Humans appear to have done a thorough job of contaminating the Earth’s rivers, oceans and atmosphere, says Rachel Nuwer. Is there anywhere pristine left on the planet?

Somewhere between 1.8 million and 12,000 years ago, our ancestors mastered the craft of fire building. Anthropologists often cite this event as the spark that truly allowed us to become human, giving us the means to cook, keep warm and forge tools. But fire also marked another important first for us: the invention of man-made pollution.

Pollution, by definition, is something introduced into the environment that harmfully disrupts it. While nature sometimes produces its own damaging contaminants – wildfires send up billows of smoke and ash, volcanoes belch noxious gases – humans are responsibile for the lion’s share of the pollution plaguing the planet today.

Wherever we go, we seem to have a knack for leaving our rubbish and waste behind. Visit even the most remote outpost on the planet and you will witness this first hand. Shredded tyres and plastic bottles punctuate the vast expanse of the Gobi desert; plastic bags ride the currents in the middle of the Pacific; and spent oxygen canisters and raw sewage mar the snows of Mount Everest.

Still, the world is a big place. Might there be some last holdouts free from the taint of our pollution? Answering that question works best if we break down the environment into four realms – the sky, land, freshwater and ocean.

Sky and land

Air pollution comes in many forms. Smog is mostly composed of particulate matter and ozone – a greenhouse gas that forms when nitrogen oxides and volatile organic compounds produced by cars and industrial plants react together in the presence of sunlight. And its impact on human health and the environment can be severe. In India alone, ozone pollution causes crop losses equivalent to $1.2 billion per year. In terms of human health, outdoor air pollution costs an estimated one million lives per year, while air pollution produced in homes – usually a by-product of cooking fires – kills around two million people annually. 

When carbon monoxide, nitrogen dioxide and other primary pollutants (those that are injected directly into the atmosphere) find their way high into the atmosphere, they often get transformed through chemical reactions into what scientists refer to as secondary pollutants. Some of these pollutants can linger for months. Others, like methane, are less reactive and may circulate the globe for years until they are eventually broken down or find their way to the ground via snow or rain. As Helen ApSimon, a professor of air pollution studies at Imperial College London, points out, this means “you don’t necessarily get away from air pollution by being further from the sources”.

Pollution expelled into the air gets transported vast distances by winds and atmospheric currents. “One thing we see very often is that pollution starts off in one place but ends up somewhere very far afield,” says David Edwards, director at the National Center for Atmospheric Research Earth System Laboratory in Boulder, Colorado.

For instance, Indonesia has recently been clearing large tracts of forest with fire to create new palm oil plantations – and Singapore now contends with significant haze problems due to its neighbour’s slash-and-burn tendencies. Smoke pollution can travel even further than that, however: fires used for farming in South America and southern Africa are a major source of air pollution for the entire southern hemisphere. On occasions, says Edwards, “pollution emitted from one source region can find its way around the globe more than once.”

So based on what we know about atmospheric currents and pollution distribution, it’s safe to say that there are no places on the planet guaranteed to be fully free from air pollution. And therefore that goes for the land surface too.

That said, however, there are places where the air is cleaner. In general, the Southern Hemisphere’s air is better than the Northern Hemisphere’s, just by virtue of the fact that fewer people live there. While pollution does move around the world, there is less mixing between the hemispheres due to barrier-like wind patterns. The South Pole, therefore, probably contains the cleanest air on Earth given its remoteness.

But as ApSimon points out, there’s still a massive pollution-caused hole in the ozone layer hovering over Antarctica, and deposits of black carbon can be readily spotted on that continent’s snow. So even if the air there is likely the cleanest, it’s by no means pristine.

Deep caves, too, could contain relatively pollution-free air, so long as they didn’t have much circulation with the outside world. “I can imagine there could be deep caves where there’s been very little air exchange for a long time,” ApSimon says. “Mind you, you don’t know what else is in that deep cave – I’m thinking there could be lots of guano.” Bat poo, in other words.

Water

Air pollution, unfortunately, also affects water, and therefore cancels out hope that perfectly clean freshwater bodies exist. “If one looks at pollution broadly, then it’s unlikely that there is a pristine catchment anywhere that hasn’t been polluted, because anthropogenic influences like air pollution have really gone all over the world,” says Thomas Chiramba, chief of the freshwater ecosystem unit at the United Nations Environment Program, based in Nairobi, Kenya.

But while pollution from the air does settle in water, it’s actually pollution from land that acts as the primary contaminant for freshwater resources. Chemicals, fertilisers and waste seep into groundwater and wash into lakes, streams and rivers, often winding up in the ocean. The result is dead zones – swathes of fresh or saltwater devoid of life. Dead zones occur when nutrient loads from land cause massive microbial blooms, which in turn deplete the water of oxygen. These tubs of death are found all over the world, but the Gulf of Mexico’s Mississippi River Delta is perhaps the most infamous example.

