TODAY’S STUDY: FARMING CARBON WITH JATROPHA AND DESALINATED SEAWATER

Carbon farming in hot, dry coastal areas: an option for climate change mitigation

Becker, Wulfmeyer, Berger, Gebel, and W. Munch, 31 July 2013 (Earth System Dynamics)

Abstract

We present a comprehensive, interdisciplinary project which demonstrates that large-scale plantations of Jatropha curcas – if established in hot, dry coastal areas around the world – could capture 17–25 t of carbon dioxide per hectare per year from the atmosphere (over a 20 yr period). Based on recent farming results it is confirmed that the Jatropha curcas plant is well adapted to harsh environments and is capable of growing alone or in combination with other tree and shrub species with minimal irrigation in hot deserts where rain occurs only sporadically. Our investigations indicate that there is sufficient unused and marginal land for the widespread cultivation of Jatropha curcas to have a significant impact on atmospheric CO2 levels at least for several decades.

In a system in which desalinated seawater is used for irrigation and for delivery of mineral nutrients, the sequestration costs were estimated to range from 42–63 EUR per tonne CO2. This result makes carbon farming a technology that is competitive with carbon capture and storage (CCS). In addition, high-resolution simulations using an advanced land-surface–atmosphere model indicate that a 10 000 km2 plantation could produce a reduction in mean surface temperature and an onset or increase in rain and dew fall at a regional level. In such areas, plant growth and CO2 storage could continue until permanent woodland or forest had been established. In other areas, salinization of the soil may limit plant growth to 2–3 decades whereupon irrigation could be ceased and the captured carbon stored as woody biomass.

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Introduction

It is now widely accepted that anthropogenic greenhouse gas emissions are causing an increase in global mean temperature and an acceleration of the global water cycle (IPCC, 2007). Unfortunately, in spite of the great threat posed by climate change to the earth’s environment and humankind, global agreements on greenhouse gas reduction have so far been largely ineffective. During the last decade, the emission rate of CO2 compared with the period 1990–2000 has even accelerated (Le Quer´ e et al., 2009). Consequently, a ´ variety of geoengineering approaches have been suggested for mitigating climate change. These options may be separated into purely technological approaches such as sun shading, increase of surface albedo by whitening of buildings, and carbon capture and storage (CCS) (Boyd, 2008) or biogeoengineering options (see, e.g., Betts, 2007). A comparison of the effectiveness of different proposals can be found in Lenton and Vaughan (2009). However, this analysis disregards feedbacks in the water cycle.

Recently, technological approaches such as CCS have become of great interest, as this technology may permit the reduction of CO2 emission rates by power plants (IPCC, 2005).

However, CCS has also been strongly questioned because of the large amounts of energy needed for its implementation, which reduces the efficiency of power plants, and the huge financial investments that this technology requires. As a matter of fact, CCS has only the potential to reduce emissions from power plants but not from other sources. Furthermore, it is not yet clear whether the long-term storage of carbon can really be guaranteed without leakage into the environment.

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Therefore, it is reasonable to explore bio-geoengineering approaches designed to change land-surface properties using the natural properties of vegetation. These include either modifications of energy partitioning by different types of vegetation or afforestation measures leading to a reduction in the levels of atmospheric CO2 and land-surface temperature. Both are options extensively investigated within IPCC (Metz et al., 2007). For instance, Ridgwell et al. (2009) and Doughty et al. (2010) studied the impact of an increase of agricultural crop albedo using global climate models. In midlatitudes, a consistent reduction of regional temperature of about 0.25 degrees per 0.01 increase in albedo was predicted.

Different relationships between these two parameters occur in other regions such as the tropics. However, global climate models are limited with respect to the correct quantitative simulation of land-surface atmosphere feedback and also to the response of the water cycle including precipitation (e.g., Hohenegger et al., 2009). These aspects call for further studies using high-resolution climate models that avoid the parameterization of convection and that improve the interaction between land-surface heterogeneities and orography with the atmosphere.

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One interesting option is afforestation which has several effects, simultaneously. First, carbon sequestration in biomass both above and below ground is a possible mitigation strategy (Metz et al., 2007). In the following discussion, we refer to this bio-geoengineering option as Carbon Farming. Secondly, daily surface temperatures may be reduced in subtropical regions due to changes in the surface energy balance. This depends critically on the partitioning of the energy balance into sensible and latent heat fluxes and its feedback to the atmospheric boundary layer (ABL), clouds, and precipitation. Thirdly, a variety of additional effects may be achieved such as the production of bio fuel and nutrients as well as the creation of a healthier environment. However, carbon farming must not compete with food production so afforestation measures should concentrate on land areas such as desert regions that are not likely to be used for conventional farming and which are not oversalted by previous unsustainable agricultural practises.

Recently, Ornstein et al. (2009) investigated this idea in desert regions on a global scale. Using a global climate model, they simulated large-scale reductions of surface temperature in the Sahara and the Australian desert. They also studied large-scale feedback processes such as teleconnection. Focusing on Eucalyptus sp. plantations, they stated that a significant mitigation of global carbon emission may be achieved if the Saharan or Australian deserts are cultivated.

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Furthermore, they found that their models predicted a large increase in precipitation in desert regions and related this to the Charney effect (Charney, 1975). With respect to irrigation, Ornstein et al. (2009) stated that the extremely valuable aquifers, which are available in some desert regions, should not be further exploited but considered instead the application of recent advances in desalination technology such as reverse osmosis. They discussed the costs and technological requirements to realize such a large-scale, international project covering areas of the order of 109 ha.

