1-4: Composição atmosférica, dióxido de carbono

Eugene S. Takle
© 1996

Uma das primeiras indicações de que o homem alterou a composição da atmosfera está nas medidas da concentração de dióxido de carbono. A figura ao lado mostra que de 1973 a 1985 a concentração de dióxido de carbono, expressa em partes por milhão em volume, na atmosfera terrestre aumentou de 320 para 350. Medidas mais recentes mostram valores ainda maiores. Verifica-se que há tendência de aumento independente de local e ocorre nos dois hemisférios. A diferênça principal nas curvas está na amplitude do ciclo anual. Exame mais cuidadoso mostra que a concentração atinge um pico em Abril ou Maio no hemisfério norte, e um mínimo em Julho, correspondendo à absorção por plantas durante a estação de crescimento e a liberação pela decomposição da vegetação fora do período de crescimento vegetal. Olhando o mapa global de vegetação em Janeiro e Julho pode-se inferir sobre as amplitudes diferentes nas diversas localidades. No entanto, para todas localidades nota-se uma tendência de que a concentração está aumentando.
Concentração de CO₂ em Ak, Mauna Loa, Havai, Samoa, e Pólo Sul. (1990: American Scientist, 78, 325. Permissão de Sigma Xi, The Scientific Research Society.)

Examinando-se os dados pode-se ter uma estimativa do nível de CO₂ ao longo do tempo. Na época da revolução industrial, no final do século 18, a quantidade de CO₂ na atmosfera era de 170 ppmv. Os registros mostram que a concentração aumentou vagarosamente até o século 20, mas nos últimos 50 anos o aumento foi grande.
CO₂ atmosférico desde o iníicio da revolução industrial. (EPA.)

Agora temos oportunidade de olhar um registro muito longo pelo exame de amostras de gêlo tiradas de 2 km de profundidade da Antartica. Uma equipe de cientistas da França e da antiga Russia analizou pequenas bolhas de ar retidos no gêlo em várias camadas abaixo da superfície atual. Camadas mais profundas correspondem a épocas mais passadas. Essas bolhas podem ser analisadas em relação aos níveis de CO₂ voltando-se no tempo até 160.000 anos atrás. Concentrações de isótopos de oxig^enio retido nessas bolhas também foi medido, e sua relação sendo dependente da temperatura, dá uma estimativa da temperatura na superfície do continente antartico quando essas bolhas se formaram. O gráfico mostra que as concentrações de CO₂ aumento de 180 para cerca de 300 ppmv nos 160.000 anos. Pode-se ver pelos dados atuais que a concentração atual excede os 350 ppmv.
Mudânça no CO₂ atmosférico e na temperatura da superfície terrestre nos últimos 160,000 years. (U.S. Global Change Research Program.)

Nossa discussão de magnitudes de emissões de dióxido de carbono numa escala global requer um sistema de unidades que não estamos acostumados a usar no dia-a-dia. A tabela ao lado define os prefixos tais como kilo e mega, que são normalmente usados, e outros termos que não são comuns. Pode ser conveniente retornar a esta tabela para refrescar sua memória do significado de Tera, Peta, ou Exa. Foi incluido algumas conversões para ajudar a captar a magnitude desses números. Uma Gigatonelada é o equivalente de Um Petagrama, e uma Gigatonelada de água corresponde a um quilometro cúbico.
Definições para discutir quantidades globais.

Concentrações de gases traços são dadas em partes por milhão em volume (ppmv), partes por bilhões por volume (ppbv), ou partes por trilhão em volume (pptv). Um ppmv significa uma molécula em um milhão. Algumas vezes as concentrações são dadas em partes por milhão em massa, mas este uso será evitado neste curso.

Sabemos a razão do aumento do dióxido de carbono na atmosfera. Deflorestamento é uma das causas, embora não seja a principal. O gráfico ao lado mostra a liberação anual atual devido ao deflorestamento, em Teragramas de carbono. Brasil contribui com a maior parte seguido pela Indonésia, Colombia, e outros países tropicais. É importante reconhecer que países desenvolvidos, como os Estados Unidos e a Europa, não aparecem nos mapas de deflorestação atual. Esses países, de fato, reduziram suas florestas, mas isto foi há muito tempo.
Liberação efetiva de carbono pelo deflorestamento tropical. (1989: EPA, Policy Options for Stabilizing Global Climate.)

