Looking at the numbers in Illustration 60, the first thing to notice is that the atmosphere is the smallest reservoir of carbon shown (soil and plant matter are combined here). By far, the largest carbon reserve is the ocean, holding a staggering 38,000 Gt, mostly in the form of dissolved CO2. CO2 doesn't dissolve easily in seawater, but Earth's oceans are very big, so even a small amount of dissolved gas per cubic foot ends up being a sizable total amount. Even Earth's plant matter, organic material in the soil and growing plants hold almost three times as much carbon as is freely available from the atmosphere. Still, 730 Gt of CO2 in the atmosphere is a respectable amount.
Another thing to notice from Illustration 60 is the amount of carbon being exchanged. Living matter gives off 119 Gt while absorbing 120 Gt. The oceans give off 88 Gt while taking in 90 Gt. The other exchanges shown are the results of human activity, changing land use gives off 1.7 Gt and consumes 1.9 Gt while fossil fuels and industrial activity pumps 6.3 Gt into the atmosphere. If everything in the diagram is added up, natural mechanisms are running a carbon deficit of around 3 Gt, almost half of the human-caused emissions. This means that the CO2 level in the atmosphere is not growing as fast as human emissions rates would indicate.
What is not shown in the carbon cycle diagram are the geologic carbon sinks that store carbon for long periods of time. Carbon in these sinks take millions of years to cycle back into the biosphere. As mentioned, there is a tremendous amount of CO2 dissolved in the oceans. Though much of the CO2 in seawater remains as dissolved gas, a portion is converted into other chemical compounds. Among the compounds that are formed are carbonate (CO3) and bicarbonate (HCO3). Many forms of sea life (labeled Aquatic Biomass) have the ability to modify bicarbonate by adding calcium (Ca), producing calcium carbonate (CaCO3). Calcium carbonate is used by these organisms to build shells and other body parts. Illustration 61 shows how carbon cycles through the oceans and sedimentary rock deposits.
Illustration 61: Carbon cycle with marine sediments. Source PhysicalGeography.net.
In this way corals build reefs, while shellfish, such as clams and oysters, make shells for themselves. Some smaller organisms, protozoa and algae, also build skeletons out of carbonate.226 When these organisms die, their shells and body parts sink to the ocean floor where they form marine sediments.
Illustration 62: Coquina. Source Florida Department of Environmental Protection.
Marine sediments can be classified as either shallow water or deep water deposits, depending on the environments that formed them.227 Shallow water deposits tend to be made up from the disintegrating skeletons of coral, mollusks, sea urchins, and the like. There also may be the remains of algae and inorganic material as well. Shallow water carbonate-rich sediments are found mostly in the tropics and subtropics. These deposits can form aggregates called coquina, consisting of whole and fragmented mollusk shells in a matrix of sand cemented by calcite. Coquina is found in large quantities in Florida and the Caribbean where it is used as a building material. It was used historically to build forts, where the soft nature of the stone allowed it to absorb cannon balls without shattering. Deposits of coquina, harder coquinite, and coquinoid limestones are found around the world.
Deep water deposits, on the other hand, tend to be made primarily from the skeletons of pelagic (Greek for “open sea”) organisms. These tiny creatures die and their remains begin the long fall to the deep ocean floor where they accumulate as carbonate-rich deposits. After long periods of time, these deposits are physically and chemically altered into sedimentary rocks. The result of this process can be seen in the large beds of limestone and dolomite present in Earth's crust today.
Illustration 63: Volcanic release of CO2. Source NASA.
Volcanoes generate CO2 when rock containing carbon, such as limestone, is melted by magma underneath an active volcanic vent (see Illustration 63). Release of CO2 from volcanoes is a minor contribution to the carbon cycle on a yearly basis. Mount St. Helens, during its 1980 eruption, had a maximum daily emission rate of 22,000 metric tons. The total amount of gas released during the non-eruptive period from the beginning of July to the end of October was 910,000 tons.228 All volcanoes combined produce about 500,000,000 tons (½ a gigaton) of CO2 per year, about 2.8% of man's yearly emissions. But, over long periods of time, the contribution of volcanoes can be significant. There have been periods in Earth's past that had higher levels of volcanic activity.
