Plastic pollution in the Arctic

Plastic pollution is now pervasive in the Arctic, even in areas with no apparent human activity, such as the deep seafloor. In this Review, we describe the sources and impacts of Arctic plastic pollution, including plastic debris and microplastics, which have infiltrated terrestrial and aquatic systems, the cryosphere and the atmosphere. Although some pollution is from local sources — fisheries, landfills, wastewater and offshore industrial activity — distant regions are a substantial source, as plastic is carried from lower latitudes to the Arctic by ocean currents, atmospheric transport and rivers. Once in the Arctic, plastic pollution accumulates in certain areas and affects local ecosystems. Population-level information is sparse, but interactions such as entanglements and ingestion of marine debris have been recorded for mammals, seabirds, fish and invertebrates. Early evidence also suggests interactions between climate change and plastic pollution. Even if plastic emissions are halted today, fragmentation of legacy plastic will lead to an increasing microplastic burden in Arctic ecosystems, which are already under pressure from anthropogenic warming. Mitigation is urgently needed at both regional and international levels to decrease plastic production and utilization, achieve circularity and optimize solid waste management and wastewater treatment. Plastic debris and microplastics are ubiquitous in the Arctic. This Review describes the sources, distribution and consequences of this pollution, and calls for immediate action to mitigate further ecosystem impact. The widespread plastic pollution in the Arctic originates from both local and distant sources. Concentrations of plastic in the Arctic vary widely, with greater accumulation in certain hotspots, but are generally similar to those of more densely populated regions. Plastic has infiltrated all levels of the Arctic food web, including many endemic species, with largely unknown organismal impacts. In the fast-changing Arctic, plastic pollution adds to the effects of climate change in terms of growing sources, transport processes, potential feedback loops and ecological consequences. Mitigation of both local and distal plastic pollution is needed to prevent further ecosystem degradation. The widespread plastic pollution in the Arctic originates from both local and distant sources. Concentrations of plastic in the Arctic vary widely, with greater accumulation in certain hotspots, but are generally similar to those of more densely populated regions. Plastic has infiltrated all levels of the Arctic food web, including many endemic species, with largely unknown organismal impacts. In the fast-changing Arctic, plastic pollution adds to the effects of climate change in terms of growing sources, transport processes, potential feedback loops and ecological consequences. Mitigation of both local and distal plastic pollution is needed to prevent further ecosystem degradation.

Industrial plastic production has grown rapidly since the 1950s, reaching 368 million tonnes globally per year by 2019 (ref. 1 ). Because of its low price, plastic has become one of the most widely used materials, especially in the packaging industry, and now forms an integral part of municipal waste. Every year, 19-23 million metric tonnes of mismanaged plastic waste are transferred from land-based sources to water globally 2 .
As plastics are designed to be durable, they persist in the environment for long periods of time and become widely dispersed 3,4 . Therefore, plastic is a planetary boundary threat 5,6 , especially once it fragments into microplastic and nanoplastic (size ≤5 mm and ≤1 μm (ref. 7 ), respectively) due to sunlight, mechanical abrasion, biotic interaction, wave action 8 and temperature fluctuations. Plastic is also extensively used in maritime operations such as fishing, aquaculture, shipping and offshore operations, leading to substantial additional leakage into marine environments. Although millions of tonnes of plastics enter the oceans annually, it is currently unknown where in the ocean 99% of the small plastic debris ends up 9 , pointing to yet unaccounted for accumulation areas 10 .
Polar regions are still perceived as pristine. However, in the past five years, high levels of plastic pollution have been found in the Arctic ( fig. 1). The formation of a sixth accumulation area in the Nordic Seas has been suggested by model projections 11 and is corroborated by an increase in marine debris over time 12,13 and comparatively high microplastic concentrations in the Arctic 14,15 . This evidence has prompted a Protection of the Arctic Marine Environment (PAME, Arctic Council) working group desktop study on marine litter and microplastics in the Arctic 16 to gauge the need for a Regional Action Plan, which, in turn, led to a mandate to assess the status and trends by the Arctic Monitoring and Assessment Programme 17 .
In this Review, we describe the sources of Arctic plastic debris, its distribution and its effects on Arctic biota, as well as knowledge gaps and mitigation with a broad, pan-Arctic view, complementing previous reviews focused on plastic pollution effects on Arctic biota 18 , such as seabirds 19 , or with differential geographic focuses [20][21][22] . We also discuss interactions between climate change and plastic pollution, as plastic pollution likely adds to the impacts of climate change, which has caused a three times faster increase in Arctic temperatures compared with the global average 23 . plastic bags and fabrics 31 . However, because such items are also used on ships, it is difficult to attribute such plastics to land-based versus sea-based sources, and input from sea-based sources was rated more important than land-based sources in the Arctic 24 . For example, large food containers amongst the household plastics found on northwest Svalbard point to the disposal of galley waste, which is of sea-based origin 32 . Litter quantities on the seafloor of the Fram Strait have been correlated with increasing activities in both the fisheries and the tourism sectors west of Svalbard 49 . The prevalence of fast-sinking glass debris on the deep Arctic seafloor also corroborates the importance of local sea-based sources 12 . Arctic ship traffic is due to increase as new and faster trans-Arctic routes open, and the shipping season extends as sea ice declines 50 , potentially leading to increased local plastic inputs.
