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The lowdown on carbon sinks

Summary

Over the past 200 years or so, human activity, such as the burning of fossil fuels and changing land-use patterns, has released about 400 Gigatonnes of carbon into the atmosphere (Gt = 1000 million tonnes). But scientists calculate that only about half of this emitted carbon has remained in the atmosphere. The implication is that the other half must have been absorbed by the Earth in some way.

The mechanisms by which this takes place are known as 'carbon sinks'. Examples of carbon sinks include the growth of forests and absorption of carbon dioxide by the oceans. Without this removal of carbon from the atmosphere, the present atmospheric concentration of carbon dioxide — which currently stands at 370 ppm (parts per million) — would be significantly higher (at about 450 ppm).

It therefore seems clear that carbon sinks are, at least for the time being, putting a brake on the accumulation of carbon in the atmosphere, and thus almost certainly on the speed and extent of climate change. In addition, it is also clear that the deliberate modification of carbon sinks, for example through changes in agricultural practices, can directly influence the overall carbon emissions of individual countries.

It is for these two reasons that the functioning of carbon sinks has become a key focal point of negotiations over the implementation of the Kyoto Protocol to the UN Framework Convention of Climate Change, the international agreement to reduce carbon emissions.

Sinks and sources: the basics

A number of questions about the nature of operation of carbon sinks are therefore directly relevant to future climate change. Firstly, how long will the natural uptake of carbon dioxide continue? Secondly, to what extent can we induce ‘man-made’ carbon sinks, for example by planting more forests or changing agricultural practices?

A third major question, relevant to both of the above, is how accurately can we measure the effectiveness of individual carbon sinks in reducing atmospheric carbon dioxide levels? This is important both to understand the main components of natural carbon sinks, as well as to verify the success of human-induced carbon sinks if and when they are implemented.

To answer such questions, we need to start with some definitions. The Earth's carbon is stored in a variety of reservoirs, or ‘stocks’. These reservoirs include living vegetation, organic matter in the soil (humus), carbonate minerals in rocks, and carbon (both dissolved and as particles) stored in the oceans.

Such reservoirs are referred to as sinks when they are accumulating carbon from the atmosphere at a faster rate than they are releasing it into the atmosphere (for example, a growing forest). Conversely, if the rate of release is higher than the rate of accumulation, (as is the case for fossil fuel reserves, which we are depleting thousands of times faster than geological processes can form them), they are considered to turn into carbon sources. (When scientists talk about carbon sinks, they tend to refer only to the uptake of anthropogenic CO2, making the assumption that the system was roughly in balance prior to human intervention, in other words that natural sinks balanced natural sources).

Some carbon stocks, such as those represented by the carbon that is in rocks and ocean sediments, change too slowly to be relevant to short-term or medium term fluctuations in the climate, only altering significantly over thousands of years or more.

More significant are those stocks, in particular the carbon stored in in the oceans, soils and vegetation, where the rate of absorption and release can vary over timescales of a century or less. At present, it seems, the rate at which carbon is being stored in these reservoirs is higher than the rate at which it is being released. In other words, the Earth is currently acting as a net global carbon sink. But how long this will continue is uncertain.

How large is the global carbon sink?

The magnitude of the global carbon sink can be estimated by comparing the rate at which carbon is being emitted into the atmosphere — currently about 7 Gigatonnes a year, which we know from inventories of fossil fuel and biomass burning — with measurements of the rate of increase of carbon dioxide in the atmosphere.

The difference between the two represents the rate at which carbon is being absorbed by the Earth. Over the 40 years since atmospheric carbon dioxide measurements began, it is estimated that the earth has been able to absorb about half of the carbon dioxide that has been emitted by human activities.

Year-to-year fluctuations in the amount of carbon dioxide that is observed to accumulate in the atmosphere, however, indicate that the rate at which the global sink operates varies over relatively short periods of time.