After examining all these complex interactions among air, water, rock and life, scientists have a mystery on their hands. If human CO2 emissions are removed, the world should be in balance—carbon in should equal carbon out. It seems that the numbers do not all add up and each year about 2.9 Gt of carbon goes missing. To quote Richard Houghton, Senior Carbon Research Scientist at the Woods Hole Research Center:
“When considered with the other terms in the global carbon equation (the atmosphere, fossil fuels, and the oceans), there is an apparent imbalance in the global accounting, and considerable effort has gone into explaining and finding this residual sink, or missing sink, of carbon.”229
The Missing Sink
For more than three decades, the attention of biologists and ecologists studying the global carbon cycle has focused on an apparent imbalance in the carbon budget. The so-called “missing sink” is a result of the following equation:
Atmospheric CO2 Increase = Human Emissions + Land Use – Ocean Uptake
This equation is simple enough: the amount of carbon produced by humans plus the carbon produced by other living things, less the amount absorbed by the oceans, must end up as atmospheric CO2. But, if actual numbers are used, the equation does not balance.
The average annual emissions of 8.5 Gt during the 1990s, 6.3 Gt from fossil fuels and 2.2 Gt from land use, are greater than the sum of the annual buildup of carbon in the atmosphere (3.2 Gt) and the annual uptake by the oceans (2.4 Gt). Here, land use includes carbon from decaying dead vegetation, soil organic matter, and wood products less the uptake by regrowing ecosystems. An additional sink of 2.9 Gt is required to balance the carbon budget. Though this is a small amount, over time, it adds up, 115 Gt of missing carbon over the period 1850-2000. The amounts involved over time can be seen in Illustration 64.
Illustration 64: Carbon flux showing sources and sinks. Source Woods Hole Research Center.
Despite the best efforts of scientists to account for the “missing” carbon, no good answer has been found. During the 1990s, the world's ecosystems are calculated to have been a net sink of 0.7 Gt of carbon per year to the atmosphere, causing speculation that plants are absorbing the missing carbon. The most popular theories revolve around an observed greening of North America, Europe and Russia. To quote again from Dr. Houghton:
“In the last few years several independent analyses based on geochemical data (data from the atmosphere and oceans) and a series of carbon budgets based on data from forest inventories have shown that carbon is accumulating in northern mid-latitude terrestrial ecosystems, although estimates of the magnitude and location of the accumulation vary among the analyses.”230
Plant life in the US has thrived over the last 100 years, and the increased vegetation growth has absorbed more atmospheric CO2. This growth is due to the recovery of ecosystems from development in the 1800s and 1900s, when prairies and forests were turned into farmland.
As farming became mechanized, the shrinking number of draft animals greatly reduced the amount of land needed for farm production. This reduction, combined with fertilizers and modern farming techniques, has reduced the amount of land needed to feed each citizen. As a result, since 1950, US forest cover has increased.231 Similar trends have been noticed in developed countries around the world.
Some think an increase in CO2 spurs plant growth232 ,233 while others say wetter weather and extended growing seasons are the cause. NASA scientists credit an 8% increase in precipitation from 1950 to 1993, combined with higher humidity, with an overall 14% increase in plant growth in the United States.234
> Scientists' best guess is that carbon is being absorbed by undisturbed or resurgent ecosystems, but they don’t know exactly where these are. Since the results of this accumulation remain unidentified the carbon remains missing and the “missing sink” remains unfound. It would seem that there are things scientists still don't understand about the carbon cycle, CO2, and Earth's climate.
The Bottom Line On CO2
With this understanding of CO2 at hand, we can begin to place human-caused global warming in some perspective. Human activity only adds around ½-1% to the amount of carbon dioxide in the atmosphere each year. Even so, that amount is growing and, as Dr. Lomborg has said, “the important question is not whether the climate is affected by human CO2, but how much.”235
Having seen how the carbon cycle works, with both short term and long term carbon sinks, we are faced with a number of new questions. How do Earth's plants respond to the higher levels of CO2?236 Why doesn't the warming caused by increasing CO2 levels result in even more CO2 release from the oceans?
