Determining the Emissions of Old Carbon Sources of Methane

The relative abundances of carbon isotopes in atmospheric carbon dioxide can be used to help deduce its origin by the following logic. The carbon in all living matter contains a small, constant fraction of a radioactive isotope, carbon-14 (14C), taken in via the carbon cycle when photosynthesis captures atmospheric C02 and when animals in turn feed off plant matter. This fact underlies the radiocarbon dating methods used by archaeologists and anthropologists: When an organism dies, its 14C decays at a known first-order rate that makes the date of its death calculable. (The assumptions justifying these methods are that biotic carbon and atmospheric carbon in C02 are balanced—in equilibrium with one another— and that the level of atmospheric *l4C is constant. The principles underlying radioactive decay are discussed in Chapter 9.)

However, in the case of atmospheric methane, the average fraction of 14C is less than the value found in living tissue. This indicates that a significant fraction of the CH4 escaping into air must be "old carbon"

that has been trapped in the pound for so long that its 14C content has diminished to almost zero as a result of radioactive decay through the ages. Most methane containing old carbon is released into the air as a by-product of the mining, processing, and distribution of fossil fuels. Methane trapped in coal is released into the atmosphere when this material is mined, as is methane in oil when it is pumped from the ground. The transmission of natural gas, which is almost entirely methane, involves losses into the air due to leakage from pipelines and is the largest of the atmospheric sources of old carbon. Measurements of the methane levels in the air of various cities have indicated that much of the loss from pipelines in the past occurred in eastern Europe. Finally, there is probably a small contribution to the old carbon source from methane trapped in permafrost in far northern latitudes; the methane was formed by the decay of plant matter that lived there many thousands of years ago when the polar climate was much warmer than it is today.

gases dissolved in crude oil are released—or incompletely flared—into the air when the oil is collected or refined. Emissions from these sources have likely leveled off in the last decade. The technique by which scientists determine the component of atmospheric methane arising from fossil-fuel sources is discussed in Box 6-3.

In summary, there are six different significant sources of atmospheric methane, of which natural wetlands make the greatest contribution (~25%). The current relative importance of the five important anthropogenic sources of atmospheric methane is thought to be:

energy production/distribution ~ ruminant animal livestock > rice production ~ biomass burning ~ landfills

Currently, the net sink for methane is thought to outweigh its sources by about 47 Tg/year, producing a slight net decrease in atmospheric methane concentration.

Methane: Concentration Trend and Possible Future Increases

Historically (i.e., before 1750), the methane concentration in air was approximately constant at about 0.75 ppm, i.e., 750 ppb. It has more than doubled since preindustrial times, to about 1.77 ppm; almost all of this increase occurred in the twentieth century because the emissions grew quickly, especially in the 1950-1980 period (see Figure 6-15). By the early 1990s, however, the rate of concentration increase had declined rapidly, and since that time it has fallen to zero in some years (see Figure 6-16b), so the methane concentration in air has been almost constant recently (Figure 6-16a). The rise in the atmospheric CH4 level that has occurred since preindustrial times is presumed to be the consequence of such human activities as increased food production and fossil-fuel use, as discussed above.

FIGURE 6-16 Atmospheric methane (a) concentration and (b) annual fluctuation in concentration in recent decades. [Source: NOAA.]

Year

Year

FIGURE 6-16 Atmospheric methane (a) concentration and (b) annual fluctuation in concentration in recent decades. [Source: NOAA.]