Outra fonte de dióxido de carbono atmosférico é a emissão pelas usinas de cimento. Material carbonáceo usada no preparo do cimento libera quantidade significativa de CO₂ durante o processamento. Essa fonte de CO₂ será incluída nas discussões de desenvolvimento sustentável, mesmo porque o cimento é usado na construção de estradas, pontes, edifícios, usinas, barragens, que são componentes essenciais para o desenvolvimento econômico. Países como China e India são importantes neste aspecto pois têm potencial muito grande para desenvolvimento.
Emissão de dióxido de carbono pela produção de cimento. (1989: EPA, Policy Options for Stabilizing Global Climate.)

Até agora, a maior causa do aumento do CO₂ é a queima de combustível fóssil. A figura ao lado mostra a emissão em Gigatoneladas por ano ao longo do tempo. Antes de 1860, as emissões eram bem inferior que 1 Gigatonelada por ano e aumentou vagarosamente até a primeira metade do presente século. A partir do final dos anos 40 essa curva subiu dramaticamente até o nível atual de mais ou menos 6 Gigatoneladas por ano.
Emissão de combustível fóssil. (1990: American Scientist, 78, 310. Permissão de Sigma, Xi, The Scientific Research Society.)

Um desdobramento do tipo de combustível fóssil contribuindo para este aumento mostra que o uso de carvão aumentou continuamente durante os últimos 100 anos e que continua aumentando significativamente até agora.
Use of oil for heating, manufacturing and automobiles started later but also is increasing at a rapid rate. Use of all types of fossil fuels is increasing, and this trend will likely continue well into the future.
Emissões de CO₂ devido ao uso de combustíveis fósseis. (EPA.)

To understand the human impact of increasing atmospheric carbon dioxide, we first must consider the natural reservoirs and fluxes of carbon in the earth/atmosphere/ocean system. We must include inventories of the carbon stored in terrestrial plants and animals, carbon in the ocean, carbon in the atmosphere, and reserves of fossil carbon that represent terrestrial carbon taken out of the biosphere at times in the distant past. Each of these reservoirs takes up and releases carbon at different rates that must be estimated to put anthropogenic emissions into perspective.

The accompanying sketch shows these reservoirs, with amounts given in petagrams of carbon and fluxes in units of petagrams of carbon per year. So, for instance, the atmosphere contains about 740 units of carbon. Terrestrial biology, including all of the plants from phytoplankton to giant sequoia trees and animals from mice to elephants, accounts for about 550 units. Even these crude numbers allow us to make simple evaluations. For instance, if humans carried deforestation to the ultimate, in other words, if we incinerated the whole terrestrial biosphere and put this carbon in the atmosphere, we would approximately double the amount of carbon dioxide in the earth's atmosphere. Seven box schematic of the carbon cycle.
(NASA.)

Soil contains molecular carbon, carbon of organisms living in the soil, and detritus (broken parts of dead plants, corn stalks, and tree leaves, etc.) in total amount of about 1200 units. This suggests that the soil contains about twice as much carbon as the terrestrial biology. The ocean presents a more complicated problem for evaluating carbon stores and fluxes. The deep ocean has about 34,000 units, and the surface ocean has about 600 units in what is considered the warm ocean and 300 units in the cold ocean. It should be noted that the deep ocean has a very large reservoir of carbon, but it's essentially stored there permanently, since the cycling time from the deep ocean is on the order of hundreds to thousands of years.

Near the ocean surface, turbulent motions promote the uptake of atmospheric carbon dioxide by the ocean through the formation of weak carbonic acid. Approximately 22 units per year go from the cold ocean into the atmosphere, and about 35 units come back, making the cold ocean a net sink for carbon dioxide from the earth's atmosphere. The warm ocean, by contrast, is a net source because it's emitting 80 units and taking in only 70, giving a net flux outward of about 10 units.

Terrestrial (land) biology consumes about 110 units per year, mostly in support of terrestrial plant growth. About 50 units of that goes back into the atmosphere in the decay process, and about 60 units goes into the soil. The soil, it turn, releases about 60 units back to the atmosphere, resulting in a balance of fluxes in and out of the natural terrestrial biological system.

