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The Arctic (above 60° N)1,2 is remote, frozen and largely undisturbed terrain that is uniquely suited for long-term biological and chemical storage. Over millennia, natural processes, accidents and intentional storage have contributed to the accumulation of the diverse compounds that are currently sequestered in northern high-latitude permafrost, ice and snow. With the Arctic warming at two to three times the mean global rate, up to 65% of the Arctic’s near-surface permafrost may be lost by 2100, releasing known and unknown hazards into the global environment3,4,5.

The majority of Arctic permafrost has been present since the Pleistocene (800 kyr  to 1 Myr ago) or the Last Glacial Maximum (~10 kyr ago; ka)6. Permafrost depth is a first-order approximation of age, where deeper layers were deposited further in the past. However, permafrost thaw changes this structure, opening new niches, closing old ones, resuscitating dormant microbial communities and enabling transport into the surrounding environment7,8. Gradual thaw releases the most recently sequestered components first, followed by increasingly older constituents as thaw penetrates deeper9,10. This top-down thaw is driven by temperature and precipitation changes and is relatively slow, typically increasing active layer depth at rates of centimetres per decade9. By contrast, abrupt thaw rapidly exposes old permafrost horizons, releasing compounds and microorganisms sequestered in deeper layers11,12. As Arctic landscape change increases with climate warming, tracking the emergence of entrained chemicals and microorganisms is a growing challenge.

Previous reviews summarized Arctic system research, focusing on important topics such as the permafrost carbon feedback and abrupt landscape change13,14. Similarly, the impacts of permafrost degradation on human infrastructure and ecosystem services are discussed in the literature15,16. However, the emergent threats from entrained viruses17,18, bacteria19,20, anthropogenic chemicals21,22, nuclear waste23,24 or other biogeochemical hazards reintroduced into the contemporary environment by thaw (Fig. 1) have received less attention. Here, we review the literature to catalogue emergent microbial, viral and chemical hazards within the new Arctic (Fig. 2), and recommend research priorities to quantify and address these risks25,26,27.

Fig. 1: Hazard transport through Arctic ecosystems.
figure 1

Conceptual diagram of material interaction and movement from the permafrost into adjacent environmental compartments. An idealized representation of first-order flow with minimal spatial variability. Landscape heterogeneity introduces complexity and flow velocity variability, which are not represented in this idealized model.

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Fig. 2: Idealized permafrost ecosystem with potential hazard storage locations.
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A representative Arctic permafrost system noting contaminants and microorganisms corresponding to specific soil horizons.

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Biogeochemical hazards from thawing permafrost

Recent degradation of the cryosphere is reintroducing ancient microorganisms that are highly adapted to Arctic conditions. Methuselah microorganisms—organisms with extremotolerant traits that have the ability to maintain viability for a million years or more—are bringing palaeoecosystem dynamics into the contemporary Arctic, with unknown consequences10,28,29. As thaw continues, increased activity from long-dormant microorganisms may already be contributing to changing Arctic ecosystem processes7,30,31,32 (Fig. 3). With atmospheric warming driving permafrost thaw from the top down (referred to as active-layer thickening or gradual thaw), microbial communities entrained within the permafrost simultaneously force change from the bottom up33,34,35 (Fig. 1). This multilevel reorganization presents an unprecedented alteration to the permafrost ecosystem, contributing to Arctic change36.

Fig. 3: Cross-section of model permafrost structure under different thaw regimes.
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An example of taxa isolated from various age categories in the permafrost for reference. Permafrost thaw regimes may alter the location and the state of taxa, these examples are taken from recovered samples discussed in the text.

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Microbial species from diverse phyla are re-entering an ecosystem that is already stressed by change and contamination37,38. Hazardous compounds transported through atmospheric deposition39,40, infrastructure leaks5, accidental release41,42 and the thawing cryosphere24,39,41,43,44,45,46,47 can cause human health problems and environmental damage when released. Amplified in the pristine conditions found across the Arctic, the convergence of climate change, re-emergence of Methuselah microorganisms and mobilization of anthropogenic chemicals may be converting the polar north into a radically different system. Our ability to forecast these emergent risks is limited by challenges in quantifying where, when, how fast and how deep permafrost will thaw.

