

Photo by Yoshikazu Tsuno/AFP/Getty

This article arises from Future Tense, a collaboration among Arizona State University, the New America Foundation, and Slate. On July 25, Future Tense will be hosting an event on agriculture’s role in climate change at the New America Foundation in Washington, D.C. For more information and to RSVP, visit the New America Foundation website.

A gigantic, steaming-hot mound of compost is not the first place most people would search for a solution to climate change, but the hour is getting very late. “The world experienced unprecedented high-impact climate extremes during the 2001-2010 decade,” declares a new report from the United Nations’ World Meteorological Organization, which added that the decade was “the warmest since the start of modern measurements in 1850.” Among those extreme events: the European heat wave of 2003, which in a mere six weeks caused 71,449 excess deaths, according to a study sponsored by the European Union. In the United States alone, 2012 brought the hottest summer on record, the worst drought in 50 years and Hurricane Sandy. Besides the loss of life, climate-related disasters cost the United States some $140 billion in 2012, a study by the Natural Resources Defense Council concluded.

We can expect to see more climate-related catastrophes soon. In May scientists announced that carbon dioxide had reached 400 parts per million in the atmosphere. Meanwhile, humanity is raising the level by about 2 parts per million a year by burning fossil fuels, cutting down forests, and other activities.

At the moment, climate policy focuses overwhelmingly on the 2 ppm part of the problem while ignoring the 400 ppm part. Thus in his landmark climate speech on June 25, President Obama touted his administration’s doubling of fuel efficiency standards for vehicles as a major advance in the fight to preserve a livable planet for our children. In Europe, Germany and Denmark are leaving coal behind in favor of generating electricity with wind and solar. But such mitigation measures aim only to limit new emissions of greenhouse gases.


That is no longer sufficient. The 2 ppm of annual emissions being targeted by conventional mitigation efforts are not what are causing the “unprecedented” number of extreme climate events. The bigger culprit by far are the 400 ppm of carbon dioxide that are already in the atmosphere. As long as those 400 ppm remain in place, the planet will keep warming and unleashing more extreme climate events. Even if we slashed annual emissions to zero overnight, the physical inertia of the climate system would keep global temperatures rising for 30 more years.

We need a new paradigm: If humanity is to avoid a future in which the deadly heat waves, floods, and droughts of recent years become normal, we must lower the existing level of carbon dioxide in the atmosphere. To be sure, reducing additional annual emissions and adapting to climate change must remain vital priorities, but the extraction of carbon dioxide from the atmosphere has now become an urgent necessity.

Under this new paradigm, one of the most promising means of extracting atmospheric carbon dioxide is also one of the most common processes on Earth: photosynthesis.

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Which is how I came to find myself plunged forearm-deep into the aforementioned mound of compost. It was a truly massive heap, nearly the length of a football field, 5 feet tall and 10 feet wide, and a second equally large pile lay nearby. It all belonged to Cornell University, one of the powerhouses of agricultural research in the United States. Michael P. Hoffmann, the associate dean of Cornell’s College of Agriculture and Life Sciences, told me it was comprised mainly of food scraps from Cornell’s dining halls and detritus from its groundskeeping operations.

The author reaches his hand into a pile of compost.
The author reaches his hand into a pile of compost.

Courtesy of Mark Hertsgaard

“You don’t want to leave your hand in there too long,” Hoffmann cautioned as I felt around inside the steaming mass of brown. Sure enough, although it was a cool, cloudy day, my forearm soon felt almost uncomfortably warm. “The microbes in there generate a fair amount of heat as they break down the organic materials,” he explained.

Compost is but one of the materials that can be used to produce biochar, a substance that a small but growing number of scientists and private companies believe could enable extraction of carbon dioxide from the atmosphere at a meaningful scale. Biochar, which is basically a fancy scientific name for charcoal, is produced when plant matter—tree leaves, branches and roots, cornstalks, rice husks, peanut shells—or other organic material is heated in a low-oxygen environment (so it doesn’t catch fire). Like compost, all of these materials contain carbon: The plants inhaled it, as carbon dioxide, in the process of photosynthesis. Inserting biochar in soil therefore has the effect of removing carbon dioxide from the atmosphere and storing it underground, where it will not contribute to global warming for hundreds of years.

Johannes Lehmann, a professor of agricultural science at Cornell, is one of the world’s foremost experts on biochar. He has calculated that if biochar were added to 10 percent of global cropland, it would store 29 billion tons of carbon dioxide equivalent—an amount roughly equal to humanity’s annual greenhouse gas emissions. This approach would take advantage of a physical reality often overlooked in climate policy discussions: the capacity of the Earth’s plants and soils to serve as a climate “sink,” absorbing carbon that otherwise would be released into the atmosphere and accelerate global warming. Oceans have been the most important sink to date, but their absorption of CO2 is acidifying the sea—threatening the marine food chain—and raising water temperatures, which is causing sea levels to rise (because warm water expands). Meanwhile, the Earth’s plants and soils already hold three times as much carbon as the atmosphere does, and scientists believe that they could hold a great deal more without upsetting the balance of natural systems.

Using photosynthesis and agriculture to extract carbon should not be confused with other methods that sound similar, such as “carbon capture and sequestration.” CCS, as experts call it, is a technology that would capture carbon dioxide released when a power plant burned coal (or, in theory, other fossil fuels) to generate electricity. A filter would collect the CO2 before it exited the smokestack; the CO2 would then be transformed into a solid and stored underground. CCS assumes that coal burning would continue; the CCS technology would simply cancel out most of the CO2 emissions this coal burning would produce—and that’s assuming the technology will actually work. So far, no nation on Earth has managed to operate a commercially viable CCS plant, despite an estimated $25 billion in subsidies.

By contrast, biochar and other photosynthesis-based methods of carbon extraction take advantage of natural processes that already help to regulate planetary health. "What we're really doing is bio-mimicry of fire," says Dr. David Shearer, CEO of Full Circle Biochar, the company that designed and built the kiln Lehmann uses at Cornell. According to Shearer:

"Historically it was fire that helped drive the carbon cycle on Earth, burning plants and trees and returning their embedded carbon to the soil in the form of charcoal. Contemporary societies have greatly restricted the use of fire. Producing biochar is a way to begin restoring the proper balance by catalyzing soil regeneration through the addition of biochar to soils."