10 CALIFORNIA EDUCATION AND THE ENVIRONMENT INITIATIVE I Unit E.7.b. I The Life and Times of Carbon

The merging of these sciences is called “biogeochemistry.” When biogeo-chemists examine the carbon cycle, they look at the history of carbon levels within the major reservoirs. For nearly half a million years, carbon

Teacher’s Background

Ocean

Carbon is the fourth most abundant element in the universe, and it is present in all life forms The attributes of the remarkable carbon atom make possible the existence of all

organic compounds essential to life on Earth Why is carbon so important to life? Carbon forms more compounds than any other element except hydrogen: scientists have discovered more than 10 million such compounds, and an estimated 10,000 new ones are reported each year

Carbon and the Carbon CycleCarbon is the second most

abundant element by mass in the human body. It is no wonder carbon is called “the building block of life”; carbon acts as the backbone of sugars, lipids, enzymes, and proteins. Living organisms use these molecules to store energy, form cell membranes, catalyze biological processes, and do everything else that defines life. Carbon-based molecules can exist as gas, liquid, or solid. The carbon cycle describes how natural processes that use carbon-based molecules move

from one portion of the ecosystem to another. Carbon atoms continually move through living organisms, the oceans, the atmospheres, and the crust of the planet.

The global carbon cycle involves the flow and exchange of carbon through different physical, chemical, and biological processes that transfer carbon between four major reservoirs: the atmosphere; the oceans; ter-restrial plants; and rocks, soils, and sediments. To follow the carbon cycle, knowledge of biological, geological, and chemical processes is necessary.

Soil

The Life and Times of Carbon

CALIFORNIA EDUCATION AND THE ENVIRONMENT INITIATIVE I Unit E.7.b. I The Life and Times of Carbon 11

the sediments continue to accumulate. Geologic processes “permanently” remove this carbon through subduc-tion of deep ocean plates into the mantle of Earth.

Human Activities Affect the Flow of Carbon

Prior to the Industrial Revolution (pre-1750), the carbon cycle was balanced by equal input and output between the atmosphere and the oceans and terrestrial environments. This natural carbon flow from one reservoir to another had, for the preceding 10,000 years, maintained atmospheric CO2 concentrations between 260 and 280 parts per million (ppm). Tremendous human population increases and industrial growth from the mid-18th century to the present have accelerated the degree to which people influence land and atmospheric reservoirs of carbon. Land use changes (for instance, soil and sediment disturbance due to deforestation) and the combustion of fossil fuels over the past 200 years have created measurable changes to two of the four major carbon reser-voirs: the atmosphere and terrestrial plants. The atmosphere now holds 36% more CO2 today than it did 250 years ago, around 380 ppm in 2005.

This level is the highest in the past 420,000 years. Data suggest this increase is causing an accelerated global warming trend.

Oceans, soils, and forests all offer some potential to be managed as a sink; that is, to promote net carbon sequestration. The role of forests in carbon sequestration probably is best understood and appears to offer the greatest near-term potential for human management. Unlike many plants and most crops, which have short lives or release much of their carbon at the end of each season, forest biomass accumulates carbon over decades and centuries. Furthermore, the carbon accumulation potential in forests is large enough that forests offer the possibility of sequestering significant amounts of additional carbon in rela-tively short periods, such as decades.

Because of their high biomass, forests are particularly efficient at removing CO2 from the atmosphere and storing it in biomass. For instance, the Amazon rainforest alone is estimated to process as much as 20% of Earth’s atmospheric CO2 on an annual basis. Yet, human civilizations have destroyed more forest than they

has been accumulating in three of these reservoirs: in the ocean, on land in forests, and in soils and sediments. A reservoir that is taking up more carbon than it is releasing is called a

“carbon sink.”

Carbon SinksThe conversion of carbon dioxide

into living matter and then back is the main pathway of the carbon cycle. Plants draw about one-quarter of the carbon dioxide out of the atmosphere and photosynthesize it into carbo-hydrates. Some of the carbohydrates are consumed by plant respiration and the rest is used to build plant tissue and growth. Animals consume the carbohydrates and return carbon dioxide to the atmosphere during respiration. Carbohydrates are oxidized and returned to the atmosphere by soil microorganisms decomposing dead animal and plant remains (soil respiration). Soil carbon, or soil organic carbon (SOC), is the carbon stored within soil. It is a key ingredient in soil organic matter.

