Methane The Other C Arbon

C02 is a sort of common currency for the carbon cycle. It is produced in abundance by all living organisms, and is the chemically stable end point for many different processes going on in the earth's atmosphere, ocean and soils. It also participates in a whole range of different processes, including photosynthesis and chemical breakdown of rocks (sec below). Methane gas by contrast is only produced under special circumstances, usually where there is almost no oxygen. It is a result of the incomplete breakdown of organic molecules which would normally be burnt to C02 and water by oxygen-breathing organisms. Under anaerobic (oxygen-free) conditions, the energy yield is measly but better than nothing; most of the chemical energy present in the original biological molecules still remains unreleased in the methane that escapes to the surface. The oxygen-free conditions that lead to methane being produced by bacteria tend to exist in the still water and muds of swamps where the diffusion of oxygen down from the surface is slow. They also occur in the guts of animals—especially the fore-stomachs of ruminant herbivores such as cattle, and in the hind-gut of termites—which together produce a large fraction of the roughly 50 million tonnes of methane that enter the atmosphere each year.

Because methane is so rich in stored energy, it docs not survive long in the atmosphere or oceans. Some bacteria at the oxygcn-rich surface of swamps live by burning the methane that comes from the anaerobic layers below. Much of the methane that bubbles up from anaerobic sea floor sediments is destroyed by methane-consuming bacteria floating in the ocean waters, before it can reach the atmosphere. The methane that docs get out into the air above is steadily broken down by reacting with oxidizing fragments of molecules—such as the hydroxyl radical— that exist in an oxygen-rich atmosphere. Most methane molecules are broken down to C02—the common denominator of the carbon cycle—within a few decades of being released to the atmosphere.

Humans arc adding to the CH4 content of the atmosphere at present by increasing the incidence of the particular environmental conditions decay without oxygen—that lead to it being released. Wherever rice is cultivated, the oxygen-free conditions of a swamp are created artificially in the flooded field. As human populations in Asia have grown, the area of these paddy fields has expanded, and output of methane to the atmosphere has increased. Cattle, pig and sheep populations have also increased as the world's population has grown, and this too has led to a large increase in methane output. Another indirect effect of humans has been partial clearance of forest land, which has led to an increased termite population and termites are also a potent sourcc of methane. The net result of all these factors has been a doubling of the methane content of the earth's atmosphere in the last 200 years.

Although methane still only occurs at very low concentrations in the atmosphere—about 1.700 parts per billion—it is important because it is a very potent greenhouse gas. A molecule of CH4 traps much more heat than a molecule of C02, so less of it is needed to make a big difference. At present, while C02 is thought to be dominating the heating up of the world due to the increased greenhouse effect, CH4 occupies a smaller but significant second place.

While most of the methane from natural decay escapes to the air and is broken down, under some circumstances it can become trapped within the sea bed or under the ground, as a strange substance known as methane hydrate. Methane hydrate resembles ice, but it is actually a mixture of methane gas and water. It can exist under pressure at cool temperatures close to freezing, but if it is warmed or if the pressure is reduced, it will fizz until it has released a huge volume of methane gas from even a small volume of hydrate. Eventually, all that is left is water, with all the methane having escaped. Methane hydrate exists in huge quantities within the sea bed in certain parts of the oceans. The quantities are uncertain, but it could exceed the amount of carbon currently in soils and vegetation. The Gulf of Mexico is one area where it is particularly abundant, for example. In some other places methane hydrate has formed beds trapped under thick permafrost (e.g., in northern Siberia).

Most of the world's methane hydrate is many thousands—if not millions—of years old. One fear for the future is that as the deep oceans warm due to the greenhouse eflcct over the next couple of ccnturies, methane hydrate on the sea floor will start to release methane gas. This might pour methane out into the atmosphere, amplifying the warming. There are some signs that sudden global warming events in the geological past—for example, one around 55 million years ago that extended subtropical vegetation far into the Arctic Circle were also caused by massive releases of methane hydrate. Some geologists have suggested that the sudden ending of the presence of organisms greatly speeds up the rate of the reactions: by lens, hundreds, even thousands of times. The products of weathering of an igneous rock—whether on Mars or on Harth—tend to be the familiar constituents of soil: clays, quartz sand grains, iron oxide and salts. Although it varies according to the different minerals found in a rock, the general chemical process is roughly as follows:

Rock silicate I C02 I water => clay I carbonate salts I silica \ metal oxides

In moist environments on earth the carbon-containing salts are dissolved in rainwater and washed by rivers down to the sea (where they form the "alkalinity reservoir", sec above), although in arid regions they accumulate inland to give carbonate-rich soils and salt lakes.