If we now consider the effects of anthropogenic changes to this natural cycle, we must focus on the two largest contributions, namely burning of fossil fuels and deforestation. Fossil fuels represent terrestrial carbon that has been taken out of the rapidly changing part of the carbon cycle and stored more or less permanently below the earth's surface where natural processes cannot release it back to the atmosphere. Fossil fuels account for about 5 to 6 units of emission into the atmosphere, and the burning of standing carbon in the form of old growth tropical forests contributes about 1 to 2 units.

The carbon cycle includes other carbon-containing compounds in addition to carbon dioxide. For instance, methane molecules, which have one carbon atom and 4 hydrogen atoms, may be produced rather than carbon dioxide in the decay process, particularly in moist soils, marshes, and boreal tundra. Methane also is produced by ruminant animals, such as cattle and sheep. These animals eat large amount of grain or forage, which goes into the first of 4 stomachs known as the rumen. The digestion process produces significant amounts of methane that are released to the atmosphere. Carbon monoxide also is released to the atmosphere from the burning of fossil fuels and from decay of plant material. Volcanoes are natural sources of both carbon dioxide and carbon monoxide.

As was previously pointed out, the ocean is both a major source and sink of carbon for the atmosphere. Inorganic carbon in the form of dissolved carbon dioxide and carbonates accounts for about 37,000 gigatons. Dissolved organic carbon contributes an additional 1,000 gigatons. Particulate carbon such as from live organisms or dead plants and animals or fragments add about 30 units. We should remember that there are large uncertainties in these estimates because of the wide variations over the planetary oceans.

Oceans regulate carbon in 3 different ways: by physical processes, chemical processes, and biological processes. Physical processes include the movement of carbon by ocean circulation from one location to another. Meteorologists and oceanographers call this process advection. Differences in temperature and salt content (salinity), in addition to the driving force of the wind and rotation of the earth, lead to bulk transport of carbon within and between major ocean basins. Another physical process is the diffusive mixing of water from one vertical level to another. Carbon dioxide dissolved in surface water is in equilibrium with CO₂ in the atmosphere because of efficient mixing in the ocean surface water.

Chemical processes transform carbon among different molecular forms. Biological processes include the production and decomposition of organic matter, which are confined to the upper layer of the ocean where photosynthesis can operate. If this biological material remains near the surface, it will continue to cycle with the atmosphere. Some carbon, such as in the form of phytoplankton that thrive in the surface water of the ocean, are eaten by small fish and eventually larger fish or animals that ultimately die, leaving skeletons or carbonate shells that sink to the ocean floor. This process, sometimes referred to as "biological pumping", takes carbon from the rapidly changing part of the cycle near the ocean's surface to the deep ocean where it may be stored for thousands of years. The deep ocean is richer in dissolved inorganic carbon, and the surface water has a predominance of organic carbon.

The increase of ultraviolet radiation, which we will discuss in connection with ozone depletion, also has implications for ocean biology. Many of the simple organisms in the ocean surface are very vulnerable to ultraviolet radiation, and increases in ultraviolet radiation due to ozone depletion could significantly affect simple organisms at the bottom of the food chain that thrive in the ocean surface water. This presents a linkage between the ozone depletion problem and the build-up of atmospheric CO₂: decreased ozone could contribute to increased ultraviolet light, which decreases ocean plant life, which decreases CO₂ consumption by the ocean, which allows for increased rates of CO₂ build-up in the atmosphere.

Sunlight penetrating the ocean surface is depleted as it passes downward, creating what is called the euphotic zone where sunlight is sufficiently intense to promote photosynthesis. In the region below the euphotic zone, the net photosynthetic rate is negative due to lack of solar energy, resulting in very little biological activity below a certain level. The ingredients in addition to sunlight that are needed for biological production are nutrients, particularly nitrogen and phosphates. The accompanying plot shows a typical nutrient deficiency in the surface layer due to consumption by micro-organisms. Deeper layers, where photosynthesis is suppressed due to lack of light, tend to have elevated levels of nutrients. If a mechanism were available to bring deep, nutrient-rich water in the euphotic zone, phytoplankton and algae would flourish, as would the marine life that live on these tiny organisms. Large-scale ocean circulation patterns in certain geographical regions and vertical motions in the ocean near continents circulate nutrient-rich water to provide the necessary nourishment for the euphotic zone.