Ancient microbial and viral diversity

Within the Arctic, permafrost is a reservoir of minimally characterized microbial life48. A single gram can contain hundreds to thousands of microbial taxa, but the composition and functionality of permafrost species is poorly sampled49. The diversity of microbial and viral species in even small areas of permafrost is considerable. For example, a recent study recovered 1,907 uncharacterized virus populations in 197 samples (10–11 cm cores) across a permafrost thaw gradient in Sweden’s Stordalen Mire18. Over 58% of these unknown microorganisms were still active, indicating a diverse virosphere18. Similarly, a team working in Svalbard found that, within the roots of two species of periglacial ectomycorrhizal plants (Bistorta vivipara and Salix polaris), 25% of the active microbial families were yet unclassified50.

Furthermore, carbon released by permafrost thaw can intensify microorganism proliferation that was previously constrained by temperatures and nutrient limitations, with unknown results31,34,49,51. This knowledge gap introduces substantial uncertainty in risk assessment and forecasting, meaning that the persistence of Methuselah microorganisms within modern ecosystems may emerge as an unfortunate surprise. Any re-emergence of Methuselah microorganisms into the modern Arctic has the potential to change the soil composition, vegetative growth and community composition—further accelerating the environmental transition from climate change7,52,53.

Recovered Methuselah viruses and microorganisms

The ability to survive in extreme environments for extended periods has kept many microorganisms in permafrost viable, regardless of when they were deposited. For example, studies in 500-kyr- to 1-Myr-old permafrost communities have described small populations of non-endospore-forming bacteria that survived through ongoing DNA repair and lessening cellular metabolic activity54. In cold methane seeps, ancient bacteria phylotypes related to Loktanella, Gillisia, Halomonas and Marinobacter spp. were successfully recovered from temperatures below −5 °C (refs. 55,56), while microorganisms from Clostridium and Psychrobacter were isolated in brine lenses from 100–120 ka (Table 1 and Fig. 3)57. In shallower permafrost, just over 30 ka, intact virus species, including mimiviruses, pandoraviruses, Pithovirus sibericum and Mollivirus sibericum, have been identified58,59. More recently, the Glaciecola, a genus of bacteria that is known for its swift consumption of phytoplankton organic material, are adapting to increasing temperatures in cold-water northern fjords by increasing carbon uptake and turnover49.

Table 1 Selected sampling studies identifying chemicals and microorganisms entrained in Arctic environments
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Research into the genomic diversity of permafrost microorganisms is an emergent field, with metagenomics making microbial catalogues and reconstructions of organisms possible8,49,58,60,61. In a recent example, the genomes of the Sclerotinia sclerotiorum (DNA virus) and the ancient Northwest Territories cripavirus (aNCV) were resynthesized from a reindeer faecal deposit entombed within 700-year-old ice62 (Fig. 2). The revival of nematodes from 30-kyr-old Siberian permafrost and the recovery of a bdelloid rotifer from 24-kyr-old permafrost demonstrate that complex, multicellular organisms have also developed effective survival strategies63.

Adaptations

Methuselah microorganisms and viruses have developed a myriad of adaptations to survive in permafrost for millennia despite static conditions in subzero temperatures with minimal water or nutrients64,65,66. Microbial adaptation strategies include the production of carotenoids67, increases in membrane fluidity64, the ability to scavenge detrital biomass30, high abundance of stress sensing and response genes66, hopanoid unsaturation after cold exposure64,67, dormancy66,68 and even surviving on trace gas constituents69.

Mechanisms to repair DNA are also important adaptations for microorganisms. In ancient permafrost, active DNA repair rather than dormancy can be an important adaptation for microorganisms. DNA damage accumulates over time in dormant cells, eventually becoming fatal, making active DNA repair important for long-term survival66. Endospore-forming cells may also persist as vegetative cells in ancient permafrost (for example, Clostridium from 33-kyr-old permafrost)70 and the upper time limits for the persistence of dormant cells are still being investigated.