Another quarter of atmospheric carbon dioxide is absorbed by the world’s oceans through direct air-water exchange. Surface water near the poles is cool and more soluble for carbon dioxide. The cool water sinks and joins with the ocean’s thermo-haline circulation, which transports dense surface water toward the ocean’s interior. Marine organisms form tissue containing carbon, and some also form carbonate shells from carbon extracted from the air. These processes remove gaseous CO2 from the atmosphere and store carbon in solid form in the oceans and terrestrial plants as biomass. As plants and corals die, they decay into smaller particles that are either used by other organisms, or are deposited as particles in sediment that accumu-lates on land and on the ocean floor. Over thousands and millions of years, Amazon rainforest

12 CALIFORNIA EDUCATION AND THE ENVIRONMENT INITIATIVE I Unit E.7.b. I The Life and Times of Carbon

have replaced. It is estimated that up to 80% of the world’s forests have been destroyed by human activities throughout history and to the present day. Recent recognition of this situa-tion has spurred aforestation efforts in North America, China, Europe, and Africa. Aforestation is the plant-ing of trees in non-forest areas, or the replanting of trees in deforested areas.

What is notable is that the release of carbon from fossil fuels, reduction in land biomass as a sink for atmospheric carbon, and the subsequent accumulation of CO2 in the atmosphere have been strikingly swift: what took millions of years to become sequestered in forests, ocean, land, and fossil fuel deposits has been released by human activities in just 200 years. The biological process of photosynthesis and the geological processes of sedimentation and subduction that naturally remove CO2 from the atmosphere and store it in terrestrial plants, the ocean, and rocks, soils, and sediments cannot keep pace with the additional burden of CO2 released to the atmosphere by human activities. By definition, the atmosphere has become a new carbon sink.

Scientists studying tiny air bubbles in ice layers in ancient glaciers and

chemical clues in ocean sediments found that during periods when more CO2 and other signature gases, such as methane were in our atmosphere, Earth temperatures were higher. It is important to note that it is not clear from the ice core record whether temperature changes or CO2 changes came first. Most scientists studying paleoclimate currently think that the temperature change triggered increases in atmospheric CO2 concentration, which in turn led to additional changes in temperature (for example, that the coincidence of atmospheric CO2 and temperature in ice core data is evidence of natural feedback loops in the climate system).

Because trace levels of CO2 and other greenhouse gases (GHGs) create Earth’s natural greenhouse effect, scientists suggest that relatively small changes in carbon reservoirs can lead to a “tipping point” that may result in abrupt climate change. It is important to consider that what we think we know about the relationship between the carbon cycle and climate may not help us in understanding what our climate future may look like. It is possible that increasing concentrations of atmospheric CO2 might stimulate plant growth and thereby result in more carbon stored

in terrestrial or ocean sinks. The logic may seem straightforward: plants need atmospheric carbon dioxide to produce food, and by emitting more CO2 into the air, our cars and factories create new sources of plant nutrition that will cause some crops and trees to grow bigger and faster. However, an unprecedented three-year experiment conducted at Stanford University is raising ques-tions about that long-held assumption. Writing in the journal Science, researchers concluded that elevated atmospheric CO2 actually reduces plant growth when combined with other likely consequences of climate change—namely, higher temperatures, increased precipitation or increased nitrogen deposits in the soil. It is also possible that climate change may result in drought, which in turn may reduce terrestrial forest and vegeta-tion growth. In short, we are not sure how the interactions between natural processes and the carbon cycle will play out. Over recent decades, both terrestrial ecosystems and the ocean have acted as sinks for some of the

“extra” carbon people have released from burning fossil fuels and land use changes. Without this uptake, scien-tists say atmospheric levels of CO2 would be twice as high as the amount we are currently measuring. Whether these reservoirs will continue to absorb and store our extra carbon emissions remains unknown.

Countries That Emit High Levels of Greenhouse Gases

The United States is one of the largest sources of greenhouse gases; China is the largest. The United States has the largest economy in the world and meets 85% of its energy needs through burning fossil fuels. The fossil fuels that Americans use to produce electricity accounts for 42% of total CO2 emissions, and transportation another 31%. In

Ice core

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California, 23% of greenhouse gases come from electricity, and 40% from transportation.

The 30 countries with major economies are among the biggest GHG emitters. The emissions from these countries comprise 78% of worldwide CO2 emissions.

Recently, the total CO2 emissions produced by developing countries (195 countries) surpassed that of the developed world, largely because of strong economic growth and increas-ing use of coal to provide energy to sustain that growth. The recent industrial rise of Asian economies, led by China and India, underlies this economic development. Current global CO2 emissions of 28.1 billion metric tons (2005) are projected to rise to 34.3 billion metric tons in 2015 and to 42.3 billion metric tons in 2030. By 2030, China and India alone are projected to account for 34% of global CO2 emissions.

Addressing the Increase of Atmospheric CO2

Current issues with atmospheric CO2 levels, concerns about global cli-mate change, the effect of our “carbon footprint,” and the impending loss

of fossil fuel supplies have spurred new technologies for addressing atmospheric CO2 levels and global energy needs. Most of these technolo-gies fall into one of four categories: restore, reduce, enhance, and resink. Aforestation is an example of restor-ing a carbon sink. CO2 reduction strategies involve developing alterna-tive energies, such as solar, wind, and wave energy; alternative fuels, such as biofuels and hydrogen fuel cells; and improved land use practices, such as “green” farming and construction. With regard to biofuels, there is much debate about the actual carbon inten-sity of a fuel: reports of CO2 emission reductions vary widely depending on the degree to which all carbon emissions related to production are factored into the accounting.