During the 1990s, for example, the annual rate at which carbon dioxide increased in the atmosphere ranged from 0.9 ppm a year to 2.8 ppm a year. To understand the causes of this variation, the sink must be broken down into its component parts: the uptake and release of carbon from both the land and oceans.

Scientists can measure the land and ocean components of the global carbon sink by making use of the fact that the uptake and release of oceanic carbon dioxide does not alter certain features of the atmosphere, such as the concentration of oxygen; this is because the ocean uptake of carbon dioxide depends upon chemical reactions between carbon dioxide and water, which do not involve oxygen.

In contrast, the processes by which carbon dioxide is exchanged on land, namely photosynthesis and respiration, do influence oxygen exchange. In these processes, carbon dioxide and oxygen are directly related to each other as products and reactants in the same chemical and biological processes.

Using this and similar approaches, scientists have calculated that over the last two decades the oceans appear to be absorbing carbon at a relatively constant rate of around 2 Gt/yr. In contrast, the rate of absorption by the land appears to fluctuate; in the 1980s it was negligible, but in the 1990s, according to the Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC), it was between one and two Gt a year.

These changes in the rate of absorption of carbon may be related to year-to-year fluctuations in the climate, perhaps associated with El Niño events, the general warming of the tropics that occurs about every four to seven years. For example, in warm and dry years typical of El Niño conditions forests act as net sources of carbon sources, whereas in the intervening cool and wet years they act as net sinks.

Furthermore even though the ocean sink is larger than usual during El Niño - mostly because outgassing of carbon dioxide from deep waters is prevented by changes in ocean circulation - the reduction in the land sink is much greater. As a result, El Niño years are in general those that see the greatest increases in the rate of carbon dioxide release into the atmosphere.

How the oceans acts as a carbon sink

Atmospheric carbon dioxide dissolves in the water close to the surface of the ocean. As the amount of carbon dioxide in the atmosphere increases, therefore, so does the concentration of carbon dioxide in these surface waters. Most of the absorbed carbon is accommodated by chemical reactions between the water and carbon dioxide . But this ‘buffering’ capacity has limits and — if this water remains at the surface — it eventually becomes saturated with carbon dioxide.

Surface water and deep water, however, are slowly but constantly overturning in a cycle of about 1000 years. As the surface waters move downwards — a process that occurs mainly in the North Atlantic and Southern oceans — it carries dissolved carbon dioxide down with it. As a result, about 75 per cent of the carbon that has been absorbed by the ocean since human activities began releasing carbon dioxide now resides in deeper waters. Overall, therefore, this process has recently been working as an important sink for carbon produced by human activity.

Such downward transport, however, is relatively slow, and so it is ocean circulation — and not dissolution of carbon dioxide in surface waters — that limits carbon dioxide uptake by the oceans. Furthermore, models of the movement of oceanic water masses predict that in a warmer climate the sinking of surface water, and hence burial of carbon dioxide, will slow down, reducing the future role of the ocean as a carbon sink.

Another way that the oceans absorb carbon is through the action of microscopic marine plants. When these organisms die, their bodies sink into deeper water. Although most of the carbon in the organisms decomposes to carbon dioxide before reaching the ocean floor, it is prevented from escaping back to the atmosphere (at least, until the oceans turn over).

This biological uptake of carbon will probably increase in future, as changes in sea surface temperatures and chemistry lead to an increase in the growth of algae. But it will not be enough, however, to compensate for the reduced downward transport of water and dissolved carbon, and hence is unlikely prevent the overall ocean sink diminishing in the future. Inedeed sinks will probably never lead to a decrease in atmospheric carbon dioxide whilst carbon dioxide emissions continue at their current level.

Carbon absorption on land

Knowledge of the behaviour of ocean sinks is important for our understanding of influences on atmospheric carbon dioxide. But the sluggishness of the underlying processes makes them slow to respond to rapid changes in carbon dioxide levels, limiting their role in climate change mitigation — at least in the short-term.

More dynamic are the land sinks, which are sensitive to the rapid climatic disturbances and changes in atmospheric carbon dioxide associated with human activities.