A major challenge to minimizing the input of waste from land into the ocean globally is the lack of adequate waste management facilities in coastal regions 51 . As Arctic population densities are low, waste collection and disposal is very basic. Recycling and baling facilities are rare and limited to large Arctic communities. Waste collection in larger communities often relies on community haul systems, whereas in small communities, it is typically by self-haul 52 , which can be less efficient in preventing waste leakage into the environment. In some communities, traditional waste management solutions are landfills and uncontrolled dumpsites, sometimes next to the sea, and simple incinerators with no or limited flue gas treatment 53 , as seen in Greenland 54 and Iqaluit, Canada 46,55 . Beach litter assessments 56 report input from inadequate waste facilities on the western shores of Greenland, where 90% of the Greenlandic population lives. In the Canadian Arctic, plastic litter densities were seven times higher near communities compared with more remote locations 43 . Open dumpsites and winter travel activities were identified as potential sources 43 . Numerous open waste disposal sites and abandoned landfills were also identified as an important source of plastic pollution distributed over the flat tundra by high winds of the Archangelsk region of Russia 57 .
Microplastics are also widely distributed in the Arctic, transported by ocean and atmospheric currents and biota from both distant and local sources. Microplastics are either manufactured directly, for example, as pre-production pellets and microbeads, or formed through weathering and breakdown of larger plastic items. Data from the east Canadian Arctic suggest primarily distal sources of microplastic 46,58 or a combination of distant and local sources 46 . Substantial quantities of microfibres are found in sediments from the Canadian Arctic (1,930 fibres per kg dry weight), 51% and 20% of which were acetate cellulose and indigo denim, respectively, indicating long-range transport from southern wastewater source regions 59 .
In other regions, local sources play a prominent role. High concentrations of microplastic in surface waters off west Greenland likely originate from the capital Nuuk, which harbours 18,000 inhabitants 60 . One local source could be effluent from sewage and wastewater treatment, which is often only mechanically treated or not

Key points
• The widespread plastic pollution in the arctic originates from both local and distant sources. • Concentrations of plastic in the arctic vary widely, with greater accumulation in certain hotspots, but are generally similar to those of more densely populated regions. • Plastic has infiltrated all levels of the arctic food web, including many endemic species, with largely unknown organismal impacts. • In the fast-changing arctic, plastic pollution adds to the effects of climate change in terms of growing sources, transport processes, potential feedback loops and ecological consequences. • mitigation of both local and distal plastic pollution is needed to prevent further ecosystem degradation.
www.nature.com/natrevearthenviron treated at all in Nuuk or Svalbard [60][61][62] . Indeed, large quantities of microplastic fibres are shed during washing of synthetic textiles 63 , which are disproportionately much worn in cold polar regions and can leak into the ocean through inadequately treated wastewater. Local wastewater could also be one of the sources of microplastic in the White Sea basin 57 . Six million microlitter particles per hour were emitted into the ocean (≥100 µm, ~1,500 particles m −3 ) by a wastewater treatment plant in Reykjavik, Iceland, that only used mechanical treatment 64 . Exceptionally high levels of microplastic were also recorded from a sandy beach near Reykjavik 65 , Iceland, which is located near a harbour and waste management facility. Therefore, even adequate waste management systems can act as sources if located close to the shore. Still, the introduction of mechanical and biological treatment at a wastewater treatment plant in Ny-Ålesund, Svalbard, has cut anthropogenic microparticle emissions by 99%, highlighting that systems are available to reduce further emissions from Arctic communities 66 .
Other potential but poorly constrained local sources of microplastic include particles shed from ship paint, skidoos and other vehicles used on ice, as well as grey water released by rising numbers of ships operating in the area 67 . Paint-derived fragments were found in southwest Greenland 46 and dominated microplastic in water samples from a National Wildlife Area on Baffin Island, hundreds of kilometres away from any major settlement, highlighting both local and distant sources in these coastal areas 68 . The expanding hydrocarbon industry could be another source of litter and microplastictube-dwelling worms and sediments taken near oil and gas platforms in the North Sea bore significantly higher microplastic burdens than those collected further away, especially the viscosity-enhancer polyacrylamide 69 . However, quantitative information on microplastic inputs from shipping and the hydrocarbon industry is lacking for the Arctic region.

