Ecosystems can be grouped into biomes, which reflect natural geographic differences in soils and climate, and consequently different vegetation types (Woodward et al. 2004). These biomes differ greatly in their capacity to assimilate and store carbon (De Deyn et al. 2008). In addition to the balance between carbon gains through growth and losses through respiration, ecosystem carbon balance is also regulated by several other factors including fire, herbivores, erosion and leaching. This section looks at carbon stores and capacity in each biome as well as at peatlands, coasts and oceans and examines the effects that human activities have on those biomes and their role in the carbon cycle.
Tundra ecosystems are dense in carbon. They have little potential to gain more carbon but a huge amount could be lost if the permafrost were to thaw. Prevention of climate change is currently the only failsafe method of minimising this loss.
Tundra ecosystems are found in Arctic and mountainous environments, particularly in Northern Canada, Scandinavia and Russia, Greenland, and Iceland. Temperatures are low or very low for most of the year with prolonged periods of snow cover and a short growing season. The active layer of soil, near the surface, tends to be waterlogged in summer and frozen in winter. Diversity of plants and animals is low. The environment selects for slow-growing hardy plants with low biomass above ground. Rates of decomposition are low and large amounts of dead plant material accumulate in the soil (approximately 218 t C per ha, Amundson 2001). The slow decomposition rate means that nutrient recycling is also slow, providing a further limitation on plant growth and leading to tundra plants allocating most of their biomass below ground (De Deyn et al. 2008). Total plant biomass is estimated to average 40 t C per ha (Shaver et al. 1992).
Below the active soil layer is a perennially frozen layer known as permafrost. Although it is difficult to estimate it is believed that carbon storage in permafrost globally is in the region of 1600 Gt, equivalent to twice the atmospheric pool (Schuur et al. 2008).
HUMAN IMPACTS AND IMPLICATIONS FOR CARBON MANAGEMENT
At present, tundra ecosystems are little used by humans and there is also little potential for more carbon capture here under current conditions. However, even a relatively small amount of global warming is expected to have major impacts on these systems. Schuur et al. (2008) estimate thawing of the permafrost as a consequence of climate change and subsequent decomposition of soil carbon could release 40 Gt CO2 into the atmosphere within four decades and 100 Gt CO2 by the end of the century, enough to produce a 47 ppm increase in atmospheric CO2 concentration.
The boreal forest biome holds the second largest stock of carbon; most of this is stored in the soil and litter. The draining of boreal forest peatlands, inappropriate forestry practices and poor fire management may all cause significant losses of the carbon stored in this ecosystem.
Boreal forests occupy large areas of the northern hemisphere and are mainly found in Canada, Russia, Alaska and Scandinavia. Biodiversity in these forests is generally low. Plant biomass is much higher than in the tundra, with roughly 60–100 tonnes of carbon per hectare, of which around 80% is in the above-ground biomass (Mahli et al. 1999; Luyssaert et al. 2007). Because of the low temperatures, decomposition in boreal forests is slow. This leads, as in the tundra, to large accumulations of carbon in the soil pool (116–343 t C per ha, Mahli et al., 1999; Amundson 2001). Fire is common in boreal forests and is one of the main drivers of the carbon balance here, with carbon being lost from the system when fire frequencies are high (Bond-Lamberty et al. 2007). There is debate about whether the very mature old-growth boreal forests are currently a carbon source or a carbon sink, though recent studies suggest that these old-growth forests may indeed be carbon sinks (Luyssaert et al. 2008). In general, due to the low decomposition rates and the extensive peatlands they can grow on, boreal forests are considered to be important carbon sinks.
HUMAN IMPACTS AND IMPLICATIONS FOR CARBON MANAGEMENT
Increasing human pressure on these forests, through logging and mining, and the draining of the peatlands these forests grow on, releases carbon to the atmosphere and significantly reduces their carbon storage capacity. Protection of boreal forests against logging and implementing best forestry practices may therefore reduce carbon emissions, sustain carbon stocks, and maintain uptake by these forests.
Temperate forests are active carbon sinks and deforestation in the temperate zone has largely stopped. Where demand for land and/or water allows, reforestation would enable carbon sequestration and could provide other benefits including higher biodiversity and recreation opportunities.
Temperate forests are found in climates with four distinct seasons, a well-defined winter and regular precipitation. They occupy large areas of Asia, Europe and North America and are found mostly in developed nations. There are many different types of temperate forests, some dominated by broad-leaved trees and others by coniferous species, and they are generally relatively high in animal and plant diversity. Because the soils they generate are often very fertile much of the area once occupied by temperate forests has been converted to croplands and pasture and is now used for food production.
