Humans and carbon sequestration

The preceding discussion highlights the role of life in carbon cycling, currently one species Homo sapiens is having unprecedented effects on this system. This is not a book on applied ecology, however, it is impossible to write a chapter on carbon sequestration without any mention of the effects of humans on the future of the Earth System (see Lovelock (2006) for an impassioned Gaia perspective on the perils of human-driven climate change). With 'global warming' we are not taking part in any new processes within the Earth system, but we are greatly altering the rate of some of the existing processes (see Box 8.2 for a brief history of human intervention in the Earth's carbon cycle). In the context of Fig. 8.2 we have greatly increased the rate of 'Fwg' by burning fossil fuels. In addition, we have altered the distribution of carbon within the 'surficial system' box by processes such as biomass destruction—which converts carbon from organic matter into atmospheric CO2.

Box 8.2: Chronology of events relating to humans and the carbon cycle (based, in part, on the appendix of Grace, 2004)

Before 1800. For thousands of years human activity, such as forest clearance, has probably caused changes in the level of greenhouse gasses (e.g. CO2 and CH4) in the atmosphere. The climatologist William Ruddiman (2003, 2005) has estimated that we started to have effects on CO2 levels some 8,000 years ago and methane levels some 5,000 years ago; however, these conclusions are somewhat controversial (Crutzen and Steffen, 2003).

1780-1820. Industrial Revolution—a dramatic increase in the use of coal and so a rise in atmospheric CO2.

1859. Oil wells sunk in Pennsylvania, USA—oil is soon being produced in many countries. Also in this year the Irish scientist and mountaineer John Tyndall discovers that water and CO2 absorb specific wavelengths of infra-red radiation and suggests that this is important in the regulation of the Earth's temperature (the 'Greenhouse effect').

1896. Swedish chemist Svante Arrhenius suggests that increasing emissions of CO2 will lead to global warming.

1903. Ford motor company founded in USA, this quickly leads to the start of the mass production of cars.

1958. Charles Keeling starts measuring atmospheric CO2 on Mauna Loa in Hawaii using an infra-red analyser, these measurements are still being made today (Heimann, 2005).

1968. First photographs of the Earth from deep space, these have a dramatic effect on how people view the Earth; it looks a lot smaller and easy to damage when seen from space. The astronomer Fred Hoyle (1950) predicted the psychological effect of such photos on our view of the planet almost 20 years before they were taken!

1979. James Lovelock publishes his first book on 'Gaia', suggesting that life may be crucial to the regulation of the Earth's climate and chemistry.

1987. Ice core data from Antarctica show a close correlation between CO2 and temperature over the past 100,000 years.

1988. Intergovernmental Panel on Climate Change (IPCC) is established.

Box 8.2: (Continued)

1997. Kyoto Protocol, international agreement to limit greenhouse gas emissions.

1998. The warmest year of the century, and probably of the millennium.

2001. President Bush announces that USA will not sign the Kyoto protocol.

2005. Kyoto Protocol formally came into force in February, but without the USA. However, some US states are taking their own action to try and meet Kyoto targets.

Although the silicate weathering feedbacks control p CO2 on geological time scales, it has little effect on time scales less than 100,000 years (Ridgwell and Zeebe, 2005)—so even though it will eventually help to reduce CO2 artificially increased by human activity it will not do so on a time scale considered useful by most people! On shorter time scales the marine carbonate cycle does play a role in atmospheric CO2 and this is currently a cause for concern, as increased atmospheric CO2 causes more CO2 to dissolve in the ocean. This lowers the ocean's pH and decreases CO32~—one of the building blocks of calcium carbonate—according to the following equation (Gattuso and Buddemeier, 2000):

This can cause decreased calcification of plankton (Riebesell et al., 2000). It is currently uncertain how a decrease in calcified plankton will feedback to p CO2 in the atmosphere. One possibility is that a reduction of carbonate production in the surface ocean would reduce the amount of CO2 in the water therefore allowing it to absorb more atmospheric CO2, and so act as a negative feedback on global warming (as carbonate production releases CO2 into the water, as described at the end of Section 8.3). However, it is also plausible that carbonate production helps remove particulate organic carbon from the surface waters to ocean sediments, so less carbonate production could reduce the removal of organic carbon to ocean sediments and so act as a positive feedback on global warming (Ridgwell and Zeebe, 2005). It would obviously be good to know which of these two processes is the most important!