Distribution and transport
Buoyant plastic can float with ocean surface currents to higher latitudes 24,[70][71][72] , with most plastic transport into the Arctic from the Atlantic 24 and modest transport of microplastic through the Bering Strait 73 ( fig. 2). Surface transport is accelerated by storms through wave-driven Stokes drift 74 or direct windage 75 . Mesoscale eddies also affect the transport of debris or other materials [76][77][78][79][80] , as can subsurface transport of less buoyant plastic at depths below 50 m (refs 81,82 Fig. 1 | Overview of the pathways of plastic pollutants into the Arctic Ocean from local and distant sources. Plastic pollution can be generated by households, traffic, agriculture, wastewater treatment, landfills, illegal dumping, industry, shipyards, tourism, ships, fisheries and offshore industry, and be transported to and/or within the Arctic via the atmosphere, rivers, ocean currents, sea ice and eroding permafrost. The seafloor and sea ice are areas of plastic accumulation. The numbers in boxes refer to the abundance of plastic debris (green) or microplastics (MP, purple) in different ecosystem compartments. The ranges are based on data from 36 peer-reviewed studies reporting from 727 locations that were compiled in the database Litterbase 84 (more details on the data extraction process are provided in the Supplementary Information). The data in each compartment were converted to common units here, but the sampling and analytical methods used in different studies varied widely, as there are currently few standardized or harmonized procedures. For example, varying size detection limits in different studies likely introduced considerable variability in the ranges shown.  Much less is known about the transport processes of plastics within the Arctic because of scarce measurements. The available data show that plastic debris (0-7.97 items km −2 ) 13,72 and microplastics (0-1,287 particles m −3 ) 27,81,85 are widely distributed in Arctic surface waters ( fig. 3). Because of pollution transport from both the south (North Atlantic Current) and the north (Transpolar Drift), plastic quantities are likely higher in the Eurasian basin 73 , which is corroborated by less weathered plastic microfibres and three times higher microfibre concentrations in the western Arctic 86 .
However, more field data are needed to verify the lower concentrations on the Amerasian side. There, Pacific water does not spread over the whole Arctic Basin, as it circulates primarily around the Beaufort Gyre before leaving with the Atlantic water via the Canadian Arctic and past west Greenland 77 ( fig. 2). Still, it has been suggested that, during this transport, microplastic from the North Pacific enters the western Arctic, concentrates in the Beaufort Gyre and is carried to the central Arctic and Eurasian basins 26 .
High microplastic loads in Arctic sea ice (31.75-12,000,000 particles m −3 ) 29,58 and models both suggest that sea ice drift supports basin-scale transport of ice-rafted plastic 26 ig. 2 | The main pathways of pollution transport to the Arctic. Plastic pollution is transported to the Arctic via atmospheric and aquatic circulation systems, which could promote their accumulation in certain areas. The main ocean currents that move pollution to and within the Arctic are shown as thin red, blue and green arrows, and the ten largest rivers that release 10% of the global river discharge into the Arctic Ocean are illustrated by thick blue arrows. Numbers in parentheses refer to average annual discharge in km 3 (ref. 198 ). The prevailing atmospheric circulation pattern is shown as translucent arrows. The solid and dashed blue lines indicate the main Arctic river basin and watershed, respectively.  fig. 2), where the ice melts and releases its legacy to the water. The presence of ice algae and sticky extracellular polymeric substances in sea ice 89 could enable heteroaggregation of particles and, thus, promote their sinking to the seafloor 90 , as could ballasting via sea-ice-derived cryogenic gypsum from under-ice Phaeocystis blooms 91 . These mechanisms could be one reason for the high quantities of microplastics (6,595 and 13,331 particles kg −1 sediment) observed in the Fram Strait near the marginal ice zone 81,92 . Backward drift trajectories of ice cores taken in the central Arctic indicate that they originated from the Siberian shelves, western and central Arctic 29,88 or circulate in the Beaufort Gyre 26 . Much of the sea ice is formed in regions 29 that receive water from Siberian rivers ( fig. 2).
Siberian rivers have huge catchment areas and cross big cities, industrial and agricultural areas, and receive wastewater effluents of unknown treatment level. Even further upstream, the Ob' and Tom rivers already contain high microplastic concentrations (44.2-51.2 particles m −3 ) 93 . The Severnaya Dvina river plays a major role in the transfer of microplastics to the White Sea 45 and river discharge was identified as the second largest source of the microplastic pollution in the Eurasian basin 27 . Yet, low levels were reported from three rivers feeding into the White Sea basin (0-6 particles m −3 ) 57 . Furthermore, litter quantities from the Russian Arctic indicate low riverine  contributions in autumn 72 . Still, during late spring, when river ice is melting and the greatest discharge into the ocean occurs, pollution levels could be higher.
Half of the plastic from municipal waste is denser than seawater 94 and sinks directly to the seafloor. However, even positively buoyant plastic is recorded in the water column and on the seafloor 81,95 . Mean quantities of 0.011 mg plastic debris m −3 prevailed in the upper 60 m of the Barents Sea 95 . Waters above the deep Arctic seafloor harboured microplastic concentrations of 0-375 particles m −3 (refs 27,81,85 ). Although no vertical trend was found in the Arctic Central Basin (0-375 particles m −3 ) 85 , in the Fram Strait, the mean MP concentration decreased sixfold towards 1,000 m depth with profiles similar to those of particulate organic carbon 81 . Hence, biological processes such as incorporation in marine snow, fast-sinking aggregates of ice algae or phytoplankton and faecal pellets likely enhance the vertical flux of microplastic 81,96 , along with vertical advection and diffusion in the water column 97 . Three-dimensional modelling of particles from the deep Fram Strait emphasized the importance of lateral advection and settling velocities in the vertical dispersal, with trajectories as long as 653 km (ref. 81 ). Most of the modelled particles likely come from the North Atlantic, but sea ice appears to be a source of microplastic tracked back from the east Greenland slope 81 . Deep-water cascading events such as the Storfjorden overflow in Svalbard could also enhance downward particle flux 98 .
