Are offsetting and trading fossil carbon with woodland and soil sequestration schemes meaningful climate change measures?

The book Climate Change: Biological & Human Aspects (Cambridge U. Press) covers many aspects of 'global warming' including the way forests and soils absorb, or sequester, carbon. Indeed there are 'carbon trading' and off-setting schemes. Are these meaningful in terms of mitigating climate change and global warming?

The below is a single extract (p378-383) from Climate Change: Biological & Human Aspects but without diagrams: just one of the book's many references to woodland, forests and soils.

Note: The book, from which this excerpt was taken, was written before the popular controversy over carbon trading and the Channel 4 Dispatches documentary ' The Great Green Smokescreen' broadcast 16th July 2007.


However, the theoretical possibility of photosynthetically drawing down such a large amount of carbon can perhaps be better understood by looking at terrestrial-atmospheric carbon exchange. Such is the present rate of carbon removal from the atmosphere that at current rates the entire atmospheric volume of carbon in theory might be exchanged within about a decade. Of course, there are mixing problems and just as carbon is drawn down in the form of carbon dioxide it is also respired back as well as returned in other ways and places (such as the oceans), not to mention turned into other forms of carbon, such as methane. Another theoretical problem of this hypothetical situation would be that actually drawing down that much carbon would serve to draw carbon out of the oceans. This is one reason why the atmospheric residence time of adding a single molecule of carbon dioxide to the atmosphere is effectively between 50 and 200 years (see Table 1.1 in the book). Nonetheless, the idea of trapping (sequestering) carbon in biomass and soils is intriguing. Indeed, we know that roughly half the carbon released into the atmosphere through human action each year remains in the atmosphere. The other half is absorbed by vegetation, soils and the oceans, so sequestration already takes place somewhere in the biosphere.

      The possibility that forests and soils might be deliberately manipulated to mitigate carbon dioxide emissions was recognised by the UN's Framework Convention on Climate Change (FCCC) in 1992 and the Kyoto Protocol (1997). (The convention and protocol will be discussed further in the next chapter.) Both these emphasise natural terrestrial sinks rather than marine ones, largely because land carbon is easier to manipulate as well as due to issues of ownership.

      There is very roughly 2.3 Tt of carbon in vegetation and soils (and a further 1 Tt in the oceans' surface layers, which we will return to later): this is very much a ball-park figure for discussion purposes (see Figure 1.3). However, carbon is not distributed equally across terrestrial biomes (categories of regional communities of species). This is not only because some biomes contain more carbon per unit area than others but because some biomes are more extensive globally than others. Figure 7.14 (in the book) provides a proportional breakdown showing in which terrestrial biomes the carbon can be found.

      Forests and forest soils contain about 47% of terrestrial-biome carbon and so this form of carbon storage is attracting considerable attention. However, broadly speaking forests that have been in existence for thousands of years (climax forest communities) tend to be greenhouse-neutral. ('Broadly speaking' because a forest's carbon balance is climate-dependent and will change with temperature and water availability, see Chapters 1, 5 and 6.) It is the establishment of new forests that has a net, short-term effect sequestering carbon from the atmosphere. If you like, this is the opposite of deforestation, which is the second anthropogenic factor contributing to the build-up of atmospheric carbon dioxide (see Table 1.3 in the book). This new forest necessity places a severe constraint on forestation as a form of carbon sequestration. Not only is land itself finite, it is also required for other uses. Furthermore, not all the Earth's land is suitable for forestation. The IPCC's second assessment report (1995) estimated that by 2050 some 60-87 GtC could be conserved or sequestered in forests. Compared to the above-estimated mean annual twenty-first-century BaU fossil-fuel emissions of 18 GtC, this represents an annual saving of around 1.2-1.7 GtC, or around 6-9% of annual emissions.

