A carbon sink is any system, natural or artificial, that takes carbon out of the atmosphere. As of the early 2000s, artificial carbon sequestration schemes were still only in the testing and study stage. This article is concerned only with natural carbon sinks.
Carbon is removed from the atmosphere by two basic mechanisms, namely: 1) photosynthesis by green plants, which obtain carbon with which to build their tissues by breaking up carbon dioxide (CO2) molecules in the air releasing the O2; and 2) absorption of CO2 by the oceans. Natural carbon sinks remove slightly more than half of the carbon from the atmosphere that is placed there every year by the burning of fossil fuels. Scientists refer to two sinks, the land sink and the ocean sink. The land sink consists of green plants, while the ocean sink consists of direct absorption of CO2 by ocean water and secondary absorption by organisms living in the sea followed by transport of the carbon in the dead organisms to deep waters or the ocean floor.
Anthropogenic (human-caused) climate change would be much greater today if it were not for natural carbon sinks. However, human activities are imperiling the efficacy of carbon sinks. The more CO2 the oceans absorb, the more slowly they absorb CO2. Also, ozone, a common air pollutant, decreases the ability of green plants to act as carbon sinks, while cutting down forests and replacing them with other land uses (such as farmland) removes acreage from the land sink.
Historical Background and Scientific Foundations
Several hundred millions of years ago, large amounts of carbon were removed from Earth's atmosphere by large swamp forests and by tiny organisms floating in the seas. Some of the dead swamp growth accumulated in thick blankets, was covered by other sediments, and was eventually transformed into coal by geological processes. Some of the dead ocean organisms drizzled to the bottom of the sea, were buried, and were eventually transformed into oil and natural gas. These billions of tons of ancient carbon, which were collected at a time when atmospheric CO2 was far higher than today, are now being returned to Earth's atmosphere in the space of only a few centuries. Human beings are adding about 29.7 billion tons (27 billion metric tons) of CO2 to the atmosphere every year, and the rate is increasing. The two major sources of CO2 in the modern atmosphere are fossil-fuel burning and land-use changes, especially the cutting down of forests.
Starting in the late 1950s, American geochemist Charles David Keeling (1928–2005) was the first scientist to measure the steadily increasing concentration of CO2 in Earth's atmosphere. Keeling found that CO2 decreases during the growing season in the Northern Hemisphere, where most of the world's vegetation— its land sink—is located. In the spring and summer, growing plants remove CO2 from the air, while in the winter, plant decay and fuel burning release more CO2 than is absorbed. The result is a series of sawtooth spikes in atmospheric CO2 concentration on top of a steadily rising line that reflects increasing average CO2 concentration.
Thanks to natural carbon sinks, the rate at which CO2 is increasing is about half of what it would be if all the CO2 being added to the atmosphere by human beings was staying there. What is more, as the amount of CO2 released by human activities has grown, tripling from about 1950 to the present, the amount taken up by sinks has grown proportionally. The whole global system of carbon sources and sinks, which is continuously releasing and absorbing carbon around the globe, is termed the carbon cycle. Carbon sinks are only one part of the carbon cycle.
The global carbon-sink picture is difficult to characterize. The atmosphere itself is the only reservoir of carbon that is easy to study, because it is so well-mixed that an air sample taken anywhere on Earth's surface gives information about global conditions. In contrast, the upper and lower layers of the ocean mix slowly— water in the deepest parts of the North Pacific has been out of contact with the atmosphere for about 1,000 years—and the composition of the ocean is not uniform around the globe. As a result, many thousands of measurements must be taken at various depths and around the world to characterize the carbon content of the oceans. Determining how much carbon goes where is even more difficult in the case of the land sink, which changes constantly and varies over different climates and landscapes.
In the early 2000s, about a third of annual anthropogenic carbon emissions were being absorbed by the ocean sink. The ocean sink has two components: the biological pump and the solubility pump. Each component transfers or pumps CO2 out of the atmosphere. The biological pump consists of tiny marine organisms, both plants and animals, which incorporate carbon into their tissues and shells and then die, sinking to deeper waters. There they either decompose, in which case their carbon is dissolved in deep waters, or settle to the bottom as sediment, where their carbon may remain isolated from the atmosphere for much longer.