Plant growth, decomposition and carbon cycling are rapid in temperate forests, with less carbon accumulating in the soil than in boreal forests or tundra. The overall carbon store for these forests has been estimated at between 150 and 320 tonnes per hectare, of which plant biomass, chiefly in the form of large woody above-ground organs and deep, coarse root systems, accounts for around 60% and soil carbon the remainder (Amundson 2001).
HUMAN IMPACTS AND IMPLICATIONS FOR CARBON MANAGEMENT
Temperate forests, notably in Europe and North America, have been increasing in extent for several decades. In many areas, current management practices, such as relatively lengthy cutting cycles and appropriate fire regimes, have led to an enhanced capacity for carbon storage. In consequence, temperate forests are currently considered to be overall carbon sinks. In Europe, forests are estimated to be taking up 7–12% of European carbon emissions (Goodale et al. 2002; Janssens et al. 2003). Further reforestation and improvements in management could increase carbon sequestration in the short term (Jandl et al. 2007).
Much of the original area of temperate grassland has been cleared for agriculture. Where natural vegetation remains, minimising human disturbance can prevent further carbon loss.
Grasslands are found across much of the world as an early successional ecosystem in forested regions. They also form the natural vegetation in climates where precipitation levels are inadequate to support trees but higher than those of deserts (Woodward et al. 2004). Extensive areas of natural temperate grassland occur in South America, the USA and Central Asia. Plant growth in these grasslands is water and nutrient limited and plants allocate much of their biomass below ground, where they produce slowly decomposing roots. Grazing animals typically play an important role in maintaining grasslands in that they accelerate carbon cycling by consuming and respiring large quantities of leaf biomass and returning some of this to the soil as dung. This is a form of organic carbon that is more decomposable than the leaf and root litter of grasses. In many areas this role is now performed by livestock.
Overall, temperate grasslands have low levels of plant biomass compared with forest or shrubland ecosystems (e.g. 0.68 and 7.3 t C per ha respectively in the temperate steppe of China, Fan et al. 2008). However, their soil organic carbon stocks tend to be higher than those of temperate forests (133 t C per ha, Amundson 2001).
HUMAN IMPACTS AND IMPLICATIONS FOR CARBON MANAGEMENT
Despite only having intermediate productivity some temperate grasslands are well suited to crop production and can produce excellent agricultural soils. In much of their natural range, e.g. the prairies of America, these have been cleared to make way for intensive agriculture.
DESERT AND DRY SHRUBLANDS
The large surface area of drylands gives dryland carbon sequestration a global significance, despite their relatively low carbon density. The fact that many dryland soils have been degraded means that they are currently far from saturated with carbon and their potential to sequester carbon can be high.
Deserts and dry shrublands occupy regions of very low or highly seasonal precipitation and can be found in numerous regions including many parts of Africa, southern USA and Mexico, parts of Asia and over large areas of Australia. The slow growing vegetation consists mainly of woody shrubs and short plants and is highly adapted to minimise water loss. Like plant diversity, animal diversity is generally low.
The lack of moisture determines the way in which these ecosystems process carbon. Plant growth tends to be highly sporadic and plants invest heavily in protecting themselves against water loss and herbivores by making their tissues tough and resistant to decomposition. Lack of water also slows decomposition rates, leading to the accumulation of carbon-rich dead plant material in the soil. Amundson (2001) estimates carbon content of desert soils as between 14 and 100 tonnes per ha, while estimates for dry shrublands are as much as 270 tonnes per ha (Grace 2004). The carbon stored in the vegetation is considerably lower, with typical quantities being around 2–30 tonnes of carbon per ha in total.
Some recent studies have suggested that carbon uptake by deserts is much higher than previously thought and that it contributes significantly to the terrestrial carbon sink (Wohlfahrt et al. 2008). However, considerable uncertainties remain and there is need for further research to verify these results, for example by quantifying above- and below-ground carbon pools over time (Schlesinger et al. 2009).
HUMAN IMPACTS AND IMPLICATIONS FOR CARBON MANAGEMENT
As these ecosystems are generally nutrient poor, they tend to make poor farmland and food production on these lands is often at a subsistence level. Land degradation, resulting from inappropriate land uses, leads to carbon loss from the soil.
SAVANNAS AND TROPICAL GRASSLANDS
Savannas cover large areas of Africa and South America and can store significant amounts of carbon, especially in their soils. Activities such as cropping, heavy grazing and increased frequency or intensity of fires can reduce carbon stored in these systems.