When the amount of CO2 in the atmosphere increases, the ocean becomes more acidic due to carbon-ate chemistry. Increasing levels of ocean carbonates reduce pH levels (increasing acidity). Since many marine organisms have a narrow tolerance to changes in pH, changes in levels of CO2 may result in altering ocean ecosystems and interrupting food webs.

Another outcome of increased atmospheric CO2 levels might be an increased rate of photosynthesis by terrestrial plants, as well as aquatic algae. Scientists are considering ways of enhancing the ocean’s natural solubility pump—a natural multi-process mechanism that results in uptake and sinking of atmospheric CO2. The solubility pump is driven by the coincidence of two natural processes in the ocean:

■ The solubility of carbon dioxide is a strong inverse function of seawater temperature (i.e., solubility is greater in cooler water).

■ The thermohaline circulation is driven by the formation of deep water at high latitudes where seawa-ter is usually cooler and denser.

In order to sink more CO2 into the deep ocean, some scientists have pro-posed fertilizing the ocean to create phytoplankton blooms. They hope to add limited nutrients, such as iron, to ocean waters to increase phytoplank-ton growth, thereby sequestering more CO2 via photosynthesis. However, scientists have serious questions about how much CO2 taken up by the bloom is drawn out of the atmosphere and transferred to the deep ocean and how long it remains sequestered there. Models and experiments suggest that the carbon taken up is not likely to sink to the deep ocean, but rather stay relatively close to the surface, and will then just come back out when we stop fertilizing. In addition, there are some serious concerns about how fertilizing might change the nutrient structure of the ocean. The largest iron fertiliza-tion research project to date (2009) produced a phytoplankton bloom, but it was rapidly consumed by copepods. As a result, little CO2 made it to the ocean floor.

Aforestation

14 CALIFORNIA EDUCATION AND THE ENVIRONMENT INITIATIVE I Unit E.7.b. I The Life and Times of Carbon

Scientists explain that most ocean uptake of carbon dioxide resulting from human activities has to do with ocean physics and chemistry. Ocean circulation is very slow, and in some places, water is upwelling that has not seen the surface in thousands of years. This water is relatively low in CO2, because it last equilibrated with the atmosphere before people increased atmospheric CO2 through

fossil fuel burning and land use change, so it takes up CO2 when it reaches the surface. This would still happen even if you killed all the living things in Earth’s oceans, and it is the biggest driver of ocean uptake of CO2 produced by humans.

ChoicesScientists and world leaders are

concerned that efforts taken by one

country to reduce atmospheric CO2 levels might result in increased carbon emissions elsewhere. Pundits call this undesirable outcome

“leakage.” For example, if the amount of CO2 emissions allowed from a particular industry, such as cement production, are tightened in one state or country, and if similar regulations are not tightened in the surrounding states or countries, cement plants may move into areas with more relaxed regulations. Thus CO2 emissions will just move from one location to another, rather than resulting in an actual reduction of emissions.

The worldwide future of energy use is going to be different, based on the realities of shrinking fossil fuel sources and increasing demand for fuels by growing human populations and economies. The choices we make to fulfill our future energy needs must reflect not only how much energy we can get out of our environ-ment, but also how we can extract and use that energy in a way that sustains our atmosphere, ocean, and land for future generations.

Aforestation: Planting or seeding forests in an area of non-forest or previously cleared forested lands.

Biogeochemistry: The study of biological, geological, physical, and chemical processes, and the cycling of matter and energy among these systems.

Carbon cycle: The process by which carbon is exchanged between organisms, such as plants, animals and humans, and the environment—the atmosphere, ocean, rocks, soil, and sediments.

Carbon flow: The movement of carbon in gas, dissolved, or solid form from one carbon reservoir to another.

Carbon footprint: The total amount of carbon gases produced directly and indirectly through human activities that use carbon-based fuels.

Carbon sequestration: The removal of atmospheric CO2 by physical or biological processes and storage in a sink, such as the ocean.

Greenhouse gas (GHG): Any gas, such as carbon dioxide, chlorine, nitrous oxide, and methane, that absorbs infrared radiation in the atmosphere and contributes to the greenhouse effect.

Leakage: Emission reductions offset by emission increases elsewhere (for instance, aforestation in one country offset by deforestation in another country).

Sedimentation: The slow and layered accumulation of detritus and weathered rocks and soils, such as on the ocean floor. Continued sedimentation leads to compression and formation of sedimentary rock and fossil fuels.

Solubility pump: The process of carbon dioxide transfer from air to sea, where it is distributed from the ocean’s surface to its interior by mixing and ocean currents.

| Glossary

Carbon dioxide emissions