In these experiments carbon plantations are established wherever they can grow and wherever they are carbon-effective compared to the baseline. Under this assumption, the six plantation types are found to be effective over large areas around the world (Figure 1). Under the A1b baseline scenario, about 3990 and 3850 Mha (i.e. 10¹⁰ m²) plantations can be established under the permanent and frequent-harvest management options, respectively up to 2100 (Table 1). Plantations of gum species (Eucalyptus spp.), for example, are projected for establishment mainly in regions that are currently covered by savanna, woodland and even some tropical forest. The potential over the next few decades is limited because much land is needed for agricultural production (this land cannot be used because of the assumption that current and future agricultural land is to be excluded). Under the alternative B2 baseline scenario less land is projected to become available for plantations than under the A1b baseline, due to greater demand for agricultural land. The projected difference between the two management options (i.e. harvested or permanent plantations) has two reasons. First, the difference results from the assumption for permanent plantations that abandoned agricultural land is not available if the re-grown natural forest is used at a later stage to fulfill the wood demand. Second, close to 2100 permanent plantations are estimated to be more widely distributed because the CO₂ emissions related to the harvest of plantations need to be compensated before harvested plantations become an effective C sink.

The projected cumulative physical C sequestration of plantations in the A1b scenario is 583 Pg C and 913 Pg C up to 2100 for the permanent and harvest options, respectively (Figure 2). Under the B2 baseline scenario, the cumulative potential is estimated to be 858 Pg C, considering frequent harvests (i.e. 6% less compared to A1b). These uptake rates equal about 37% and 58% of the projected overall CO₂ energy and industry emissions in the A1b scenario for the permanent and harvest options, respectively. Under the B2 baseline, the estimated uptake is even 67% of the energy and industry emissions. Hence, the projected long-term physical potential of carbon plantations for slowing down the atmospheric CO₂ increase is large. However, it will take more than 20 years to compensate for carbon emissions related to the establishment of the plantations. The projected physical potential up to 2020 is negligible where the cumulative potential up to 2030 is about 100–150 Pg C (Figure 2).

The two management options show a higher C sequestration potential in the case of harvested carbon plantations, especially beyond 2050 (Figure 2). This is caused by a decreasing sequestration rate for permanent carbon plantations, whereas the uptake potential remains high if a carbon plantation is frequently harvested harvests. This difference is induced by the C sequestration of plantations decreasing with age. The average age increases in permanent plantations but remains low in the frequent harvest case. This difference is projected specifically for plantations in Latin America and Africa.

Geographically speaking, the highest physical sequestration rates have been projected for plantations in tropical regions like South America and Africa, dominated by the two Eucalyptus plantation types (Figure 2). The projected sequestration potential is relatively low in high latitudes, because of low growth rates. In various parts of Canada and Russia, the net cumulative carbon sequestration even remains negative for about 50 years.

We assessed the social sequestration potential of C plantations up to 2100 using wood availability and nature conservation as main constraints in addition to the food security criterion. These constraints have been implemented by estimating the potential on abandoned agricultural land only. Assuming permanent carbon plantations (Experiment 4), 181 and 831 Ma are projected in the A1b scenario potentially to be established around the world up to 2050 and 2100, respectively (Table 2). In the case of harvested carbon plantations, the area available in 2100 is projected to be 1014 and 695 Mha under the A1b and B2 baseline scenarios, respectively (Experiments 5 & 6). The difference between the baseline scenarios is caused by a larger land abandonment under the A1b baseline scenario than under the B2 baseline. The difference between the two management options is caused by the assumption for permanent carbon plantations that abandoned agricultural land is not available if the re-grown natural forest is needed at a later stage to fulfill the wood demand. For frequently harvested carbon plantations, the timber from the plantations is used to fulfill the wood demand, reducing the pressure on existing forests. Similar to the physical potential, the difference between the management options is projected to decrease near to 2100 because the CO₂ emissions related to the harvest need to be compensated before the plantations become an effective C sink. As a consequence, fewer harvested plantations will be established.

