Ocean's capacity to store carbon may alter because of climate change

Ocean's capacity to store carbon may alter because of climate change

All life is based on the element carbon. Carbon is the major chemical constituent of most organic matter, from fossil fuels to the complex molecules (DNA and RNA) that control genetic reproduction in organisms. Yet by weight, carbon is not one of the most abundant elements within the Earth's crust. In fact, the lithosphere is only 0.032% carbon by weight. In comparison, oxygen and silicon respectively make up 45.2% and 29.4% of the Earth's surface rocks.

Carbon is stored on our planet in the following major sinks (Figure 9r-1 and Table 9r-1): (1) as organic molecules in living and dead organisms found in the biosphere; (2) as the gas carbon dioxide in the atmosphere; (3) as organic matter in soils; (4) in the lithosphere as fossil fuels and sedimentary rock deposits such as limestone, dolomite and chalk; and (5) in the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate shells in marine organisms.

Figure 9r-1: Carbon cycle.

Table 9r-1: Estimated major stores of carbon on the Earth.

Sink	Amount in Billions of Metric Tons
Atmosphere	578 (as of 1700) - 766 (as of 1999)
Soil Organic Matter	1500 to 1600
Ocean	38,000 to 40,000
Marine Sediments and Sedimentary Rocks	66,000,000 to 100,000,000
Terrestrial Plants	540 to 610
Fossil Fuel Deposits	4000

Ecosystems gain most of their carbon dioxide from the atmosphere. A number of autotrophic organisms have specialized mechanisms that allow for absorption of this gas into their cells. With the addition of water and energy from solar radiation, these organisms use photosynthesis to chemically convert the carbon dioxide to carbon-based sugar molecules. These molecules can then be chemically modified by these organisms through the metabolic addition of other elements to produce more complex compounds like proteins, cellulose, and amino acids. Some of the organic matter produced in plants is passed down to heterotrophic animals through consumption.

Carbon dioxide enters the waters of the ocean by simple diffusion. Once dissolved in seawater, the carbon dioxide can remain as is or can be converted into carbonate (CO₃^-2) or bicarbonate (HCO₃^-). Certain forms of sea life biologically fix bicarbonate with calcium (Ca⁺²) to produce calcium carbonate (CaCO₃). This substance is used to produce shells and other body parts by organisms such as coral, clams, oysters, some protozoa, and some algae. When these organisms die, their shells and body parts sink to the ocean floor where they accumulate as carbonate-rich deposits. After long periods of time, these deposits are physically and chemically altered into sedimentary rocks. Ocean deposits are by far the biggest sink of carbon on the planet (Table 9r-1).

Carbon is released from ecosystems as carbon dioxide gas by the process of respiration. Respiration takes place in both plants and animals and involves the breakdown of carbon-based organic molecules into carbon dioxide gas and some other compound by products. The detritus food chain contains a number of organisms whose primary ecological role is the decomposition of organic matter into its abiotic components.

Over the several billion years of geologic history, the quantity of carbon dioxide found in the atmosphere has been steadily decreasing. Researchers theorized that this change is in response to an increase in the Sun's output over the same time period. Higher levels of carbon dioxide helped regulate the Earth's temperature to levels slightly higher than what is perceived today. These moderate temperatures allowed for the flourishing of plant life despite the lower output of solar radiation. An enhanced greenhouse effect, due to the greater concentration of carbon dioxide gas in the atmosphere, supplemented the production of heat energy through higher levels of longwave counter-radiation. As the Sun grew more intense, several biological mechanisms gradually locked some of the atmospheric carbon dioxide into fossil fuels and sedimentary rock. In summary, this regulating process has kept the Earth's global average temperature essentially constant over time. Some scientists suggest that this phenomena is proof for the Gaia hypothesis.

Carbon is stored in the lithosphere in both inorganic and organic forms. Inorganic deposits of carbon in the lithosphere include fossil fuels like coal, oil, and natural gas, oil shale, and carbonate based sedimentary deposits like limestone. Organic forms of carbon in the lithosphere include litter, organic matter, and humic substances found in soils. Some carbon dioxide is released from the interior of the lithosphere by volcanoes. Carbon dioxide released by volcanoes enters the lower lithosphere when carbon-rich sediments and sedimentary rocks are subducted and partially melted beneath tectonic boundary zones.

Since the Industrial Revolution, humans have greatly increased the quantity of carbon dioxide found in the Earth's atmosphere and oceans. Atmospheric levels have increased by over 30%, from about 275 parts per million (ppm) in the early 1700s to just over 365 PPM today. Scientists estimate that future atmospheric levels of carbon dioxide could reach an amount between 450 to 600 PPM by the year 2100. The major sources of this gas due to human activities include fossil fuel combustion and the modification of natural plant cover found in grassland, woodland, and forested ecosystems. Emissions from fossil fuel combustion account for about 65% of the additional carbon dioxide currently found in the Earth's atmosphere. The other 35% is derived from deforestation and the conversion of natural ecosystems into agricultural systems. Researchers have shown that natural ecosystems can store between 20 to 100 times more carbon dioxide than agricultural land-use types.

A study released today provides some of the first solid evidence that warming-induced changes in ocean circulation at the end of the last Ice Age caused vast quantities of ancient carbon dioxide to belch from the deep sea into the atmosphere. Scientists believe the carbon dioxide (CO2) releases helped propel the world into further warming. The research is significant to understand how oceans with their large carbon storage capacity will react to human induced climate change.