Euphotic and Aphotic Zones.

Essential nutrients for ocean plants.

The accompanying satellite picture shows ocean biological activity in the vicinity of Antarctica during the Southern Hemisphere spring. The color coding indicates the level of biological activity, going from a magenta, which represents essentially no biological activity, to blue, yellow, green, and finally red, which represents the highest observed concentration of phytoplankton. The arrival of sunshine to the arctic region in the spring and early summer leads to a rapid "bloom" of phytoplankton in this region. Notice that the regions of highest biological activity are along continental coastlines and around the Antarctic Continent. Equatorial ocean areas far from continents, on the other hand, are virtual biological deserts by comparison. Certainly sunlight is not lacking in these areas, so we must conclude that lack of nutrients prevents these regions from becoming biologically productive. Upwelling near continents creates rich biological regions, but closer examination reveals that not all coastal areas are equally productive. Again, we can conclude that differences in upwelling of nutrient-rich water must be the cause of these differences in ocean biology. Biological activity in the oceans of the southern hemisphere.
(July/Aug 1990: American Scientist. Permission granted by Sigma Xi, The Scientific Research Society.)

The next photograph of the Northern Hemisphere, centered on the North Pole shows that the whole North Atlantic Ocean is a very biologically productive region. In the North Pacific Ocean, coastal regions between Alaska and Russia also show high levels of phytoplankton. Considering that these small organisms cover such a large area and consume CO₂ , we must conclude that the polar oceans are tremendous sinks (removal mechanisms) for atmospheric carbon dioxide. In contrast to lower latitudes where lack of nutrients limits biological activity except near coasts, in the polar regions the supply of nutrients is persistent, but the lack of sunlight during winter periods shuts down the phytoplankton production beginning in autumn. However the large extent of the summer blooms means that a tremendous amount of carbon dioxide is used seasonally by these organisms. A closer look at coastal California in the next figure shows that northern California experiences a rich phytoplankton bloom, which is less pronounced in the ocean off southern California. Ocean currents (to be discussed later in the course) produce upwelling preferentially in the northern part of the state, a situation that also is responsible for ocean temperatures off San Francisco being much colder than those off Los Angeles.

Biological activity in the oceans of the northern hemisphere.
(July/Aug 1990: American Scientist. Permission granted by Sigma Xi, The Scientific Research Society.)

Mean near-surface phytoplankton pigment concentrations off the California coast. (NASA.)

A limiting factor for the growth of phytoplankton and algae around Antarctica, even during the spring bloom, is insufficient amounts of iron. A proposal has been made that by fertilizing the Antarctic ocean with iron, the growth in ocean marine plants would sequester large additional amounts of carbon dioxide from the atmosphere and counteract the anthropogenic increases. More information on this "iron solution" is given on the electronic dialog, where you are encouraged to voice your opinion on the wisdom of such "engineering approaches" to solving environmental problems.

Terrestrial plants consume carbon dioxide during the daytime period and respire some of this CO₂ back to the atmosphere at night. The accompanying sketch shows typical levels of CO₂ within a vigorously growing plant canopy at different times of day and night. At night, plants tend to respire or give up carbon dioxide, raising the ambient level well above the global mean (which was about 330 ppm when this sketch was printed). During the day however, if the plants are not limited by lack of water or nutrients, photosynthesis may draw down the amount of CO₂ to approximately 300 parts per million. The carbon taken up by the plants is converted to plant carbon and remains in this form until the decay process begins. For typical agricultural plants, the biomass is broken down during the dormant period and is converted to soil carbon and atmospheric carbon dioxide at rates that depend on temperature and moisture conditions. Vertical distribution of carbon dioxide in the air around a forest varies with time of day.
J. D. Butler, Air Pollution Chemistry, 1979.

Woody plants play a special role in the capture (sequestration) of carbon from the atmosphere. Trees lose their leaves (deciduous) and some or all needles (conifers) every year and return a portion of the season's carbon capture back to the atmosphere. The woody parts of the tree, however, may persist as plant carbon for several decades until the tree dies and decay begins. Trees, therefore, like annual plants, participate in the rapid part of the carbon cycle by cycling CO₂ back to the atmosphere in 1-3 years, but also store some carbon that is not recycled to the atmosphere for 50 to 100 years. Tropical rain forests have very large amounts of carbon sequestered in the trunks of trees as a part of this longer term cycle. Deforestation interrupts this natural long term cycle and puts woody material back into the atmosphere before the natural decay process would recycle it into CO₂ .