Savannas are a major component of the Earth’s vegetation and occupy large areas in Sub-Saharan Africa and South America. The savanna biome is characterised by the co-dominance of trees and grasses, but ranges from grasslands where trees are virtually absent to more forest-like ecosystems where trees are dominant. Most of the savanna areas are natural ecosystems; however, they can also be formed by the degradation of tropical forests from burning, grazing and deforestation. In Africa savanna areas support a charismatic fauna of large mammals and opportunities for eco-tourism are significant.
The amount of carbon stored above ground depends on how much tree cover there is, and can range from less than 2 tonnes of carbon per ha for tropical grasslands to over 30 tonnes per hectare for woodland savannas. Root carbon stocks tend to be slightly higher, with estimates of 7–54 tonnes of carbon per ha. Soil carbon stocks are high compared to those of the vegetation (~174 t C per ha, Grace et al. 2006). Savannas and tropical grasslands are naturally subject to frequent fires, which are an important component in the functioning of these ecosystems. Fire events in savannas can release huge amounts of carbon to the atmosphere (estimated at 0.5–4.2 Gt C per year globally). However, the carbon lost is mostly regained during the subsequent period of plant regrowth, unless the area is converted to pasture or grazing land for cattle (Grace et al. 2006) and these ecosystems are considered currently to act overall as carbon sinks, taking up an estimated 0.5 Gt C per year (Scurlock and Hall 1998).
HUMAN IMPACTS AND IMPLICATIONS FOR CARBON MANAGEMENT
Human pressure on these ecosystems is still increasing and it is estimated that more than one percent of global savanna is lost annually to anthropogenic fires, cattle raising and agricultural activities.
Tropical forests hold the largest terrestrial carbon store and are active carbon sinks. Reducing emissions from deforestation and degradation is a vital component of tackling dangerous climate change. In addition, tackling illegal and ill-managed logging will be an important part of reducing emissions from forestry.
Tropical forests occupy large areas of central and northern South America, western Africa, South-East Asia and north-eastern Australia. Most tropical forests are moist forests, found in areas where annual rainfall normally exceeds 2000 mm per year and is relatively evenly distributed. Such forests have extremely high levels of plant, mammal, insect, and bird diversity and are considered to host the greatest biodiversity of all the Earth’s biomes.
The warm and wet climate of tropical moist forests results in rapid plant growth and most of the carbon can be found in the vegetation, with biomass estimates of 170–250 t C per ha (Malhi et al. 2006; Chave et al. 2008; Lewis et al. 2009). Tropical moist forests can vary considerably in their carbon stocks depending on the abundance of the large, densely wooded species that store the most carbon (Baker et al. 2004). On average, they are estimated to store around 160 tonnes per hectare in the above-ground vegetation and around 40 tonnes per hectare in the roots. Soil carbon stocks are estimated by Amundson (2001) at around 90-200 tonnes per hectare, and are thus somewhat lower than biomass stocks.
Globally, tropical forests are considered to be currently carbon sinks, with recent research indicating an annual global uptake of around 1.3 Gt of carbon. Of this forests in Central and South America are estimated to take up around 0.6 Gt C, African forests somewhat over 0.4 Gt and Asian forests around 0.25 Gt (Lewis et al. 2009). To put this figure into context, the carbon uptake of tropical forests is equivalent to approximately 15% of the total global anthropogenic carbon emissions. Tropical forests therefore make a significant contribution to climate change mitigation.
HUMAN USE AND CONVERSION OF TROPICAL FORESTS
Tropical forests are being converted to industrial and agricultural (food and biofuel production) land uses at high rate. The causes for tropical deforestation are complex and range from underlying issues of international pressure and poor governance to local resource needs (Geist and Lambin 2001). Global tropical deforestation rates are currently estimated to be between 6.5 and 14.8 million ha per year and these deforestation activities alone release an estimated 0.8–2.2 Gt carbon per year into the atmosphere (Houghton 2005a). Deforestation not only reduces vegetation carbon storage but can also significantly reduce soil carbon stocks.
In addition to deforestation, tropical forests are also being used for the extraction of timber and other forest products. This leads to degradation of the forest and is estimated to contribute globally to a further emission of around 0.5 Gt carbon per year into the atmosphere (Achard et al. 2004).
In logging of tropical moist forests, typically only one to twenty trees per ha are harvested. Conventional logging techniques damage or kill a substantial part of the remaining vegetation during harvesting, resulting in large carbon losses. Reduced-impact logging techniques can reduce carbon losses by around 30% during forestry activities compared with conventional techniques (Pinard and Cropper 2000).