These results are encouraging and the interest in afforestation for production of biodiesel and application of the Clean Development Mechanism (CDM) is steadily increasing (see, e.g., cdm.unfccc.int, Kumar et al., 2011). However, several caveats remain with respect to technological and scientific aspects: the technologies for realising huge afforestation efforts such as irrigation with desalination plants are still in their infancy. It is not clear whether the carbon sequestering potential of suitable plants such as Eucalyptus sp. and Jatropha curcas can be maintained over large plantation areas, but ultimately the only way to find out will be to try.

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Furthermore, it is well known that coarse-scale global climate models have severe deficiencies when it comes to simulating land-surface–cloud–precipitation feedback. For instance, Hohenegger et al. (2009) demonstrated that coarsescale models, which require a convection parameterization, and convection-permitting models (grid resolution < 4 km) even give feedbacks between soil moisture and precipitation of different sign. This is a critical issue for the credibility of climate simulations. These results have been refined by Rotach et al. (2009a, b) and Wulfmeyer et al. (2008, 2011) who demonstrated severe deficiencies in models with convection parameterization when they are applied to mountainous regions or areas with strong land-surface heterogeneity. This is also the case in coastal desert regions. Therefore, it is highly questionable whether resilient quantitative results concerning land-surface feedback and precipitation can be achieved with models that use convection parameterization.

Consequently, we are convinced that an analysis of afforestation measures should be based on a thorough transdisciplinary scientific study on a local scale combining an analysis of the costs, the carbon binding potential, and the economic efficiency of these plantations in connection with the CDM. Here, the technological challenges can be studied in more detail and may be complemented and verified by results from plantations. Furthermore, land-surface–atmosphere feedback processes can be studied more realistically using high-resolution models. This combination of modeling efforts is also essential for studying the sustainability of carbon farming.

This work is intended to extend previous work in this area and to close an important gap in the analysis of afforestation projects in dry coastal areas. We focus on Jatropha curcas because we consider this plant to be one of the more promising and robust plants suitable for desert regions. Also, the authors of this paper have much specialized knowledge of and relevant data for this plant, its potentialities and requirements.

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That said, we are also well aware that other tree crops, especially Eucalyptus sp. or fixtures of various species, may be more suitable in many cases. Mixed crops can include food and fodder crops and also have the advantage that they produce more diverse ecosystems and thus reduce the danger of epidemics and large scale attack by pests. However the methodology and analysis that we apply here to Jatropha curcas could, with suitable raw data, be adapted to these other species and mixtures. With this approach and the application of available data, we are aiming at an analysis of the performance of the plantation over a time period of 1–3 decades. This may also provide an appropriate basis for the assessment of the fate of large-scale plantations for up to a century in future research.

This paper is organized as follows: in Sect. 2 we introduce the project strategy and explain the goals and the interactions of the project partners. In Sect. 3, the results of the study are presented. The biomass production and carbon sequestration potential of Jatropha curcas plantations is presented in Sect. 3.1; the irrigation, desalination, and energy supply costs in Sect. 3.2; and the impact on the regional climate in Sect. 3.3. An overview of other expected impacts is presented in Sect. 4 followed by some conclusions in Sect. 5…

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Conclusions

We have introduced a transdisciplinary project for simulating the technological, economic and climatological impacts of carbon farming by Jatropha curcas plantations in dry coastal areas as well as their usefulness with respect to the Clean Development Mechanism (CDM). We have determined both by estimations and by field measurements that plantations of Jatropha curcas if established in hot, dry coastal areas around the world – should be capable of capturing 17–25 t of carbon dioxide per hectare per year from the atmosphere (averaged over 20 yr). We found that a project to implement these ideas is technically feasible using recent advances in desalination methods such as reverse osmosis. Economically, carbon farming is competitive with carbon capture and storage (CCS). The total cost for carbon farming were estimated to be 42–63 EUR t−1 CO2, which is similar to that of CCS technology (54 EUR t−1 CO2; IPCC, 2005). In extensive sensitivity tests, we simulated the carbon sequestration cost in response to changes in market prices, labor requirements and biomass production (factor-by-factor approach).

The most sensitive economic factors in our tests were the price of possible carbon credits and, not surprisingly, the underlying biomass growth curve. In the worst-case scenario, assuming no tree growth after year 12 and all economic factors to take on their most unfavourable values, sequestration costs would about double. In the best-case scenario, in contrast, sequestration costs could decrease by half. Climatologically, using a plantation size of 100 km × 100 km, for simulations with an advanced land-surface–vegetation–atmosphere model, we found a decrease of annual mean temperature over the plantations of the order of 1◦K and an occurrence and/or enhancement of precipitation by approximately 11 mm and 30 mm averaged over the plantations during summer time in Oman and the Sonora, respectively. Particularly in Oman, formation of dew was predicted during spring, fall, and winter. Although this paper concentrates on the growth of Jatropha in hot, dry areas, the models devised for the agronomic, economic and climatic aspects of this study are sufficiently flexible and transparent that calculations could be made for alternative scenarios provided relevant data were available.

Our ex ante assessment had to be based on many data sources and on simulation methods of different degrees of reliability. Particularly lacking were, information on the soil nutrients and water dynamics of Jatropha curcas, long-term (up to 20 yr) comprehensive data on its growth and data that could lead to a complete life cycle analysis. We would therefore strongly recommend establishing a pilot project using sea-water desalination in order to gather more precise on-field data. This would help us to optimize irrigation, cultivation and carbon monitoring and improve the assessment of possible environmental risks. Reflecting on the urgent need to take action on climate change we strongly recommend including carbon farming in dry coastal areas in the portfolio of mitigation strategies. Overall, we hope that we have demonstrated that carbon farming is a promising mitigation strategy deserving at least as much attention as many of the other geoengineering options, which are currently being discussed.

The interdisciplinary combination of simulations presented in this work can be considered as a starting point for studying the sustainability of carbon farming…

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