As soon as any life evolved on earth, there was probably some sort of living film of microbes covering rocks on land, and it is likely that these microbes accelerated weathering by producing acids and other by-products that etched into mineral surfaces. Another step-up in the weathering rate likely occurred as the first lichens (Figure 7.4) appeared on land, perhaps 600 million years ago from the few tentative fossils. These symbiotic organisms, combining a fungus and an alga, have been shown to produce acids and chelating (ion-binding) agents that can increase the weathering rate by several times compared with a bare rock surface (Figure 7.5). From the point of view of the carbon cycle, the important thing here is that carbon is taken up into the weathering process, ending up ultimately as calcium carbonate and

Figure 7.4. One of the thousands of species of lichens symbiotic combinations of a fungus and alga. Lichens are thought to accelerate the chemical weathering rate by hundreds or even thousands of times, compared with lifeless rock surfaces. Source: Author.

bicarbonate—the "alkalinity" in the oceans. The faster the rate of weathering, the faster C02 is removed from the atmosphere. If other sources such as volcanoes do not replenish it as quickly as it is taken up, the C02 level in the atmosphere will fall.

The bigger land plants, with roots, shoots and leaves, have likely led to a further increase in weathering rate and in C02 uptake. Their roots arc very good at insinuating themselves into cracks in rocks and between mineral grains. As well as producing their own exudates, plants employ fungi (mycorrhizae) living on their roots to help with the process of breaking down minerals and sucking in the nutrients that are released. From experiments and observations, it seems that vascular plants can increase the weathering rate ten-fold over the simpler lichens and algae which preceded them on the land surface. Mosses arc also likely to be good at promoting weathering, because they can form a dense spongy mass over the rock surface, as well as accumulating dead parts that break down to release acids.

By promoting weathering, it is thought that land plants of various sorts may act as a sort of thermostat on the earth's temperature. If a burst of volcanic activity causes C02 levels to increase over several million years, the warming that results from it should affect the rate of growth and the mass of plant material around the world. 'I'hc plants may also benefit from the direct effect of fertilization by the increased C02 (Chapter 8). The more vigorous the plants, the greater the rate of weathering that takes C02 down. The decrease in atmospheric C02 should tend to cool the planet, and as the planet cools the weathering rate should also case off. This negative feedback may be part of the reason that the earth's temperatures have stayed within the general band that they have, since the origin of complex plant life on land. Without this living thermostat, fluctuations in volcanic activity would sometimes have tilled the atmosphere with C02 gas and made it burning hot, too hot for life. By turning up its activity as the temperature increased, the weathering thermostat would have taken more C02 out and kept a moderate temperature. At other times, if the sun somehow became fainter or ice sheets began to spread, weathering rate would decrease allowing more C02 to accumulate in the atmosphere and warming the climate.

Nevertheless, at times the weathering feedback can perhaps be thrown off-bal-ance if it is suddenly presented with too many rocks to weather. When the 1 lima lay as, the Andes, the Alps and other mountain belts grew up almost simultaneously over the past few tens of millions of years, the huge volumes of easily weatherable igneous rocks that they exposed may have led to most of the C02 being sucked out of the atmosphere. While the alkalinity store in the oceans would tend to replenish it (as bicarbonate dissociates to yield carbon plus C02, which can leave the ocean), there are limits to how much extra C02 it can provide. Maureen Raymo of MI T proposed that the eventual outcome of plants weathering rocks from the I limalayas and other mountains may have been a precipitous decline in C02 and the drastic global cooling trend that ended in the ice ages of the last 2 million years.

The carbon taken up by weathering does not stay forever in the seawater in the alkalinity reservoir. On the timescale of millions of years, sea creatures extract calcium from the bicarbonate in sea water, to build their shells and skeletons. When they die, this calcium carbonatc gets deposited in the mud on the sea floor. This

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Figure 7.6. (a. b) History of temperature and atmospheric CO:, deduced from polar ice cores. Temperature history is derived from analyzing isotopes in the ice. CO> history comes from analyzing bubbles of ancient air trapped within the ice. Both temperature and C02 show the same "sawtooth" pattern of fluctuations on approximately a MX).000 year timescale. From work by Barnola et al. (CDIAC).