A priority in terms of climate change expected in the coming decades to century is therefore to understand the operation of carbon sinks on land. For the same reason, land carbon sinks are more amenable to human modification, which is why they are the focus of current negotiations to help individual governments achieve the emission reduction targets agreed in principle in Kyoto.

The role of land as a carbon sink — or source — depends on the balance between plant growth and photosynthesis, which absorb carbon dioxide, and the process of respiration by both plants and microbes in the soil, which releases it.

We already know that changes in land-use are one of the most important factors influencing the size of the terrestrial carbon sink. This is especially so in North America, for example, through the re-growth of forests on abandoned agricultural land and increased fire control in existing forests, both of which lead to significant increases in stocks of carbon on land, which would otherwise reside in the atmosphere.

Increases in carbon dioxide concentration in the atmosphere (as well as more nitrogen entering natural systems, from agricultural fertilisers) also augment the role of land as a carbon sink by stimulating plant growth.
It must be remembered, however, that these processes will not continue indefinitely. The effect of carbon dioxide and nitrogen on plant growth is expected to saturate as other limiting factors, such as water availability, take over. Additionally, the amount of abandoned land on which re-growth can occur is finite.

Furthermore, while some environmental changes can boost the role of land as a sink, others will ultimately diminish the overall land sink. For example, large stocks of carbon are currently preserved in frozen soils of the polar regions. Climate warming would melt these soils, stimulating breakdown and release of this ‘locked up’ carbon to the atmosphere and so form a carbon dioxide source that would offset carbon dioxide sinks elsewhere.

Carbon absorption on land

Knowledge of the behaviour of ocean sinks is important for our understanding of influences on atmospheric carbon dioxide. But the sluggishness of the underlying processes makes them slow to respond to rapid changes in carbon dioxide levels, limiting their role in climate change mitigation — at least in the short-term.

More dynamic are the land sinks, which are sensitive to the rapid climatic disturbances and changes in atmospheric carbon dioxide associated with human activities.

A priority in terms of climate change expected in the coming decades to century is therefore to understand the operation of carbon sinks on land. For the same reason, land carbon sinks are more amenable to human modification, which is why they are the focus of current negotiations to help individual governments achieve the emission reduction targets agreed in principle in Kyoto.

The role of land as a carbon sink — or source — depends on the balance between plant growth and photosynthesis, which absorb carbon dioxide, and the process of respiration by both plants and microbes in the soil, which releases it.

We already know that changes in land-use are one of the most important factors influencing the size of the terrestrial carbon sink. This is especially so in North America, for example, through the re-growth of forests on abandoned agricultural land and increased fire control in existing forests, both of which lead to significant increases in stocks of carbon on land, which would otherwise reside in the atmosphere.

Increases in carbon dioxide concentration in the atmosphere (as well as more nitrogen entering natural systems, from agricultural fertilisers) also augment the role of land as a carbon sink by stimulating plant growth.
It must be remembered, however, that these processes will not continue indefinitely. The effect of carbon dioxide and nitrogen on plant growth is expected to saturate as other limiting factors, such as water availability, take over. Additionally, the amount of abandoned land on which re-growth can occur is finite.

Furthermore, while some environmental changes can boost the role of land as a sink, others will ultimately diminish the overall land sink. For example, large stocks of carbon are currently preserved in frozen soils of the polar regions. Climate warming would melt these soils, stimulating breakdown and release of this ‘locked up’ carbon to the atmosphere and so form a carbon dioxide source that would offset carbon dioxide sinks elsewhere.

Kyoto and the carbon sink controversy

Considering the various factors described above, the overall role of land as a carbon sink is expected by most researchers to diminish over the next few decades. Indeed according to some predictions it could disappear altogether as early as 2050.

Furthermore, as the additional stocks of carbon accrued in plants and soils over the past 200 years or so begin to be released, the land itself may actually become a source of carbon dioxide. Combined with the decrease in the role of the ocean sink, these two factors may only increase the impact of human carbon dioxide emissions.