Plastic pollution has been recorded from various regions of the Arctic seafloor, including the Norwegian However, unlike bottom trawls from the Chukchi and Kara Seas, trawls from the East Siberian and Laptev Seas returned no litter 33 . The former was attributed to fishing activities in the Barents Sea 33 . Quantities of plastic debris on the seafloor range between 0 and 24,500 items km −2 (refs 12,49,100,101 ) and have increased from 813 to 5,970 items km −2 between 2004 and 2017 in the Fram Strait 12 . The absence of light, low temperatures and stable conditions lead to degradation rates that are particularly low in the deep sea, as indicated by 30-year-old plastic recovered from the Sea of Japan without any signs of deterioration 106 . Bottom currents can carry microplastics on the seafloor to accumulation areas that also happen to be biodiversity hotspots 107 . In the Arctic deep sea, microplastic concentrations range between 0 and 16,041 particles kg −1 sediment 58,108 and rank amongst the highest measured concentrations globally.
Atmospheric transport is also an important transport pathway, as indicated by the presence of microplastic in snow samples from ice floes in the east Canadian Arctic 58 , western Arctic 26 , Svalbard, Fram Strait 109 and Icelandic ice cap 110 ranging from 0 to 14,400,000 particles m −3 (refs 58,109 ). Atmospheric transport could also be a pathway to lakes, although early evidence from four lakes in the Archangelsk region of Russia suggests low pollution levels (0-2 particles m −3 ) 20 . As with mercury pollution, atmospheric circulation patterns including the Icelandic Low, North American High, Aleutian Low and Siberian High could carry air masses with microplastic and nanoplastic from urban eastern and western Europe, North America, East Asia and Siberia to the Arctic, where they can fall out by wet and dry deposition and accumulate in the ocean, cryosphere and permafrost 111 (fig. 2). Airborne microplastic emissions from car tyres and brakes could be as high as riverine or direct inputs of these sources to the ocean 112 . Models suggest that tyre-wear particle concentrations in Arctic snow range between 6 and 150 ng kg −1 for particles ≤10 µm and that Greenland and the Arctic Ocean are important receptor regions 112 . The ocean itself also appears to be a conduit of atmospheric transport, as indicated by microplastic in sea spray mist and onshore winds from the open Atlantic 113 .
Interactions with Arctic wildlife Pervasive contamination of plastic pollution in the Arctic has led to wildlife exposure to both macroplastic and microplastic pollution ( fig. 4). Wildfire and plastics interact through colonization or rafting on marine debris, ingestion, entanglement and smothering, affecting a total of currently 131 species in the Arctic (based on available information as of November 2021) 84 . Interactions can occur both at sea and on land, either with beached debris or with waste from open dumpsites 31,39,114 .

Ingestion of plastics among Arctic species.
Ingestion of plastic debris by organisms does not always lead to direct harm, but it creates the potential for malnutrition, internal injury, obstruction of the intestinal tract causing starvation or rupture, and potentially death 83 . Plastic ingestion has been reported across various regions of the Arctic ( fig. 4) across several levels of the food web, including in zooplankton from the east Canadian Arctic 58 and the Fram Strait 115 . A range of other marine invertebrates also ingest microplastic such as sea anemones, starfish, brittlestars, shrimps, crabs, whelks, bivalves [116][117][118] , amphipods 119 and tube worms 69 . Plastic has been found in Arctic fish such as sculpin (Triglops nybelini) 120 , saithe (Pollachius virens) 121 , polar cod (Boreogadus saida) 120,122 , Atlantic cod (Gadus morhua) 121,123,124 and Greenland shark (Somniosus microcephalus) 125,126 . Because fish are indicators of ecosystem health, important links in Arctic food webs and part of the human diet, further research on plastic contamination in Arctic fish is warranted.
Seabirds are amongst the most studied biota in terms of plastic pollution, both globally and in the Arctic. Early reports of plastic ingestion by herring gulls (Larus smithsonianus) and parakeet auklet (Aethia psittacula) date back to the 1970s 19 . A total of 51 species of seabirds breed in the Arctic region and the ingestion of plastic is widespread among them 19 . It was common among 12 seabird species from the Russian Arctic, for instance, ~60% of Chaun Bay gull nests containing boluses with plastic likely from a nearby dumpsite 31 . The northern fulmar (Fulmarus glacialis) is the most widely studied species for plastic ingestion in the Arctic and globally, and has been sampled in a handful of Arctic regions www.nature.com/natrevearthenviron 0123456789();: repeatedly since 2001. Plastic ingestion levels vary with latitude, with fulmars sampled closer to the pole having lower levels (87% of the birds examined) 127 then their counterparts from other regions 128 , which could reflect lower pollution levels in their feeding grounds.