Age of entrapped air (kyr bp)

Figure 7.6. (a. b) History of temperature and atmospheric CO:, deduced from polar ice cores. Temperature history is derived from analyzing isotopes in the ice. CO> history comes from analyzing bubbles of ancient air trapped within the ice. Both temperature and C02 show the same "sawtooth" pattern of fluctuations on approximately a MX).000 year timescale. From work by Barnola et al. (CDIAC).

geological studies of the ancient occan have focused on, is around and underneath the sea ice off Antarctica. This area is already biologically very productive in the modern-day world. If the sea ice area extended and became even more productive than it is now, this could explain how extra carbon was sent to the bottom of the ocean. However, the picture from observations of ocean sediments is rather complex, with some indicators of past productivity supporting the idea of more vigorous phyto-plankton growth during icc ages, and others contradicting it.

2 v C02 Atmospheric carbon dioxide

Fixed by ocean C plankton c c C C C

Fixed by ocean C plankton

Dead cells and other organic materials fall to deep ocean, taking carbon with them

Interglacial ocean

Fast circulation of deep water does not allow carbon to accumulate

Glacial ocean

Glacial ocean

Figure 7.7. (a,b) How plankton activity may have decreased the CO: concentration during glacials, (a) An increased "biological pump" puts more carbon down into the deep ocean, (h) A slower circulation of the deep ocean means that more carbon can accumulate there. Source: Author.

Slower circulation of deep water lets more carbon accumulate

Figure 7.7. (a,b) How plankton activity may have decreased the CO: concentration during glacials, (a) An increased "biological pump" puts more carbon down into the deep ocean, (h) A slower circulation of the deep ocean means that more carbon can accumulate there. Source: Author.

□ Extreme desert (<2% vegetation cover)

□ Extreme desert (<2% vegetation cover)

□ Closed forest

□ Extreme desert

Figure 7.8. The distribution of forest and desert in (a) the present natural world and (b) the last alacial maximum or LGM (18,000 14C years ago). The LGM world is much colder and drier, resulting in less forest and more desert. Source: Author.

been there on land during an intcrglacial), would actually have been enough to raise the global C02 twice over. The major reason that glacial C02 was not higher but in fact lower was that most of this (about seven-eighths of it) would have dissolved in the ocean anyway as part of the alkalinity reservoir, leaving only about one-eighth above the water. However, even this amount would be enough to raise up C02 level by some 15%. We know of course that the C02 level in the atmosphere actually fell, by some 30% during glacials. I lowcvcr, if the land system had not pushed out carbon

INTERGLACIAL

Carbon stored in vegetation and soils

INTERGLACIAL

Carbon stored in vegetation and soils

GLACIAL

Ocean takes

GLACIAL

Land releases CO LAND

Figure 7.9. How the land reservoir of carbon may help keep up C02 concentrations in the atmosphere when the oceans are dragging carbon down.

Ocean takes

Land releases CO LAND

Figure 7.9. How the land reservoir of carbon may help keep up C02 concentrations in the atmosphere when the oceans are dragging carbon down.

it is reasonable to suppose that glacial C02 would have gone even further down, adding to the severity of the ice age through a weaker greenhouse effect. Climate models suggest that the earth is always on the edge of a runaway glaciation during ice ages, w ith an ice albedo feedback on temperature that would bring ice sheets down to the tropics just as happened once already about a billion years ago (see above). Part of what has kept this from happening in more recent geological history may be the readily-released reservoir of carbon in plants and soils, that prevents C02 from declining too far despite the best efforts of the ocean carbon pumps to drag it down. This is then a negative feedback on the intensity of ice ages: as temperature declines, carbon is released from the land tending to prevent a further descent into a deeper ice ase.

As life has diversified and spread across the earth's surface, the potential for such "checks and balances" within the system seems to have increased and has likely kept the climate more stable. I lowever, it is worth bearing in mind that the land vegetation is also working in another very different way to destabilize the earth's climate during ice ages: in Chapters 4-6 we considered how vegetation affects temperature through its albedo. As vegetation cover diminished during an ice age it would not only be releasing C02 (which tends to keep the earth warmer) but leaving behind bare soil and exposed snow-covered ground which reflects sunlight straight back into space, cooling the world in a positive feedback. So, as the earth cools this feedback only tends to make things cooler. In the global system, nothing is ever simple; the same component—vegetation—can be participating in both positive and negative feedbacks at the same time!

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