It is for this reason that the main aim of the Kyoto protocol is to enforce the reduction in carbon dioxide emissions. However, while the role of natural carbon sinks is not addressed in the protocol, it does accept that artificial manipulation of carbon sinks can help governments reach national emission reduction targets in the relatively short term.

The precise way in which governments can exercise this option, however, has become a major point of contention. In November 2000, for example, the international negotiations over implementation of the protocol temporarily broke down largely over objections from a US-lead bloc of countries. They disagreed with restrictions that other governments — particularly from Europe — were seeking on the extent to which emissions reductions that could be exempted by carbon sinks.

They also disputed whether carbon sink projects needed to be national, or could be financed by another country. Other countries worried that, by including human-induced carbon sinks in meeting emission targets, little or no overall reductions in fossil fuel emissions would be achieved.

To some extent, these concerns have been met in the text agreed at the subsequent meeting of negotiators in Bonn in July 2001. Part of the agreement reached at this meeting was a compromise on the issue of carbon sinks, namely that countries can use forestry schemes — albeit with strict limitations — as a contribution to net reduction in carbon emissions, some of which can come from overseas projects.

The negotiators, however, keen to generate broad ratification of the Kyoto protocol by national parliaments, agreed to allow Australia, Russia, Japan and Canada special allowances until 2010, where they can offset over half of their emissions with carbon sinks. In practice, as many environmental groups quickly pointed out, this could mean that of the 5.2 per cent emissions reduction (relative to 1990 values) required by 2012, only 2 per cent may actually come from reduced fossil fuel burning.

What the Kyoto protocol allows—and what it doesn’t

As the current wording stands, the exact nature of human-induced carbon sinks recognised by the Kyoto Protocol as potential contributions to meeting carbon emission reductions are both limited and ambiguous.

Article 3.3 of the protocol, which sets out the guidelines for using carbon sinks, only refers to “afforestation, reforestation and deforestation” as allowable activities. This obviously includes forest creation projects, either on deforested land (reforestation) or land that is not naturally forested, such as grassland (afforestation). But the extent to which this includes management of existing forests and other current carbon-sequestering management practices, such as reduced ploughing of agricultural land, is unclear.

Furthermore all changes in carbon stock in these projects must be verifiable, in other words it must be possible to independently measure the amount of carbon sequestered. This poses its own problems. Carbon stocks in vegetation above ground are fairly straightforward to assess, from ground and satellite surveys. But below-ground carbon stocks, which can represent up to 90% of the total carbon stocks in some forest systems, are far harder to determine.

If management of existing forests were to be included under emission reduction strategies, methods would be needed to determine which sinks are natural and which are additional (for example, attributable to management and not natural changes in growth and sequestration due to climate). High costs or poor accuracy of these verification procedures could exclude many projects from being viable or recognisable under the Kyoto Protocol — particularly in developing countries.

Yet the potential for new carbon-absorbing forestation schemes is greatest in sub-tropical and tropical areas, where forest growth is fastest, and land and water availability is greater than temperate regions. This is reflected in the projected cost of such strategies, estimated to be between 0.1—20 $ per tonne of carbon in the tropics, as compared to 20-100 $per tonne of carbon for non-tropical countries, according to the Second Assessment Report (SAR) of the IPCC on Mitigation.

It is estimated by the IPCC that up to 100 Gt of carbon in total could come from such biological mitigation projects. Allowing for uncertainties, this could account for 10-20% of the carbon release up to 2050 according to SAR and the IPCC Special Report on Land Use, Land-use Change and Forestry.

A number of such projects are already under way, such as those organised by the World Resources Institute (WRI). The predicted total impact of these projects alone is estimated to be the absorption of between 0.1 and 0.15 Gt of carbon over the next 40 years or so.

Carbon sequestration projects can have other benefits, such as providing rural employment, enhancing land sustainability, and improving the management of watersheds. At the same time however, there can be negative effects, including the loss of biodiversity, groundwater pollution, and the disruption of communities, for example, if new projects lead to a major breakdown of established social structures.

