Methane The Other Carbon

CO2 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 (see 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 CO2 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 does not survive long in the atmosphere or oceans. Some bacteria at the oxygen-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 does 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 CO2—the common denominator of the carbon cycle—within a few decades of being released to the atmosphere.

Humans are adding to the CH4 content of the atmosphere at present by increasing the incidence of the particular environmental conditions—decay without oxy-gen—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 source 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 atmo-sphere—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 CO2, so less of it is needed to make a big difference. At present, while CO2 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 effect over the next couple of centuries, 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 last ice age was boosted by methane released from hydrate layers on the sea floor and under permafrost.

7.3.1 Carbon and the history of the earth's temperature

There is a fair amount of evidence that for the first 2.5 billion years of its existence (out of roughly a 4.5 billion year history), the earth was much hotter than it is now, probably because its atmosphere was packed full of greenhouse gases such as CO2

and CH4. Studies on the oxygen isotopes in silica—which precipitated out of oceans and fresh waters—before about 1.5 billion years ago indicate the earth's average temperature may have been above 50 °C. Although this is too hot for multicellular animals and plants, there are many types of archean bacteria (known as thermo-philes) that thrive in extreme temperatures in hot springs in the present-day world. Some of them will even grow in water kept under pressure above boiling point. If the temperature reconstructions are accurate, presumably, back then the ancestors of these thermophiles were the major life forms on earth, floating in the ocean and working away in its sediments. Around 1.5 billion years ago, a long time after the time that oxygen-producing photosynthesis appeared 3.5 billion years ago, the earth seems to have undergone a dramatic cooling. This culminated in a series of massive ice ages that brought polar ice sheets right down to the tropics. The jumbled sediments that form just offshore from an ice sheet are found for example in Namibia, a subtropical country which was even closer to the equator at that time. It has been suggested that the relentless extraction of carbon by photosynthesizers in the ocean took so much C02 from the atmosphere that the earth's temperature dropped dramatically to around freezing on several occasions, each cold phase lasting a few million years. Then the earth would be seized by a sudden warming that melted back the ice. Exactly what brought the temperature back up again is a matter of conjecture. 0ne idea has it that as global temperature declined to around freezing the uptake of C02 by rock-weathering (see below) virtually ceased, and allowed carbon dioxide added by volcanoes to build up in the atmosphere and warm the world up. Eventually, by around 700 million years ago the earth's temperature settled down to stable warmer temperatures, for reasons that are unclear. Animal life first appears in the fossil record after these great ice ages ended, suggesting that the more stable climate was more important in the evolution and survival of complex life forms.

7.3.2 Plants, weathering and C02

0ver the last several hundred million years, plants have progressively spread out of the seas and rivers, and across the continents. This has meant more living carbon stored in vegetation, and more dead carbon in soils. It has also probably led to a dramatic increase in another route by which carbon is taken out of the atmosphere: weathering. Weathering is the gradual breakdown of the minerals in rocks that are exposed near the surface. In terms of the long-term carbon cycle, the most important chemical reactions of weathering tend to occur on igneous rocks—the products of solidified magmas from within the earth such as granites and basalts. Even in a lifeless world, weathering reactions would occur naturally where there is any water, plus carbon dioxide which acts as an acid to dissolve the silicate minerals in rocks. 0n the planet Mars, which is apparently without life, chemical weathering was once able to break down the rocks to yield the red iron oxide that gives the planet its characteristic color. Although weathering can occur under lifeless conditions, the signs are that the presence of organisms greatly speeds up the rate of the reactions: by tens, hundreds, even thousands of times. The products of weathering of an igneous rock—whether on Mars or on Earth—tend to be the familiar constituents of soil: clays, quartz sand

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 several-fold, compared with lifeless rock surfaces. Source: Author.

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 + C02 + water ^ clay + carbonate salts + 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'', see 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 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.

■ Lichen site 20L o Lichen site 20A

Figure 7.5. Results of an experiment that compared the amounts of salts (derived from weathering) turning up in rainwater that had run off lichen-covered rocks vs. bare rock surfaces. Weathering rate—as indicated by the magnesium ion or silicon content of the water—is several times higher on the lichen-covered areas of rock. After Schartzman.