It is not known with certainty why the growth rate in methane concentration decreased recently. Since the rate of change in concentration is proportional to the difference between the rate of change in the emission rate and the rate of change in the destruction rate, change in either one or both rates could be responsible; neither can be measured directly very accurately. Natural gas pipelines carry -90% methane, about 1.5% of which is lost to the atmosphere. Part of the decline in the emission rate of methane into the air was likely due to much-decreased emissions from pipelines in the former Soviet Union, which a few decades ago leaked much more gas than at present. However, increasing use of fossil fuels in northern Asia has probably now replaced some of these emissions. The draining, and drying out from global warming, of wetlands has resulted in decreased methane emissions from this natural source in recent decades. Some scientists have speculated that the declines in the early 1990s were related to the air temperature decreases associated with the explosion of Mount Pinatubo. The rate of oxidation of CH4 by OH would also have increased if the concentration of hydroxyl radical increased overall. ;

PROBLEM 6-9

The concentration of atmospheric methane is about 1.77 ppm, and the rate constant for the reaction between CH4 and OH is 3.6 X 10~15 cm3 mole-cule~~1 s~1. Calculate the rate, in Tg per year, of methane destruction by reaction with hydroxyl radical, the concentration of which is 8.7 X 105 molecules cm-3. See Problem 6-6 for additional data.

Some scientists have speculated that the rate of release of methane into air could greatly increase in the future as a consequence of temperature rises from the enhanced greenhouse effect. For instance, higher temperatures would accelerate the anaerobic biomass decay of plant-based matter, as occurs in a common landfill. In turn, the additional release of methane would itself cause a further rise in temperature. This is another example of positive feedback.

Methane release from biomass decay among the extensive bogs and tundra in Canada, Russia, and Scandinavia could also increase with increasing air temperature and would also constitute positive feedback. However, the rate of biomass decay and of plant growth, and thus of CH4 production, also depends on soil moisture and therefore on rainfall, which probably would be affected by climate change in an as yet uncertain direction, so the net feedback from these sources could be positive or negative.

There is much methane currently immobilized in the permafrost of far northern regions; it was produced from the decay of plant materials during warm spells in the region but became trapped due to glaciation as temperatures became lower and lower at the start of the last ice age. Melting of the permafrost due to global warming could release large amounts of this methane. Melting would also allow the decomposition of organic matter currently present in the permafrost, with the consequent release of more methane.

In addition, there are monumental amounts of methane trapped at the bottom of the oceans, on continental shelves, in the form of methane hydrate. This substance has the approximate formula CH4 • 6 H?0 and is an example of a clathrate compound, i.e., a rather remarkable structure that forms when small molecules occupy vacant spaces (holes) in a cage-like polyhedral structure formed by other molecules. In the present case, methane is caged in a 3-D ice lattice structure formed by the water molecules. The melting point of the structure is + 18°C, somewhat higher than that of pure ice. Clathrates form under conditions of high pressure and low temperature, such as are found in cold waters and under ocean sediments. The methane was produced over thousands of years by bacteria that facilitated the anaerobic decomposition of organic matter in the sediments.

If seawater warmed by the enhanced greenhouse effect penetrates to the bottom of the oceans, the clathrate compounds could decompose and release their own methane, as well as reservoirs of pure methane currently trapped below them, to the air above. Methane trapped far below the permafrost in northern areas and in offshore areas in the Arctic also exists in the form of clathrates; it would be released eventually if the Arctic warmed sufficiently. Measurements made thus far do not indicate any significant emissions from these sources. It has been suggested by some scientists that CH+ released from clathrates may be oxidized to C02 before it reaches the air, thereby greatly reducing the global warming potential.

Although the uncertainties concerning methane feedback are large, the stakes are higher than with any other gas. A few scientists believe that several positive climate feedback mechanisms, including those involving methane, could possibly combine to trigger an unstoppable warming of the globe. This worst-case scenario is called the runaway greenhouse effect. Such climate change would threaten all life on Earth, as the temperature would rise markedly, ocean currents would probably shift, and rainfall patterns would be very different from those we know. The possibility that the North Atlantic ocean current, which brings warm water from the south and thereby warms Europe, could cease to operate because of rapid global warming— induced by rapid increases in methane or carbon dioxide—is one of the most dramatic predictions about the possible consequences of the enhanced greenhouse effect.

PROBLEM 6-10

Calculate the mass of methane gas trapped within each kilogram of methane hydrate.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

Get My Free Ebook


Post a comment