Other sequestration strategies

Other techniques for sequestering carbon have been suggested, although none of these has so far been accepted under the Kyoto protocol as a legitimate contribution to reducing overall carbon emissions.

One suggestion has been to increase the role of agricultural land as a carbon sink by reducing the depth of ploughing by farmers. This practice, it is argued, would reduce the rate at which organic material contained in soils (such as dead plant material) is moved closer to the surface, where it is more likely to decompose and release carbon dioxide back into the atmosphere.

As well as being investigated in the semi-arid tropics, where decomposition rates on heavily ploughed soil is relatively high, the argument that such practices should be considered as legitimate ways of reducing overall carbon emissions is being heavily promoted by the United States. Such a move is very attractive to many US politicians, as large tracts of agricultural land exist, and practices could be easily implemented by providing farmers with government subsidies to change their ploughing practices.

Another technique already being explored would be to pump carbon dioxide into geologically stable layers deep underground. Indeed one such project has already been under way for three years, financed by Statoil in Norway. Here, carbon dioxide produced as a by-product of the extraction of natural gas is being pumped back into an off-shore underground reservoir, below the sea floor, that is capped by a layer of clay that prevents carbon dioxide escaping into the atmosphere.

Others are investigating the possibility of burying carbon dioxide at sea. Several large-scale experiments into ways of doing this are currently being investigated, based on the fact that carbon dioxide released at high pressure more than 1000 meters below the surface of the ocean can form a solid carbon dioxide-hydrate (an ice-like crystal of carbon dioxide and water) that slowly dissolves into seawater. In principle, and given the appropriate conditions, carbon dioxide could remain trapped in the deep oceans in this way for centuries. But the technological costs are likely to be high, and the environmental consequences remain uncertain.

Finally, one widely discussed proposal to increase the uptake of carbon dioxide by the oceans is to add iron to certain regions of the worlds' oceans — in particular the Southern Ocean and Equatorial Pacific — that currently have a relatively low concentration of the mineral. The idea is that, much like applying fertilisers to land plants, this would increase the growth of photosynthetic algae and thus lead to a massive uptake of carbon dioxide.

So far, although various experiments have been carried out on large tracts of ocean (up to 50 km2) to test this idea, there is no evidence that it has led to carbon being transferred to or stored in deeper waters. Furthermore, the impact on marine systems remains unknown.

Despite such uncertainties and concerns, however, the prospect of ocean fertilisation is being actively discussed between scientists, government agencies and politicians in the United States. At the same time private attempts are already being made to secure their rights to fertilise large tracts of ocean, on the basis that such moves could be used within the Kyoto negotiations to compensate for carbon emissions elsewhere.

Conclusion

Politicians in several countries — particularly those that stand to benefit, such as Japan, Canada, Australia and the United States — have responded enthusiastically to the idea that the role of natural phenomena such as forests in storing carbon can be enhanced, thus counteracting the effects of CO2 emissions arising from the use of fossil fuels.

Many such individuals are keen to find ways of meeting their commitment to reduce overall carbon emissions while minimising the economic impact of the policies needed to do so. The use of carbon sinks to mitigate carbon emissions, a suggestion included as Article 3.3 in the Kyoto Protocol, offers one way of doing this.

The long-term impact of such mitigation strategies, however, is likely to be limited. For example, carbon can leak from locations in which it has been stored as a result of natural disturbances — such as fires, storms or the effects of insects — or of human activity. Similarly carbon sequestration projects can eventually become saturated, while the amount of land available for implementing sequestration strategies will slowly diminish.

Providing such potential limitations are acknowledged, however, the manipulation of natural carbon sinks could play an important role in reaching the short-term emission-reduction targets specified in the Kyoto protocol. Perhaps most importantly, they could help buy the time needed to implement other options that tackle emission reductions more directly, and with a greater long-term impact.

The author is a former physical sciences editor at Nature. He can be contacted at: [email protected]