Plastic Pollution and its Effect on the Thermal Capacity of Seawater

The findings of this study indicate that as expected the natural albedo of seawater is susceptible to positive and negative forcing by pollution and natural agents. Comparison of oil and gas pollutants showed inverse temperature change profiles, with the oil sample heating more rapidly and cooling more slowly than seawater, while the plastic sample heated slower and cooled faster than the control. Regarding oil pollution, reports have shown that while a rainbow film of oil over the surface of the ocean will dampen wave action and can increase albedo, its larger impacts comes from its ability to transfer heat to depth, by acting as an absorbent of radiation, altering the scattering profile and increasing sub-surface refraction with the water column (Pershin, 2011).

Additionally, an oil film also will also severely reduce evaporative cooling rates, acting essentially as a greenhouse over the ocean surface (Pershin, 2011). Estimates indicate that 20% of global oceans are now contaminated with oil. Given that the presence of oil pollution can significantly affect the ocean’s optical properties, areas of high pollution exposure such as coastal zones, harbors, and shipping routes are likely to experience localized oceanic heating (Drozdowska, et al., 2011).

Looking at the Gulf of Mexico, Pershin (2011) related the anomalous 200km shift in the gulf stream following the BP oil spill to a large body of oil-slicked ocean water that had been unnaturally warmed due to the interference of the radiative heating capacity of seawater. Interestingly, the plastic pollution was found to inversely affect the heating profile of seawater, which can be assumed to be the result of increased surface albedo and plastic’s lower specific heat capacity dissipating radiation back into the atmosphere. While the consequences of marine plastics have become an emergent field of study, their impact on the heating capacity of the ocean has not yet be explored. Given that a number of factors such as chemical composition, color, and age of plastic affect its ability to reflect solar radiation, studies may find that impacts are localized rather than global. One paper suggests that increased plastic loading may reduce out-going radiative heat transfer at the surface, inducing heat retention due to internal refraction (Fdez-Sevilla, 2014). However, this time the greatest threat may result from plastics' ability to float over seawater, where it is subjected to chemical breakdown by solar radiation, leading to carbon dioxide production, fueling atmospheric warming. Additionally, preliminary studies have shown that at this point the lighter degraded plastic could then be carried to depth by vertical mixing processes where it would decrease the albedo and lead to higher reflection and sub-surface heating (Fdez-Sevilla, 2014). It should be noted that for this study the plastic sample contained pieces that both sank and floated, corroborating plastic’s ability to disperse throughout the water column. Exploring the natural variables, it was found that the sediment sample, did not exhibit an effect on the heating profile of seawater, which is in contrast to the literature that suggests sediment loading in marine waters absorbs incoming radiation at a higher rate than water due to its lower specific heat capacity and those dark particles such as the black topsoil would reduce ocean surface albedo (Fondriest Environmental Inc., 2014).

A potential explanation for this may have been due to the large particle size relative to the volume of seawater as caused most of the sediment to sink. Correcting for this in future studies would involve blending the sediment into the sweater using a blender, to better simulate the turbulent mixing process of coastal waters. The mangrove samples collectively buffered the seawater from excessive heating and cooling due to their ability to absorb the incoming solar radiation and prevent it from entering the water column. In nature mangroves are important nursery habitats, rich in biological life, therefore their buffering capacity is essential for vulnerable juvenile species (de Lima, Gilma and Galvani, 2013). Additionally, dependent on the coverage characteristics, mangroves intercept precipitation, which further influences the incoming and outgoing radiation ability of the marine environment below (de Lima, Gilma, and Galvani, 2013). This variability in influencing heating and cooling profiles was noted between the two different coverage samples, with the 50%-coverage having less of an effect on the heating and cooling rates. The slower cooling profile of the 100%-coverage may have been due to increased evaporation by the mangrove heating the air above the sample, reducing the temperature gradient at the water surface, thus limiting the sample's cooling rate. While there were potential issues with this study, such as the small volume of water, which was reduced from 100ml to 50ml in order to observe changes over the total 30-minute sampling interval completed separately for each sample. Interpreting temperature measurements required removing the sample temporally from the light source in order to record a value, this would have affected the heating process. Additionally, the same vessel size may have caused heat from the small glass vessel to also be transferred into the recording. Further studies would aim to correct for this by scaling up the experiment and or moving it out to the ocean to record in situ thermal effects. Conclusion Although this study found unpredicted effects of marine plastic on the heating and cooling rates of seawater, the findings regarding oil were as hypothesized.

These findings suggest implications for climate change, as the ocean’s absorptive heat capacity is not infinite, and as suggested the greatest impacts of marine pollution are still unknown. However, as a result of rising atmospheric carbon dioxide levels, sediment run-off, and thermal effluents, oceans have already absorbed more than 90% of the extra anthropogenic heat generated in the last century (Kitman et al., 2013). This has led to nonlinear ocean warming trends as rapidly increasing atmospheric and sea surface temperatures, have led to increased heating at depth, with 30% of the ocean warming now occurring at depths below 700m (Kitman et al., 2013). This states that a major factor in the rate of climate change is the rate at which ocean heat is transferred from the surface to the interior (Kitman et al., 2013). Acting initially to resist climate change, beyond a threshold sub-surface heating may actually accelerate it. This will have long-term effects, even if atmospheric carbon levels are reduced as the internal heat will continue cycling throughout the oceans for centuries. This unsettling fact highlights the need for continued studies to understand the ocean heat budget as the full hysteresis of ocean warming is yet to be determined. References de Lima, B., Gilma, N., & Galvani, E. (2013). Mangrove microclimate: A case study from southeastern brazil. Earth Interactions, 17(2), 1-16. doi:10.1175/2012

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