Lichen-covered

Lichen-covered

1/Precipitation (cm-1)

1/Precipitation (cm-1)

Lichen site 20L Lichen site TW

o Lichen site 20A + Lichen site 20B

The bigger land plants, with roots, shoots and leaves, have likely led to a further increase in weathering rate and in CO2 uptake. Their roots are 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 are 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 CO2 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. The plants may also benefit from the direct effect of fertilization by the increased CO2 (Chapter 8). The more vigorous the plants, the greater the rate of weathering that takes CO2 down. The decrease in atmospheric CO2 should tend to cool the planet, and as the planet cools the weathering rate should also ease 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 filled the atmosphere with CO2 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 CO2 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 CO2 to accumulate in the atmosphere and warming the climate.

Nevertheless, at times the weathering feedback can perhaps be thrown off-balance if it is suddenly presented with too many rocks to weather. When the Himalayas, 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 CO2 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 CO2, which can leave the ocean), there are limits to how much extra CO2 it can provide. Maureen Raymo of MIT proposed that the eventual outcome of plants weathering rocks from the Himalayas and other mountains may have been a precipitous decline in CO2 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 carbonate gets deposited in the mud on the sea floor. This becomes buried, compressed and turned into limestone and other carbonate-containing rocks. On an even longer timescale, over tens or hundreds of millions of years, the carbonate-containing rocks may be folded and heated to the point where the carbonate begins to break down and give off C02. The C02 percolates up through cracks in the rocks or gets dissolved in the molten magma of volcanoes, and returns to the atmosphere from hot springs or volcanic explosions. Thus, in addition to the carbon cycle of the surface world of plants, atmosphere, soil, and oceans there is a deeper and slower carbon cycle of rocks and volcanoes. Much of the C02 which enters the atmosphere from volcanoes and hot springs was previously part of the living world of plants and soils, from which it was taken up in biological weathering processes, and formed into the skeletons of marine organisms before it ended up on the sea floor. It turns out that the biological world is unexpectedly and inextricably linked to the geological world.

7.3.3 Plants, C02 and ice ages

0nce the time of ice ages had got started about 2.5 million years ago (Chapter 1), marine plants acting on the carbon cycle may also have played an important role in altering the detailed course of both C02 and temperature. Analyses of bubbles of the ancient atmosphere trapped in ice cores through the Greenland and Antarctic ice caps show that there were fluctuations of around 30% in the atmospheric C02 level. Each time the earth slipped into a major glaciation, the C02 level paralleled the climate change, reaching a low point in concentration at just about the time that ice sheets were at their most extensive (Figure 7.6a, b). And each time the world warmed, the C02 concentration shot up just about as fast. Essentially, the only way such large and relatively rapid fluctuations in C02 could have occurred is if something in the oceans was storing up carbon and then releasing it again. There are various ideas to explain what was occurring, but they all involve phytoplankton as an integral part of the mechanism.

0ne idea is that the plankton somehow became more productive during glacial conditions, perhaps because there was more upwelling in certain parts of the oceans, bringing nutrients to the surface where they could be used. The increased rain of dead phytoplankton cells and other remains of the food chain dragged more organic carbon down into the deep ocean. There it accumulated as organic carbon in the sediments or dispersed through the water, or perhaps it was oxidized into C02 and became part of the alkalinity reservoir (the bicarbonate and carbonic acid combination) in the deep ocean (Figure 7.7a). Either way, part of the underlying cause of the lower C02 levels during ice ages would have been the continual "pumping" of carbon down into the deep ocean by phytoplankton; C02 was taken up in photosynthesis and (in effect) sent downwards where it accumulated. Somehow, the nutrients got separated out along the way and recycled up to the surface by upwelling, whereas the carbon remained in the abyss. Various models of ocean circulation during ice ages have been devised to explain what might have brought about this increase in plankton productivity, and at least some observations seem to support the idea that the oceans were more productive during glacials. A key area, which both the models and geological studies of the ancient ocean 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

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Age of entrapped air (kyr bp)

Age of entrapped air (kyr bp)

History of temperature and atmospheric C02, deduced from polar ice cores.

Temperature history is derived from analyzing isotopes in the ice. C02 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 100,000 year timescale. From work by Barnola et al. (CDIAC).

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 phytoplankton growth during ice ages, and others contradicting it.

