Enhancement of Terrestrial Carbon Sink Potential: A Possible Contribution to Mitigating Global Warming

- By:

Courtesy of


Climate researcher Klaus Hasselmann, Director of the Max-Planck-Institut (MPI) for Meteorology in Hamburg and a project co-ordinator of EC’s Environment and Climate Programme, was one of the first scientists to warn that recently observed global warming trends have a discernible human related forcing component. Climate model calculations show, that global warming is closely related to rising atmospheric concentrations of greenhouse gases (GHGs) as consequence of man’s activity. Since pre-industrial times the atmospheric concentration of CO2, the most important GHG, has increased from 280 to 360 ppm and will rise further. According to the Intergovernmental Panel on Climate Change (IPCC) total anthropogenic emissions add up to 7-8 GtCy-1 (1 GtCy-1 = 1,000,000,000 tons) of carbon per year. Burning fossil fuels and deforestation are two of the largest contributors to the emissions figures. If recent global warming trends continue, the impact on natural and agriculture and ecosystems in many regions of the world can be expected to be severe and to affect almost all sectors of human life, from tourism to water supply. To avoid these potentially devastating consequences, both the climate research community and the public are calling for urgent political action to cut GHG emissions.

The balance of evidence of human interference with climate supported by climate research has forced policy makers to take the threat of climate change seriously. Finally, after lengthy negotiations, the efforts culminated in the third session of the Conference of the Parties to the United Nations Framework Convention on Climate Change (UNFCCC) in Kyoto. Here, for the first time the parties agreed on legally binding commitments (Annex I countries only) to reduce GHG emissions by 5% on average (the EU has unilaterally committed itself to a voluntary 8% reduction), compared to the 1990 level in the commitment period 2008-2012.

The scientific uncertainties

In order to meet the GHG reduction targets there are basically two options: either to cut atmospheric emissions or to enhance GHG sinks in the terrestrial biosphere. The sink option is based on the assumption that the terrestrial biosphere is able both to take up and store significant portions of CO2 from atmosphere. Estimates by the IPCC suggest that the terrestrial biosphere currently takes up about 25% (1.8 GtCy-1) of the global annual emissions of CO2. The margin of error associated with this estimate, however, is of the same order, which to some extent undermines the credibility of the approach. Even less is known about the storage capacity and possible saturation levels of the terrestrial biosphere.

A third alternative is to make use of the GHG storage potential of the oceans. There are a number of direct and indirect options available. Deep-sea disposal of CO2 is one possibility but production of liquid CO2 or dry ice is expensive and transport costs are high. Moreover, deep-sea injection of CO2 would acidify the water into which it was injected and create CO2 lakes at the bottom of the sea. The environmental implications could be severe. A second (indirect) method often discussed is the enhancement of the biological activity in the upper ocean layer by fertilizer. By this method the concentration of organic particles transported into the deeper sea (‘biological pump’) would be increased thus enhancing carbon flux in parallel. Although the carbon in the deep-sea is ‘safely buried’, decomposition processes will reduce the oxygen content of deeper ocean layers. In summary, these options are costly and the environmental impacts and risks associated with these options are unpredictable. The precautionary principle makes using the terrestrial biosphere the most pragmatic way of mitigating the greenhouse gas problem for the time being.

Although enhancing the carbon sequestration potential of the terrestrial biosphere poses less of a risk to the environment, it is nevertheless difficult to quantify the sources and sinks. There are three reasons for this uncertainty:

The amount of carbon accumulated by the terrestrial biosphere is small compared with the overall carbon turnover (the exchange between the terrestrial biosphere and the atmosphere is about 60 GtCy-1 in both directions).
The processes in the soil, plants and atmosphere controlling the gas exchange between the reservoirs are complex and not very well understood and therefore difficult to model.
There is a mismatch between the size of the problem and scales involved. The size is global, but measurements dealing with the problem are mainly local. Local measurements have to be extrapolated (called ‘up-scaling’) taking into account the large geographical and temporal variations (dimensions have to be upscaled from metres to continental scale, times from hours to years). Unfortunately, records of consistent observations of carbon fluxes with sufficient temporal, horizontal and vertical resolution (also required to calibrate the models) do not yet exist.
On the other hand the implementation of new measurement technologies and methodologies has now made it possible to separate ocean and land uptake. The initial results indicate that the forests in the Northern Hemisphere present a strong sink in the early 90’s. The magnitude is of the order of 0.8 GtCy-1 but varies from year to year. The origin of the sink in the Northern Hemisphere is not fully understood. It could be related to increased nitrogen deposition associated with industrial and agricultural activities. Nitrogen plays an important role in the nutrient balance of ecosystems. It acts as a fertilizer and enhances productivity. The fertilization effects of increasing atmospheric CO2 concentrations could also contribute, but how the biosphere responds to this fertilization from the species level all the way up to the ecosystem level is not known. Re-growth of forests and the lengthening of the growing season (observed by satellites) provide another possibility. Most of these effects have occurred simultaneously during recent years and it remains difficult to identify and quantify the contribution of each process to the regional or global carbon budget.

The largest source of uncertainty, however, is the response of the carbon pools of the terrestrial biosphere to climate change. Past records show that the annual atmospheric growth rate of CO2 is not steady with time. Climate fluctuations following El-Niño events, change of ocean circulation and volcanic eruptions have modulated the CO2 growth rate in the past (equivalent to an annual uptake/release variation of 2-3 GtCy-1).

