Carbon Sinks

According to the World Resources Institute, the world's forests are natural regulators of CO2 in the atmosphere. While forests are effective sinks for CO2, if they are being cut down, then they cannot be counted on in the future to absorb the CO2 being released by fossil fuels and other sources.

According to a study conducted by NASA, scientists estimate that there are roughly 1.1 to 2.2 billion tons (1 to 2 billion metric tons) of carbon generated each year that are "missing" from the global carbon budget. They have determined that 15 to 30 percent of the carbon released into the atmosphere from the burning of fossil fuels is currently unaccounted for in the Earth's carbon budget.

According to Steven C. Wofsy, an environmental scientist at Harvard University, scientists assume that the "missing carbon" is being absorbed by vegetation, and it is important to be able to predict how much CO2 is generated so that questions such as how much will temperatures rise and how will this affect humans and ecosystems can be answered.

Trees consist of about 50 percent carbon. Immense amounts can be stored in forests, such as these giant redwoods in California. (NPS)

Scientists to date have calculated that globally humans release around 7 billion tons of carbon each year. Of that amount, 3 billion tons stay long-term in the atmosphere, and 2 billion tons are absorbed into the oceans. What they do not know is where the rest goes; they assume it is stored long-term in vegetation. And if it is stored in vegetation, are the 2 billion tons stored temporarily or permanently?

In order to answer that question, NASA dispatched an interdisciplinary scientific team called the Boreal Ecosystem-Atmosphere Study (BOREAS) from 1994 to 1997 to cover two Canadian provinces. Teams from 85 nations participated in a quest to find an answer to this question.

CO2 is measured at different latitudes worldwide, and the data is entered into models that track the circulation of air over a period of time. Through analysis of the models, NASA scientists determined that most of the atmospheric CO2 was being absorbed somewhere above 40°N latitude in the Northern Hemisphere. NASA scientists also estimate that the boreal forest covers 21 percent of the Earth's forested land surface, contains 13 percent of all the carbon stored in biomass, and holds about 43 percent of all the world's carbon that is stored in soil.

According to Forrest G. Hall, one of the BOREAS project scientists, "For the past 7,000 years, the boreal forest floor has been accumulating carbon at a rate of about 30 grams (roughly one ounce) per square meter per year. When you walk through the Canadian boreal forest, you can literally go from ankle-deep to in over your head in carbon litter. In some places where there were once holes or ravines, we have measured peat several meters deep."

Hall also says that the boreal ecosystem has been taking in an average of 0.66 billion tons (0.6 billion metric tons) of carbon per year for roughly the last 7,000 years. As straightforward as all this sounds, however, it is not. The boreal forest is both a sink (storage area) and a source of carbon because it both absorbs and releases CO2. According to Hall, on whether the boreal forests could be the net sink of carbon, "We won't know until we add up the contributions from all the kinds of forest that make up the boreal ecosystem. That will require satellite-derived maps of the entire circumpolar boreal region that enable us to sort out the different types of forests and assess their productivity."

What they determined was surprising. From late May through July (spring/summer), the boreal forest "inhaled" 1 to 1.5 grams of carbon per square meter per day. In August and September (the hottest, driest period), carbon intake was almost zero. In October, the forest "exhaled" carbon back into the atmosphere at 0.6 to 0.8 grams per square meter per day.

According to Steven Wofsy, "We found that what's going on in the soils is more important than what is going on in the trees. We found that the boreal peat is drying and thawing out and that the soils will release CO2 faster than the trees can absorb it."

Hall agrees, stating, "If the boreal soils were to dry, it might increase soil carbon decomposition, but the carbon loss might be offset by the intake of carbon by trees as they grow."

The general trend they observed was that years with warmed springs and falls were better for plant uptake of carbon but resulted in increased rates of carbon loss. It is during these years that boreal forests become a net source rather than a sink. The scientists on the team think that the boreal carbon budget is very sensitive to climate change, and subtle changes in the climate can make the difference between the boreal forest as a carbon source or sink; this difference can show up on a global scale.

This study has major implications for the global climate. Because of boreal forests' climate sensitivity, it responds quickly to changes in climate. In addition, temperature increases are happening the fastest at the high latitude continental interiors—exactly where most of the boreal forests are located. Over the past 40 years, average temperatures have increased by as much as 2.08°F (1.25°C) per decade. Therefore, if the Earth's "missing carbon" is being stored there, and if the recent warming trend is turning the boreal forests into a source of carbon, then there could be an alarming rate of carbon buildup in the atmosphere. In fact, scientists are concerned that by 2100, the amount of CO2 in the atmosphere might double preindustrial levels—to 560 ppm (in 1990 it was 353 ppm). If this kind of warming happened, it would thaw the frozen boreal soil and permafrost, releasing CO2 and methane, thereby increasing levels in the atmosphere.

Carbon sinks have been proposed as one way to proactively tackle global warming by "soaking" up excess CO2. One alternative—planting new forests in deforested areas (reforestation)—is a popular one in locations with huge logging industries and large forests, such as the United States, Canada, some Latin American nations, and Indonesia.

Some environmentalists are critical of simply relying on reforestation as a solution for eliminating large areas of natural forest, however. According to Global Issues Organization, this approach legitimizes continued destruction of old growth and pristine forests that have diverse, rich, well-established ecosystems. The organization also contends that it only provides a "quick fix" but does not directly solve the problem by reducing CO2 emissions to begin with.

Global Issues also points out that the use of carbon sinks in general is a major "loophole" in the Kyoto Protocol—that if carbon sinks can be counted toward emissions reductions credit, then industrialized countries would be able to meet their commitments while reducing emissions by less than would otherwise be required. Carbon sinks also have the caveat that when vegetation naturally dies or is burned, it releases all the stored carbon to the atmosphere—which turns it quickly from a sink to a source without much warning.

According to Simon Retallack, author of The Kyoto Loopholes, "As biologists point out, there is not yet enough data on natural carbon cycling to establish full accounting and verification procedures for carbon sinks. The science simply does not exist to enable prediction of exactly how much carbon is being absorbed by a country's sinks and whether the carbon moving into forests or soils will actually stay there."

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