In contrast to microorganisms, known viruses recovered from permafrost employ substantially different adaptations. As they are metabolically inert, persistence in subzero temperatures depends on infecting cold-adapted hosts56. This requirement limits the long-term viability of viral pathogens in permafrost, in which microbial communities are relatively sparse. Little is known about permafrost adaptations of viruses, but studies from other subzero and highly saline environments suggest a propensity towards lysogenic (the viral genome is integrated into the host chromosome) rather than lytic (active infection) cycling54,71,72. Thaw may cause virus activity to change to lytic, or active, infection, producing viral particles or modifying host physiology. Subsequently, the virus can encode genes to enhance host cold adaption or promote infection, assembly and release18,59,73. Considerable diversity in virus species and limited linkages between metagenomes and hosts present ongoing challenges in identifying virus adaptation strategies.

Although forecasts and interpretations of the impacts of emergent microorganisms and viruses are forthcoming, prediction uncertainty is large, as the composition and density of permafrost species are largely unknown. Some microorganisms are already transforming the landscape through carbon transformation and emission74, while others are adapting to warmer environments by moving to new environments49. However, it is becoming clear that the emergent microbial composition of a warmer Arctic may alter ecosystem dynamics and introduce new risks to the system.

Anthropogenic contamination

Heavy metals21,75, black carbon76 and other by-products of fossil fuel combustion have been introduced continuously into permafrost environments by long-range atmospheric transport since the beginning of the industrial age77,78,79,80 (Fig. 2 and Table 1). In the last 80 years, anthropogenic contamination has grown to include organochloride species—polychlorinated biphenyls (PCBs)81,82,83, dichloro-diphenyl-trichloroethane (DDT), hexachlorohexane (HCH)39,84—and nuclear waste41,85. Once released back into the environment, these chemicals can negatively impact ecosystem stability4,86, human and animal health87,88,89. For example, persistent organic pollutants (POPs) are known to cause cancer90, congenital disabilities91,92 and long-term health issues93 after human exposure, posing risks to Arctic populations. Moreover, heavy metals lead (Pb), mercury (Hg) and cadmium (Cd) are all known to cause organ damage, developmental issues, cancer and even death94,95,96. It is clear that the chemical footprint of global anthropogenic activities over the last century is unequivocally re-emerging in the Arctic, as studies across the globe have identified the increasing re-release of sequestered anthropogenic compounds in ice, snow, meltwater and sediment97,98. However, the real and future costs of Arctic chemical pollution remain unquantified.

Heavy metals

The Arctic contains natural metal deposits that have been mined for decades. Within mining areas, a variety of heavy metal refuse, including arsenic (As), Cd, nickel (Ni) and Hg, is dispersed through the environment, with concentrations highest in soils below 10 cm (refs. 27,75,79,99). In one example, open-pit mining above the permafrost in the Yamal Peninsula has resulted in dangerous levels of Cd (1–4 mg kg−1), Ni (1,000–1,500 mg kg−1) and Hg (40–120 mg kg−1) in the local soils79. With tens of millions of hectares dedicated to mining across the Arctic, heavy metal contamination from mine waste poses a notable exposure risk to communities and wildlife in regions spanning Scandinavia, Russia, Alaska, Canada and Greenland100.

Direct atmospheric deposition also increases heavy metals in permafrost. Studies of Arctic peat date the first atmospheric deposition of elemental Pb to over 3 ka101. Subsequent atmospheric Pb deposition persisted at relatively constant levels until around 1900 when it began to grow steadily, peaking in 1970 (refs. 101,102). Since then, the Pb concentrations in Arctic soils have decreased. It is speculated that this decline is due in part to the global phase-out of leaded gasoline, but it may also be a result of increased surface erosion and hydrological transport101. Any diffusion of stored Pb throughout the immediate environment may lead to redeposition and biotic uptake downstream, which can then be stored or transferred through the lipid-heavy plant and animal biomass, characteristic of Arctic biota, for decades97,102.