There are only a few records of plastic ingestion by Arctic mammals 18 , most of which are from whales, including sperm whales (Physeter macrocephalus) 129 , belugas (Delphinapterus leucas) 130 , fin whales (Balaenoptera physalus), bowhead whales (Balaena mysticetus) 131 and Stejneger's beaked whales (Mesoplodon stejnegeri) 132 . Only a handful of pinnipeds (seals, sea lions, walrus) have been examined in the Arctic region. No plastic pieces above 425 µm were detected in the stomachs of ringed seals (Phoca hispida), bearded seals (Erignathus barbatus) and harbour seals (Phoca vitulina) 133 . Similarly, no plastic pieces larger than 5 mm were found in harp seals (Pagophilus groenlandicus) in Greenland, but two plastic sheets were reported in a 20-day-old hooded seal pup (Cystophora cristata) from the Greenland Sea 134 . Seventy percent of walrus faeces in Svalbard contained microfibres larger than 1 μm (ref. 135 ). Although current knowledge suggests relatively low plastic ingestion levels of mammals overall, no firm conclusion can yet be drawn from the current data.

Effects of plastic debris on Arctic wildlife.
Entanglement in plastic debris can have deleterious effects, such as injury, restrained movement, starvation, strangulation and suffocation if air-breathing animals cannot return to the sea surface 83 . Entanglement has been reported for Arctic terns on Svalbard (Sterna paradisaea) and seven other seabird species in the Russian Arctic 31,39 . Thirteen seabird species have also been found to incorporate plastic debris in their nests 31 , which can cause entanglement. Notably, almost all nests of two of the existing northern gannet (Morus bassanus) colonies at the Murman coast and 10% of an ivory gull (Pagophila eburnea) colony from the Kara Sea contained plastic 31 . Polar bears (Ursus maritimus), Arctic foxes (Alopex lagopus), bowhead whales, reindeer (Rangifer tarandus), bearded seals, harbour seals, Greenland halibut (Reinhardtius hippoglossoides), Atlantic cod and snow crabs (Chionoecetes opilio) 31,39,144-147 also experience entanglement.
Plastic debris can also act as a raft to transport animals from one location to another. Six percent of the plastic items stranded on Svalbard were colonized by bryozoans (Membranipora membranacea) and barnacles (Semibalanus balanoides) 148 . Macroalgae, bryozoans, barnacles (Semibalanus sp., Lepas anatifera) and blue mussels (Mytilus sp.) also inhabited beach debris on Svalbard 41 . Rafting of adult groups could favour dispersal over larval transport and be one of the drivers behind the reappearance of Mytilus after 1,000 years of Impacts of plastic pollution on human health are poorly understood but microplastics have been recorded in the lungs, intestines and placenta Many Arctic species have not been examined for plastic ingestion Species on land exposed to plastic have been found to ingest and become entangled in plastic pollution Surface-feeding seabirds are reported to have the highest levels of plastic ingestion Several mid-water-feeding species have been reported to have low levels or no detectable ingested microplastics www.nature.com/natrevearthenviron absence 149 . The invasion of xenobionts via rafting can have population-level or community-level effects 150 , posing a potential threat to Arctic ecosystems.

Ecological effects of plastic.
Because of the widespread contamination of plastic pollution in wildlife, there is urgency to answer questions related to ecological impact 150,151 . In non-Arctic systems, there is overwhelming evidence of detrimental effects from macro plastics to individuals and compelling evidence for effects to populations, communities and ecosystems 150 . For microplastics, impacts have been demonstrated across several levels of biological organization 150,152 , including oxidative stress 153 , changes in gene expression 136,154 , inflammation 155 and reduced growth 156 and reproduction 157 rates. Although these effects could apply to closely related Arctic species, too, there has been little research on the ecological effects of plastic debris in Arctic ecosystems, which are already under stress due to climate change 158 .
One of the few studies available on the effects of plastic on benthic species is in the deep Fram Strait, where 45% of the plastic debris observed showed interactions with epibenthic megafauna, such as entanglement in up to 31% of the sponge colonies 12 . Although data on effects are lacking in this case, entangled fishing gear caused tissue abrasion and (partial) mortality in sponges from Florida, rendering the organisms more susceptible to pathogens, predation and overgrowth 159 . As with cold-water coral 160 , coverage of the sponge's feeding apparatus could impair water-exchange processes, prey capture and growth. Another frequent observation was the colonization of plastic debris by sessile biota such as sea anemones 12,103 , which affects diversity. In general, the presence of plastic debris in benthic sediments can alter community structure 161 . Plastic items covering sediments can also affect biogeochemical processes, which could alter bottom-dwelling communities, as shown in an intertidal zone in Ireland with anoxic conditions, reduced organic matter and lower densities of sediment-inhabiting invertebrates nine weeks after coverage with plastic bags 162 . Although sediments from the Fram Strait and Canada contain up to 13,000 and 16,000 small-sized micro plastics kg −1 sediment 58,81 and are, thus, amongst the most polluted in the world, the effects on deposit-feeding organisms such as sea cucumbers, nematodes or other worms are currently largely unknown.