Probably the first direct human intervention in the short-term carbon cycle was by forest clearance. For example, 5,000 years ago the dominant vegetation type over most of Britain was various sorts of forest: ranging from Pine Pinus sylvestris and Birch Betula spp dominated forests in the north to Lime Tilia spp dominated forest in the southeast of England and Oak Quercus spp dominated ones over much of southwest England (Bennett, 1989). As agriculture developed most of this forest was lost, a trend that has been repeated in many parts of the world. Agriculture first appears approximately 10,000 years ago at the eastern end of the Mediterranean; although just as with the ages of the first life or first eukaryotes there are disagreements about the details of exact timing (Lev-Yadum et al., 2000). Over the past 10,000 years the idea of agriculture has been invented on a minimum of five occasions around the world—and possibly on nine or more independent occasions (Diamond, 1997 and 2002). William Ruddiman (2003, 2005) has suggested that deforestation associated with agriculture was affecting global CO2 levels some 8,000 years ago and CH4 production (presumably from rice paddies etc.) rises about 5,000 years ago. As is well known there are still concerns about forest clearance in areas such as the Amazon basin.

Since the removal of forests has been a source of CO2 replacement of forests could act as a CO2 sink (But see Box 7.2 for possible implications for CH4 production). For example, Rowantree and Nowak (1991) estimated that the urban trees of the USA sequestered approximately 6.5 million tonnes of carbon per year. Provided rainfall levels are still adequate forests can cover a denuded landscape remarkably quickly (Fig. 8.3). The classic example of this are the forests of eastern USA and Canada much of which were cleared during the eighteenth and nineteenth centuries to make room for European-style agriculture. However, during the mid-nineteenth century, as people moved from the farms to seek their fortunes in the newly expanding cities, much of this land went out of agricultural production and returned to forest (Foster, 1992). Although the North American example is the most well studied, giving rise to many detailed studies of 'old field succession' described in the ecology textbooks, similar things have happened in many other places. For example, between 1960 and 1980 large areas of forest were cleared in the Chorotega region of Costa Rica— this area includes the Santa Rosa National Park well known to ecologists through decades of groundbreaking work by Daniel Janzen and colleagues. Much of the forest was cleared for beef production and when, in the 1980s, beef prices fell large areas were allowed to revert to forest (Arroyo-Mora et al., 2005). Both the north American and the Costa Rican examples are of areas which have returned to forest after earlier human-caused deforestation. However some areas have never had forest because they were too isolated for trees to arrive and have not had enough time for the tree growth-habit to evolve from scratch, a fascinating example being Ascension Island in the tropical south Atlantic. Much of the island is very arid, however, on Green Mountain there is much higher rainfall and considerable occult precipitation. Through human introductions of a large

Fig. 8.3: Examples of new or re-grown forests. (a) Regenerating forest in southern Ontario, Canada. This is typical of much of eastern USA and Canada in having been largely cleared of forest for agriculture in the eighteenth and nineteenth century, with later forest re-growth after many people moved to the developing sites of eastern north American during the later nineteenth and early twentieth centuries. (b) The summit of Green Mountain on Ascension Island. Prior to human

Fig. 8.3: Examples of new or re-grown forests. (a) Regenerating forest in southern Ontario, Canada. This is typical of much of eastern USA and Canada in having been largely cleared of forest for agriculture in the eighteenth and nineteenth century, with later forest re-growth after many people moved to the developing sites of eastern north American during the later nineteenth and early twentieth centuries. (b) The summit of Green Mountain on Ascension Island. Prior to human number of plant species, the mountain has gone from a treeless system in the early nineteenth century to one which today is best described as tropical cloud forest (Wilkinson, 2004a).