Permafrost Hg deposition and re-emergence also pose a growing risk. Permafrost regions store an estimated 600 Gg Hg (384–750 Gg)103 to 1,656 ± 962 Gg Hg in the top 3 m of soil, of which an estimated 800 ± 500 Gg Hg is frozen in permafrost104. Arctic Hg emission from thawing permafrost is estimated to increase 14–200% by 2300, driven by Arctic warming105. Already, up to 70% of mobilized Hg is absorbed by vegetation when deposited onto the soil surface104,106. Ensuing ecosystem uptake through bioaccumulation in fish (an increase of up to 222% by 2300), vegetation and sediment (82.5% of total uptake) increases with logarithmic thaw rates, exceeding the US Environmental Protection Agency water quality criteria by 2100 (ref. 105).

Migratory seabirds represent another notable biovector of heavy metals in remote Arctic environments and move recently exposed metals regionally107,108,109,110. Pond sediment in areas with frequent bird use shows high concentrations of Hg (25× the background), with colony proximity further increasing sediment contamination in the local area107. At Devon Island, Canada, ecosystems less than 1 km from colony breeding grounds had the highest contamination of As (9.20 μg g−1 maximum), Cd (8.40 μg g−1 maximum) and Zn (343.20 μg g−1)108. Bioaccumulation within specific species drives concentration, with the large glaucous gull species maintaining high levels of Hg (4.9 μg g−1), PCB (3,326 ng g−1) and DDT (2,367 μg g−1) in both tissue and shell samples110. In heterogeneous wetland and pond environments that are reliant on the influx of nutrients from seabird guano, this toxic input may create additional ecosystem stress and degradation over time108,111.

Legacy organic chemicals

Research from across high alpine and polar ecosystems globally has identified chemicals that were banned in the early 2000s that are still stored in permafrost and glacier ice39,112,113. Preferential chemical attachment to snow (snow scavenging) and polar magnification in cold regions concentrates POPs well above background levels98,114,115. In the Arctic, these physical dynamics combine to trap atmospherically transported organic and inorganic pollutants in ice, snow and permafrost116,117.

Cryospheric release of organic compounds is an exposure route of growing concern and is predicted to increase over the next 20 years, proportional to permafrost and ice loss118,119,120. The high concentrations of DDT (10 ng l−1)121, PCB (~4.5 ng l−1)122 and HCH (~20 ng l−1)121 recovered from glacial ice cores are also observed in below-glacier watersheds, confirming wider ecosystem transport98,123,124. After secondary emergence from ice and snow, chemical exposure expands through transport in the food web (Fig. 1), where the abundance of fatty tissue required by animals to survive frigid Arctic temperatures increases their contaminant load and subsequent biomagnification potential117,125.

Uptake of POPs by invertebrates34, seabirds107,110, salmon125,126,127 and mammals81,128,129 reflects the penetration of these chemicals into all trophic levels. Currently, the concentrations of PCBs and organochlorine pesticides are higher in Arctic vegetation than in the surrounding soils, suggesting that the lipophilicity of chemicals has already made trophic transport a primary dispersal mechanism75,130,131. As thaw elevates chemical release, uptake by organisms will mirror environmental output116,128. The consequences of this uptake and increasing contaminant load include decreasing populations as the multigenerational impacts of high contamination levels may limit the population size and offspring health, creating cascading system vulnerabilities43,117.

Despite the known consequences for ecosystem health, the impacts to Arctic inhabitants and ecosystems as POPs and other hazardous organic compounds move through adjacent environmental compartments (Fig. 1) are largely undetermined. Risk assessments of chemicals released by glacier melt show that the potential for human uptake increases with exposure and time39,119,125. However, few studies have traced POP transport and risk, and it is probable that the impact of these chemicals within Arctic systems is underestimated.