The solubility pump is driven by the overturning global circulation of the oceans. Surface waters move toward the polar regions, cooling as they go. As they cool, they become capable of absorbing more CO2 from the atmosphere. Near the poles, they sink and begin to journey back toward the tropics along the ocean floor. Eventually, after as many as 1,500 years, the water rises to the surface in the tropics and is heated. When heated, the water gives up CO2 to the atmosphere again.
The Southern Ocean, the ring-shaped body of water that surrounds Antarctica south of 60° south latitude, accounts for about half of all absorption by the oceans, that is, about 15% of annual anthropogenic carbon releases.
The land sink is about the same size as the ocean sink, but there are many uncertainties about its size and nature. For decades, most scientists assumed that the land sink's increasing uptake of CO2 was being driven by the fertilizing effect of increased CO2 in the atmosphere (most plants grow faster when there is more CO2). However, in the early 2000s, studies of forest growth in the United States showed that this fertilization effect was far too small to account for the large size of the land sink in North America. In the United States, at least, it now seems more likely that the regrowth of abandoned farmland and formerly logged lands probably accounts for the relatively large size of the land carbon sink. Increased tree growth in areas where forest fires have been suppressed also contributes.
WORDS TO KNOW
ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.
BIOSPHERE: The sum total of all life-forms on Earth and the interaction among those life-forms.
DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
OZONE: An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes, on average, less than one part per million (ppm) of the gases in the atmosphere. (Peak ozone concentration in the stratosphere can get as high as 10 ppm.) Yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface, where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems.
PHOTOSYNTHESIS: The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process, oxygen and water are released. Increased levels of carbon dioxide can increase net photosynthesis in some plants. Plants create a very important reservoir for carbon dioxide.
More than half the total (land plus ocean) sink for anthropogenic carbon is in the Northern Hemisphere, and most of this northern-hemispheric sink is terrestrial (on land). Partly due to global warming, which has made for longer growing seasons, the amount of carbon being taken up by the terrestrial biosphere increased from about 220 million tons (200 million metric tons) per year in the 1980s, with large uncertainty, to about 1.5 billion tons (1.4 billion metric tons) per year in the 1990s.
Impacts and Issues
Despite the enhancement of the land carbon sink in the 1980s and 1990s due to longer growing seasons, scientists predict that the negative effects of climate change on the land biosphere will soon be dominant, and that global warming will slow CO2 uptake by both the ocean and land carbon sinks. This will increase the fraction of anthropogenic CO2 that remains in the atmosphere, making climate change more severe and rapid, other factors being equal.
The global scientific consensus as expressed in 2007 by the United Nations' Intergovernmental Panel on Climate Change (IPCC) is that it is more than 90% likely that terrestrial ecosystems will become net sources of CO2 between 2050 and 2100. That is, the land carbon sink will shrink, and land-based carbon sources, such as deforestation, will grow until they are emitting more CO2 than the land sink is absorbing. Deforestation, higher temperatures, and shifts in rainfall patterns will all contribute to the shrinkage of the land sink. Ozone (O3) pollution is also reducing the efficacy of the land sink by slowing plant growth.
The ocean sink will slow its absorption of carbon as the amount of CO2 dissolved in the water increases and lowers the water's ability to take up still more CO2. In 2007, an international science team announced that the Southern Ocean's absorption of CO2 has decreased by about 15% per decade since 1981. This decrease was caused by global climate change, but not (yet) by increased carbon dissolved in the ocean; rather, increased wind strength due to climate shifts was the cause, altering ocean mixing patterns and decreasing carbon uptake.
Primary Source Connection
Forests, prairies, marshlands, and other densely vegetated areas that compose “carbon sinks” help Earth reabsorb carbon dioxide. John Roach, a correspondent for National Geographic News, reports on new research on the North American carbon sink and why it may not help offset human-made emissions in the future.
STUDIES MEASURE CAPACITY OF ‘CARBON SINKS.’
After years of wide disagreement, scientists are getting a better grip on how much carbon Earth's forests and other biological components suck out of the atmosphere, thus acting as “carbon sinks.” New research in this area may be highly useful in efforts to devise international strategies to address global warming.