The majority of the carbon plantations is projected in all the experiments to be established after 2050, because land only becomes available then, due to decreasing population and increasing efficiency. The projected cumulative global social C sequestration potential remains low in the coming decades (Figure 3), and, up to 2050, reaches 12–17 Pg C for the different baselines and harvest regimes (Table 3). Under the A1b scenario the potential increases up to 93 and 133 Pg C in 2100 for permanent and harvested plantations, respectively (Figure 3 and Table 3). This is 5–7% of the projected cumulative emissions up to 2100 coming from the energy and industry sector (i.e. about 1740 Pg C). The potential uptake up to 2100 under the B2 scenario is 68 Pg C, implying 5% of the energy and industry emissions (i.e. 1272 Pg C). The net C sequestration potential can be higher under a frequent harvest regime due to a higher area-based uptake and the broader distribution. Comparing the 2 baseline scenarios, the projected global sequestration of carbon plantations in 2100 is 95% higher under the A1b scenario than in the B2 baseline (Table 3). This is mainly due to the higher establishment rates.

Geographically speaking, most plantations are projected for establishment in tropical regions (Figure 1 and Table 2). The consequences for the C sequestration are that under the A1b baseline scenario, 40–50% of the global potential can be sequestered in plantations in Africa, 10–20%, in China, 10% in Latin America, and 10% in Oceania (Table 3). Although a considerable amount of abandoned agricultural land is projected for Europe, Canada and the FSU as well, the effectiveness of establishing C plantations here is projected as being rather limited. For example, 6% of the global potential area can be established in the FSU up to 2100, sequestering only 4% of the global potential.

With respect to the social potential, evaluating the effectiveness of carbon plantations in slowing down the build-up of CO₂ in the atmosphere shows that the concentration in 2100 under the A1b scenario can be reduced from 752 to 713 ppm (i.e. a 39 ppm reduction) when planting permanent carbon plantations, whereas it reaches 700 ppm (i.e. a 52 ppm reduction) assuming frequently harvested plantations (Table 3). The two management options differ because of the broader distribution of carbon plantations when planting frequently harvested plantations and because of the additional C that will be stored in the soil compartment. The lower social sequestration potential projected under the B2 baseline scenario results, obviously, in a lower effectiveness. Assuming frequently harvested carbon plantations, we project a CO₂ concentration of 579 ppm in 2100, which is 27 ppm less than in the baseline.

Here we have presented a methodology to assess the global and regional sequestering potential of carbon plantations established after 2000. Based on ecological and environmental constraints alone, carbon plantations can be effective in large parts of the world with a projected cumulative sequestering potential of 913 Pg C up to 2100. In the A1b baseline scenario this equals 52% of the total cumulative CO₂ emissions from energy and industry from 2000 to 2100. In the B2 scenarios it is even 67%. The social sequestration potential is much lower but still considerable. The annual average global potential is projected at 0.1 – 0.2 Pg C yr^-1 up to 2050, and 0.68–1.3 Pg C yr^-1 up to 2100 (Table 3). In 2100 this leads to a 27–52 ppm smaller increase in the atmospheric CO₂ concentration and compensates for 5–7% of the total energy and industry related CO₂ emissions. The sequestration potential is likely to considerably increase beyond 2100, because many plantations are projected to be established only close to the end of the 21^st century. This holds especially for regions where large areas of arable land are expected to be abandoned towards 2100, such as China.

The social sequestration potential of the plantations projected up to 2050 is at the low end of ranges found in the literature, whereas values for the coming 100 years are more in line (Table 4). Geographically, the most effective plantations are located in tropical regions, whereas due to low growth rates the C sequestration in high latitudinal plantations is limited (Table 3). This is in line with the findings of Masera et al. 21 and Cannell 22. Many other estimates are especially useful in a comparison with our area-based potentials, because the studies often focus on the C sequestration potential in existing forests (Table 4). For example, the projected social C sequestration potential of tropical plantations of Latin America and Africa (1.6–1.9 Mg C ha^-1 yr^-1 for 2000–2100), is found at the low end of the range given by Silver et al. 23. Our projections for Europe up to 2100 – between 0.3 and 1.1 Mg C ha-1 yr-1 – are well in line with the projected area-based uptake of 0.52 Mg C ha-1 yr-1 given by Liski et al 24. Note that we have used a particular definition of "social potential", possibly causing differences with other studies in either direction. Areas that we excluded, for example, because of competition with other needs may be converted in reality, while areas that we included could not be appropriate for the establishment of plantations because of other social or institutional factors.