Nuclear materials

Radioactive materials have been sequestered in the Arctic since the beginning of nuclear testing in the 1950s132,133,134, with additional inputs from accidents, weapons tests and intentional dumping of nuclear waste that have since increased point-source concentrations23,24,135.

Between 1955 and 1990, the Soviet Union conducted 130 nuclear weapons tests in the atmosphere and near-surface ocean of the Novaya Zemlya Archipelago135,136. The tests used 224 separate explosive devices, releasing ~265 megatons of nuclear energy135. In the nearby Kara and Barents seas, over 100 decommissioned nuclear submarines were also scuttled136,137,138. Dispersed contamination from these radionuclides persists in multiple substrates across the region. For example, sediments in the bottom of the Chernaya Inlet of the Kara Sea contain 2,500–11,000 Bq kg−1 of plutonium, 3–4 orders of magnitude larger than the background level (0–3 Bq kg−1)139. The neighbouring Severny Island ice sheet also yielded high levels of radioactive caesium (137Cs; 450–650 Bq kg−1) in boreholes and snow pits140. These concentrations were further reflected in the vegetation (610 Bq kg−1) and soils (450 Bq kg−1) below the glacier140. The recent implementation of a strategic clean-up plan has led to the disposal of submarines and spent nuclear fuel, although sunken ships excluded from the plan account for ~8,860 TBq of radiation138. These remaining vessels may add to ongoing problems as radionucleotides in sediment mobilize into the greater environment141,142.

Camp Century (~240 km from Thule, Greenland) was a nuclear-powered Arctic research centre carved into the ice cap >9 m below the ice surface in 1959 by the US Army Corps of Engineers24. The research and laboratory facilities were powered by a portable nuclear reactor (Alco PM-2A) and produced considerable nuclear and diesel waste143. When the site was decommissioned in 1967, all wastes were left below the accumulating ice, which is now rapidly receding (~268 tons of ice loss per year)23,24,42. These combined wastes have an estimated bulk radioactivity of ~1.2 × 109 Bq, representing ~9.2 × 103 tons of physical waste, 2.0 × 105 l of diesel fuel and PCBs, and 2.4 × 107 l of sewage41,144. While there still remains a small window of opportunity for removal, ongoing meltwater infiltration could disperse materials widely145 (Fig. 1).

Near Camp Century, the 1968 Thule bomber crash scattered >4.6 × 1012 Bq of uranium and plutonium on the surface of the Greenland ice sheet41. Though it is not entirely clear how far the bomb debris dispersed, bioturbation, sediment leaching and hydrological transport are expected to increase the radioactivity’s range144,145.

Taken together, nuclear contamination in the Arctic may be considerable and mobile. Research from the 1990s into the transport and uptake of 137Cs suggested that harmful levels of radioactivity may be present until the year 2500 (ref. 85), but critical analysis has not incorporated recent changes from climate warming.

Interactions between emerging hazards in thawing permafrost

Microorganisms and chemicals released from thawing permafrost may interact, with the potential for partial risk mitigation through biodegradation146,147. Methuselah microorganisms may experience additional protection under contaminated conditions due to their extremophilic adaptations, as severe, limiting conditions are not new for these species. For example, in Arctic soils contaminated by oil, researchers discovered the new species Flavobacterium petrolei sp., which is both cold-adapted and able to biodegrade oil148. F. petrolei is notable for its ability to uptake almost 60% of the oil in its immediate environment, suggesting that it could have a potential remedial use in Arctic oil spills147. Moreover, recovered Acidobacteria and Sphingomonadales species degrade dioxins and furans, making them dominant in contaminated conditions149. Other new species isolated from soil, permafrost, streams and wetlands may stem from existing genera, including Hymenobacter. Hymenobacter psychrophilus sp. nov. is one of the more recently discovered members of the genus, the psychrophilic properties and lipid membrane of which increase its survival in both harsh and polluted Arctic soil54,150. It is unclear how the interaction between permafrost microorganisms and anthropogenic chemicals will progress, and the consequences of these new interactions across the Arctic remain unknown.

