Christopher Ruf, University of MichiganPlastic is the most common type of debris floating in the world’s oceans. Waves and sunlight break much of it down into smaller particles called microplastics – fragments less than 5 millimeters across, roughly the size of a sesame seed.
To understand how microplastic pollution is affecting the ocean, scientists need to know how much is there and where it is accumulating. Most data on microplastic concentrations comes from commercial and research ships that tow plankton nets – long, cone-shaped nets with very fine mesh designed for collecting marine microorganisms.
But net trawling can sample only small areas and may be underestimating true plastic concentrations. Except in the North Atlantic and North Pacific gyres – large zones where ocean currents rotate, collecting floating debris – scientists have done very little sampling for microplastics. And there is scant information about how these particles’ concentrations vary over time.
To address these questions, University of Michigan research assistant Madeline Evans and I developed a new way to detect microplastic concentrations from space using NASA’s Cyclone Global Navigation Satellite System. CYGNSS is a network of eight microsatellites that was launched in 2016 to help scientists predict hurricanes by analyzing tropical wind speeds. They measure how wind roughens the ocean’s surface – an indicator that we realized could also be used to detect and track large quantities of microplastics.
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Looking for smooth zones
Annual global production of plastic has increased every year since the 1950s, reaching 359 million metric tons in 2018. Much of it ends up in open, uncontrolled landfills, where it can wash into river drainage zones and ultimately into the world’s oceans.
Researchers first documented plastic debris in the oceans in the 1970s. Today, it accounts for an estimated 80% to 85% of marine litter.
The radars on CYGNSS satellites are designed to measure winds over the ocean indirectly by measuring how they roughen the water’s surface. We knew that when there is a lot of material floating in the water, winds don’t roughen it as much. So we tried computing how much smoother measurements indicated the surface was than it should have been if winds of the same speed were blowing across clear water.
This anomaly – the “missing roughness” – turns out to be highly correlated with the concentration of microplastics near the ocean surface. Put another way, areas where surface waters appear to be unusually smooth frequently contain high concentrations of microplastics. The smoothness could be caused by the microplastics themselves, or possibly by something else that’s associated with them.
By combining all the measurements made by CYGNSS satellites as they orbit around the world, we can create global time-lapse images of ocean microplastic concentrations. Our images readily identify the Great Pacific Garbage Patch and secondary regions of high microplastic concentration in the North Atlantic and the southern oceans.
Tracking microplastic flows over time
Since CYGNSS tracks wind speeds constantly, it lets us see how microplastic concentrations change over time. By animating a year’s worth of images, we revealed seasonal variations that were not previously known.
We found that global microplastic concentrations tend to peak in the North Atlantic and Pacific during the Northern Hemisphere’s summer months. June and July, for example, are the peak months for the Great Pacific Garbage Patch.
Concentrations in the Southern Hemisphere peak during its summer months of January and February. Lower concentrations during the winter in both hemispheres are likely due to a combination of stronger currents that break up microplastic plumes and increased vertical mixing – the exchange between surface and deeper water – that transports some of the microplastic down below the surface.
This approach can also target smaller regions over shorter periods of time. For example, we examined episodic outflow events from the mouths of the China’s Yangtze and Qiantang rivers where they empty into the East China Sea. These events may have been associated with increases in industrial production activity, or with increases in the rate at which managers allowed the rivers to flow through dams.
Better targeting for cleanups
Our research has several potential uses. Private organizations, such as The Ocean Cleanup, a nonprofit in The Netherlands, and Clewat, a Finnish company specializing in clean technology, use specially outfitted ships to collect, recycle and dispose of marine litter and debris. We have begun conversations with both groups and hope eventually to help them deploy their fleets more effectively.
Our spaceborne imagery may also be used to validate and improve numerical prediction models that attempt to track how microplastics move through the oceans using ocean circulation patterns. Scholars are developing several such models.
While the ocean roughness anomalies that we observed correlate strongly with microplastic concentrations, our estimates of concentration are based on the correlations that we observed, not on a known physical relationship between floating microplastics and ocean roughness. It could be that the roughness anomalies are caused by something else that is also correlated with the presence of microplastics.
One possibility is surfactants on the ocean surface. These liquid chemical compounds, which are widely used in detergents and other products, move through the oceans in ways similar to microplastics, and they also have a damping effect on wind-driven ocean roughening.