Peatland soils store a large amount of carbon but there is a grave risk that much of this will be lost as peatland ecosystems worldwide are being converted for agriculture, plantations and bioenergy. Conservation and restoration of tropical peatlands should be considered a global priority.
While not a true biome, peatlands represent a special case in the management of the global carbon cycle. Peatlands are associated with a range of waterlogged environments in which the decomposition of dead plant material and soil carbon is extremely slow, resulting in the fossilisation of litter inputs and soil with an organic carbon content of over 30%. Although some peat soils can be found in productive ecosystems such as reed and papyrus swamps and mangroves, peat soils are often seen in unproductive environments where plant growth is very slow. Their capacity for storage is huge; with estimates suggesting that ~550 Gt of C is stored globally in peat soils (Sabine et al. 2004), and a worldwide average of 1450 t C per ha (Parish et al. 2008). These areas are globally widespread but cover a tiny proportion of land area making peatland among the most space effective carbon stores of all ecosystems.
Great quantities of carbon are currently being lost from drained peatlands and unless urgent action is taken this loss will increase further as the area of drained peatlands is steadily increasing. At least half of these losses are currently happening in tropical peatlands. In these areas, which are concentrated in Malaysia and Indonesia, large areas of tropical forest are being drained for palm oil and pulpwood production (Verwer et al. 2008). Drainage of peat soils produces an aerobic environment in which peat carbon is respired by soil organisms. Carbon losses are further exacerbated by the increased likelihood of fire outbreak on drained peatlands, with drained peat acting as a fuel source for underground fires.
There is uncertainty over the degree of carbon losses from drained peatlands (Parish et al. 2008; Verwer et al. 2008) but in all probability losses are already significant (0.5–0.8 Gt C per year) and a significant fraction of overall anthropogenic emissions of greenhouse gasses. Because of these losses, biofuels grown on drained peat soils have a negative impact on the global carbon balance. It is estimated for instance that combustion of palm oil produced on drained peatland generates per unit energy produced 3–9 times the amount of CO2 produced by burning coal, equating to a carbon debt requiring 420 years of biofuel production to repay (Fargione et al. 2008). Such a figure highlights the false carbon economy of cultivating biofuels on drained peatland, the need to conserve pristine peatlands and highlights the potential for emission reduction by rewetting. Rewetting of peatlands restores them to their waterlogged state, re-imposing the anaerobic conditions in which the decomposition of dead plant material is halted, greatly reducing the release of CO2 and the risk of fire outbreaks.
OCEANS AND COASTS
Without the contribution of oceans and coastal ecosystems to global biological carbon sequestration today’s CO2 concentration in the atmosphere would be much larger than it is. But the uptake capacity of oceans and coasts is both finite and vulnerable. Minimisation of pressures, restoration and sustainable use are management options that can help these ecosystems maintain their important carbon management function.
The oceans play a hugely important part in both the organic and inorganic parts of the carbon cycle. They contain dissolved in them about fifty times as much inorganic carbon as is found in the atmosphere, as a complex mixture of dissolved carbon dioxide, carbonic acid and carbonates (Raven and Falkowski, 1999). Carbon dioxide is considerably more soluble in cold water than in warm water, and the relationship between the concentration of carbon dioxide in the atmosphere and of dissolved inorganic carbon in the oceans is therefore heavily dependent on water temperature and ocean circulation. Typically, cold surface waters at high latitudes absorb large amount of carbon dioxide. As they do so they become denser, and sink to the sea-floor, carrying dissolved inorganic carbon with them and creating the so-called solubility pump. As the concentration (or partial pressure) of carbon dioxide increases in the atmosphere, so the oceans absorb more of it. Because of this, the oceans are believed to have absorbed around 30% of human carbon dioxide emissions since industrialisation (Lee et al. 2003). The ocean is thus the second largest sink for anthropogenic carbon dioxide after the atmosphere itself (Iglesias-Rodriguez et al. 2008). One impact of the extra uptake of carbon dioxide has been a small but measurable acidification of the ocean over this period (Orr et al., 2005).
Dissolved inorganic carbon is translated into dissolved or particulate organic carbon in the open ocean through photosynthesis by phytoplankton. In total, the oceans are estimated to account for just under half of global biological carbon uptake (Field et al. 1998). The majority of this fixed carbon is recycled within the photic zone (the depth of the water column that is exposed to sufficient sunlight for photosynthesis to occur), supplying microorganisms that form the basis of the marine food web. Photosynthetic activity in much of the ocean is limited by nutrient availability. Notable exceptions are upwelling zones, where cold nutrient-rich waters are brought to the surface, leading to abundant plankton growth. Phytoplankton here can form large-scale ‘blooms’ covering hundreds of thousands of square kilometres of the sea surface and influencing important ecological and carbon cycle processes. When remnants of dead plankton sink to the sea floor, organic matter from their biomass is buried as sediments exceptionally enriched in organic carbon – this transfer of carbon from surface waters (and therefore indirectly from the atmosphere) to the deep ocean floor and ultimately through subduction, into the earth’s crust, is referred to as the biological pump. Only 0.03% to 0.8% of organic matter in the sea forms sediment (Yin et al. 2006), and in order for this to be permanently sequestered, it is necessary that it is not recycled back into the trophic exchange system.