      Soils can also be managed to increase their carbon content (although it has to be said that we have a lot to learn about soil-carbon mobility and fixation). The potential here might at first appear quite considerable, especially as roughly three times the amount of carbon is stored in soils than as vegetation above ground. Indeed, previously noted is that sub-arctic soils already hold a considerable volume of carbon (albeit a small proportion of terrestrial carbon in both vegetation and soils). Of the various soil types, peatlands represent a huge store of carbon (see Chapter 4). As previously noted, high-latitude peat-lands (which include parts of tundra, boreal and semi-desert soils and wetlands), with between 180 and 455 GtC, represent up to about a third of the global soil carbon pool. Some 70 GtC has been sequestered since the last glacial maximum (LGM) but, since the LGM was over 15 000 years ago and with the current interglacial Holocene only lasting 11 500 years so far, this represents a small annual sequestration rate which would be difficult to enhance to meaningful levels. So, despite being a large carbon pool, high-latitude soils do not have a commensurate potential for further sequestration. If anything, there is concern for the opposite. As with forests these high-latitude peatlands and permafrost soils are climate-sensitive. It is quite likely that they might release their store of carbon if warmed. In short, with global warming they could become a carbon source.

      Conversely, agricultural soils do have more potential. Soils that are currently used for agriculture are regularly ploughed, so bringing organic carbon to the surface and destroying plant root networks that physically trap carbon compounds. Ploughing also greatly aerates soils, so facilitating oxidation of carbon compounds. On the other hand natural and semi-natural grassland and forest systems are not subject to regular ploughing but additionally provide the soil with carbon compound inputs. The IPCC (2001a) notes that some agricultural land, such as set-aside (which represents some 10% of agricultural land in the EU), could be managed to enhance soil carbon. In total the IPCC's second assessment report (1995) estimated that up to 2050 some 23-44 GtC could be sequestered by agricultural soils, or around 2.6-5% of estimated mean annual twenty-first-century BaU emissions. This excludes the potential saving of fossil-fuel emissions (mitigation) from biofuels.

      The IPCC's 2001 estimate for carbon mitigation from both vegetation and soil management up to the year 2050 is 100 GtC. This is equivalent to 10-20% of fossil-fuel emissions estimated for that period. The IPCC (2001a) also notes that 'hypothetically' if all the carbon released due to historic land-use change (forest clearance and such) could be reversed then projected 2100 atmospheric concentrations (approximately 800 ppm under BaU scenarios) could be reduced by 40-70 ppm. This last would be a one-off gain from the finite land area historically affected by land use to release carbon dioxide (see Chapter 5), for once a new forest is fully established it ceases to be a net absorber of carbon.

      Managing the terrestrial short-term carbon-cycle carbon can offset a small, albeit significant, proportion of carbon likely to be emitted from twenty-first-century fossil-fuel burning. However, it is not risk-free. The more carbon built up in an ecosystem the more the likelihood of possible carbon leakage. Clearly, if an ecosystem is devoid of carbon then there is none to leak. On the other hand, a system rich in carbon, be it through peaty soils or woodland, has plenty of carbon that can escape. For instance, a forest fire can release nearly all the above-ground carbon extremely quickly, whereas ploughing grassland managed to maximise soil carbon can undo much of the benefit in a single season. Leakage can also take place in soils whose carbon pools have adjusted to the comparatively stable Holocene climate of the past 10,000 years or so, but which could well release their carbon with warming anticipated for the twenty-first century that takes them beyond temperatures seen in this past time (see Chapter 1).

      The potential for carbon leakage should be of concern to policy-makers concerned with climate change. As we shall see in the next chapter, some countries (typically nations that use a lot of fossil fuels) have insisted that it be possible to trade, using permits, the right to emit carbon dioxide for carbon-sequestering ecosystem-management schemes. Trading in such greenhouse permits at best can have a short-term mitigating effect. However, it comes with a risk, and there is the question as to what would happen in this permit game if after a number of decades a forest - created, say, by funding from a fossil-fuel station - burned down? Or if its soil carbon was released with global warming? How would the trading permit be paid back and the permit's environmental impact nullified? (Note: Underlining added for this web page and is not in the book which has the same text in a normal font.) These questions have not been resolved. Indeed the IPCC (2001a) warn that 'larger carbon stocks [in ecosystems] may pose a risk for higher CO2 emissions in the future'. It says that 'if biological mitigation activities are modest, leakage is likely to be small. However, the amount of leakage could rise if biological activities became large and widespread'.