We know that the ocean gives off CO2 when it heats up. Why this happens can be demonstrated with a bottle of carbonated water. Soda water and other carbonated beverages contain dissolved CO2, just like ocean water, but under greater pressure. If you open a warm bottle of soda water there will be a hiss of escaping gas and the liquid will bubble up, possibly overflowing. If the bottle is placed in a refrigerator to cool prior to opening, uncapping the bottle results in a slight psst, few bubbles and no overflow. The reason for the difference is that cold water can hold much more dissolved gas than warm water.
The same is true of ocean water. As the oceans grow warmer, huge volumes of CO2 are released—not quite as dramatically as a foaming bottle of warm soda, but for precisely the same reason. Scientists have come to believe that the oceans may have released large quantities of CO2 in ages past, causing periods of intense global warming.237 This is particularly troubling if CO2 is the primary driver of earthly temperatures.
If mankind's release of extra CO2 into the atmosphere causes the oceans to warm, more CO2 will be released. This will cause further warming, which will cause the release of even more CO2 and so on, in an ever-increasing spiral of rising temperatures and CO2 levels. This is an example of what engineers call a positive feedback loop, where a process feeds on itself, spinning out of control. But this doesn't seem to be happening, temperatures have not spiraled wildly upwards.
A clue as to why Earth doesn't have a runaway greenhouse effect might be hidden in the missing carbon sink that has been confounding scientists for the past 30 years. The missing CO2 hints at other mechanisms actively helping to control Earth's temperature. Whatever the reason, it is fortunate that other forces are at work beside CO2.
We do know that the planet is warming around 1.8°F (1°C) per century. The obvious question is how much of that is due to added CO2, and how much might be due to other factors? Emissions from ruminants (cud chewing animals) have been recently cited by the European Parliament as “the greatest threat to the planet.”238 These emissions, making up 18% of all greenhouse gases, are mostly in the form of methane and nitrous dioxide. More recently, a study of the pervasive, low-level particulate pollution, found over much of Asia, has revealed that the ubiquitous “brown clouds” are having an unsuspected warming effect. Based on air samples taken by drone aircraft, researchers have concluded that man-made haze is warming the lower atmosphere by as much as 1.4°F (0.8°C).239 Given these latest findings, perhaps the case for CO2 being the major driver of Earth's climate needs to be re-evaluated.
Much of the carbon cycle involves interaction between land, sea and air—and the huge geologic carbon sink that is directly related to both ocean life and Earth's active geology. We will next examine those areas in detail. One of the factors that sets Earth apart from its sister planets is Earth's ever-changing face, which we examine in the next chapter. The changes are caused, not by meteor impacts, but by the moving continents.
Moving Continents & Ocean Currents
“Rocks crumble, make new forms, oceans move the continents, mountains rise up and down like ghosts yet all is natural, all is change.”
— Anne Sexton
Continental drift has dramatically reshaped the face of Earth during the Phanerozoic, causing significant impact on climate by affecting ocean and atmospheric currents. As the continents move, warm ocean currents can be rerouted or blocked altogether, keeping tropical waters from warming colder regions. This can cause changes in precipitation, desertification and weathering patterns. In turn, these changes may affect the release of CO2 into the atmosphere. It is probably not a coincidence that during the periods of coldest climate, all of the land masses were gathered into single giant super-continents; Rodinia during the Cryogenian Period (850-635 mya, snowball Earth) and Pangaea during the Permo-Carboniferous Ice age (350-260 mya).
Collisions between continental plates raise mountain ranges, which in turn affect air currents and precipitation patterns. High mountains make snow fall in temperate regions year round and cause glaciers to form, providing islands of cold in otherwise tropical latitudes. The bright white of snow and glaciers reflect more sunlight causing Earth to cool. Mt. Kilimanjaro has kept a permanent snow cap throughout recorded history despite being only 120 miles (300 km) from the equator. The snows of Kilimanjaro have been visibly retreating over the past 100 years, and are expected to disappear by 2020.240 The melting of mountain glaciers and snowfields is normal during an interglacial period.