Risks and impacts arising through transport

Researchers have only begun to understand the impacts of biogeochemical hazards remobilized from thawing permafrost. What is clear is that established transport pathways could rapidly disperse hazardous compounds through atmospheric, hydrologic and terrestrial systems after emergence151,152,153. When materials move through adjacent environments, they are subject to biotic and abiotic transformation, creating secondary and tertiary impacts throughout the Arctic system52,153,154 (Fig. 1). Permafrost carbon may further serve as a nutrient source and vector, increasing the range of pollutants and microorganisms as thaw increases7,53. As the cryosphere collapses under climate change, higher temperatures alter dispersion pathways, heighten transport, accelerate chemical degradation and increase bioaccumulation of hazardous materials34,155,156. As such, international efforts to reduce the release of anthropogenic chemicals into the environment must account for secondary remobilization from degrading systems116,157,158.

Quantifying the risks to rapidly changing Arctic ecosystems from entrained permafrost biochemical compounds is an ongoing challenge9. The dominant Arctic transport mechanisms of atmospheric remobilization159,160, hydrological transport83,161,162, direct transport (through human or animal migration)110,163 and trophic-level flow all need to be assessed for dispersion potential86,88,111. The transport rates and efficiency differ tremendously between these mechanisms, releasing sequestered compounds into recently thawed ecosystems160,164,165.

Hydrologic and atmospheric transport

Permafrost thaw opens new hydrologic transport pathways, increasing connectivity and providing more efficient delivery for chemicals and microorganisms49 (Fig. 1). Freshwater lakes and ponds currently cover ~16% of the Arctic landscape166, but thaw and glacier melt further activates groundwater pathways, linking lakes, rivers and the coastal ocean167,168. Carbon9, mercury105, microorganisms169, legacy pollutants39,77 and any soluble materials can then flow between ecosystem compartments, increasing potential interaction. Microbial genes also move in meltwater between ecosystems. For example, a recent study of pelagic Chloroflexi (a diverse phylum of aerobic microorganisms) within the Arctic Ocean identified a lateral transfer of genes that were previously found only in terrestrial genomes169. These terrestrial genes markedly increase aquatic Chloroflexi’s efficiency in metabolizing terrestrial carbon, a new adaptation for the Arctic Ocean species169.

As the Arctic warms and permafrost thaws, atmospheric dynamics will continue to fluctuate, transferring volatile chemicals and particulated microorganisms into near-surface air155,170,171,172. It is unclear whether this flux will increase the number of ice nucleating particles from the soil, changing the density, range and precipitation from Arctic clouds172,173 (Fig. 1). Changes in albedo and circulation patterns will continue to drive fluctuations in Arctic atmospheric conditions, but lesser-understood drivers such as ice nucleating particles may have a growing role.

Human transport

The probability of direct anthropogenic transport increases with increased tourism, commerce and resource extraction in the warming Arctic. Reductions in sea ice enable cruise ships and commercial operations to bring more visitors to the north174. Over 1,000 settlements—including those focused on resource extraction, military and scientific endeavours—have been erected on permafrost in the last 70 years175. This growing non-resident population creates more opportunities for both intentional and accidental transport of Methuselah microorganisms and viruses throughout the Arctic and to lower latitudes176.

The scientific extraction and regeneration of ancient DNA, pathogens and microorganisms from permafrost further increases opportunities for accidental contact or release49,62,65. Although the immediate risk of release is low, a growing human population moving minerals, fossil fuels and biota out of the Arctic could increase the risks for both the local population and more heavily populated regions to the south.

Risks to human populations

As of 2017, approximately 3 million people in the high northern latitudes lived on permafrost, where degradation imperils local infrastructure and increases the risk of exposure to emergent constituents175. As the new Arctic evolves, increasing pollution, shipping, natural resource exploitation, tourism and population growth leave resident communities vulnerable to even small ecosystem disruptions125,177,178.