The "greenness index" of the planet, shown in the accompanying figure for July and August shows very rich terrestrial biological activity in the Northern Hemisphere continental areas, particularly in the boreal forests of northern Russia and Canada. At this time of year, the Southern Hemisphere, of course, is experiencing the biologically dormant winter period. Major desert regions, located approximately 30^o north and south of the equator, show lack of vegetation in both hemisphere (both summer and winter). During the Northern Hemisphere winter, shown in the next figure, the boreal forest regions are seen to have shut down their biological production. In the Southern Hemisphere the smaller amount of land mass does not reveal the dramatic seasonal variation of the Northern Hemisphere. This suggests that global seasonal cycles in atmospheric carbon dioxide will be dominated by the seasonal cycle in the Northern Hemisphere.

July-August global plant biological activity as determined by the Global Vegetative Index.
American Scientist. Permission granted by Sigma, Xi, The Scientific Research Society. January-February global plant biological activity as determined by the Global Vegetative Index.
American Scientist, 78, 322 (1990). Permission granted by Sigma, Xi, The Scientific Research Society.

The accompanying figure shows biological production potential for the entire planet, including both ocean and land areas. An important conclusion to be drawn from this figure is that relatively large regions of the planet, including tropical oceans, great deserts on land, and Antarctica, have very low potential for biological production. Essentially all the biologically productive land that is not in forests or urban use is under human agricultural management, and most of the productive ocean areas are heavily fished by global fishing fleets from many countries. Increased food production to feed a growing population on the planet will require higher production on present agricultural land or more conversion of forest land to agricultural land, both of which present problems and challenges. This figure reminds us of the finiteness of our planet and the need for preservation and stewardship of its food-producing resources. Productive potential of the Earth's vegetative biomass.
(NASA)

Traduzido por Antonio Roberto Pereira

Sunlight penetrating the ocean surface is depleted as it passes downward, creating what is called the euphotic zone where sunlight is sufficiently intense to promote photosynthesis. In the region below the euphotic zone, the net photosynthetic rate is negative due to lack of solar energy, resulting in very little biological activity below a certain level. The ingredients in addition to sunlight that are needed for biological production are nutrients, particularly nitrogen and phosphates. The accompanying plot shows a typical nutrient deficiency in the surface layer due to consumption by micro-organisms. Deeper layers, where photosynthesis is suppressed due to lack of light, tend to have elevated levels of nutrients. If a mechanism were available to bring deep, nutrient-rich water in the euphotic zone, phytoplankton and algae would flourish, as would the marine life that live on these tiny organisms. Large-scale ocean circulation patterns in certain geographical regions and vertical motions in the ocean near continents circulate nutrient-rich water to provide the necessary nourishment for the euphotic zone.
	Euphotic and Aphotic Zones.
	Essential nutrients for ocean plants.

The next photograph of the Northern Hemisphere, centered on the North Pole shows that the whole North Atlantic Ocean is a very biologically productive region. In the North Pacific Ocean, coastal regions between Alaska and Russia also show high levels of phytoplankton. Considering that these small organisms cover such a large area and consume CO₂ , we must conclude that the polar oceans are tremendous sinks (removal mechanisms) for atmospheric carbon dioxide. In contrast to lower latitudes where lack of nutrients limits biological activity except near coasts, in the polar regions the supply of nutrients is persistent, but the lack of sunlight during winter periods shuts down the phytoplankton production beginning in autumn. However the large extent of the summer blooms means that a tremendous amount of carbon dioxide is used seasonally by these organisms. A closer look at coastal California in the next figure shows that northern California experiences a rich phytoplankton bloom, which is less pronounced in the ocean off southern California. Ocean currents (to be discussed later in the course) produce upwelling preferentially in the northern part of the state, a situation that also is responsible for ocean temperatures off San Francisco being much colder than those off Los Angeles.
	Biological activity in the oceans of the northern hemisphere. (July/Aug 1990: American Scientist. Permission granted by Sigma Xi, The Scientific Research Society.)
	Mean near-surface phytoplankton pigment concentrations off the California coast. (NASA.)