Raw sewage and industrial waste are primary culprits wreaking havoc on freshwater. In many countries, “sanitation” refers only to removing waste from homes – not treating it before returning it to the environment. By some estimates, 80% of wastewater generated in developing countries is discharged directly into local waterways. That figure can be worse on a case-to-case basis: New Delhi dumps 99% of its wastewater into the Yamuna River, for example, while Mexico City pumps all of its liquid refuse into the Mezquital Valley. “That is the main source of pollution all over the world,” says Asit Biswas, founder of the Third World Centre for Water Management in Mexico, and a distinguished visiting professor at the Lee Kuan Yew School of Public Policy in Singapore. “As a result, the rivers become polluted, and people living downstream are forced to drink that water.”

According to Biswas’s research, none of South Asia’s 1.65 billion people have access to clean, safe tap water; more than half of China’s rivers and lakes are too polluted to drink; and 72% of samples collected from Pakistan’s water supply system were found to be unfit for human consumption. What’s bad for humans is also bad for the environment. According to a report recently issued by the WWF, animal populations living in freshwater have declined by 75% over the last 40 years, thanks largely to pollution.

As with the air, freshwater bodies furthest from humans are probably also the cleanest. Canada’s far northern lakes and rivers, along with the Arctic and Antarctic’s freshwater are likely candidates for least-polluted bodies of water. Glacial layers that formed prior to the Industrial Revolution as well as sub-glacier lakes trapped far below the surface could in fact be pristine. Antarctica’s Lake Vostok, for instance, is buried under ice that is 400,000 years old. But these water bodies are clean because humans cannot physically get to them – other than by using drills. When it comes to more accessible areas, remote corners of the Congo Basin and the Amazon rainforest could be close contenders for second place. “Where you have the smallest human populations, you’ll also find increasingly pristine freshwater resources,” Chiramba says.

Ocean

Even the oceans, which remain largely unexplored and occupy a whopping 70% of the Earth’s surface, has not escaped our pollution’s reaches. Today, an estimated 60-80% of marine pollution originates from land, reaching the water through harbours, dirty beaches and polluted waterways that drain into the sea. Of that pollution, plastic is the most pervasive. That’s because most plastic takes centuries – perhaps even longer – to completely disappear. Paper, on the other hand, disintegrates quickly, and glass isn’t nearly as common as it used to be.

Surprisingly, some of the remotest places in the ocean are also some of the most polluted, thanks to the patterns of the currents. Midway Atoll, a speck of land in the middle of the North Pacific, for example, is uninhabited save for scientists who visit for a few weeks at a time. But it’s covered in washed up debris, which often fatally finds its way into the digestive system of seabirds living there.

Likewise, the deep sea was once thought to be largely cut off from the human world, but the more we explore, the more we are coming to terms with the fact that that is not the case. “I’ve done a lot of work on the bottom of the ocean with submarines and ROVs [remote operated vehicles], and there’s human debris everywhere,” says Lisa Levin, a biological oceanographer at Scripps Institution of Oceanography in San Diego, California. “It brings home the fact that human beings are an integral part of marine ecosystems now.”

On the deep-sea floor, the most readily identifiable pollution tends to be cans and bottles, though discarded fishing gear, ropes, metal objects, military ammunition and even old shoes regularly turn up, too. The diversity of garbage represents the fact that, historically, “people used the ocean as a dumping ground”, Levin says. In addition to the things we can see, much more is likely buried under the sediment, she adds, while other forms of pollution cannot be spotted by the human eye, such as microplastic – former bottles and bags that have broken down into ever smaller particles. Those tiny plastic pieces fill the ocean and “are probably impossible to ever clean up”, says Jenni Brandon, a graduate student in biological oceanography at the Scripps Institution, who specialises in the Great Pacific Garbage Patch. “A lot of people think those particles can really be around forever.”

Plastic pollution is not the only man-made waste contaminating the ocean, however. Oil spills regularly occur all over the world, even if the majority of them escape the notice of Western media. Persistent chemicals such as polychlorinated biphenyls (PCBs) also leach into the water from land, and then travel up the marine food chain.

And not all marine pollution is physical. Noise pollution caused by things like ship engine noise and sonar is becoming an increasing problem that has been implicated in whale, dolphin and squid deaths. “There are some places that don’t have physical debris – or at least where we haven’t found physical debris,” Brandon says. “But it would be hard to find anywhere that hasn’t had any human impacts.”

Some human impacts on the marine realm can also be completely unexpected. In 2007, for example, several amphipod crustaceans scooped up from water 11km (6.8 miles) below the surface of the Pacific Ocean turned out to have cow DNA within their guts. “How do you get cow to the bottom of the Kermadec Trench?” Levin says. “I’m sure it was just a ship dumping its leftovers.”

While a burger for lunch may or may not harm those trench-dwelling creatures, it does demonstrate just how deeply our influence on the planet reaches. Whether our contaminants take the form of a discarded lunch, human excrement or billions of metric tonnes of airborne pollutants, we’re left with an unfortunate but clear answer: there probably is no place on Earth without pollution. In other words, as Biswas says, “We human beings have done a wonderful job of contaminating the environment around us.”

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Update: The forest-clearance behind Singapore's haze problem is predominantly in Indonesia, not Malaysia.