Further study is needed to identify how the smooth areas that we identified occur, and if they are caused indirectly by surfactants, to better understand exactly how their transport mechanisms are related to those of microplastics. But I hope this research can be part of a fundamental change in tracking and managing microplastic pollution.
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Katherine Seto, University of Wollongong; Johann Bell, University of Wollongong; Quentin Hanich, University of Wollongong, and Simon Nicol, University of CanberraSmall Pacific Island states depend on their commercial fisheries for food supplies and economic health. But our new research shows climate change will dramatically alter tuna stocks in the tropical Pacific, with potentially severe consequences for the people who depend on them.
As climate change warms the waters of the Pacific, some tuna will be forced to migrate to the open ocean of the high seas, away from the jurisdiction of any country. The changes will affect three key tuna species: skipjack, yellowfin, and bigeye.
Pacific Island nations such as the Cook Islands and territories such as Tokelau charge foreign fishing operators to access their waters, and heavily depend on this revenue. Our research estimates the movement of tuna stocks will cause a fall in annual government revenue to some of these small island states of up to 17%.
This loss will hurt these developing economies, which need fisheries revenue to maintain essential services such as hospitals, roads and schools. The experience of Pacific Island states also bodes poorly for global climate justice more broadly.
Island states at risk
Catches from the Western and Central Pacific represent over half of all tuna produced globally. Much of this catch is taken from the waters of ten small developing island states, which are disproportionately dependent on tuna stocks for food security and economic development.
These states comprise:
- Cook Islands
- Federated States of Micronesia
- Marshall Islands
- Papua New Guinea
- Solomon Islands
Their governments charge tuna fishing access fees to distant nations of between US$7.1 million (A$9.7 million) and $134 million (A$182 million), providing an average of 37% of total government revenue (ranging from 4-84%).
Tuna stocks are critical for these states’ current and future economic development, and have been sustainably managed by a cooperative agreement for decades. However, our analysis reveals this revenue, and other important benefits fisheries provide, are at risk.
Climate change and migration
Tuna species are highly migratory – they move over large distances according to ocean conditions. The skipjack, yellowfin and bigeye tuna species are found largely within Pacific Island waters.
Concentrations of these stocks normally shift from year to year between areas further to the west in El Niño years, and those further east in La Niña years. However, under climate change, these stocks are projected to shift eastward – out of sovereign waters and into the high seas.
Under climate change, the tropical waters of the Pacific Ocean will warm further. This warming will result in a large eastward shift in the location of the edge of the Western Pacific Warm Pool (a mass of water in the western Pacific Ocean with consistently high water temperatures) and subsequently the prime fishing grounds for some tropical tuna.
This shift into areas beyond national jurisdiction would result in weaker regulation and monitoring, with parallel implications for the long-term sustainability of stocks.
What our research found
Combining climate science, ecological models and economic data from the region, our research published today in Nature Sustainability shows that under strong projections of climate change, small island economies are poised to lose up to US$140 million annually by 2050, and up to 17% of annual government revenue in the case of some states.
The Intergovernmental Panel on Climate Change (IPCC) provides scenarios of various greenhouse gas concentrations, called “representative concentration pathways” (RCP). We used a higher RCP of 8.5 and a more moderate RCP of 4.5 to understand tuna movement in different emissions scenarios.
In the RCP 8.5 scenario, by 2050, our model predicted the total biomass of the three species of tuna in the combined jurisdictions of the ten Pacific Island states would decrease by an average of 13%, and up to 20%.
But if emissions were kept to the lower RCP 4.5 scenario, the effects are expected to be far less pronounced, with an average decrease in biomass of just 1%.
While both climate scenarios result in average losses of both tuna catches and revenue, lower emissions scenarios lead to drastically smaller losses, highlighting the importance of climate action.
These projected losses compound the existing climate vulnerability of many Pacific Island people, who will endure some of the earliest and harshest climate realities, while being responsible for only a tiny fraction of global emissions.
What can be done?
Capping greenhouse gas emissions, and reducing them to levels aligning with the Paris Agreement, would reduce multiple climate impacts for these states, including shifting tuna stocks.
In many parts of the world, the consequences of climate change compound upon one another to create complex injustices. Our study identifies new direct and indirect implications of climate change for some of the world’s most vulnerable populations.
Katherine Seto, Research Fellow, University of Wollongong; Johann Bell, Visiting Professorial Fellow, University of Wollongong; Quentin Hanich, Associate Professor, University of Wollongong, and Simon Nicol, Adjunct professor, University of Canberra