The Trouble with Trees: Volatile Organic Compounds Exacerbate Climate Change and Air Pollution

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Scientists are using sophisticated emission models and monitoring techniques to better understand the relationship between volatile organic compound production, climate change, and air pollution. As rainforests are cut down and replaced with isoprene-emitting oil palm plantations, data suggest that genetically modified trees may provide a hopeful alternative.

Oil palm fruit. Image Credit: Wiki Commons

Oil palm fruit. Image Credit: Wiki Commons

A gleaming dome shines like a beacon in the desert set against the backdrop of the Santa Catalina Mountains near Tucson, Arizona. The dome, called Biosphere 2, is a steel and glass structure that houses seven complete ecosystems designed to mimic those on Earth. Within parts of Biosphere 2, scientists monitor gas emissions from plants to better understand their impact on the atmosphere and air quality in hopes of finding effective remedies for climate change.

‰ÛÏThe air that we breathe and that surrounds us is full of millions of interesting compounds. Some of these compounds can be harmful to our health and some of them can affect things like weather and climate,‰Û explains Dr. Alex Archibald, an atmospheric chemist and lecturer for the Department of Chemistry at the University of Cambridge. Archibald‰Ûªs research focuses on the relationship between atmospheric chemistry and Earth systems.

This relationship is often uncertain and riddled with complexity. Advances in climate science reveal elaborate biogeochemical feedback systems that can positively or negatively affect air quality and global temperatures. Many of these feedback systems involve volatile organic compounds (VOCs). According to the U.S. Environmental Protection Agency, VOCs are small particles emitted as gas that become pollutants in the air. They are typically associated with chemicals found in household cleaning products and building materials, but it turns out that trees also emit VOCs.

Plant cells naturally synthesize VOCs as insect and animal attractants or repellents and emit them almost exclusively through their stomata, or leaf pores. For example, the odor of many fragrant plants results from a subset of VOCs. Emission levels may increase in response to environmental and biological stressors.

Although VOCs play crucial roles in plant defense and survival, different plant species emit varying levels and types of volatile organic compounds that also frequently act as natural pollutants. ‰ÛÏPlants and trees emit almost the same mass as the human population in the form of the VOC isoprene,‰Û says Archibald. Isoprene is the main natural volatile organic compound related to atmospheric pollution.

Many deciduous trees naturally produce isoprene, including members of the sweet gum and oak family. However, trees especially known to emit high levels of the VOC pollutants, such as poplars and oil palms, are a major agricultural resource for some developing countries.

Many governments and corporations promote oil palm plantations as a developmental agent for poor, rural regions, especially in Borneo and Sumatra. According to the European Palm Oil Alliance (EPOA), Malaysia and Indonesia export 85 percent of oil palm products. As of 2012, global consumption of palm oil was 52.1 million tons. The growth of the oil palm industry relates to the global demand for vegetable oils, which has increased by more than 5 percent per year over the last decade. Used primarily in the cosmetic and food industries, palm oil is likely to remain in high demand due to its high yield, versatility, and low cost.

When free in the atmosphere, VOC pollutants can interact with nitrogen oxides (NOx) to form ozone. Ozone is beneficial high in the Earth‰Ûªs atmosphere, where it acts as a shield against harmful ultraviolet radiation. However, at ground level it causes increased greenhouse effect, breathing problems, and plant damage. VOCs also may compete with atmospheric oxidants and prevent them from destroying methane, a powerful greenhouse gas.

‰ÛÏThere is the potential then for VOCs to inadvertently affect climate by allowing methane levels to rise,‰Û says Archibald.

Ground ozone can be harmful to human health. Image credit: NASA

Ground ozone can be harmful to human health. Image credit: NASA

The VOC isoprene contributes to the largest fluxes in Earth‰Ûªs atmosphere, and has estimated annual emissions even greater than those of some common anthropogenic pollutants, according to research by Dr. Alex Guenther [3], a professor at University of California, Irvine, and Squire et al. [6]. The high isoprene emissions of oil palms link the industry to air pollution and climate change. According to a report by Accenture for Humanity United [1], oil palm plantations also contribute to deforestation and human rights violations, including child labor in some remote areas, and oil palms replace diverse ecosystems with acres of high-VOC emitting trees.

In Malaysia there are 4.7 million hectares of palm oil plantations, covering about 71 percent of all agricultural land. As demand continues to grow, so will the coverage of oil palm plantations in areas like Malaysia, Indonesia, and Papua New Guinea.

Although replacing native rainforests with oil palm plantations maintains, or possibly increases, the number of trees in the area, these trees emit drastically higher levels of VOCs than the same area of rainforest. According to a recent three-year project funded by the Natural Environment Research Council in the Danum Valley in Borneo, Malaysia, the increase of VOCs could lead ozone concentrations to rise to levels perilous to human health.

The findings of the Danum Valley study [4] show that the types of trees planted in an area matters significantly and could have drastic impacts on tropical forest biodiversity, climate, and human health.

The problems presented by these oil palm plantations in these developing regions reveal a need for site-specific approaches and the inclusion of models of regional, local, and global VOC emissions when considering possible solutions to counteracting air pollution and climate change. ‰ÛÏIncluding VOCs is paramount for global climate,‰Û says Archibald. He predicts that future climate models will include VOC chemistry.

However, monitoring emissions is not a solution‰ÛÓit is a step toward better understanding the intricate relationship influencing climate. Although monitoring is vital to future research and policymaking, the global demand for palm oil poses a quandary.