Sea ice also harbours high concentrations of microplastics 29 , which likely affect this ecosystem. Experimental evidence suggests that the presence of microplastic reduces the colonization of already formed sea ice by ice algae, a process that is important to transfer sea ice species from multi-year to first-year ice 90 . If added during the process of ice formation, however, microplastic did not affect algal concentrations in sea ice.
Data on contamination are often collected before digging deeper into effects. Here, we suggest that it is time for a new research priority: understanding the effects of plastics in the Arctic across organismal and ecosystem scales. These efforts are especially important, as the Arctic is vulnerable to a combination of many stressors (for instance, fast warming and a sink for organic pollutants), and the addition of microplastics raises concern about multi-stressor effects to wildlife.
Plastic pollution and climate change Although they are often thought of separately, climate change and plastic pollution are directly and indirectly linked, and both are amongst the biggest ecological challenges faced today globally and in the Arctic ( fig. 6), not least they share the same fossil origin, oil and gas. Global heating is three times faster in the Arctic compared with the rest of the planet 23 , such that Arctic ecosystems are already under severe stress 158 . One of the most prominent effects of climate change is the melting of the cryosphere. Sea ice entrains microplastic during its formation 90,163 and releases it during melting 26,29,61 . Changes in ice properties and its distribution will, therefore, affect the levels and spatial distribution of microplastics in the environment. Increasing quantities of released plastic particles in the water column, along with extracellular polymeric substances from ice algae 90 , could promote the formation of heteroaggregates, affecting the nutrient availability and turbidity in habitats of cyanobacteria and phytoplankton communities 6 . A decline in their populations could reduce the sequestration of carbon from the atmosphere and, thereby, fuel climate change instead 6,164 . On a smaller scale, a positive correlation has been found between salinity and microplastic concentrations in sea ice brine 90,163 . The microplastic levels reported in Arctic sea ice could increase the albedo effect by 11% and alter both the permeability of sea ice and the absorption of solar radiation, with a feedback on sea ice melting 29,163 . However, it is also conceivable that high concentrations of particles darker than the cryosphere promote solar absorption and, thus, melting.
In the atmosphere, airborne microplastic and nanoplastic can also enhance ice nucleation and, thereby, cloud formation and climate change 165 if they contribute to atmospheric trapping of infrared radiation from the Earth surface, instead of enhancing the reflection of sunlight. This process is important for the hydrological cycle, as more than 50% of the Earth's precipitation is induced in the ice phase 165 . Through atmospheric fallout and glacial meltwater, microplastics could also penetrate and affect permafrost, and be released to rivers and the Arctic Ocean with accelerating permafrost thaw 166 . Airborne microplastics have also infiltrated snow on glaciers, potentially affecting their light absorbance, structural and general rheological properties, and could, thereby, promote the ongoing fast melting of glaciers, the greatest cause of rising sea levels 110 .
Growing inputs of freshwater to the Arctic Ocean lead to a decrease in the relative buoyancy of plastics debris 167 and a weakening thermohaline circulation 168 , which could eventually slow down the poleward transport of plastic pollution (fig. 6). Global warming also amplifies poleward winds 169 , which define convergence zones and surface currents and, thus, influence plastic transport, as convergence zones are accumulation areas for plastic debris 10 . Furthermore, higher wind speeds promote the vertical mixing of small plastics into deeper waters 170 . In addition, warming surface waters result in a higher frequency of storms 171 , which break up the sea ice and enhance melting 172 . Sea level rise and storm events bring about higher inputs of plastic debris from land to the ocean via water runoff 173 and wind transport. Over time, these processes could also lead to higher pollution levels in the Arctic Ocean 11, 15,24 . In addition to direct effects, there are many indirect links between plastic pollution and climate change. For example, climate change causes a decrease in the sea ice thickness and extent 174 . As a result, maritime traffic in the Arctic is on the rise 175 , leading to higher levels of plastic pollution, for example, from fishing vessels, merchant shipping or tourist activities 49 .
Plastic production also fuels climate change, as it accounts for 6% of the global oil consumption and could reach 20% by 2050 (ref. 176 ). Fossil-based plastics produced in 2015 emitted 1.8 gigatons of equivalent CO 2 over their life cycle 177 . Under the current trajectory, plastic-related CO 2 emissions could rise to 6.5 gigatons by 2050, which will accelerate climate change and could use up 10-13% of the remaining SR15 carbon budget of 570 gigatons to limit warming to a 66% chance of staying below 1.5 °C (ref. 178 ). Furthermore, greenhouse gases such as methane, ethylene, ethane and propylene are released during degradation of some common plastic polymers throughout their lifetime 179 . Polyethylene, the most produced plastic polymer 1 , releases the highest levels of methane and ethylene. Once initiated by solar radiation, such as in the surface ocean, this process continues in the dark 179 . The scale of greenhouse gas emissions from these processes are currently unknown.