Although forests are generally believed to be carbon sinks, this may not be true in all cases. There is recent evidence that boreal forests are highly vulnerable to climate change and can switch from being a carbon sink to a carbon source depending on climatic conditions. Recent results also indicate that tropical forests accumulate larger amounts of carbon than previously thought, but again, estimates show a wide spread depending on forest type and climatic conditions. The underlying processes, in particular those affecting the soil, need to be better understood.

The sink approach

Prior to the Kyoto conference there was a broad consensus within the European climate research community that the problem of global warming should be tackled at its roots by cutting emissions and not to go for the sink enhancement strategy. The main concern is that the carbon sequestration potential of the terrestrial biosphere is limited and that the carbon sequestered is not ‘buried safely’ over the long term. It will, sooner or later, reach a saturation level and re-emission to the atmosphere within a few decades becomes likely. Therefore the sink enhancement strategy provides only a temporary ‘political’ solution, but could in fact simply shift the problem to later generations. Furthermore, as discussed above, the carbon exchange between the atmosphere and the terrestrial biosphere and the bio-geochemical processes involved are complex, highly variable in space and time and are still not very well understood. This is the reason why both measurements as well as model calculations of the carbon sequestration of the terrestrial biosphere show a wide spread. At the current state, detailed estimates of changes in the terrestrial carbon stocks, as requested by the Kyoto Protocol are available for some local areas but not at a global scale. Science knows even less about the long-term consequences, feedback mechanism and possible ‘surprises’ related to distortions of the global carbon cycle and its impacts on marine and terrestrial ecosystems.

A large source of carbon dioxide emissions, directly related to the human interference with the terrestrial biosphere is often forgotten in discussions. Land-use change and deforestation, especially the conversion of natural forest into farmland, significantly contribute to the overall rise of atmospheric CO2. On average 1.6 Gt of CO2 are released to the atmosphere annually, accounting for more than 20% of global anthropogenic carbon emissions. These facts made most scientists recommend made GHG emission cuts without the sink option. Alternatively, the conservation of natural forest should have highest priority, summarized in the session statement of Greenhouse gas Workshop in Orvieto, Italy, 10-13 November 1997, organized by the European Commission:
‘Although probably accumulating carbon at a lower rate, the large carbon stocks of pristine forests represent carbon accumulated over many centuries. This carbon can only be replaced over a similarly long time-scale. Therefore, preservation of pristine forests should take priority over afforestation programmes where possible.’

Scientific agenda after Kyoto

Although the parties agreed in Kyoto to include GHG source and sink options in the Protocol this was strictly limited to the forest-related categories of afforestation, reforestation and deforestation. The consequences of the Protocol have been analysed e.g. during a workshop organized by the European Commission and the IGBP Terrestrial Carbon Working Group. The scientific community came to the conclusion that the partial sink categories agreed upon are insufficient. Instead they recommended using the full carbon budget of the terrestrial biosphere, monitored over sufficiently long time scales as the appropriate basis for a carbon accounting system. Furthermore, the sink approach of Kyoto Protocol has a number of loopholes and opens ways for a ‘creative’ accounting system. (Refer to the articles listed in the references for a more detailed analysis of the Kyoto Protocol). In summary, the contribution of the carbon sequestration potential of terrestrial biosphere, even towards a temporary solution of the global warming problem will remain small due to the limited carbon sink categories agreed upon.

Although climate scientists still have reservations, and still give preservation of natural forests highest priority, the Kyoto agreement on terrestrial sinks is also a big challenge for climate research to fill gaps in our understanding of their characteristics. The EC is supporting a number of research projects such as EUROFLUX, ESCOBA and Eurosiberian Carbonflux in the framework of the Environment and Climate Programme in the field of GHG research. The aim of these projects is to develop tools and methodologies to make it possible to understand the processes and quantify the sources and sinks better. Within EUROFLUX a network of carbon monitoring stations has been established along a European axis at a number of representative forest sites. The long-term measurements of the carbon exchange between forests and the atmosphere together with the application of new model-based methodologies will allow better estimates of the European carbon balance in the future. The EUROFLUX methodology provides the basis for a global carbon monitoring network to be established. The integration of carbon flux data between the terrestrial biosphere and the atmosphere at a continental scale is the aim of the Eurosiberian Carbonflux project. This will be achieved by joint field experiments carried out by Russian and EU research teams over Europe and Siberia. Aircraft measurements at different heights in the atmospheric boundary-layer, complemented by ground-based observations, provide the basis for data integration. The objective of the ESCOBA project is the investigation of the global carbon budget by using sophisticated measurement techniques and inverse modelling methods. These techniques will help to better quantify and distinguish carbon up-take between the terrestrial biosphere and the ocean. Since more data of better quality on a global scale will soon become available, inverse modelling techniques will help to identify the carbon sources and sinks more precisely.

That the European Commission (EC) has taken the challenge of climate change research on board is shown once more in the 5th Framework Programme, which includes the Key-action ‘Global change, climate and biodiversity’ as part of the programme ‘Preserving the ecosystem’. This focuses on climate-related environmental problems and gives GHG research a high priority. This key-action also supports new elements such as infrastructure and long term monitoring programmes of environmental parameters to meet the demands of researchers in this area more closely. This new approach will put European research at the forefront of international efforts on this crucial issue.

Customer comments

No comments were found for Enhancement of Terrestrial Carbon Sink Potential: A Possible Contribution to Mitigating Global Warming. Be the first to comment!