The push for renewable energy has led to the generation of biofuels from cellulose-rich biomass, algae, and crops. Currently, crop-based biofuels are limited to those derived from agricultural products: corn, soybean, rapeseed, and surgarcane. An increase in the demand for crop-based biofuels will require either an increase in the amount of agricultural land or an increase in crop production on existing land.

An expansion of agricultural land can only occur if whatever is presently on the land is sacrificed—this can mean abandoned lands, pastures, or natural systems. Natural systems such as grasslands and forests store large amounts of carbon; if turned into agricultural lands, this carbon could be released into the atmosphere. Though crops also store carbon as biomass during their growth, regular harvests do not allow for long-term carbon storage. From a climate perspective, this could be problematic.

Do the carbon and nitrogen emissions that result from the deforestation and land-use intensification offset the environmental benefits of displacing fossil fuels? One way to assess this issue is by calculating carbon payback times, which represent the period over which the total greenhouse gas savings due to the displacement of fossil fuels equals the initial losses in ecosystem carbon stocks caused by land conversion.

Recently, an international team of scientists improved this process through the development of a greenhouse gas payback time (GPBT), which assesses the impact of biofuel production on multiple classes of greenhouse gas emissions. The GPBT essentially indicates the number of years it takes before the greenhouse gas savings due to biofuels equals the initial losses of carbon and nitrogen stocks from the original ecosystem.

The greenhouse gas payback times depend on the amount of carbon dioxide and dinitrogen oxide emitted into the atmosphere due to the removal of biomass both when space is cleared for agricultural lands and when fertilizer is applied during cultivation. When calculating the GPBTs, the amount of greenhouse gasses emitted due to the production of bioenergy and the use of fossil energy are both considered. Finally, the amount of bioenergy gained through biofuel production, which depends on the feedstock yield, feedstock-to-biofuel conversion efficiency, and energy content of the biofuel, plays a role.

Previous studies have demonstrated that greenhouse gas emission involved with production of crop-based biofuels varies depending on the type of crop that is cultivated, the location, and the intensity of farm management practices. However, most previous studies have been focused on specific countries and regions; the limited scope of these investigations has kept the analysis from effectively addressing global biofuel productions. To address this issue, these scientists derived spatially explicit, high-resolution GPBTs that accounted for the conversion of natural vegetation to feedstock cropland.

GPBTs were calculated for the production of bioenthanol from corn grain, sugarcane sucrose, and winter wheat grain, as well as the production of biodiesel from rapeseed and soybean oil. A global crop model was used to simulate spatially explicit cultivation of biofuel crops. The influence of farming intensification for individual crops, which occurs by increased labor input or use of technology, was also assessed. Finally, the influence of geographic location, management regime, and crop type on the GPBTs was evaluated.

The types of crop-based biofuel production studied in this investigation produce large quantities of byproducts that emit greenhouse gases. An energy-based method was used to allocate the appropriate amount of greenhouse gas emissions to these byproducts. Without this allocation, GPBTs are on average 61 percent shorter.

When starting from natural vegetation, GPBTs varied anywhere from one to 162 years, depending on the crop, management intensity, and location. GPBTs were longest in the tropical regions of South America, Africa, and Southeast Asia. The scientists found a median GPBT of 51 years when converting tropical moist forest to cropland for biofuels and 27 years when replacing tropical grasslands. For the temperate and boreal regions, the median GPBT was between six and 20 years, depending on what types of temperate vegetation were replaced with biofuel cropland.

Next, the team looked at how farming intensification reduced the GPBTs. When looking at GPBTs under no-input farming, rapeseed-based biodiesel production yielded the shortest GPBTs, with a median of 21 years. The longest GPBTs came from bioethanol production from sugarcane under no-input farming, where the median was 60 years. GPBTs increased for all crops in the tropical regions. In contrast, under high-input farming, the median GPBTs for all crops was 79 percent shorter compared with no-input farming having a median value of 45 years. Under these conditions, corn and winter wheat yielded the lowest median GPBTs of six years and eight years, respectively.

Overall, the scientists found more variability in the GPBTs than previous studies, which they think can be attributed to the inclusion of more regions with relatively low yields due to their global simulation. This study underscores the importance of using appropriate assessment tools to predict future trends in order to guide biofuel policy.

DOI: 10.1038/NCLIMATE2642