The emission of carbon dioxide from the combustion of fossil fuels is the leading cause of the buildup of greenhouse gases in the atmosphere, which many people believe is the main culprit behind an increase in Earth's temperatures.
For a long time, scientists have known that forests, crops, soils, and other organic matter soak up some of that carbon, thereby slowing down the rate of global warming. Yet their calculations of how much carbon is absorbed have differed, in some cases significantly.
A team of scientists led by Stephen Pacala, a professor of ecology and evolutionary biology at Princeton University in New Jersey, set out to resolve this discrepancy in calculations. Their research is reported in the June 22 issue of Science.
Different Measuring Techniques
While some carbon is absorbed by organic matter such as trees and shrubs, carbon is also regularly emitted into the atmosphere by activities on land such as the burning of fossil fuels.
Researchers' lack of agreement on how much carbon is “stored” has been rooted in the use of two different methods of measurement—one atmosphere based, the other land based.
The first method involves measuring concentrations of carbon dioxide in the air as the air moves across land-masses from Point A to Point B. The second method entails making an inventory of all the carbon in a given area of ground and calculating the difference between the levels of carbon recorded from year to year.
Although there is wide variation among different atmospheric models of carbon measurement, their results have consistently indicated that higher levels of carbon are absorbed than the land-based models show.
Pacala said his team's land-based analysis was more thorough than earlier studies. “We did the first exhaustive analysis of the land sink,” he said.
Previous land-based models inventoried mainly the amount of carbon absorbed by trees, he explained. He and his colleagues included measures of carbon absorbed by landfills, soils, houses, and even silt at the bottom of reservoirs.
“We found out that the land sink was bigger than had been reported by other analyses, about twice as big, and the atmosphere [models] gave numbers that were consistent,” he said.
The researchers used their results to help answer a major question that has been a subject of much contention:How big is the entire “carbon sink” of the continental United States?
According to their findings, the scientists estimate that U.S. forests and other terrestrial components absorb from one-third to two-thirds of a billion tons of carbon each year.
At the same time, reliable figures indicate that the United States emits more than two to four times that amount of carbon each year, about 1.4 billion tons.
Taking into account the carbon sink effect, 800 million to 1.1 billion tons of carbon accumulates annually in the atmosphere, the researchers say. This refutes the idea that the U.S carbon sink is big enough to equal the amount of carbon that U.S. factories emit through the burning of fossil fuels, as some studies have concluded.
The results of the Princeton-led study are particularly interesting because the 23 scientists who participated in the research and agreed on the conclusions initially held strongly differing views about the size of the U.S. carbon sink.
Pacala and his colleagues say the main reason the United States is drawing in a large volume of carbon is because many forests and areas of land that were logged or converted to agriculture in the last 100 years are now recovering with the growth of new vegetation.
These trees and shrubs absorb carbon dioxide from the air and channel it into the growth of massive tree trunks, branches, and foliage. This, in turn, gradually expands the overall size of the U.S. carbon sink.
Pacala emphasizes, however, that the U.S. absorption of carbon does not fully offset the emissions of carbon from fossil fuels and should not be seen as a license to release more carbon. A large part of the current sink effect, he said, is the land re-absorbing large quantities of carbon that were released during heavy farming and logging of the past.
“When we chopped down the forests, we released carbon trapped in the trees into the atmosphere. When we plowed up the prairies, we released carbon from the grasslands and soils into the atmosphere,” said Pacala. “Now the ecosystem is taking some of that back.” But, he added, the sink effect will steadily decrease and eventually disappear—as U.S. ecosystems complete their recovery from past land use.
“The carbon sinks are going to decrease at the same time as our fossil fuel emissions increase,” he said. “Thus, the greenhouse problem is going to get worse faster than we expected.”
Carbon Sink in China
In a separate study in Science, researchers reported on a similar carbon sink effect in China, which they attribute to the regrowth of logged forests and intensive planting of new forests.
Jingyun Fang, an ecology professor at Peking University in Beijing, and his colleagues noted that Chinese forests were heavily exploited from 1949 to the end of the 1970s. Since then, however, the government has undertaken wide-scale forest planting and reforestation, mainly to combat erosion, flooding, desertification, and loss of biodiversity.
An unintended consequence of this increase in vegetation was the growth of a carbon sink that is estimated to be on par with that of North American forests.
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