The archetypal pristine forest system which has become an icon for much of the environmental movement are the tropical rainforests. The examples of both dry and moist forests from Costa Rica and the Ascension Island cloud forest suggest that tropical forests may be able to recover quickly from serious human disturbance, in a similar way to the forests of eastern north America. Palaeoecological studies are now starting to show that many areas currently covered by tropical rainforest have had significant disturbance in the past. A good example of this is the Darian area of Panama, rich in endemic species, it gives the impression of untouched tropical wilderness. However, Bush and Colinvaux (1994) used pollen and other remains from sediment cores to show a long history of human disturbance. Their data suggested significant forest clearance for agriculture which was abandoned after the Spanish conquest, so this apparently primeval tropical forest was largely 350 years old. Similar evidence for widespread human disturbance of apparently 'virgin' rainforest has recently been described from Africa and Asia, as well as South and Central America (reviewed by Willis et al., 2004); clearly rainforest systems may sometimes be much more resilient than has previously been assumed. It is interesting to compare this rapid recovery of recent forests with the evidence for rapid recovery of terrestrial biomass following the supposed impact event at the end of the Cretaceous (Section 6.4).

Probably the most important factor in this fast recovery, and its associated carbon sequestration, is adequate rainfall. In the Ascension Island example, forest was able to colonize Green Mountain because of the trade winds blowing large amounts of moisture onto the summit (Ashmole and Ashmole, 2000; Wilkinson, 2004a). Modelling suggests that destruction of large areas of the Amazon forest could greatly reduce rainfall due to reduced transpiration and so lead to a climate unsuitable for tree growth (Betts, 2004). Therefore a large biomass of vegetation in the Amazon is crucial to the continual existence of a climate able to support a large biomass of vegetation. Presumably the clearances of tropical forest now known from the archaeological record were not extreme enough to have such wider climatic effects.

intervention the largest common plants here were ferns (although the ferns visible in the photograph are introduced species). Now the summit is cloud forest dominated by Bamboos with trees covering much of the rest of the mountain. The sunny conditions in the photograph are deceptive, for much of the time the summit is enveloped in cloud which provides the water supply for this lush growth of vegetation on an otherwise arid island.

One important result of thinking about the carbon cycle from a more geological perspective is that it draws attention to the crucial role of soil. In the long-term carbon cycle this soil is important because of its effects on silicate weathering, in the shorter cycle it is the amount of organic matter contained in the soil which is important; how this will change with global warming is currently a large and important unknown in the behaviour of the Earth System (Cox et al., 2000; Lenton and Huntingford, 2003). The importance of soils in carbon sequestration is illustrated by a survey of Monks Wood, a well-studied deciduous woodland in southeast England, where it was estimated that approximately 129 tha-1 of carbon was contained in the vegetation (including roots) and 335 tha-1 in the soil (Patenaude et al., 2003).

The extent to which land use changes have affected soil carbon content is not well quantified but it is potentially very important, for example some cultivated soils are known to have lost over half their soil organic carbon (Lal, 2004). Therefore the management of soils, as well as above-ground biomass, is important in the context of increasing atmospheric CO2 and climate change. The effect of past human land use on soils is also a potential complication for the ideas of Ruddiman (2003, 2005) on an early start to human effects on the global carbon cycle and climate. Many of the past modifications of the land will have reduced soil organic matter which is consistent with Ruddiman's hypothesis; however, there are important exceptions. For example, in Britain clearance of forest from wet, often upland, parts of the country triggered the formation of blanket bog (Moore, 1975, 1993), such systems can sequester more carbon than the previous forest. However, the anaerobic nature of peat bogs creates an additional complication as microbes within them produce CH4 (a greenhouse gas) as well as sequestering carbon. Peatlands are important in the global short-term carbon cycle as they contain around 455 Gt of carbon, which equates to the amount of carbon in all living organisms on the surface of the Earth (Moore, 2002). As such destruction of peatlands—be they the temperate bogs described in this chapter's introduction or tropical peatlands such as those in southeast Asia— is clearly important for the CO2 content of the atmosphere.

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