With such a high demand for palm oil, is there a feasible solution? In tropical areas, the push for reforestation was included in agreements made during the recent Paris Climate Conference, which took place in December of 2015. But, considering the surging demand, expansive industry, and livelihood of millions of people, policymakers face convoluted obstacles in the search for a panacea.

With increasing demand for bioenergy, demand for palm oil production will likely increase even more, because trees that emit high levels of isoprene, such as poplars and oil palms, are frequently the best candidates for biofuel. Currently, designated areas of oil palm plantations grow partially as an alternative fuel source‰ÛÓone that could reduce fossil-fuel dependency and thus cut down on greenhouse gas emissions.

However, the authors of the study in Danum Valley warn that without NOx emissions control or the genetic modification of oil palms, the palm oil industry will have negative consequences on human health and crop yield. It seems that ‰ÛÏthe utility of palm oil as an environmentally friendly fuel therefore may be severely time-limited.‰Û

Should there be a boycott on the palm oil industry, ending jobs and ignoring the potential benefits of biofuel? Or should the expansion of an industry with such a complicated list of problems, including links to human rights violations, deforestation, and potentially detrimental VOC emissions, be allowed to continue?

A complex problem requires complex solutions, and recent research on this conundrum reveals a potentially promising answer‰ÛÓgenetically modified organisms (GMOs) that emit significantly less harmful VOCs, specifically isoprene [2].

Within the last two decades, this area of research has increased in popularity. In 2003, joint research from The University of Colorado at Boulder and Biosphere 2 affirmed the possibility of producing GMO poplars that would lessen their isoprene output. In an article published in Nature [5], the researchers write, ‰ÛÏas almost all commercial agriforest species emit high levels of isoprene, proliferation of agriforest plantations has significant potential to increase regional ozone pollution and enhance the lifetime of methane, an important determinant of global climate.‰Û

Researchers grew poplar trees in greenhouses with the hope of isolating the chloroplasts of leaf cells. Both groups in Colorado and Arizona discovered that increasing carbon dioxide in the laboratory lead to a decrease in isoprene emissions from the leaf cells. Now scientists create modified trees by inhibiting enzymes responsible for isoprene production.

Biosphere 2‰Ûªs informational system represents a powerful, sensitive monitoring system, part of which has been dedicated to monitoring relationships between plants and atmospheric chemistry. Image Credit: The University of Arizona/ Biosphere 2

Biosphere 2‰Ûªs informational system represents a powerful, sensitive monitoring system, part of which has been dedicated to monitoring relationships between plants and atmospheric chemistry. Image Credit: The University of Arizona/ Biosphere 2

Because of the increased demand for sustainable bioenergy, oil palms and poplar tree species are currently receiving global attention. Poplars are fast-growing pioneer trees, allowing for high production and short-rotations. Poplar biomass, like oil palm, produces heat and power, making it a practical substitute for fossil fuels.

‰ÛÏClimate impacts food, energy, and water availability as well as ecosystem health,‰Û says Guenther, who investigates the reactive trace gases responsible for air pollution and climate. Even though the advent of low-or non-isoprene emitting trees provides a hopeful alternative, implementing regional, local, and global biogenic VOC monitoring is imperative to understanding how these intricate interactions alter the atmosphere.

According to Guenther, ‰ÛÏWe need to develop a quantitative understanding of the processes that control climate, including VOC emissions, in order to enable science-based decisions regarding resource management.‰Û With a complicated problem like climate change, it is important not to oversimplify. Yet as climate change continues to drastically impact Earth systems, the threats to human health increase. Effective solutions are imperative and in the realm of biofuel and agriforests, genetically modified trees may unlock part of the answer.

Kayla Townsley is an undergraduate at Portland State University majoring in molecular biology and a student science writer with the Earthzine Writing Club.

References

[1] Accenture for Humanity United. (2013). Exploitative Labor Practices in the Global Palm Oil Industry. humanityunited.org/pdfs/Modern_Slavery_in_the_Palm_Oil_Industry.pdf

[2] Behnke, K. et al. (2011). Isoprene emission-fre poplars – a chance to reduce the impact from poplar plantations on the atmosphere. New Phytologist, V. 194, Issue 1.åÊonlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2011.03979.x/full

[3] Guenther, A. et al. (2006). Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature), Atmos. Chem. Phys., V. 6, pp.3181-3210. atmos-chem-phys.net/6/3181/2006/acp-6-3181-2006.html

[4] Hewitt, C. N. et al. (2009). Nitrogen management is essential to prevent tropical oil palm plantations from causing ground-level ozone pollution. Proceedings of the National Academy of Sciences, V. 106, no. 44, pp. 18447-18451.åÊdx.doi.org/10.1073/pnas.0907541106

[5] Rosenstiel, T. et al.. (2003). Increased Co2 uncouples growth from isoprene emission in an agriforest ecosystem. Nature, V. 421. nature.com/nature/journal/v421/n6920/full/nature01312.html

[6] Squire, O. J., Archibald, A.T, et al. (2013). Influence of future climate and cropland expansion on isoprene emissions and tropospheric ozone, Atmos. Chem. Phys. Discuss., V. 13, pp. 18307‰ÛÒ18344, atmos-chem-phys-discuss.net/13/18307/2013/doi:10.5194/acpd-13-18307-2013