      Boreal and tundra soils hold considerable carbon stocks, and of these, as noted in Chapters 1 and 4 high-latitude soils currently have up to a third of the global soil carbon pool. These, as per the IPCC warning, pose a risk for higher carbon dioxide emissions with warming. Already there is some experimental evidence suggesting that warming will result in carbon loss from these high-latitude soils and indeed carbon loss from temperate and tropical soils (see Chapter 1). Yet soil carbon loss may also increase another way as the climate warms.

      The amount of carbon held in a soil should really be viewed as a balance between the rate of carbon entering the soil and the rate at which it leaves. Such a consideration should include the living plant dimension, as it is the photo-synthetically driven primary productivity (plant growth) that largely draws down carbon, with plants creating roots within the soil and plant remains lying on the soil surface, from which carbon compounds can leach into the soil beneath. Therefore, if something were to hinder primary productivity then the balance of carbon flows would shift. In 2005 it was reported that this indeed happened in Europe during the 2003 heatwave. Then the July temperature in places was up to 6 C above long-term means and rainfall showed deficits of up to 30 cm/year. This extreme weather event impeded primary productivity and reversed these ecosystems' former net carbon sequestration.

      The work (Ciais et al., 2005) was conducted as part of the EU CarboEurope research programme and, though not as detailed an assessment as many ecologists would like, as a partial snapshot it gives cause for thought. The researchers looked at the programme's monitoring of carbon dioxide, water and energy from one grassland and 14 forest sites for 2003 (the heatwave year) and 2002 (the control year). They also analysed crop-harvest data at country level and compared 2003's figures with the 1998-2002 annual averages. Finally, they linked these field data to a sophisticated ecosystem computer model that necessitated supercomputing facilities (which were provided by the French Commissariat l'Energie Atomique). The results suggested that the 2003 heatwave months resulted in a 30% reduction in gross primary productivity for the year and that carbon release across Western Europe was some 500 GtC. Up to then it had (tentatively) been thought that Europe's soils had slowly been accumulating carbon. This carbon loss roughly equalled some 4 years of such accumulation. What was not known is the knock-on effect of this on the following year's carbon balance. However, what is known is that the 2003 European heatwave temperatures, while those of an extreme event early in the twenty-first century, are destined to be average summer temperatures later in the century (and that heatwave summers then will be correspondingly warmer; see Chapter 6). In short, Western Europe's 2003 carbon loss was not a one-off event.

      All of this does not bode well for a system of trading of greenhouse permits that uses carbon sequestration in natural systems, because it assumes that these systems are known and can be taken for granted given a certain management regime in the future. As a way of mitigating fossil emissions, it can at the very best only be as good as however long is the commitment to ecosystem management. ('At the very best' because, as noted, a warmer world facilitates ecosystem carbon release regardless of commitment and present carbon savings could easily turn into future carbon losses). The success of such trading schemes also depends on our knowledge (currently not sufficiently complete) as to agricultural sinks and especially soils, and our ability to monitor them, as well as the schemes' robustness lest they be open to gamesmanship by fossil-energy producers and consumers. Nonetheless, in the 1990s and early 2000s many policy-makers welcomed such permit-trading schemes.

      However, whereas permit-trading schemes that rely on soils as carbon sinks may not function properly per se, carbon sequestration can still play a role in helping (albeit temporarily) slow the build-up of atmospheric carbon dioxide. This last could well help slow the rate of climate change and may (marginally) help both ecosystems and human systems adapt even if a hectare of ecosystem cannot be equated with the consumption of a specific amount of fossil fuel.


Climate Change: Biological & Human Aspects is available from Cambridge University Press, Cambridge UK and its offices overseas including in New York (US), Melbourne (Australia), Madrid (Spain), Cape Town (South Africa) and elsewhere. ISBN 978-0-521-87399-4 (hardback) and ISBN 978-0-521-6919-7 (paperback). See also details at CUP.   It is illustrated with around 70 diagrams and a score or so of tables. It is fully referenced and has a number of explanatory appendices. Aimed at those with differing expertise, it is an introductory text but, at around 500 pages, comprehensively covers a wide range of climate-related issues.

Climate Change: Biological & Human Aspects 2nd edition (2013).

Bioscience review Climate Change: Biological and Human Aspects


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