Mitigation
Plastic pollution is a transboundary problem, especially in the Arctic, where it stems from both distant and local sources. The problem, thus, needs to be tackled both regionally and internationally. Plastic pollution is a function of increasing plastic production coupled with inadequate waste management. Therefore, an effective upstream reduction in the global production of plastic waste via binding targets set in international treaties similar to the Paris Agreement or Montreal Protocol 2,180 is warranted. In addition, a circular use of plastic and of sustainable and truly biodegradable alternatives are needed alongside improved municipal waste collection and management to help reduce leakage to the environment 2,180 . Manual clean-ups on shorelines, harbours and riverbanks can help to mitigate pollution if impact assessments show that benefits outweigh environmental cost 181 , such as disturbance and increased mortality of biota due to incidental by-catch caused by non-selective removal technologies or operational greenhouse gas emissions.
Emissions from sea-based sources lead directly to marine pollution because of the direct input pathways. As much of the plastic debris in the Arctic region stems from local and distant commercial fisheries, mitigation in this sector would reduce plastic pollution particularly efficiently. Gear-marking schemes can prevent fishing gear www.nature.com/natrevearthenviron loss and discarding 182 , along with incentives for adequate waste disposal 183 . Programmes for reporting and recovery of lost fishing gear are already in place in Norway and should be extended to other regions 184 , as should be schemes to recycle fishing gear, which are currently practiced in Iceland. In the long run, the use of fully biodegradable material for nets 185,186 , along with bans on particularly short-lived components, such as dolly ropes, that become abraded during a trawl's passage on the seafloor, could help to reduce leakage to the environment. Education awareness campaigns designed for fishers, for example, during mandatory sea survival courses, help to shift perception in the industry but must be accompanied by institutionalized and well-organized waste facilities at fish landings and harbours to foster behavioural change 184 . The disposal of plastic in the Arctic Ocean and adjacent areas could be reduced through improved port reception facilities following a regional reception facilities plan, as is currently underway under the International Maritime Organization in the Pacific region. Lower harbour fees for ships with better waste facilities on board, a 'No Special Fee' system similar to HELCOM 187 and on-port recycling hubs could help to alleviate illegal dumping of waste at sea. Given that ship traffic has already increased and will further increase in the Arctic due to vanishing sea ice, this sector deserves particular attention, including improved surveillance schemes. In many locations throughout the Arctic, open landfills are still in use 56 , and it is clear that investments in local waste management solutions will reduce the leakage of plastic pollution to the environment. Rural Arctic communities that desire efficient waste collection and management schemes need financial and logistical support, for instance, through extended producer responsibility schemes or governments to establish or improve waste management and treatment. Importantly, coupled with community-based monitoring programmes 57 , sources and effectiveness of policy changes can be detected at the local scale relatively quickly 16 . Waste management studies and investments must be a priority to stem the tide of plastics from sources within the Arctic.
Reducing emissions from diffuse sources is necessary but challenging. Improved material design could reduce emissions from automotive vehicle tyres and brakes, which is one of the most important sources of microplastics globally 112 , as well as from ship paint from (ice-breaking) vessels. Collection schemes of road runoff could mitigate some of the pollution as well. New regulation aimed at improvements of wastewater treatment on land, offshore and on ships could help reduce inputs of plastic microfibres.
Finally, communication and community action are needed. Global audiences must be taught about plastic pollution in the Arctic, as distant sources contribute to the plastics burden in the Arctic. It is important to include local voices in both research 46 and actions aimed at reducing plastic pollution. Listening to indigenous voices has been recognized as a critical part of communication strategies under the Arctic Council 188 . For many, plastic pollution is affecting their way of life. In northern Canada, the community focus on understanding plastic pollution in the Arctic is illustrated by the variety of community-based research programmes on litter and microplastics funded under the Northern Contaminants Program. For this reason, a course including plastic pollution as a contaminant in the Arctic has been taught at Nunavut Arctic College in Iqaluit, Canada, each year since 2009. The students learn, share stories and knowledge, and participate in local research on plastic pollution. As stated by Aggeuq Ashoona, a college student who participated in this course from Kinngait, Nunavut, "This is affecting Inuit very much […]. To find plastic in their [wildlife] stomach is heart-breaking, because these are our food".
Summary and future perspectives Regardless of its remoteness, plastic pollution has infiltrated the Arctic from the atmosphere to the deep ocean floor, with pollution levels sufficiently high for some regions to be considered accumulation areas 15,24,81 . Despite recent advances in research, there is still a lack of understanding of the importance of different transport processes within the Arctic and the role of local sources, rivers and the atmosphere. It is clear, however, that plastic pollution exacerbates the impacts of climate change. These effects seem particularly clear in the Arctic, where not only are climate change effects occurring faster than elsewhere 23 but where these changes likely strongly influence the sources and transport of plastic debris, perhaps more so than in other regions. Still, we have barely scratched the surface when it comes to impacts on Arctic life, including human communities in the Arctic, requiring further and urgent research.