The coastal zone (inshore waters up to 200 metres in depth, which includes coral and seagrass ecosystems) also has an important role in the oceanic carbon cycle. Various estimates indicate that the majority of mineralisation and burial of organic carbon, as well as carbonate production and accumulation takes place in this region, despite the fact that it covers less than 10% of total oceanic area (Bouillon et al. 2008). Organic carbon burial here is estimated at just over 0.2 Gt C per year (Duarte 2002).
Coastal wetlands have the potential to accumulate carbon at high rates over long time periods because they continuously accrete and bury organic-rich sediments. For example, Chmura et al. (2003), calculated that, globally, mangroves accumulate around 0.038 Gt C per year, which, when taking area of coverage into account, suggests that they sequester carbon faster than terrestrial forests (Suratman 2008). However there is widespread agreement that if current patterns of use, exploitation and impacts persist, coastal wetlands will become carbon sources rather than sinks (Hoojier et al. 2006; Jaenicke et al. 2008; Cagampan and Waddington 2008; Uryu et al. 2008; Neely and Bunning 2008; Parish et al. 2008). Duarte et al. (2005) estimate that widespread loss of vegetated coastal habitats has reduced carbon burial in the ocean by about 0.03 Gt C per year.
Some engineering solutions have been proposed to increase the sequestration potential of oceans. Some, such as ocean fertilization using iron, phosphorus or nitrates, increase the biological uptake of carbon. Others, such as injection of CO2 into the deep sea, use geophysical stores. The rationale for engineering the oceans, which are estimated to have a combined storage capacity of several thousand Gt C, is to accelerate the transfer of CO2 from the atmosphere to the deep ocean, a process that occurs naturally at an estimated rate of 2 Gt C per year (Huesemann 2008). Some researchers warn that these are unlikely to succeed on a global scale, with many questions remaining over the potential ecological side effects, and the direct impacts these may have on local marine life. Large-scale ocean fertilization experiments are proceeding, but it is difficult to determine the quantity of carbon that is actually sequestered on the ocean floor. With too many unknown variables and the current limitations with models, some are urging a cautious approach be taken with any ocean engineering intervention.
SUMMARY – NATURAL ECOSYSTEMS
The world’s terrestrial ecosystems are a vast store of carbon containing more than 2000 Gt C and are acting as a net sink of approximately 1.5 Gt C per year, of which tropical forests account for a large proportion (Luyssaert et al. 2007; IPCC 2007b). Sequestration at these levels would be equivalent to a 40–70 ppm reduction of CO2e in the atmosphere from anthropogenic emissions by 2100 (Canadell and Raupach 2008).
As well as maintaining these stores and sinks, there is significant potential for reducing future emissions of greenhouse gases through restoring degraded environments, for example through re-wetting peatlands and re-planting forests in areas that have been deforested, and reducing the rates of deforestation and loss of peatlands.
Without implementation of effective policies and measures to slow deforestation, clearing of tropical forests is likely to release an additional 87 to 130 Gt C by 2100, corresponding to the carbon release of more than a decade of global fossil fuel combustion at current rates (Houghton 2005b; Gullison et al. 2007). Of course if deforestation could be eliminated, these emissions would be avoided. However, even using more conservative assumptions for reductions in deforestation (deforestation rates observed in the 1990s decline linearly from 2010–50 by 50%, and deforestation stops altogether when 50% of the area remains in each country that was originally forested in 2000), a cumulative emission reduction of 50 Gt C could be achieved by 2100 (Gullison et al. 2007).
Peatlands are another ecosystem that offers great potential for reducing future emissions. It is estimated that 65 million ha of the global peatland resource is currently degraded, largely as a result of drainage. Peat oxidation from this area is believed responsible for annual carbon emissions of about 0.8 Gt, equivalent to 20% of the total net 2003 greenhouse gas emissions of the Annex 1 Parties to the UNFCCC. Peat fires in Southeast Asia (primarily Indonesia) are responsible for half of these global peatland emissions (Parish et al. 2008).