Despite the estimated considerable C sequestration potential up to 2100, the uptake potential for the coming decades is projected to be limited (Figure 3). It can take about 20 years to compensate for the emissions related to the establishment of the plantations. Moreover, not much agricultural land will likely be abandoned in coming decades due to the current and projected agricultural pressure. The limited potential in coming decades is in line with findings of Marland & Schlamadinger 25, who showed that the sequestration potential in forests established since 1990 is mainly relevant in the long term. As such, we do not confirm the suggestion of Kirschbaum 26 that plantations may help to buy some time in initiating emission reductions already in the next few decades.

The limited role of plantations in the coming decades might be caused by our assumptions that C plantations can only be established after 2000. Various other studies report afforestation activities in different locations around the world, even before 2000. Brown 27 and FAO 28, for example, reported that globally 124 Mha and 187 Mha forest plantations have been established up to 1995 and 2000, respectively. More than 90% of these plantations have been established in 30 countries only, mainly in such Asian countries as China (45 Mha), India (32 Mha), and Japan (11 Mha). Furthermore, various studies report existing afforestation activities, but seldom account for deforestation in the same region (the so-called leakage effect). This has also been shown by others (e.g. 29) by estimating an annual afforestation rate in the tropics of 2.6 Mha yr^-1 throughout the 1980s, but at the same time a deforestation rate of 15.4 Ma yr^-1. In our methodology, leakage is not possible because we only establish plantations on land that is available for the entire simulation period (i.e. up to 2100). Finally, our projections are lower than in other studies that account for the C sequestration in forests planted for various other reasons (e.g. recreation, agroforestry and soil restoration). For India, for example, we have project a negligible afforestation potential up to 2030 because of the large pressure on the land for food production. Nevertheless, Ravindranath & Somashekhar 30 reported an afforestation rate of India of 1.6 Mha yr^-1, mainly for agroforestry purposes. Again, these afforestation rates are partly counterbalanced by deforestation activities in India 3031.

The methodology presented is aimed at quantifying the sequestration potential of carbon plantations around the world, in consideration of the requirements mentioned in different conventions and protocols. The UN Framework Convention on Climate Change 3 and its underlying Kyoto Protocol, which opened the possibility for developed countries to use afforestation programs in achieving their reduction commitments, clearly stress that C plantations are only effective in the long term if (see also 103233):

• they are additional to a baseline;

• all C fluxes are considered (i.e. full C accounting);

• they are permanent. If not, a carbon plantation has little value in terms of actually reducing the concentration of GHG in the atmosphere, since carbon sequestered over various years will return to the atmosphere;

• the credited C sequestration in one region is not to be compensated by C losses elsewhere (i.e. no leakage 34),

• the C sequestration in plantations exclude 'indirect human influences' in terms of, for example, climate and CO₂ change.

The additionality issue has been taken into account in the methodology presented by considering the sequestration potential of both plantations and natural ecosystems. Furthermore, the methodology considers all C fluxes by keeping track of fluxes in both vegetation and soil, plus the carbon losses due to the establishment of the plantations. The permanency concern is taken into account by comparing the C plantation option with various other land-use options. Alternative land-use options pose a main threat to the permanency of a carbon plantation, especially in the long term (e.g. when the demand for agricultural land fluctuates or prices of land-use products change). Since permanency is more certain if plantations are established in areas that are not used for food, fodder and timber production, areas needed for agriculture or wood up to 2100 have been excluded in the all experiments. As mentioned earlier, leakage is not possible in the methodology presented because we only establish plantations on land available for the entire simulation period (i.e. up to 2100). Finally, the methodology accounts only for carbon sequestered directly by the plantations, corrected for climate change and CO₂ fertilization (i.e. indirect human influences). This has been done both for the historical uptake – where we corrected 1995 growth rates for observed changes in CO₂ and climate (see Equation 2) – as well as the projected future (reducing the projected social potential in the supply curves for climate and CO₂ changes in the baseline).