Atmospheric CO2, also produced by burning of fossil fuels, is thought to be largely responsible for current warming. However, scientists have known for some time that the gas also goes through natural cycles. By far most of the world's mobile carbon is stored in the oceans - 40 trillion metric tons, or 15 times more than in air, soil and water combined. But how this vast marine reservoir interacts with the atmosphere has been a subject of debate for the last 25 years.

The new study shows carbon that had built up in the ocean over millennia was released in two big pulses at about 18,000 years ago and 13,000 years ago, says Dr. Thomas Marchitto of the University of Colorado at Boulder, who jointly led the study with colleague Dr. Scott Lehman.This is some of the clearest evidence yet that the enormous carbon release into the atmosphere during the last deglaciation was triggered by abrupt changes in deep ocean circulation.

The study, done by researchers at the University of Colorado, Kent State University and Columbia University's Lamont-Doherty Earth Observatory, appears in the May 10 advance online version of the leading journal Science.

While much of the CO2 released by the ocean after the end of the last ice age about 18,000 years ago was taken up by the re-growth of forests in areas previously covered by ice sheets, enough remained in the atmosphere to pump up CO2 concentrations significantly, the authors said. Today, CO2 levels are higher than at any time in at least the past 650,000 years because of increased fossil fuel burning.

“The timing of the major CO2 release after the last ice age corresponds closely with deep sea circulation changes caused by ice melting in the North Atlantic at that time. So our study really underscores ongoing concerns about the ocean’s capacity to take up fossil fuel CO2 in the future, since continued warming will almost certainly impact the mode and speed of ocean circulation.” - Dr. Scott Lehman, University of Colorado at Boulder.

The researchers found the evidence in a core of Pacific Ocean sediment brought up from 705 meters off the coast of Baja California, Mexico. The core held the remains of bottom-dwelling protozoa called foraminifera, which take up carbon from surrounding water and use it to build their shells. The isotope carbon 14 - normally used to date organic remains such as wood and bones - can also be used to date the water in which the foraminifera grew . Going back through layers built up over the past 38,000 years, the researchers found the shells contained expected levels of C14 in all but two brief periods, beginning roughly 18,000 years and 13,000 years ago. That meant the protozoa were using older sources of carbon, long isolated from the atmosphere:

The carbon could come from only one place: upwelling of the deep sea, from depths of 3 kilometers (nearly two miles) or more. The researchers believe the water came not from the Pacific, but from the faraway Antarctic Ocean--the only part of the world where great upwelling can occur, due to the bottom topography and wind patterns. Most of the rising C02 probably poured out into the air in southern latitudes, but some carbon-rich water traveled on currents at intermediate depths to the north, where the foraminifera recorded its C14 signature.

The upwelling and release of this carbon dioxide matches well with rapid warming and rises in atmospheric CO2 shown in glacial ice cores from Antarctica and other far-flung records. The researchers believe that largely as a result of these episodes, CO2 in the atmosphere went from 190 parts per million (ppm) during glacial times to about 270 ppm, and remained at that level until recently. A similar but much more rapid rise, to 380 ppm, has taken place since the Industrial Revolution - most of it in the last few decades. Both rises almost certainly stoked climate warming.

Exactly what caused the upwelling is not clear, but many scientists believe the world was already undergoing a natural warming cycle, possibly due to a slight periodic change in earth's orbit. This suddenly ended the last Ice Age, in turn changing ocean currents and wind patterns. The hypothesis favored by paper's authors is that sudden disintegration of northern ice sheets during this initial warming slowed or halted deep Atlantic Ocean circulation. This in turn warmed the Antarctic, causing massive retreats of sea ice and allowing deep Antarctic waters to surface. Thus, it is possible that the signal detected in the Pacific ultimately originated on the other side of the world.

"Once the CO2 started rising, it probably helped the warming process along - but exactly how much, we can't say," said Robert Anderson, a Lamont-Doherty expert in ocean circulation who was not involved in the study. "And there is still huge uncertainty as to how the oceans will respond to current warming." Anderson says the study should be a wake-up call to the scientific community to expand studies of the oceans' relationship to climate change.

“If the oceans were not such a large storage ‘sink’ for carbon, atmospheric CO2 increases in recent decades would be considerably higher,” Lehman says. “Since the uptake of CO2 on Earth’s land surface is being offset almost entirely by the cutting and burning of forests, any decrease in the uptake of fossil fuel CO2 by the world’s oceans could pose some very serious problems,” he says.

“This study provides strong indicators of just how intimately coupled the connection between the ocean and atmosphere can be,” Ortiz says. “The findings should give us pause to consider the impact that fossil fuel release will have on ocean circulation and future climate change.”

“When the ocean circulation system changes, it alters how carbon-rich deep water rises to the surface to release its carbon to the atmosphere,” says the University of Colorado at Boulder’s Dr. James White, a climate scientist who was not involved in the study. “This is important not only for understanding why glacial times came and went in the past, but it is crucial information we need to understand how the oceans will respond to future climate change.”

Studies in the past several years have shown sharp declines in Arctic sea ice in recent decades and a loss in ice mass from Greenland, which some believe could combine to alter North Atlantic circulation and disrupt ocean circulation patterns worldwide.