Antibiotic resistance

Deep permafrost (>3 m) is one of the few environments on Earth that has not been exposed to modern antibiotics, yet antibiotic-resistant bacteria are prevalent in these soil horizons179,180,181. In Siberia, over 100 diverse microorganisms, including Firmicutes, Arthrobacter and Bacteroidetes, were determined to be resistant to aminoglycoside, tetracycline and chloramphenicol antibiotics181 (Table 1). These antibiotic-resistant bacteria were identified in permafrost dated 15–290 ka, with species even more abundant in active layers in older, deeper permafrost181,182. Subsequently, concerns have been raised about the potential for the exchange of genetic material between antibiotic-resistant permafrost microorganisms and contemporary bacteria to create new antibiotic-resistant strains183. To date, bacteria resistant to chloramphenicol, streptomycin, kanamycin, gentamicin, tetracycline, spectinomycin and neomycin have been recovered from permafrost soils18,184.

Pathogen emergence

Changing Arctic ecology combined with permafrost thaw raises the prospect of disease emergence. Stable conditions in permafrost have preserved fragmented genomic material from smallpox and influenza viruses, pre-dating the first known human smallpox case by over 1 kyr (ref. 61). While these viruses may not be preserved in infectious forms, recovery and reconstruction of viral genomes is ongoing185,186. The release of intact viral pathogens from permafrost are limited by host community density and longevity, preventing long-term persistence for many virus species. However, any potential research reconstructing ancient viruses must be weighed against the risks of release or mutation185,187. As the pathogenic risks from all Methuselah microorganisms and viruses are not fully characterized, laboratory security is critical.

Although the direct infection of humans from permafrost pathogens is currently improbable, questions abounded after a 2016 outbreak of anthrax in Russia188,189. The hardy endospores of anthrax (Bacillus anthracis) endure entombment within permafrost for decades to centuries, raising concerns about re-emergence. While the 2016 outbreak was most likely a result of reindeer overpopulation and undervaccination, increasing vector transfer potential is a growing concern20,189,190. Similarly, a new pox virus termed Alaskapox has recently been identified in the Alaskan Arctic, with its origins, rate of spread and risk of illness still poorly quantified191.

Legacy pollution

Chemical and heavy metal wastes left a legacy of health problems for northern populations, and these risks are exacerbated by the impacts of permafrost degradation27,79,119,125,169. The accumulation of hazardous chemicals in the top layers of ice and permafrost can rapidly reach harmful levels if mobilized101. The bioaccumulation of POPs in salmon and large marine mammals poses a critical problem for subsistence communities across the Arctic125. Organochlorine concentrations in Arctic fish already show non-geographically constrained bioaccumulation trends, transporting DDT, HCH and PCB throughout the local environment192,193. One study indicated that the probability of biomagnification was nearly 100% for slowly metabolized compounds (such as PCB and DDT) in freshwater systems, substantially elevating the risk to human consumers101. Lipophilic chemicals are difficult to trace in humans, so the full consequences of POP re-emergence in the Arctic may not be understood for a generation194,195,196. This is also true for other waste stored in the Arctic. For example, the health risks from the uptake of nuclear materials are substantial and range from radiation sickness and burns to lung scarring and cancer197,198.

The worst effects of known toxins on human populations in the Arctic may be yet to come. The increasing load of chemical mixtures158 from permafrost can bioaccumulate across generations, weakening the immune system and increasing vulnerability to new pathogens37,38,88,111,119,125.

Future research priorities

For most of human history, the Arctic has been dominated by the year-round persistence of snow, ice and permafrost. Inaccessible for the majority of the Earth’s population, it was considered to be ideal by far-away decision-makers for the ‘permanent’ disposal of chemicals, biological hazards and even military assets of lower-latitude regions. Thus, the risks from emergent microorganisms and chemicals sequestered in permafrost are poorly understood and largely unquantified. Under climate change, a new Arctic devoid of a perpetual cryosphere has the potential to distribute previously sequestered compounds at large scales (Fig. 1). These shocks present an additional challenge to fragile Arctic stability, and establishing methodologies for locating and assessing emerging risks is critical. Synthesizing the current state of knowledge on the emergent biogeochemical risks from thawing permafrost has revealed three critical research priorities—improving permafrost models, quantifying the abundances and spatial distributions of permafrost pollutants and microbial communities, and developing rapid risk assessment tools tailored for the Arctic.