Plastic pollution research is particularly challenging in the Arctic because of its remoteness, lack of infrastructure and harsh environmental conditions. Conventional scientific sampling is often restricted to summer months and requires the use of aircraft, research bases and/or ice-class ships. Even then, fieldwork can be jeopardized by low visibility, polar bears, ice and low temperatures defying technology. Arctic landscapes are often characterized by coarse sediments, permafrost, snow and/or ice, which lack coherent survey guidelines, and, overall, these environments are currently undersampled 189 . Another common approach to quantify plastic pollution, which is to count litter floating at the sea surface by ship-based observers, is often difficult or impossible due to fog or sea ice, which can also impede sampling by surface trawls. These examples highlight that we currently lack the basic methodology to determine pollution levels in certain areas of the Arctic and during significant periods of time. In some areas, these challenges can be overcome by the use of year-round moored sampling devices 190 , drones or collaborative research with citizen scientists 39,57,109,191 or local communities 192 . For example, many scientists work directly with local Inuit communities in the Canadian Arctic to design sampling schemes, sample and interpret results 46,133,193 . During the COVID-19 pandemic, many researchers in Canada could not access field sites in the Arctic, and, in some cases, local communities were compensated to undertake annual sampling. In Russia, a programme was developed to enable monitoring by local school children and students 57 . Such schemes complement professional science and should be expanded to fill knowledge gaps.
In addition to difficulties that arise while conducting fieldwork in the Arctic, there is currently a lack of standardized sampling and analytical methodologies or even harmonized procedures, especially in terms of microplastics. This lack of standardization is concerning, as different analytical approaches can cause several orders of magnitude differences in the results obtained 60,194 . Therefore, despite a surge in plastic research in the Arctic, the results are often not comparable between studies, hampering efforts to describe the sources, sinks and large-scale distribution patterns of Arctic plastic pollution. However, the research and monitoring recommendations recently set out by the Arctic Monitoring and Assessment Programme (AMAP) 17 could inform a more harmonized research approach, which would also benefit from a common database for the upload of recorded pollution data.
Nanoplastics in the Arctic have largely not been investigated, including their distribution amongst different ecosystem compartments and how they interact with microplastics as the sea ice forms and melts. It is conceivable, for example, that nanoplastic interacts with sea ice in a similar way as, for example, salt and is rejected from the ice matrix as sea ice forms. Data on nanoplastic are particularly important, as particles of this size fraction can pass biological membranes and, thus, translocate to organs, where they could elicit a strong biologic response 195 . Progress in the development of sampling and analytical methods have not only demonstrated the presence of nanoplastic in glacial ice from Greenland but will also help us to fill this knowledge gap 196 .
Currently, there are no plastic budget data on relative contributions of various sources of plastic to the Arctic, such as local versus long-distance sources. Current understanding suggests that, along with local emissions, inputs of Atlantic origin could be most important, but data from the Amerasian Arctic have only begun to emerge, so no firm conclusions can yet be drawn. Information on the sources of pollution is needed to assess pan-Arctic exchange -how much plastic debris leaks from North America to Europe and vice versa. As outlined in this Review, such assessments are currently hampered by the lack of harmonized data. Another major knowledge gap pertains to atmospheric transport, which allows microplastic and nanoplastic to infiltrate even the most remote ecosystems on our planet via precipitation. Although this pathway is important for other pollutants such as mercury 111 , its contribution to the Arctic's overall plastic burden is unknown. Integrating microplastic sampling into research cruises and ongoing air pollution observation programmes could improve our understanding of the role of airborne microplastics 197 .
The amount of plastic debris entering the Arctic Ocean through rivers is unclear, but could be important, owing to their enormous catchment areas that lie beyond the Arctic borders, some of which pass through big cities. Arctic rivers are a conduit of land-based plastic pollution into the ocean, and their massive discharge every spring or summer makes the impact potentially substantial. With over 37 million people living along these waterways 16 , understanding plastic pollution in rivers that drain into the Arctic Ocean is crucial. It also increases our knowledge of terrestrial sources, which can help mitigate its input in the long run. Especially as local people depend on freshwater and land for subsistence and culture, understanding the effects of plastic pollution in these systems is a priority. Given the interest in litter and microplastics in northern and indigenous communities, and the breadth of community-based research and monitoring projects across the Arctic, locally designed and implemented projects should be prioritized within research planning strategies 46,57 . This strategy will ensure that local and regional research needs are included, and local communities are engaged in result discussions throughout the process and can relay this information as directly into policy solutions as needed.
The propagation and impact of microplastic within the Arctic food web ( fig. 5), which is already under pressure from fast climate forcing, is another source of major uncertainty. Targeted work that examines plastic pollution throughout the food web is needed in order to understand where plastic pollution accumulates and the actual effects on biota. Although studies have focused hitherto on single species, future studies should take an ecosystem approach, with sampling of biota across trophic levels 153 , and in relation to environmental compartments where they feed 17 . This knowledge will help tease apart questions relating to bioaccumulation, biomagnification, excretion and, thus, cycling of both plastic pollution and contaminants that are both sorbed and derived from plastic pollution.
We are also only beginning to investigate the effects of microplastic and nanoplastic on important physical processes, such as soil functions, biogeochemistry, ice properties (melting, UV reflectance and attenuation), weather (condensation, precipitation) and particle flux through the water column (biological pump), all of which have repercussions for the functioning of our Earth system, especially in a changing Arctic. However, it is already clear that effective mitigation is urgently needed to prevent further deterioration of Arctic ecosystems and communities.