An avoidable methane hotspot: pan-tropical estimates of greenhouse gas emissions and energy potential in tropical palm oil plantations

Background/Question/Methods

The production of palm oil is one of the fastest growing sectors of modern agriculture, and represents an industry that holds promise for economic growth in developing countries.  And yet, new palm oil plantations often come at the cost of old-growth rainforests, creating negative consequences for both biodiversity and climate.  While the release of greenhouse gases (GHGs) following deforestation for palm plantations receives more attention, a secondary but still significant source of GHGs is methane from palm oil mill effluent (POME).  We first assessed the potential for – and controls over – GHG emissions from POME in a typical palm oil processing operation in SW Costa Rica.  Along a sequence of seven settling ponds, we measured fluxes of CO₂, CH₄ and N₂O, along with sediment and water column nutrient levels, chemical oxygen demand (COD), and redox potential.  Next, we estimated pan-tropical emissions of CH₄ from POME over the last 35 years using a variety of published data sources on CH₄ emissions, POME volume and effluent COD.  Finally, we extended the approach to estimate CH₄ loss from effluent in two other widespread agricultural systems in tropical latitudes:  sugar cane and cassava.

Results/Conclusions

Within our Costa Rican study site, POME was strongly anaerobic throughout all settling ponds, and displayed COD values up to ~370,000 mg/l.  In tandem with such high COD values, we measured extraordinarily high rates of CH₄ efflux, reaching rates that are among the highest in the literature for all system types.  And yet, comparisons with COD and/or CH₄ efflux values from POME across the tropics suggest that such rates are not uncommon in palm oil processing operations.  We estimate that palm oil wastewater lagoons emitted 3.62 ± 1.1 Tg of methane in 2012, a value on par with present-day estimates for the release of methane hydrates in the Arctic. Palm oil production is forecast to reach 156 million tons by 2050, which in turn could produce ~12 Tg of CH₄ per year. Estimates for emissions from sugarcane and cassava also suggest high effluent emissions of CH₄, and the potential for strong future growth.  Critically, these emissions could be averted via widespread adoption of readily available methane flaring or waste-to-energy technologies, but the present-day policy and economic environment impedes significant progress, thereby hindering clean development mechanisms that represent both economic potential for poorer regions and effective climate mitigation.