The effectiveness of harvesting plantations and using the biomass to displace fossil fuels and/or timber, compared to having carbon stored in a permanent plantation, depends to a great extent on the displacement factor (i.e. the extent to which wood from carbon plantations can be effectively used to replace fossil fuels) 35. Here, a displacement factor of 'one' is assumed. Theoretically this can be achieved if fossil fuels are displaced by harvested wood 2236. However, if the displacement factor is (much) smaller than 'one', the environmental effectiveness of harvested plantations decreases sharply. Likewise, establishing carbon plantations is, in general, less effective than avoiding deforestation (especially in tropical regions, 3716). This, however, is associated with various social difficulties and avoiding deforestation in one region may be counterbalanced by additional deforestation elsewhere.

The effectiveness of carbon plantations in especially high latitudes is questioned because of the effect on different biophysical processes (i.e. changed radiation balance) that may counterbalance the additional C sequestration 383940. On the basis of the albedo effect and the projected low net sequestration potential for high latitudinal plantations (i.e. in parts of Canada and Russia the net C sequestration even remains negative for about 50 years), the establishment of carbon plantations in high latitudes is only favorable if the objective to sequester carbon is combined with other environmental considerations. For example, plantations may also contribute to water protection and soil erosion control 2141.

An environmental constraint often mentioned for large-scale C plantations is the availability of water and nitrogen 414243. Also in the methodology presented, the high growth rates of the carbon plantations (compared to natural forests) rely on a high level of management, including nitrogen fertilization for plantations situated on poor or degraded soils. The additional use of water and fertilizer should indeed be a concern in the planning and management of the plantation, especially because a (higher) fertilizer use could imply additional emissions of N₂O, which were neither accounted for in our study, nor in most other studies. Likewise, afforestation activities have recently also been questioned in the context of possible additional methane emissions from trees – the second-most important greenhouse gas 44. Although this issue is currently still under scientific debate, the effectiveness of afforestation programs would be reduced by a maximum of 10%. This has been confirmed by others (see, for example, 45 for a more detailed discussion).

The methodology to assess the C sequestration potential in carbon plantations, as presented here, is a rule-based approach that is implemented on a geographical explicit -0.5° longitude × 0.5° latitude-grid (Figure 4). The time horizon is 2000 – 2100. This facilitates the quantification of the long-term potential of carbon plantations in different parts of the world in mitigating the build-up of CO₂ in the atmosphere. We distinguish different potentials, defined according IPCC definitions 46. The methodology consists of three steps (Figure 4). The first step is to determine the physical sequestration potential of C plantations, accomplish by adding carbon plantations as a new land cover class in IMAGE 2 (see below for a general description of the IMAGE 2 model). All carbon pools and fluxes of the potential carbon plantations (e.g. Net Primary Production – NPP and Net Ecosystem Productivity – NEP) are calculated by the IMAGE-2 terrestrial C-cycle model, taking environmental (e.g. climate and atmospheric CO₂) and local conditions (e.g. soil) into consideration. In the second step, the social potential of plantations is determined using the restriction 'no interference with food supply and nature conservation'. In the third step, the social potential is transferred into the economic potential by linking the C sequestration potential to establishment and land costs. The resulting marginal abatement cost curves can be used to compare the potential of carbon plantations with other mitigation strategies using cost minimization (e.g. 47). The focus of this paper is on describing and analyzing steps 1 and 2 of the methodology. We will also summarize step 3 (i.e. economic potential), but refer for details on this to the companion paper by Strengers et al. 19.