Because of the contradictory nature of the evidence, the idea that increased plankton productivity was the key to low glacial C02 has rather fallen out of favor. Instead, attention has tended to focus on a second mechanism that involves a slowed-down circulation in the deep ocean (Figure 7.7b). Basically, the oceans are made up of different layers of water stacked up like a layer cake, each moving in different directions, and originating in particular places where surface water folds down into the deep (see Chapter 1). Each of these layers tends to disperse upwards over time, and they also tend to well up to the surface more rapidly in particular places near coasts

Interglacial ocean

Interglacial ocean

Glacial ocean

Figure 7.7. (a, b) How plankton activity may have decreased the C02 concentration during glacials. (a) An increased "biological pump" puts more carbon down into the deep ocean. (b) A slower circulation of the deep ocean means that more carbon can accumulate there. Source: Author.

Figure 7.7. (a, b) How plankton activity may have decreased the C02 concentration during glacials. (a) An increased "biological pump" puts more carbon down into the deep ocean. (b) A slower circulation of the deep ocean means that more carbon can accumulate there. Source: Author.

(such as off the coast of Peru, see Chapter 1). As these water layers move sideways through the ocean on their journey away from the place where they originated, they accumulate a rain of carbon-containing debris from the plankton and other sea life above. The longer that these moving water layers spend in the deep, the more carbon gets loaded up within them. Although the deep waters will eventually well up or mix up into the surface waters and release the carbon they accumulated as CO2 back into the atmosphere, if they move more slowly they will always end up holding more carbon in the deep ocean. Many questions remain about exactly what was different in the ocean circulation during glacials, but plausible models of the ice age ocean circulation tend to suggest that slower-moving deep ocean water is the best explanation for the low CO2 of ice ages. So, in this scenario decreased CO2 during ice ages occurs because of a basic change in the ocean circulation, but it is a mechanism that only works against a background of plant productivity always sending carbon down. Once again, we see plants are playing an integral role in the global carbon cycle.

However, while plants in the ocean were working in such a way as to help lower the CO2 level of the atmosphere, plants on land seem to have been doing the opposite. During glacials when plankton were loading up the deep ocean with carbon, the world's land surfaces were much colder and drier. A vegetation map of any region of the world at that time makes the point when compared with a modern vegetation map (Figure 7.8a, b). Many areas that would now naturally be forest were scrub or grassland, and what are now grassland areas tended to be semi-desert or desert. In the glacial world, peat deposits (which nowadays make up about a quarter of the organic material stored on land) were almost non-existent. Making some reasonable guesses about how much carbon would be present in each biome in its natural state, there can be basically no doubt that there was far less carbon stored on land as vegetation and in soils during glacials, than during interglacials—such as the one we are in at present. The shift in carbon storage at the start of a glacial would have occurred gradually, as growth of new trees and productivity of new plant parts declined, while the respiration that breaks down dead plant parts and the humus in soil continued slowly but relentlessly. When breakdown of organic carbon on the forest floor and in soils is not equaled by new production, the carbon reservoir shrinks and the CO2 floods out to other parts of the carbon cycle. So, just as the oceans were tending to drag down the CO2 level of the atmosphere, land ecosystems were releasing carbon out into the atmosphere (Figure 7.9). The land carbon must have had the effect of keeping the CO2 level higher overall than it would have been if the oceans had worked unopposed. The amount of carbon which left the land system during glacials, about 1,000 billion tonnes (about half the carbon that would have been there on land during an interglacial), would actually have been enough to raise the global CO2 twice over. The major reason that glacial CO2 was not higher 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 CO2 level by some 15%. We know of course that the CO2 level in the atmosphere actually fell, by some 30% during glacials, because the ocean had soaked up the extra carbon. However, if the land system had not pushed out carbon it is reasonable to suppose that glacial CO2

Figure 7.8. The distribution of forest and desert in (a) the present natural world and (b) the last glacial 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.

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, with 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 CO2 from declining too far despite the best efforts of the ocean

INTERGLACIAL

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INTERGLACIAL

Carbon stored in vegetation and soils ccccccccc

LAND

Carbon stored in vegetation and soils

GLACIAL

GLACIAL

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.

LAND

OCEAN

LAND

OCEAN

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.

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 age.

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. However, 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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