Though continents seem rock solid, they do move. Continents are made primarily of granitic rock that is lighter than the basaltic rock that forms the ocean floors and the molten mantle. Though it is hard to think of granite as light, it is in a comparative sense. Continents float like corks on top of heavier, molten rock. Plate Tectonics, the geological theory that describes the movement of Earth's crust and uppermost solid material, attributes continental motion to the mechanism of sea floor spreading. In Greek, tectonics means “to build,” and, in geological terms, a plate is a large, rigid slab of solid rock. According to this theory, new rock is created by volcanism at mid-ocean ridges and returned to Earth's mantle by subduction at ocean trenches. Earth's crustal plates are in constant, slow motion as new ocean floor is created at submarine ridges. As this happens, the continents go along for the ride.
How was continental drift discovered? Francis Bacon, Benjamin Franklin, Alexander von Humboldt and many others had noted that the shapes of continents on either side of the Atlantic Ocean, Africa and South America, seem to fit together like pieces of a puzzle. But they could not explain why this should be so.
Drifting Continents
Friedrich Wilhelm Heinrich Alexander Freiherr von Humboldt (1769-1859) was a Prussian naturalist and explorer. The son of a Prussian Army officer belonging to a prominent Pomeranian family, he was well-educated but had a sickly childhood. In 1789, he produced his first scientific treatise, Mineralogische Beobachtungen über einige Basalte am Rhein (Mineralogical Observations over some Basalts on the Rhine), after an excursion up the Rhine river.
As a result of his experience on the Rhine, Humboldt decided to pursue a career as a scientific explorer. He set himself to learning astronomy, geology, anatomy and botanical science. His early travels were mostly confined to Europe but, after the death of his mother in 1792, he began to explore more distant realms.
Illustration 65: Alexander von Humboldt, painted by Georg Weitsch.
The name of the Jurassic Period derives from Humboldt's use of the term “Jura Kalstein” for carbonate deposits he found in the Jura region of the Swiss Alps in 1799. Later that year, after a false start on a world-circling expedition, Humboldt traveled to the Americas.
During this trip, which was to last until 1804, Humboldt recorded observations on geography and wildlife. He commented on the Leonid meteor shower, the transit of Mercury, electric eels and searched for the source of the Amazon river. By the end of his journey he was well-positioned to make f
uture contributions to meteorology, geography and oceanography.
In 1817, he developed the concept of “isothermal lines,” providing a way of comparing the climatic conditions of various countries. He was the first to investigate the decrease in temperature with increasing altitude above sea level. By investigating the origin of tropical storms, Humboldt laid the groundwork for the future discovery of more complicated laws governing atmospheric disturbances in higher latitudes.
One of his many contributions to geology was his observation that volcanoes seemed to fall into linear groups. Based on his study of the New World volcanoes, he suggested that these bands of volcanoes were a reflection of extensive subterranean fissures, thousands of miles long. Humboldt had discovered an important clue that would eventually help resolve the mystery of continental drift, but he pursued the idea no further.
Though Humboldt investigated and wrote about many different aspects of nature, for the purposes of this chapter we will note only one more of his discoveries—the Humboldt Current. This cold ocean current, consisting of low salinity, nutrient-rich water, extends along the West Coast of South America, from Northern Peru to the southern tip of Chile. The waters of the Humboldt Current flow in the direction of the Equator and extend 1,000 kilometers offshore.
The Humboldt Current is one of the major upwelling systems of the world, supporting an extraordinary abundance of marine life. We will return to the discussion of ocean currents and their effect on climate after continuing with our exploration of the discovery of continental drift.
Illustration 66: Alfred Wegener. Source USGS.
Alfred Wegener (1880-1930), a German physicist and meteorologist, was the first to use the phrase “die Verschiebung der Kontinent,” German for “continental drift.” In 1912, he formally published the hypothesis that the continents were once all together in one large land mass and had somehow drifted apart. Unfortunately, he couldn't convincingly explain what caused them to drift. His best explanation was that the continents had been pulled apart by centrifugal force resulting from Earth's rotation. This force caused the continents to “plow” through the sea floor to new locations. Wegener's explanation was considered unrealistic by the scientific community.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 14