First, permafrost thaw mechanisms are poorly understood. Knowledge of abrupt thaw is evolving rapidly9,199,200, yet questions remain about where permafrost is most vulnerable201, how biophysical mechanisms drive thaw202,203 and how fast thaw (or abrupt thaw) will occur12,13. In particular, studies that quantify the rates of permafrost degradation from fire disturbance and thermokarst are urgently needed. Accurate models of the dynamics driving thaw are required to forecast the evolution of post-permafrost ecosystems. Without a clearer understanding of permafrost structure and underlying thaw processes, forecasting risk from emerging permafrost constituents will be nearly impossible.

Transport and permafrost thaw models should be used in tandem to characterize the transition from frozen to thawed permafrost and quantify potential risks from emergent biogeochemical hazards. This is particularly urgent for nuclear waste and heavy metals, which are very likely to have immediate health impacts shortly after release. Transport models spanning scales are essential to quantify the potential dispersal of permafrost components beyond the Arctic39,204.

Second, the heterogeneity of much of the Arctic has contributed to major uncertainties concerning the location, function and composition of permafrost microorganism and virus communities49,70. Owing to this vastness and the lack of access to deep permafrost, the diversity of microbial life has yet to be systematically quantified17,49,160. To forecast hazard emergence and routes of exposure, systematic studies of the biogeochemical constituents sequestered in permafrost are required.

In particular, it is unclear whether Methuselah microorganisms will be able to thrive in or colonize modern conditions and adapt to climate change. To answer these questions and understand their functions and potential threats to contemporary ecosystems, emergent microorganisms must be characterized using metagenomic sequencing and taxonomic analyses.

Similarly, the presence of high-risk anthropogenic materials across the Arctic has yet to be systematically catalogued, minimizing the opportunity for forecasting ecosystem change or potential health impacts as long-sequestered hazards emerge. As the Arctic warms, it is unclear whether chemical degradation rates will change or whether environmental transport will continue unabated. Additional questions about the emission rate and range of point source and non-point source pollutants must be answered.

Third, the Arctic is home to millions of people, many of whom engage in subsistence practices and live in settlements that have been populated continuously for millennia. Arctic peoples are immediately impacted by landscape changes, industrial resource extraction and, now, potential emergent health threats. The health and safety of the resident populace and connected populations at lower latitudes are first-order priorities. There is an urgent need for rapid risk assessment tools tailored to the Arctic. Methodologies to swiftly assess health risks from emergent pollution125 and to forecast interactions between emergent microorganisms and humans are critical. This may require a combination of field, laboratory and modelling tools, integrating knowledge from disciplines as diverse as microbiology, environmental toxicology and Earth systems modelling. It also entails proper valuation of Arctic lifestyles, cultures and natural resources, indexing the rapid changes that Arctic warming will bring to local regions and the entire Earth system. Collaboration between scientists in lower latitudes and the Arctic must integrate intergenerational knowledge and guardianship. The long-ranging and first-hand knowledge of Arctic residents is an underutilized resource that is invaluable to future research. Thus, future research endeavours must incorporate the input and knowledge of all regional groups affected by Arctic changes.

The Arctic has often been considered to be one of the last remaining pristine environments, far from the impacts of global industrialization and commerce. However, geographical asymmetries between anthropogenic greenhouse gas emissions and their projected warming impacts reveal steep inequalities, with warming especially amplified above 60° N. The resulting collapse of the Arctic cryosphere and the subsequent potential for exposure to emergent biogeochemical hazards highlights the fact that no element of the Earth system remains isolated. Climate-driven instability will continue to affect the Arctic Ocean, atmosphere and terrestrial systems. Clearly, the primary strategy for mitigating risk from biogeochemical hazards must be to take action to slow permafrost thaw.